The Effects of Neurotransmitters in Chondrocytes and Cartilage Biology

Abstract

Neurotransmitters are key factors for cell communication within the central and peripheral nervous systems. Recent evidence indicates that they also regulate the function of non-neuronal tissues, including cartilage, as chondrocytes express neurotransmitter receptors, transporters, and other signaling molecules, enabling them to respond to neurochemical cues affecting homeostasis and disease. This review synthesizes the current literature on the roles of major neurotransmitters in chondrocytes and cartilage biology, focusing on norepinephrine, glutamate, γ-aminobutyric acid (GABA), acetylcholine, and serotonin. Specifically, we conducted an extensive literature review within major databases such as PubMed and Scopus, which led to 2748 papers being screened based on various inclusion/exclusion criteria. A total of 42 relevant manuscripts were further selected and are presented to describe the role of neurotransmitters in chondrocytes/cartilage. The data demonstrate that neurotransmitter signaling regulates diverse aspects of chondrocyte biology, including proliferation, differentiation, endochondral ossification, mechanotransduction, extracellular matrix remodeling, inflammation, apoptosis, autophagy, and osteoarthritis (OA). Norepinephrine is the most extensively studied neurotransmitter and exhibits context-dependent effects on cartilage development, OA progression, and repair through adrenergic receptor signaling and neuroendocrine pathways. Glutamatergic signaling, particularly through N-methyl-D-aspartate (NMDA) receptors, is implicated in mechanotransduction, inflammatory activation, circadian regulation, and OA-associated phenotypic changes, whereas GABAergic signaling primarily regulates growth plate maturation and chondrogenesis. Finally, acetylcholine signaling appears to promote chondrocyte survival and anti-inflammatory responses, while serotonin is linked to altered calcium signaling, inflammatory mediator production, and cartilage catabolism. The biological effects of these neurotransmitters are highly dependent on receptor subtype, disease stage, inflammatory status, and mechanical environment, highlighting the complexity of neuro-cartilage communication. Although significant knowledge gaps remain regarding receptor-specific mechanisms and translational significance, current data support neurotransmitter signaling as an important regulator of cartilage and a promising avenue for the development of novel therapeutic strategies for OA.

1. Introduction

Articular cartilage is a vital component in providing a low-friction surface between articulating bone surfaces of diarthrodial joints and facilitating load transmission with a low frictional coefficient [1]. Articular cartilage is a specialized type of connective tissue composed of hyaline cartilage and is relatively thin (2–4 mm). Unlike most tissues, it is devoid of blood vessels, nerves, or lymphatics, but it is composed of a dense extracellular matrix (ECM) with a sparse distribution of chondrocytes [1]. The cartilaginous ECM is primarily composed of water, glycosaminoglycans (GAGs), and proteoglycans, and exists as a gel to provide frictional resistance to flow and pressurization of water, and to provide the joint with the ability to withstand significant mechanical loads [1]. Chondrocytes are the resident cells of articular cartilage that are metabolically active during development, maintenance, and repair of cartilage ECM through their ability to synthesize matrix components such as proteoglycans and GAGs [1]. However, the metabolic activity of chondrocytes can be altered through changes in their chemical and mechanical environment, as proinflammatory cytokines such as interleukin-1 and tumor necrosis factor-α have catabolic and anabolic effects on the degradation and synthesis of ECM macromolecules [1,2]. Essentially, chondrocytes are responsible for cartilage homeostasis through maintenance of the ECM, but without active blood supply and low proliferative activity, they render articular cartilage with a very limited capacity to regenerate [3]. As the articular cartilage ages and becomes stiffer, and in some cases with defects, chondrocytes are unable to meet the demands of cartilage regeneration to provide a stable ECM to withstand the imposed mechanical loads and provide a low-friction articular surface, thus leading to pathological states such as osteoarthritis (OA) [1,2,3,4,5].

Traditionally, neurotransmitters were studied in the context of the nervous system (NS), with different types demonstrating critical functions within the central and peripheral nervous system [6] (CNS, PNS). Catecholamines are composed of tyrosine-derived neurotransmitters such as dopamine, norepinephrine, and epinephrine. Dopamine was shown to be involved in neural reward pathways, emotional learning, and voluntary motor functions [7]. Norepinephrine (NE) modulates attention, arousal and is a critical component of the sympathetic nervous system, whereas epinephrine acts primarily as a hormone but has established roles in certain neural pathways [7]. Glutamate and γ-aminobutyric acid (GABA) serve as the CNS’s primary excitatory and inhibitory neurotransmitters, respectively [8]. Acetylcholine is a major neurotransmitter in both the CNS and PNS, involved at the neuromuscular junction and in muscle contraction, and regulates memory, attention, and learning in the brain [9]. Serotonin, a monoamine neurotransmitter, operates in both CNS and PNS to regulate mood, sleep, digestion, and cardiovascular functions [10]. Although these neurotransmitters were described extensively in the NS, growing evidence indicates that they are active in other tissues. For example, serotonin was shown to be a critical regulator of inflammation, as it signals to mast cells and macrophages to modulate the inflammatory response in either direction [11]. Acetylcholine has been shown to be anti-inflammatory by inhibiting macrophages from releasing inflammatory cytokines, and NE creates an immunosuppressive environment by upregulating regulatory T cells [12]. In addition, neurotransmitters also have a role in skeletal and connective tissues [13,14,15]. The evidence is growing and indicates that neurotransmitters operate in non-neural tissues and may act as local signaling molecules or neuroimmune mediators involved in cellular survival, inflammation, differentiation, and tissue remodeling.

In cartilage, the literature has identified neurotransmitter receptors, transporters, and signaling pathways associated within chondrocytes [16,17,18,19,20,21,22,23]. Further investigations into the neurotransmitter-associated machinery and pathways within chondrocytes have implicated them in chondrocyte differentiation, mechanotransduction, inflammation, ECM metabolism, apoptosis, autophagy, and OA progression [17,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56]. With growing evidence that neurotransmitter signaling has a role in chondrocyte biology, this review aims to present evidence related to whether neurotransmitter signaling in chondrocytes primarily contributes to cartilage homeostasis, degeneration, inflammation, repair, or whether these effects are dependent on the receptor subtype expressed or the pathological state of cartilage. As such, this systematic review aims to synthesize the available literature on neurotransmitter roles in chondrocytes and cartilage with a focus on catecholamines, glutamate, GABA, acetylcholine, and serotonin. Specifically, the review evaluates the reported data on the effects of neurotransmitters in chondrocyte differentiation, maturation, mechanotransduction, inflammation, ECM metabolism, survival, and cartilage pathology.

2. Materials and Methods

2.1. Search Strategy

The literature search was conducted from 6 April 2026 to 6 August 2026 using specific terms and database search methods across all databases. Specific databases (i.e., PubMed, Scopus, Cochrane, CINAHL, and Epistemonikos) required explicit terms to ensure a wide yet relevant search (Figure 1, Table S1).

Terms such as ‘tiab’ were used in the PubMed search as search tags, indicating that the database should look for terms in the title and abstract, similar to the search in Scopus, where the search term “TITLE-ABS-KEY” served the same function. The main search terms consisted of the neurotransmitter (i.e., serotonin, dopamine, norepinephrine, epinephrine, acetylcholine, glutamate, and GABA), followed by a Boolean operator such as “AND” or “OR” (See Table S1 for exact search terms used). Neuropeptides were included in the search to ensure that any relevant papers investigating neurotransmitters were also included (See Table S1). Further, to ensure all aspects of neurotransmitters, including their receptors, were included, we used search terms for specific receptor types, such as “adrenergic receptors” (See Table S1). Lastly, chondrocyte and cartilage search terms were expanded to include other relevant investigations, such as “growth plate” and “articular cartilage,” as well as chondrocyte markers [57].

Figure 1. PRISMA Flow Diagram. Number of articles identified, screened, and included in this review. PRISMA flow diagram generated using an R package and Shiny app for producing PRISMA 2020-compliant flow diagrams [58].

Figure 1. PRISMA Flow Diagram. Number of articles identified, screened, and included in this review. PRISMA flow diagram generated using an R package and Shiny app for producing PRISMA 2020-compliant flow diagrams [58].

2.2. Study Selection

Following the completion of the initial search (which resulted in 5342 manuscripts) and removal of duplicates (2594), we used a two-step screening process. The first step consisted of title and abstract screening and focused primarily on reading only the title and abstract to determine if the study met the eligibility criteria. Following the title and abstract screen, a full-text screening was conducted to confirm eligibility criteria had been met. Rayyan.ai, a mobile and desktop software developed to increase efficiency and accuracy at title and abstract screening, was used to conduct the first stage of the screening process [59].

2.3. Data Extraction

Once the screening process was complete and all studies used in this investigation were compiled, specific data parameters were obtained and recorded (Figure 1). The species and cell source, neurotransmitter and its applicable receptor investigated, study methods and results, and the overall direction of effect were noted and recorded. The online dataset of unpublished raw data was recorded and stored in Google Sheets, where the data was subsequently used to determine the results of our investigation. Also, inclusion and exclusion criteria were used as shown in Table 1. Lastly, no new data was created or analyzed.

Table 1. Inclusion and Exclusion Criteria for articles.

Criteria	Decision
Cells/Tissue: Chondrocytes, cartilage (human, animal, or cell line)	Inclusion
Exposure/Intervention: Any neurotransmitter or neurotransmitter receptor	Inclusion
Outcome: Any biological effect on chondrocytes or cartilage (proliferation, differentiation, inflammation, matrix metabolism, apoptosis, etc.)	Inclusion
Study type: Original research (in vitro, in vivo, ex vivo)	Inclusion
Language: English (or other languages if translatable)	Inclusion
Publication type: Peer-reviewed journal articles	Inclusion
Other tissues: Non-cartilage cells/tissues only	Exclusion
Exposure/Intervention: No neurotransmitter involvement	Exclusion
Outcome: No chondrocyte/cartilage outcome reported	Exclusion
Study type: Reviews, editorials, commentaries, conference abstracts without data	Exclusion
Publication type: Retracted articles	Exclusion
Language: Non-translatable languages	Exclusion

2.4. Risk of Bias

Following PRISMA 2020 guidelines, we assessed the risk of bias in the included manuscripts to evaluate the validity of the reported results. We systematically evaluated each study’s design, conduct, and analysis to assess the validity of the results. Despite the outcome of the assessment, we excluded no studies based on risk of bias; instead, we used the risk of bias assessment to inform the interpretation and synthesis of the results presented.

All studies that met our inclusion criteria were original preclinical research, and we appraised those using in vivo models with SYRCLE’s risk-of-bias tool for animal models to assess the methodological quality and internal validity of their experimental methods [60]. We selected SYRCLE because it is the only known risk-of-bias tool developed and validated for animal intervention studies. The tool evaluates ten domains spanning six bias types: sequence generation and baseline characteristics and allocation concealment (selection bias); random housing and blinding of caregivers/investigators (performance bias); random outcome assessment and blinding of outcome assessors (detection bias); incomplete outcome data (attrition bias); selective outcome reporting (reporting bias); and other sources of bias. Each entry was judged and rated as “low risk” (criterion met), “high risk” (criterion not met), or “unclear risk” (insufficient detail reported to permit a judgment) [60]. One benefit of the SYRCLE tool is its flexibility, which we used when judging studies that employed a fixed genotype as their “intervention,” which cannot be randomly allocated to animals. As such, we judged the selection and performance bias items based on the randomizable elements of the experimental design. Two reviewers independently assessed each study across all ten domains, with disagreements resolved by discussion. In keeping with the recommendation of the tool’s developers, no composite scores were given to each study; rather, the results are presented as a traffic light plot showing the distribution of low, high, and unclear judgments for each domain, generated using the robvis tool [61]. A high proportion of “unclear” judgments, particularly with sequence generation, allocation concealment, random housing, and blinding, were anticipated and are interpreted as reflecting incomplete methodological reporting rather than poor study conduct (Supplemental Figure S1).

In vitro and cell-line studies were assessed with a modified version of the Office of Health Assessment and Translation (OHAT) RoB tool. We selected the OHAT tool for the in vitro and cell-line studies because it was designed to evaluate the credibility of experimental and observational evidence-based judgments [62] and because OHAT has been recommended and previously adapted for reviews of in vitro studies [63,64]. OHAT items that apply to whole-animal or human control designs were either removed or re-specified for a cell-based context, and additional signaling questions relevant to in vitro validity were added. Two reviewers independently assessed the nine domains of our modified OHAT too, with disagreements resolved through discussion. Our modified OHAT evaluated internal validity across nine key domains: randomization and treatment allocation; concurrent and identical group handling; adequacy of controls; characterization of the exposure/intervention (identity, purity, concentration, and consistency); outcome assessment validity and blinding; data completeness; selective outcome reporting; appropriate statistical unit definitions (biological versus technical replicates); and overall threats to study validity. Consistent with criteria identified as frequently under-reported in in vitro reviews [63], we included signaling items pertaining to the authentication of the cell line or primary cell source and documentation of independent biological replicates. Each signaling item was rated on the standard four-level OHAT scale: ‘definitely low’ risk of bias (RoB), ‘probably low’ RoB, ‘probably high’ RoB, or ‘definitely high’ RoB [65]. Items that were not applicable to a study (for example, assessor blinding for an objective, instrument-read outcome) were recorded as ‘not applicable’ and excluded from the roll-up for that domain rather than penalized. Items for which a study provided insufficient information were rated as high risk. Overall study-level RoB was determined using a tiered approach adapted from OHAT guidance, where key domains carried greater weight [62,66]. Studies were classified into three tiers based on these ratings: low risk (Tier 1), intermediate risk (Tier 2), or high risk (Tier 3). Following OHAT principles, we did not exclude studies with high RoB; instead, we weighted the confidence ascribed to findings during evidence synthesis (Supplemental Figure S2).

3. Results

3.1. Catecholamines

NE is the primary neurotransmitter of the sympathetic nervous system, classically regulating cardiovascular and systemic physiological functions, including heart rate, myocardial contractility, and blood pressure [67]. The studies identified in this review indicate that NE signaling also extends to cartilage and chondrocyte biology. Specifically, NE was primarily shown to act through adrenergic receptors [17,31,32,34,35,37,39,42,45,47,48,51,53,55] and was implicated in chondrocyte differentiation [17,45,51], cartilage remodeling [32,37,48], inflammatory responses [42,51], and cellular metabolism [33,42]. Collectively, these findings suggest that adrenergic signaling represents an important neurochemical pathway through which sympathetic activity may influence cartilage development, maintenance, degeneration, and repair in a context-dependent manner.

Several studies demonstrated that NE regulates chondrocyte differentiation and skeletal development through β2-adrenergic receptor signaling [17,45,51]. In mouse models, β2-adrenergic receptor activation promoted embryonic growth plate chondrocyte proliferation while inhibiting differentiation [17]. β2-adrenergic receptor signaling also showed the ability to regulate the transition from proliferative to hypertrophic chondrocytes during long bone growth and fracture healing [39]. Mechanistically, NE-mediated β2-adrenergic receptor activation was linked to ERK1/2 and PKA signaling [45] (Figure 2). PKA activation was reported to stimulate the AP-1 transcription factor Jun-B, leading to reduced expression of chondrogenic markers, including Sox6 and type II collagen [45] (Figure 2). Related findings with epinephrine further suggest that β2-adrenergic receptor signaling interferes with chondrogenic differentiation through suppression of Sox6, which ultimately results in the downregulation of the Sox9 family of proteins, the master regulators of cartilage differentiation [53].

Figure 2. NE-stimulated β2-adrenergic receptor chondrocyte signaling (adapted from Mitchell et al. [45]). ((Top Left) panel): (A–D), β2-AR inhibition of Col IIa1 is mediated by protein kinase A and ERK1/2 MAPK in primary chondrocytes. ((Top right) panel): (A–D), β₂-AR stimulation of activator protein 1 (AP-1) proteins and activity in primary chondrocytes. ((Bottom left) panel): (A–D), β₂-AR inhibition of Sox-6 is mediated by Jun-B in primary chondrocytes. ((Bottom right) schematic): The β2-adrenergic receptor signaling pathway in chondrocytes was created in BioRender.

NE also appears to have effects on cartilage remodeling and OA progression. Loss of peripheral NE release in osteoarthritic C57BL/6J mice was associated with exacerbated disease progression and increased cartilage calcification; however, there was no effect on cartilage surface degradation or synovial inflammation, suggesting that adrenergic signaling may contribute to cartilage homeostasis in a tissue-dependent manner [47]. Subsequent work in C57BL/6J mice further implicated loss of NE-mediated β2-adrenergic receptor signaling in cartilage, synergistically with increased leptin levels through promotion of osteoblast activity and suppression of osteoclast activity [48]. This was suggested as a key mechanism underlying cartilage calcification and subchondral bone changes during OA [48]. In contrast, sympathetic innervation of aging mouse joints was reported to promote cartilage degeneration through β1-adrenergic receptor signaling, which induced chondrocyte-derived exosomal miR-125 release and promoted osteoblast differentiation in subchondral bone [32]. These findings suggest that the effects of NE on cartilage are highly dependent on receptor subtype, tissue compartment, disease stage, and the local inflammatory or mechanical microenvironment.

In addition to its role in disease-related cartilage degeneration, NE signaling may participate in cartilage repair and regeneration through central-peripheral neural circuits [34,55]. Studies identified a hypothalamic paraventricular nucleus–mediated circuit that promotes mature articular cartilage formation, in part through inhibition of β2-adrenergic receptor signaling [34]. Additional experiments with the hypothalamic neural circuit suggested that mechanical stimulation of articular cartilage reduces hypothalamic-driven NE release, thereby supporting renewal of superficial zone chondrocytes [55]. The study further expanded on how NE promotes superficial zone (SFZ) chondrocyte anabolism by demonstrating that NE acts on the Adrb2 receptor on the chondrocytes to drive internalization of an Adrb2-TβRIII (TGF-β type II receptor) complex to decrease TGF-β signaling, which is responsible for lubricin and ECM protein synthesis [55]. This mechanism was further explored in the study by showing that physical activity creates joint friction that degrades the superficial membrane of articular cartilage, which is regenerated by SFZ chondrocytes under the influence of the hypothalamic neural circuit [55]. Specifically, during mechanical loading of the joint, there is an increase in PGE2 from osteoblasts activating the prostaglandin E2 receptor, EP4, on sensory nerves and sending an interoceptive signal from the skeleton to the paraventricular nucleus of the hypothalamus. Ultimately, this signal serves as negative feedback, reducing hypothalamic sympathetic output. This decrease in sympathetic NE release near articular cartilage reduces β2 signaling in SFZ chondrocytes, thus allowing for the restoration of anabolic cartilage ECM synthesis via TGF-β and lubricin signaling [55]. Interestingly, the same study performed an inducible Adrb2 deletion in SFZ chondrocytes, which damaged the superficial membrane. The authors suggested that since Adrb2 is exclusively expressed in SFZ, it serves as a defining feature of the cell population. Therefore, complete abolition of the receptor within the population causes the loss of maintenance and survival input necessary for SFZ chondrocytes to retain their identity [55]. The discrepancy between decreasing NE signaling and abolition of the Adrb2 receptor suggests that lowering NE helps establish optimal balance for SFZ anabolism, whereas deleting Adrb2 removes a constitutive maintenance signal that eliminates SFZs and the superficial membrane they produce [55]. Collectively, these findings indicate that NE signaling is not uniformly catabolic or anabolic but instead may regulate cartilage remodeling through both local receptor-mediated mechanisms and broader neuroendocrine feedback pathways.

NE has also been implicated in chondrocyte inflammatory responses, and its effects appear to be dose- and receptor-dependent. For example, at lower concentrations in human articular cartilage, NE was reported to promote chondrocyte apoptosis through α-adrenergic receptor signaling, whereas at higher concentrations it promoted chondrocyte proliferation through β-adrenergic receptor signaling by modulating cell-cycle progression [42]. Also, in monolayer cultures of human osteoarthritic articular chondrocytes under inflammatory conditions, NE did not demonstrate synergy with IL-1β to promote chondrocyte dedifferentiation toward a fibroblast-like phenotype, with increased pro-catabolic mediator release and downregulation of chondrogenic markers [51]. The dedifferentiation effects driven by IL-1β were independent of NE signaling. Co-treatment with NE did not further potentiate the IL-1β-induced downregulation of fibroblastic/chondrogenic markers (SOX9, type II collagen, COMP, aggrecan) or the upregulation of matrix metalloproteinases (MMP13, ADAMTS4) [51]. These findings suggest that the role of NE in inflammatory and catabolic changes in chondrocytes toward dedifferentiation, particularly in the presence of cytokine-mediated low-grade inflammation, remains poorly understood. Adrenergic receptor subtypes appear to be central to the direction of NE-mediated chondrocyte responses. Although many effects of NE in cartilage were attributed to β2-adrenergic receptor signaling, α-adrenergic receptors have also been linked to catabolic pathways [33,37]. Activation of α2-adrenergic receptors was associated with increased catabolic factor release, reduced anabolic factor production, and enhanced mitochondrogenesis in rat chondrocytes [33]. In a rat temporomandibular joint model, α2-adrenergic receptor activation promoted degenerative remodeling through chondrocyte catabolism [37]. Additional work in human articular cartilage and chondrocytes showed that IL-1β altered adrenergic receptor activity and contributed to catabolic signaling, including changes in glycosaminoglycan release [35]. Together, these findings support a model in which NE-mediated cartilage effects are shaped by inflammatory status, receptor subtype expression, and downstream metabolic signaling.

The literature also suggests a relationship between sympathetic stress signaling and impaired cartilage and skeletal growth. Seven-week-old C57BL/6J mice were placed in a chronic psychosocial stress environment through chronic subordinate colony housing, exhibiting reduced tibia and femur lengths as determined by microCT and histological analyses. The reduced limb lengths were due to altered endochondral ossification, as the growth plate thickness had increased along with decreased growth plate mineral deposition. Interestingly, bone marrow cells located at the growth plate had increased tyrosine hydroxylase expression, indicating that local catecholamine synthesis or sympathetic activity is altered in growth plate cartilage, thus interfering with physiological endochondral ossification [31]. Lastly, this was accompanied by a decreased hypertrophic chondrocyte population and reduced Runx2 expression, further providing evidence that chronic social stress alters local adrenergic signaling at the growth plate, impairing overall limb growth [31]. These findings suggest that psychosocial stress-associated adrenergic signaling may impair endochondral ossification by altering growth plate chondrocyte maturation.

Although NE was the primary catecholamine identified in this section, studies related to catecholaminergic mediators are also present in the literature. Dopamine was reported to reduce cartilage degradation and promote cartilage stability by inhibiting cartilage matrix-degrading factors while increasing ECM content in eight-week-old C57BL/6J mice [44]. Epinephrine was implicated in ATDC5 chondrocyte dedifferentiation through β2-adrenergic receptor signaling [53] and was reported to be chondrotoxic at high concentrations under low-pH conditions in human chondrocytes [30]. These findings suggest that catecholaminergic regulation of cartilage extends beyond NE alone, but the distinct roles of NE, epinephrine, and dopamine should be interpreted separately. Lastly, all of the aforementioned studies are summarized in Table 2.

Table 2. Data from investigations into the effects of catecholamine neurotransmitters or their respective receptors in cartilage cell lines or animal models. Note: GPCs = growth plate chondrocytes, AR = adrenergic receptor, SFZ = superficial zone, AAV = adeno-associated virus, OA = osteoarthritis, Syx = sympathectomy, Col II = collagen type II, GAG = glycosaminoglycans, DA = dopamine, OARSI = Osteoarthritis Research Society International.

Reference	Neurotransmitter	Cells/Tissue/ Animal Model	Study Design	Summary Findings
Lai and Mitchell, 2008 [17]	Norepinephrine	Embryonic Mouse GPCs	Embryonic E18.5 mouse GPCs were used to determine the role of β-AR signaling.	The sympathetic nervous system is an important regulator of cartilage development, as β2-AR signaling stimulates GPC growth but inhibits their differentiation, as seen with β2-AR agonism leading to decreased expression of chondrocyte differentiation markers (Ihh, COLX) and increased chondrocyte growth.
Wang et al., 2025 [55]	Norepinephrine	Mouse SFZ Chondrocytes	Mouse study using pharmacologic interventions, conditional knockout models, AAV-mediated gene knockdown, treadmill running, and primary superficial zone chondrocyte assays to examine neuroimmune and prostaglandin-mediated regulation of articular cartilage, with outcomes assessed by histology, immunostaining, ELISA, RT-qPCR, Western blotting, immunoprecipitation, in vitro kinase assays, and single-cell RNA sequencing.	Physical activity promotes the renewal of articular cartilage’s superficial membrane through downregulation of hypothalamic norepinephrine, as Adrb2-deficient SFZ chondrocytes damage the superficial membrane, which becomes exacerbated with physical activity.
Lorenz et al., 2016 [42]	Norepinephrine	Human OA Articular Chondrocytes	OA chondrocytes were investigated for the expression patterns of TH and ARs, along with 3D-cell cultures used to determine the effect of NE on chondrocyte metabolism and signaling factors.	NE, in a dose-dependent manner, affects the inflammatory response that subsequently has a role in OA chondrocytes by preserving the stable chondrocyte phenotype via β-AR signaling and promoting the OA chondrocyte phenotype via α-AR signaling. At lower doses, NE induced apoptosis via α1-AR, and at a higher concentration, chondrocyte cell cycles slowed down and proliferation increased via β-AR signaling.
Rösch et al., 2021 [48]	Norepinephrine	Murine OA model	OA was induced in either wild-type or Adrb2-deficient mice to determine the effects of β2-AR deficiency in OA.	β2-AR plays a major role in OA-related subchondral bone remodeling, as β2-AR-deficient mice had increased calcified cartilage and subchondral bone plate thickness.
Rösch et al., 2022 [47]	Norepinephrine	Murine OA model	C57BL/6 mice underwent peripheral sympathectomy and OA induction to investigate effects on cartilage.	Peripheral sympathectomy promoted OA-specific cartilage calcification but did not influence cartilage degradation, as cartilage degradation was comparable between Syx and control mice, and AR numbers decreased in cartilage but increased in synovium in both groups after OA induction.
Dragoo et al., 2010 [30]	Epinephrine	Human Articular Chondrocytes	Controlled laboratory study investigating chondrotoxicity of differing pH and epinephrine concentrations.	Epinephrine-mediated chondrotoxicity is typically a function of low pH and thus requires awareness when using epinephrine in clinical medicine.
Huo et al., 2026 [34]	Norepinephrine	Human and Mouse Models	Using multiple techniques to characterize the brain-cartilage neural circuit regulating cartilage regeneration in human and mouse models.	Characterization of a brain-cartilage circuit that regulates cartilage regeneration, as inhibition of paraventricular nucleus neurons led to the production of stable mature articular cartilage mediated by the inhibition of β2-AR in PRG4+ cells.
Jiao et al., 2016 [37]	Norepinephrine	Rat TMJ Chondrocytes	TMJ degeneration was induced in rats to investigate the role of chondrocyte-ARs in TMJ degenerative remodeling.	α2A-AR signaling within chondrocytes promotes degenerative remodelling by increasing chondrocyte pro-catabolic activities, as seen with α2A-AR inhibition leading to increased cartilage thickness and decreased pro-catabolic factors.
Mitchell et al., 2011 [45]	Norepinephrine	Primary Mouse Chondrocytes	Embryonic E18.5 mice were investigated for the effects of β2-AR inhibition on Col II expression in GPCs and the molecular pathways involved.	β2-AR mediates chondrocyte differentiation through ERK1/2 and PKA stimulation of the AP-1 factor Jun-B that inhibits the expression of Sox-6 and Col II.
Hwang et al., 2021 [35]	Norepinephrine	Human Articular Chondrocytes	IL-1β-stimulated human chondrocytes were investigated for the effect on β-AR activity with the addition of β-AR ligands.	β-ARs are regulators of IL-1β-stimulated cartilage catabolism, as inhibition of β-AR signaling led to catabolic signaling, while NE antagonized the catabolic response and inhibited the release of GAG from cartilage explants.
Speichert et al., 2019 [51]	Norepinephrine	Human Articular Chondrocytes	Investigation of NE contributions to human articular OA chondrocyte dedifferentiation in the presence or absence of IL-1β.	Low-grade inflammation exerts a significant effect on chondrocyte dedifferentiation, as NE alone did not change gene expression patterns nor influence IL-1β effects, but in combination with IL-1β caused a significant change in morphology, causing chondrocytes to resemble fibroblasts and decreasing chondrocyte markers while promoting pro-catabolic factors.
Takarada et al., 2009 [53]	Epinephrine	ATDC5 cells and Mouse Metatarsal Chondrocytes	Preclinical experimental study using murine ATDC5 cells, primary skeletal cell cultures, embryonic metatarsal organ cultures, and a mouse femoral fracture model to investigate adrenergic/cAMP-CREB signaling in chondrogenesis and skeletal repair, with outcomes assessed by gene and protein expression analyses, staining assays, proliferation assays, luciferase reporter assays, in situ hybridization, and fracture-callus analysis.	Epinephrine interferes with chondrogenic differentiation through β2-AR signaling, which downregulates Sox6 expression through the suppression of gene transactivation by Sox9
Foertsch et al., 2017 [31]	Norepinephrine	C57BL/6N mouse GPCs	PTSD model mice were used to investigate the effects of chronic psychosocial stress on bone and cartilage.	Chronic psychosocial stress reduces endochondral ossification in the growth plate and thus longitudinal bone growth, as there was reduced Runx2 expression in hypertrophic chondrocytes in the growth plate and increased tyrosine hydroxylase expression at the growth plate, indicating a role of local adrenergic signaling in skeletal phenotype.
Guan et al., 2025 [32]	Norepinephrine	Naturally Aging OA Mouse Chondrocytes	Investigation of sympathetic regulation on aging-related OA through multiple mouse models	Cartilage damage in aging mice was exacerbated through sympathetic innervation, facilitating the transfer of exosomal miR-125 from OA chondrocytes. β1-AR knockout attenuated OA progression in aging mice, and degraded cartilage explants produced more exosomes than the nondegraded explants, along with NE-mediated activation of β1-AR, facilitated the release of exosomes and miR-125 from chondrocytes, promoting osteoblast differentiation in subchondral bone.
Kuhn et al., 2026 [39]	Norepinephrine	Primary Mouse Chondrocytes	Cartilage and bone phenotypes were investigated in β2-AR-deficient mouse chondrocytes.	β2-AR plays a significant role during long bone growth, bone homeostasis in older animals, and fracture healing, as β2-AR-deficient mouse chondrocytes resulted in reduced long bone growth from a faster transition of proliferative to hypertrophic chondrocytes and delayed fracture healing due to a reduced cartilage area during healing.
He et al., 2024 [33]	Norepinephrine	Primary Rat Chondrocytes	Chondrocytes were treated with either NE or α2-AR antagonists after NE treatment to investigate chondrocyte metabolism and mitochondrial dynamic changes.	The upregulation of mitochondrial dynamics in chondrocytes is responsible for the activation of α2-AR-mediated chondrocyte catabolism, as NE stimulation increased pro-catabolic factors while decreasing anabolic factors, which was accompanied by upregulated mitochondriogenesis and the number of mitochondria, which was attenuated when an α2-AR antagonist was applied after NE treatment.
Lu et al., 2019 [44]	Dopamine	OA mouse model	OA mouse model underwent histological analysis, PCR, and Western blotting to determine cartilage degradation and factors involved.	Dopamine has a potential role in OA treatment as DA treatment suppressed cartilage degradation and improved OARSI scores through inhibition of pro-degradation factors and increased cartilage matrix content.

3.2. Glutamate/GABA

Glutamate is the primary excitatory neurotransmitter, whereas γ-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the CNS [8]. Both neurotransmitters signal through ionotropic and metabotropic receptors and are implicated in multiple neurological and psychiatric disorders [68]. The studies identified in this review suggest that glutamatergic and GABAergic signaling are not restricted to the nervous system but are also present in cartilage and chondrocytes. Several studies demonstrated that glutamate and GABA may have roles in chondrocyte differentiation and maturation, mechanotransduction, endochondral ossification, inflammatory response, and chondrogenesis [18,19,20,21,22,26,28,29,36,38,46,50,52,54].

Specific studies demonstrated that chondrocytes express the molecular machinery required for glutamatergic signaling. In cultured rat costal chondrocytes, AMPA receptor subunits and vesicular glutamate transporter-1 (VGLUT1) were expressed, and endogenous glutamate release was prolonged following AMPA receptor activation [54]. Using a similar rat chondrocyte model, Hinoi et al. demonstrated expression of the excitatory amino acid transporters GLAST, GLT-1, and EAAC1, with glutamate uptake showing sodium- and membrane–potential–dependent transport characteristics [19]. Further, in embryonic mouse cartilage models, excitatory amino acid transporters were expressed in a differentiation stage–specific pattern, suggesting a potential role for glutamate transport in chondrocyte maturation and endochondral ossification [19]. Glutamate was also implicated in OA pathology, as rat knees with experimentally induced OA showed significantly elevated glutamate levels in joint dialysates compared with controls [36]. In addition, chondrocytes were shown to express machinery for autocrine glutamate signaling, and N-methyl-D-aspartate receptor (NMDAR) activity was linked to inflammatory processes associated with cartilage degradation [46]. Collectively, these studies support the presence of functional glutamatergic signaling in chondrocytes and suggest potential roles in cartilage maturation, inflammation, and OA pathology.

Studies also focused on NMDAR-mediated glutamate signaling. Human articular chondrocytes respond to mechanical stimulation through NMDAR-dependent mechanisms [21]. In normal chondrocytes, NMDAR antagonism inhibited the hyperpolarization response to mechanical stimulation, whereas OA chondrocytes showed an altered depolarization response that was not affected by NMDAR antagonism [21]. These findings suggest that NMDAR-mediated mechanotransduction is disrupted in OA. Additional studies demonstrated that normal and OA human chondrocytes differ in NMDAR subunit expression [20]. In OA chondrocytes, NMDAR signaling required extracellular calcium and was associated with PSD-95 and nNOS activity. Downstream signaling activated tetrodotoxin-sensitive sodium channels and small-conductance calcium-activated potassium channels, a response not observed in normal chondrocytes [20].

NMDAR signaling was also linked to inflammation, apoptosis, and catabolic phenotypic changes. In human and rat chondrocytes, NMDAR signaling contributed to inflammatory pathways associated with cartilage damage, and IL-1β altered glutamate metabolism while inducing pro-inflammatory gene expression [46]. In SW1353 chondrocytic cells, inhibition of both NMDAR signaling and complement C5 reduced apoptosis and inflammatory mediator expression, suggesting interaction between glutamatergic and complement-mediated inflammatory pathways [43]. NMDAR signaling was also shown to regulate chondrocyte circadian clock genes and phenotype [38]. In OA chondrocytes, NMDAR inhibition restored BMAL1 and PER2 expression toward control levels and reduced pro-catabolic factor expression, whereas GluN2B expression produced the opposite effect [38]. Further work demonstrated that IL-1β-mediated regulation of PER2 and BMAL1 involved downstream NMDAR/CREB and NF-κB signaling pathways [26]. Together, these findings suggest that NMDAR activity may couple inflammatory signaling, circadian dysregulation, apoptosis, and catabolic cartilage remodeling.

NMDAR signaling also appears to contribute to chondrogenesis. In chicken limb bud–derived chondroprogenitor cells, NMDAR activation induced transient intracellular calcium release, which was required for chondroprogenitor differentiation into matrix-producing chondroblasts [22]. Inhibition or knockdown of NMDAR signaling reduced calcium signaling and impaired chondrogenesis, suggesting that NMDAR-mediated calcium transients may support early cartilage differentiation [22].

Although NMDARs were the most frequently studied glutamate receptors, AMPA and metabotropic glutamate receptor pathways were also implicated in chondrocytes [23,54]. Rat costal chondrocytes expressed the GluR3 AMPA receptor subunit and VGLUT1, indicating that chondrocytes possess machinery similar to that used for vesicular glutamate release in neurons [54]. These cells also release endogenous glutamate, and AMPA receptor activation prolonged this release, supporting an autocrine or paracrine glutamatergic signaling mechanism [54]. Glutamate also promoted chondrocyte (mice and rats) differentiation toward mineralization through pathways involving group III metabotropic glutamate receptor activation, retrograde cystine/glutamate antiporter activity, intracellular glutathione depletion, and apoptosis-mediated mineralization [23]. The authors also suggested that at physiologic glutamate concentrations, glutamate activates group III mGluR, inhibiting cAMP synthesis and preventing chondrocyte differentiation toward mineralization. However, at supraphysiological extracellular glutamate concentrations, the cystine/Glu antiporter reverses direction, contributing to the pathway that induces chondrocyte differentiation toward mineralization. Thus, Wang et al. demonstrate a role for glutamate in chondrocyte differentiation and maturation through changes in the cystine/Glu antiporter [23] (Figure 3).

Figure 3. Glutamate’s role in cartilage differentiation (adapted from Wang et al. [23]). ((Top left) panel (a–e)) shows the effect of Glu on cell viability in cultured chondrocytes. ((Top right) panel (a–c)) shows the effect of Glu on ROS production and reduced glutathione (GSH) levels in cultured chondrocytes. ((Bottom left) schematic) depicts the effects of glutamate on chondrocytes at physiologic concentrations based on results from Wang et al. [23]. ((Bottom right) schematic) represents the proposed mechanism by Alihali et al. [26] of how the NMDAR/CREB and NF-κB signaling pathways regulate the core circadian clock components PER2 and BMAL1 in chondrocytes (created in BioRender).

Glutamatergic signaling was repeatedly associated with inflammatory and OA-related chondrocyte responses. In human and rat chondrocytes, NMDAR activity interacted with IL-1β-mediated inflammatory signaling and altered glutamate metabolism in ways that promoted pro-inflammatory and cartilage-degrading pathways [46]. In SW1353 cells, inflammatory stimulation increased NMDAR subunits NR1 and NR2A and complement C5, while inhibition of these pathways reduced apoptosis and inflammatory mediators including TNF-α, IL-6, and NLRP343. In primary equine chondrocytes, inflammatory stimulation altered calcium signaling to glutamate and increased glutamate release [50]. Under inflammatory conditions, glutamate-induced calcium signaling shifted from single-peak transients toward oscillatory calcium responses, while glutamate transporters GLAST and GLT-1 were upregulated [50]. A subsequent equine chondrocyte study confirmed that inflammatory stimulation increased glutamate release in both healthy and osteoarthritic chondrocytes within 24 h, suggesting that glutamate release may represent an early response to acute inflammatory stimulation [49]. These findings are supported by a rat OA model in which glutamate levels were elevated in joint dialysates after OA induction [36].

Compared with glutamate, fewer studies examined GABAergic signaling in cartilage; however, the available evidence suggests roles in chondrocyte differentiation, growth plate physiology, calcium-sensing receptor signaling, and osmotic adaptation [18,28,29,52]. In rat growth plate chondrocytes and ATDC5 cells, GABAA and GABAB receptor subunits were expressed, and receptor-specific agonist and antagonist experiments suggested that GABA signaling may regulate chondrogenic differentiation and chondrocyte proliferation [18]. These findings support a potential autocrine or paracrine role for GABA in growth plate cartilage.

GABAB receptor signaling was particularly associated with chondrogenesis and endochondral ossification. Specifically, GABAB receptor subunit-1 (GABAB-R1) was expressed in chondrocytes, especially in prehypertrophic and hypertrophic cartilage regions and GABAB-R1 deficiency was associated with reduced body size, delayed skeletal calcification, and reduced Indian hedgehog (Ihh) expression [52]. Mechanistic experiments suggested that GABAB-R1 promotes chondrogenesis by enhancing ATF4 nuclear translocation and ATF4-dependent Ihh promoter activation [52]. GABAB-R1 was also shown to interact with the extracellular calcium-sensing receptor in mouse growth plate chondrocytes [28]. These receptors were coexpressed, colocalized, and physically associated, and loss of GABAB-R1 impaired calcium-mediated signaling, reduced proliferation, increased apoptosis, and shifted marker expression toward late differentiation [28], suggesting that GABAB-R1 regulates growth plate chondrocyte maturation partly through interaction with calcium-sensing receptor pathways.

GABA-related transport was also implicated in the osmotic regulation of articular chondrocytes. In primary porcine articular chondrocytes, hypertonic culture conditions induced a time-dependent shift in transport activity, with early activation of sodium-dependent amino acid transport system A followed by delayed induction of betaine–GABA transport [29]. This response required new RNA and protein synthesis, but BGT-1 mRNA was not detected, suggesting that chondrocytes may use an alternative or unidentified osmolyte transport mechanism [29].

Collectively, these studies suggest that glutamate and GABA signaling pathways are active in chondrocytes and cartilage but appear to regulate partially distinct biological processes. Glutamate signaling is most strongly associated with mechanotransduction, inflammatory activation, OA-related phenotypic changes, apoptosis, mineralization, and chondrogenesis, particularly through NMDAR-, AMPAR-, transporter-, and cystine/glutamate exchange–dependent mechanisms. In contrast, GABAergic signaling appears more closely linked to growth plate chondrocyte proliferation, chondrogenesis, endochondral ossification, calcium-sensing receptor interactions, and osmotic adaptation. Lastly, all of the aforementioned studies are summarized in Table 3.

Table 3. Findings from studies investigating glutamate, GABA, or their respective receptors in cartilage animal models or cell lines. Note: NMDAR = N-methyl-D-aspartate receptor, PSD-95 = postsynaptic density 95, SK channels = small-conductance calcium-activated potassium channels, EAATs = excitatory amino acid transporters, LPS = lipopolysaccharide, GABA = ɣ-aminobutyric acid.

Reference	Neurotransmitter	Cells/Tissue/ Animal Model	Study Design	Summary Findings
Wang et al., 2006 [23]	Glutamate	Mouse Embryonic Metatarsal Chondrocytes, Rat Costal Chondrocytes	Embryonic mouse metatarsal organ cultures, newborn mouse tibial sections, and primary rat costal chondrocytes were used to investigate glutamate-related effects on chondrocyte maturation, mineralization, oxidative stress, apoptosis, and viability. Outcomes were assessed using histology, Alizarin red and Alcian blue staining, ALP activity assays, TUNEL and Hoechst staining, ROS imaging, MTT viability assays, glutathione quantification, RT-PCR, in situ hybridization, and immunohistochemistry.	At physiological concentrations glutamate inhibits chondral mineralization whereas at supraphysiological extracellular glutamate promotes chondrocyte differentiation toward mineralization. The effects of glutamate occurred via an apoptosis-mediated mechanism and the bidirectional transport of the cystine/Glutamate antiporter, along with group III mGluR activation.
Lu et al., 2023 [43]	Glutamate	Human SW1353 Cell Line	SW1353 cells were induced first with LPS and then treated with an NMDAR antagonist and complement inhibitor, where RT-qPCR was performed to determine NR2A subunit and C5 RNA expression, including inflammatory cytokines. An apoptosis assay was performed through flow cytometry.	Inhibition of NMDAR and complement C5 suppressed apoptosis and the inflammatory response in SW1353 cells, as NMDAR antagonism reduced subunit levels of NR1 and NR2A, and reduced the apoptosis rate and inflammatory cytokine expression, with C5 complement inhibition enhancing the effects of NMDAR inhibition.
Ramage et al., 2008 [20]	Glutamate	Primary Human Chondrocytes	Human knee joint chondrocytes underwent RT-PCR and Western blotting to determine the expression levels of NMDAR subunits and PSD-95. NMDAR activity was analysed via calcium uptake and membrane potential changes in the presence or absence of NMDAR agonists or antagonists.	Human OA chondrocytes differ in their NMDAR subunit expression. OA chondrocytes require extracellular calcium for NMDAR signaling, along with an association with PSD-95 and nNOS activity. NMDAR downstream signaling leads to the activation of tetrodotoxin-sensitive sodium channels and SK channels, which is not seen in normal chondrocytes.
Piepoli et al., 2009 [46]	Glutamate	Human Chondrocyte Cell Line SW1353, Primary Rat Articular Chondrocytes.	Chondrocytes were used to determine glutamate release and uptake through fluorimetric and radiometric methods, along with gene expression analysis and NMDAR pharmacology investigations.	Chondrocytes express machinery for autocrine glutamate signaling, and NMDAR has a potential role involving the inflammatory process that leads to cartilage damage. NMDAR antagonism reduced chondrocyte proliferation, and IL-1β altered glutamate metabolism and induced the gene expression of pro-inflammatory factors involved in cartilage degradation.
Wang et al., 2005 [54]	Glutamate	Rat Costal Chondrocytes	Cultured rat costal chondrocytes were used to assess chondrogenic marker expression, AMPA receptor/VGLUT1 expression, and endogenous glutamate release.	Rat costal chondrocytes expressed GluR3 AMPA receptor subunits and VGLUT1 and released endogenous glutamate, with AMPA receptor activation prolonging glutamate release, therefore proposing a glutamatergic signaling mechanism in chondrocytes.
Kalev-Zylinska et al., 2018 [38]	Glutamate	Human OA & MN Chondrocytes, H5 Chondrocytes	Chondrocytes from normal or human OA cartilage were treated with NMDAR antagonists or GRIN2A or GRIN2B siRNA to measure clock genes and chondrocyte phenotypic markers using RT-qPCR to determine NMDAR’s role in chondrocyte clock and phenotype.	NMDAR regulates the chondrocyte clock and phenotype, and GluN2B expression in OA may contribute to OA pathology by altering the chondrocyte clock, as NMDAR inhibition in OA chondrocytes restored PER2 and BMAL1 expression to control levels and reduced pro-catabolic factors, and GluN2B expression reduced clock gene expression and increased pro-catabolic factor levels.
Hinoi et al., 2005 [19]	Glutamate	Rat Costal Chondrocytes, ATDC5 cells	Investigation into rodent cartilage for the expression of EAATs and their role in glutamatergic signal termination using RT-PCR, Western blotting, IHC, and [3H]glutamate accumulation analysis.	Rat chondrocytes expressed functional excitatory amino acid transporters, particularly GLAST, GLT-1, and EAAC1, with glutamate uptake showing sodium- and membrane–potential–dependent transport characteristics. In embryonic mouse cartilage, GLAST, GLT-1, and EAAT4 displayed differentiation stage–specific expression, suggesting that glutamate transport systems may contribute to chondrocyte maturation and endochondral ossification.
Jean et al., 2005 [36]	Glutamate	Rat Knees and Ligaments	ACL transection was used to induce early OA, with the contralateral sham-operated knee serving as the control. Twenty weeks after surgery, knee joint dialysates were collected by microdialysis and analyzed for excitatory amino acids using high-performance liquid chromatography, followed by histopathological assessment with hematoxylin/eosin and Safranin-O staining.	ACL-transected knees showed histological features of early OA, including cartilage fibrillation, chondrocyte clustering, and reduced proteoglycan staining. Glutamate and aspartate levels were significantly elevated in osteoarthritic knee dialysates compared with sham-operated controls, suggesting that increased excitatory amino acid release may contribute to early OA pathogenesis.
Salter et al., 2004 [21]	Glutamate	Human Articular Chondrocytes	Investigation into the roles of glutamate receptors in chondrocytes in response to mechanical stimuli via IHC, RT-PCR, and membrane potential monitoring in response to mechanical stimulation.	NMDARs are involved in the regulation of the response to mechanotransduction in human articular chondrocytes, which is altered in OA, as NMDAR antagonism inhibited the hyperpolarization response in normal chondrocytes and had no effect on the depolarization response in OA chondrocytes when mechanically stimulated.
Alhilali et al., 2021 [26]	Glutamate	Primary Human Chondrocytes	Investigation into the mechanism behind chondrocyte proliferation and altered differentiation mediated by IL-1β through analysis of NMDAR/CREB and NF-κB signaling.	NMDAR/CREB and NF-κB signaling pathways regulate PER2 and BMAL1 in chondrocytes, and these core clock components are implicated in the pathology of OA. CREB antagonism prevented the increase in IL-1β-induced PER2 expression, and NF-κB antagonism prevented the IL-1β-induced reduction in BMAL1 expression.
Matta et al., 2019 [22]	Glutamate	Chicken Embryonic Limb-Bud-Derived Chondroprogenitor Cells	Investigation into glutamatergic signalling during the course of in vitro chondrogenesis in high-density chondrifying cell cultures using single-cell fluorescent calcium imaging, electrophysiological studies, gene silencing, and Western blotting.	NMDARs play a role in the transition of chondroprogenitor cells to cartilage matrix, producing chondroblasts, as NMDAR-mediated signaling stimulates transient intracellular calcium release and is critical in chondrogenesis, as knockdown models lead to the inhibition of differentiation of chondroprogenitor cells.
Skiöldebrand et al., 2019 [49]	Glutamate, Serotonin	Primary Equine Chondrocytes	An equine chondrocyte study comparing healthy and osteoarthritic cells under IL-1β- or LPS-induced acute inflammatory stimulation, with assessment of Ca2+ signaling, glutamate release, actin organization, and opioid receptor expression.	Glutamate, serotonin, and opioid signaling may contribute to equine OA progression, as OA chondrocytes responded differently to acute inflammation than healthy chondrocytes, showing enhanced serotonin-induced Ca2+ signaling and increased μ- and κ-opioid receptor expression, with inflammation increasing glutamate release in both groups.
Skiöldebrand et al., 2018 [50]	Glutamate, Serotonin	Primary Equine Chondrocytes	Primary equine articular chondrocytes were cultured in monolayer and stimulated with IL-1β or lipopolysaccharide to model inflammatory conditions. Calcium responses to glutamate, serotonin, and ATP were assessed using Fura-2/AM calcium imaging, while chondrocyte phenotype and inflammatory/catabolic responses were evaluated by immunocytochemistry, Western blotting, MMP-13 activity assays, and glutamate-release measurements.	Chondrocyte response to inflammatory stimuli resembles that previously described in astrocyte and cardiac fibroblast networks. Glutamate and 5-hydroxytryptamine induced the release of Ca2+ intracellularly and were released in an oscillatory pattern following inflammatory insults, along with inflammatory insults causing an increase in the expression of glutamate transporters and pro-catabolic factors.
Takahata et al., 2012 [52]	GABA	Primary Mouse Chondrocytes, ATDC5 cells	Cultured murine costal chondrocytes, newborn mouse tibial sections, GABAB receptor subunit-1 knockout mice, cultured metatarsal explants, and gene/transcriptional assays were used to examine the role of GABAB receptor signaling in chondrogenesis and endochondral ossification.	GABAB receptor subunit-1 was expressed in chondrocytes, particularly in prehypertrophic to hypertrophic regions, while GABA_B receptor subunit-1 knockout mice showed reduced body size and delayed skeletal calcification. Knockout metatarsals showed reduced Indian hedgehog expression, and mechanistic assays suggested that GABAB receptor subunit-1 promotes chondrogenesis by enhancing ATF4 nuclear translocation and ATF4-dependent Ihh promoter activation.
Cheng et al., 2007 [28]	GABA	Mouse GPCs	Mouse growth plate chondrocytes were used to assess calcium-sensing receptor and GABA_B receptor expression, interaction, and effects on calcium-mediated chondrocyte signaling and differentiation.	GABA_B-R1 regulates growth plate chondrocyte maturation through calcium-sensing receptor interactions. Calcium-sensing receptors and GABA_B-R1 were coexpressed, colocalized, and physically associated in growth plate chondrocytes. GABA_B-R1 knockout impaired calcium-mediated signaling, reduced proliferation, increased apoptosis, and shifted marker expression toward late differentiation.
de Angelis et al., 1999 [29]	GABA	Porcine Articular Primary Chondrocytes	Primary porcine articular chondrocytes were used to examine the effects of isotonic versus hypertonic culture conditions on amino acid and betaine transport activity. Uptake of selected amino acids and betaine was measured after incubation in isotonic or hypertonic media, with additional inhibitor and Northern blot analyses used to assess mechanisms of transporter induction.	Hypertonic conditions induced a time-dependent shift in chondrocyte transport activity, with early activation of Na+-dependent amino acid transport system A followed by increased betaine–GABA transport activity. Betaine–GABA transport induction depended on the duration of hypertonic exposure and required new RNA and protein synthesis, but BGT-1 mRNA was not detected, suggesting involvement of a distinct or unidentified osmolyte transport mechanism.
Tamayama et al., 2005 [18]	GABA	Rat GPCs, ATDC5 cells	Rat growth plate tissue and murine ATDC5 chondrocytes were used to examine GABA_A and GABA_B receptor expression and their effects on chondrogenesis and cell proliferation. Outcomes were assessed by RT-PCR, immunohistochemistry, confocal microscopy, and BrdU proliferation assays following treatment with receptor-specific agonists and antagonists.	GABA has a role in ATDC5 chondrogenic differentiation through signaling via GABAA and GABAB receptors, with a potential role in chondrocyte growth. GABA also serves as an autocrine and paracrine signaling factor that is involved in the regulation of the physiological state of growth plate chondrocytes

3.3. Acetylcholine

Acetylcholine is a major neurotransmitter of the CNS and PNS and is classically involved in processes such as voluntary muscle contraction, autonomic regulation of heart rate, memory consolidation, and attention [9]. Studies suggest that acetylcholine-related signaling also extends beyond the nervous system and may contribute to chondrocyte survival, inflammatory regulation, and OA pathology [16,27,40,41,56,69].

Evidence for cholinergic signaling in cartilage was demonstrated in studies of human hip OA and rheumatoid arthritis (RA) [16]. Beckmann et al. identified several proteins involved in choline import and acetylcholine release, including Organic Cation Transporter 1 (OCT1), Organic Cation Transporter 3 (OCT3), Organic Cation/Carnitine Transporter 1 (OCTN1), and members of the choline transporter-like family [16]. These findings suggest that cartilage contains the molecular machinery capable of supporting local cholinergic signaling, although transporter expression did not appear to differ substantially in pathological cartilage [16].

Several studies focused on the α7 nicotinic acetylcholine receptor (α7nAChR) and its role in OA-related cartilage degeneration. In human and murine chondrocytes, α7nAChR stimulation was reported to induce anti-inflammatory and anti-catabolic responses, suggesting a protective role in OA pathology [69]. Additional experiments with human articular cartilage and α7nAChR-deficient chondrocytes suggested that it regulates the balance between autophagy and apoptosis [41]. Specifically, human OA cartilage showed reduced α7nAChR expression compared with healthy cartilage, which was associated with dysregulated autophagy and apoptosis [41] (Figure 4). Further, α7nAChR-deficient chondrocytes had reduced LC3 (Microtubule-associated protein 1A/1B-light chain 3) levels and greater sensitivity to chemically induced arthritis, supporting a role of α7nAChR in maintaining chondrocyte homeostasis and survival [41] (Figure 4). However, the results from Liu et al. do not definitively demonstrate that α7-nAChR activation promotes autophagic flux because the LC3-II levels reflect a single time point of steady-state levels between autophagosome synthesis and lysosomal turnover. The steady-state data cannot be conclusively attributed to an increase in autophagic flux because no turnover assays were performed to definitively demonstrate this. In an α7nAChR-deficient mouse model of induced arthritis through type II collagen immunization, α7nAChR-deficient mice developed milder arthritis, reduced cartilage destruction, and had lower measures of adaptive immunity compared with controls [56]. Therefore, α7nAChR signaling may have disease-context–dependent effects, with potentially significant roles in degenerative OA and immune-mediated inflammatory arthritis. However, additional confirmatory assays and studies are required to determine the causative relationship of α7nAChR signaling in cartilage disease models.

Figure 4. Effects of α7-nAChRs deficiency on apoptosis and autophagy in primary cultured chondrocytes (adapted from Liu et al. [41]) (A–E). Increased apoptosis in α7-nAChR knockout chondrocytes compared with wild-type controls, as demonstrated by nuclear staining, FACS analysis, and Western blotting. (F–J), Demonstrates autophagy as a measurement of various markers and their ratios. The right figure represents a working model proposed by Liu et al. [41] showing that α7-nAChRs play a role in switching from autophagy to apoptosis in cartilage degradation of OA. It also shows that activation of α7-nAChRs could inhibit phosphorylation of mTOR, promote autophagic flux, and prevent cartilage degradation.

Although the majority of studies focused on α7nAChR, muscarinic acetylcholine receptors were also implicated in cartilage and arthritis models [27,40]. M3 muscarinic receptor (M3R)-deficient mice subjected to collagen antibody-induced arthritis were not protected against disease and were associated with a greater inflammatory response and increased type II collagen degradation in articular cartilage [27]. These findings suggest that M3R signaling may contribute in a protective role in arthritis. In chick scleral chondrocytes, muscarinic receptor antagonism reduced sulfate incorporation, a proxy for glycosaminoglycan synthesis, suggesting a role for muscarinic receptor signaling, particularly M1 receptor activity, in regulating cartilage ECM metabolism [40]. However, because this study used scleral rather than articular chondrocytes, its relevance to joint cartilage should be interpreted cautiously.

Collectively, these studies suggest that cartilage contains components of a non-neuronal cholinergic signaling system and that acetylcholine receptors may regulate chondrocyte survival, autophagy, apoptosis, inflammation, and ECM metabolism. The strongest evidence centers on α7nAChR, which appears to support anti-inflammatory, anti-catabolic, and survival-related pathways in OA models and collagen antibody-induced arthritis. Muscarinic receptor studies further indicate that cholinergic signaling in cartilage is receptor subtype–specific and may vary according to tissue type, disease model, and inflammatory context. Lastly, all of the aforementioned studies are summarized in Table 4.

Table 4. Findings from studies investigating acetylcholine or the respective receptors in cartilage animal models or cell lines. Note: nAChR = nicotinic acetylcholine receptor, CAIA = collagen-antibody-induced arthritis, RA = rheumatoid arthritis, mAChR = muscarinic acetylcholine receptor.

Reference	Neurotransmitter	Cells/Tissue/ Animal Model	Study Design	Summary Findings
Liu et al., 2021 [41]	Acetylcholine	Human knee articular cartilage, α7nAChR-deficient chondrocytes	Comparison of α7nAChR expression in human knee articular cartilage from healthy and OA patients to investigate autophagy and apoptosis.	α7nAChR has a role in regulating the balance of autophagy and apoptosis in cartilage, as OA patients had decreased α7nAChR expression with dysregulated autophagy and apoptosis, and α7nAChR-deficient chondrocytes had reduced LC3 levels (autophagy measurement) and were more sensitive to monosodium iodoacetate-induced arthritis.
Westman et al., 2010 [56]	Acetylcholine	a7nAChR- deficient mouse model	Experimental mouse study using α7-nAChR–deficient mice to examine the role of α7-nAChR in collagen-induced arthritis through type II collagen immunization development and adaptive immune responses, with outcomes assessed by histological staining and ovalbumin stimulation.	α7nAChR has been shown to have important roles in arthritis development, along with regulation of immunological mechanisms, as α7nAChR-deficient mice had markedly milder arthritis and less cartilage destruction and lower adaptive immunity measures.
Beckmann et al., 2016 [27]	Acetylcholine	M3R-/- mouse models	Collagen antibody-induced arthritis was induced in M3R-deficient and wild-type mice, where histopathological changes, gene expression changes, and inflammatory response were investigated.	M3R-deficient mice were not protected from CAIA compared to wild-type mice and tended to have a higher inflammatory response, as demonstrated by marked degradation of collagen II in the articular cartilage of M3R-deficient mice
Beckmann et al., 2015 [16]	Acetylcholine	Human Hip OA & /RA cartilage	In vitro analysis of OA and RA human cartilage to determine the expression patterns of choline and acetylcholine transporters	All components for choline import and acetylcholine release are present in the human joint. Cartilage contains OCT1, OCT3, OCTN1, and all members of the CTL family, with no difference in pathological states.
Courties et al., 2020 [69]	Acetylcholine	Human OA and Murine chondrocytes	Investigation into α7nAChR biochemical machinery in chondrocytes and its role in OA development.	α7nAChR has a protective role in OA, as stimulation of nAChR induces anti-inflammatory and anti-catabolic activities through α7nAChR.
Lind et al., 1998 [40]	Acetylcholine	Chick scleral chondrocytes	Chick scleral chondrocytes underwent treatment with several types of mAChR-ligands and then were analyzed for DNA content, cell viability, and cell metabolic activity.	The sclera is a possible site of action for mAChR antagonists used to influence myopia, most likely mediated by M1, with toxicity achieved at high doses, as evident by sulfate incorporation (GAG measurement), which was inhibited by mAChR antagonists, consistent with M1 regulation.

3.4. Serotonin

Serotonin is a major neurotransmitter and modulatory neurohormone involved in neurological functions such as mood, appetite, and sleep, as well as gastrointestinal motility [10]. Serotonin also functions as an immune-modulating mediator that can either suppress or promote inflammation depending on the tissue type and receptor subtype [70]. The studies identified in this review suggest that serotonin-mediated signaling in chondrocytes may similarly be tissue-, receptor-, and disease-dependent.

For example, in primary equine chondrocytes, inflammatory stimulation altered intracellular calcium signaling in response to serotonin [50]. Under inflammatory conditions, calcium transient events shifted from a single-peak response to an oscillatory pattern, a response previously described in astrocytes [50]. This finding suggests that inflammatory conditions may alter chondrocyte calcium signaling through overstimulation of Gq protein–coupled receptors (GqPCRs), including mGluR5 and 5-HT2A [50]. Additional work in primary equine chondrocytes showed that osteoarthritic chondrocytes exhibited stronger serotonin-induced intracellular calcium release than healthy chondrocytes under inflammatory conditions [49] (Figure 5). These findings suggest that serotonin signaling may contribute to altered calcium responses in osteoarthritic chondrocytes and may be relevant to OA-associated pain-related processing [49] (Figure 5).

Figure 5. Ca²⁺ release in chondrocytes adapted from (Skiöldebrand et al. [49]). ((Left) panel) displays calcium responses evoked by serotonin in cultured chondrocytes from healthy and osteoarthritic joints stimulated with either IL-1β, lipopolysaccharide (LPS), or was untreated. CC BY-NC-ND 4.0 ((Right) panel) depicts the action of the 5HT3 receptor once activated in chondrocytes and how its promotion of inflammatory mediators influences pain processing in arthritis (based on Stratz et al. [24] and created in BioRender).

Other studies also implicate specific serotonin receptors in inflammatory and catabolic chondrocyte responses. For example, overstimulation of 5-HT2A was suggested to contribute to inflammation-associated chondrocyte pathology through altered calcium signaling [50]. In human chondrocytes, antagonism of the 5-HT3 receptor produced anti-inflammatory effects by reducing IL-6, prostaglandin E2, and free radical production, with indirect inhibition of COX-2 activity24, suggesting that endogenous 5-HT3 signaling may contribute to pro-inflammatory responses in these cells [24].

Serotonin was also linked to catabolic activity in human osteoarthritic articular chondrocytes as serotonergic receptors were identified on chondrocytes, and receptor activation was associated with phospholipase A2 activation, prostaglandin E2 synthesis, and increased collagenolytic activity [25]. These findings suggest that serotonin signaling may contribute to OA-related cartilage degradation through inflammatory prostaglandin pathways and collagenolytic enzyme (collagenase) activity [25].

Collectively, these aforementioned studies suggest that serotonin signaling may influence chondrocyte inflammatory responses, calcium signaling, pain-associated OA mechanisms, and cartilage matrix degradation. These effects appear to depend on receptor subtype and inflammatory context, with 5-HT2A implicated in altered calcium signaling and 5-HT3 signaling associated with pro-inflammatory mediator production. However, the current evidence remains limited and should be interpreted cautiously until receptor-specific mechanisms are more fully delineated in articular chondrocytes/cartilage and OA models. Lastly, all of the aforementioned studies are summarized in Table 5.

Table 5. Findings from studies investigating serotonin or the respective receptors in cartilage animal models or cell lines.

Reference	Neurotransmitter	Cells/Tissue/ Animal Model	Study Design	Summary Findings
Stratz et al., 2014 [24]	Serotonin	Human Primary Chondrocytes	Human primary chondrocytes were studied to assess the effects of 5-HT3 receptor antagonists on IL-1β–induced inflammatory and oxidative responses.	5-HT3 may have an inflammatory role as 5-HT3 receptor antagonists, especially dolasetron, reduced IL-1β–induced PGE2, 8-isoprostane, and IL-6 production.
Richard et al., 1991 [25]	Serotonin	Human OA Primary Chondrocytes	Primary human osteoarthritic chondrocytes were isolated from joint replacement cartilage to assess the effects of short-term compound stimulation on cartilage-degrading enzyme activities, including phospholipase A2, collagenolytic, and proteoglycanase activities.	Serotonin, in a dose-dependent manner, stimulated phospholipase A2, enhancing collagenase type II activity, but did not affect proteoglycanase activity through induced prostaglandin E2 synthesis.
Skiöldebrand et al., 2019 [49]	Glutamate, Serotonin	Primary Equine Chondrocytes	An equine chondrocyte study comparing healthy and osteoarthritic cells under IL-1β- or LPS-induced acute inflammatory stimulation, with assessment of Ca2+ signaling, glutamate release, actin organization, and opioid receptor expression.	Glutamate, serotonin, and opioid signaling may contribute to equine OA progression, as OA chondrocytes responded differently to acute inflammation than healthy chondrocytes, showing enhanced serotonin-induced Ca2+ signaling and increased μ- and κ-opioid receptor expression, with inflammation increasing glutamate release in both groups.
Skiöldebrand et al., 2018 [50]	Glutamate, Serotonin	Primary Equine Chondrocytes	Primary equine articular chondrocytes were cultured in monolayer and stimulated with IL-1β or lipopolysaccharide to model inflammatory conditions. Calcium responses to glutamate, serotonin, and ATP were assessed using Fura-2/AM calcium imaging, while chondrocyte phenotype and inflammatory/catabolic responses were evaluated by immunocytochemistry, Western blotting, MMP-13 activity assays, and glutamate-release measurements.	Chondrocyte response to inflammatory stimuli resembles that were previously described in astrocyte and cardiac fibroblast networks. Glutamate and 5-hydroxytryptamine induced the release of Ca2+ intracellularly and were released in an oscillatory pattern following inflammatory insults, along with inflammatory insults causing an increase in the expression of glutamate transporters and pro-catabolic factors.

4. Discussion

Although cartilage is an avascular and aneural tissue, it is hard to conceive that neurotransmitters would have a role or effects on chondrocytes and cartilage biology. But, published data clearly shows that they do. Specifically, this review demonstrates that classical neurotransmitters exert diverse and biological effects on cartilage and chondrocyte function, extending well beyond their established roles in the nervous system. Across the literature, norepinephrine, glutamate, GABA, acetylcholine, and serotonin were each shown to be involved with distinct yet overlapping aspects of cartilage biology, including chondrocyte differentiation, mechanotransduction, inflammatory signaling, extracellular matrix homeostasis, cartilage degeneration, and repair. Collectively, these findings support the concept that cartilage is an active neuroresponsive tissue capable of integrating neurotransmitter-mediated signals through locally expressed receptors, transporters, and intracellular signaling pathways.

4.1. Catecholamines

Among the neurotransmitters examined, NE has the most extensive evidence linking sympathetic nervous system activity to cartilage biology. While NE is classically recognized for regulating cardiovascular physiology [67], the reviewed studies demonstrate that adrenergic signaling also influences chondrocyte differentiation, cartilage remodeling, OA progression, inflammatory signaling, and cell metabolism. These effects appeared to be highly receptor- and tissue-dependent, with β-adrenergic and α-adrenergic pathways producing distinct and sometimes opposing effects. Predominantly, β2-adrenergic receptor signaling was strongly associated with chondrocyte differentiation and skeletal development. Several studies suggested that norepinephrine regulates chondrocyte maturation through β2-adrenergic receptor activation of ERK1/2 and PKA signaling, leading to induction of the AP-1 transcription factor Jun-B and reduced expression of chondrogenic markers such as Sox6 and type II collagen [45]. Similarly, β2-adrenergic effects were also observed with epinephrine [53], suggesting that catecholamine-mediated regulation of chondrocyte phenotype may extend beyond norepinephrine alone. However, these findings are largely derived from in vitro and in vivo animal models thus, further studies are needed to determine whether the same β2-adrenergic signaling mechanisms regulate chondrocyte differentiation in human cartilage.

NE was also implicated in cartilage remodeling and diseased states such as OA, although its effects differed by adrenergic receptor subtype. β2-adrenergic signaling appeared to support cartilage homeostasis in some models, whereas β1-adrenergic signaling [32] was associated with cartilage degeneration, including chondrocyte-derived exosomal signaling and subchondral bone remodeling. These findings suggest that adrenergic signaling may influence OA progression through both cartilage-intrinsic and osteochondral mechanisms. Additional studies should focus on human osteoarthritic cartilage in order to define the receptor-specific roles of β- and α-adrenergic signaling in disease progression.

A particularly notable finding was the identification of a hypothalamic–skeletal neural circuit regulating articular cartilage homeostasis [34,55]. In this model, mechanical loading promoted skeletal PGE2-mediated interoceptive signaling through EP4 receptors on sensory nerves, which relays information to the paraventricular nucleus of the hypothalamus [55]. This mechanical signaling reduced sympathetic norepinephrine output to the joint, thereby decreasing β2-adrenergic receptor activation in chondrocytes of the superficial zone and promoted anabolic matrix activity, lubricin expression, and superficial cartilage maintenance [55]. Although this circuit provides a compelling mechanism linking joint loading, neural regulation, and cartilage regeneration, the evidence remains limited to a small number of preclinical studies and further research is needed to determine whether this circuit is active in loaded human joints and whether it can be therapeutically modulated in OA.

Adrenergic signaling was also implicated in chondrocyte inflammatory responses And the reviewed studies suggest that α- and β-adrenergic receptors may exert different effects depending on receptor subtype, norepinephrine concentration, and inflammatory state. α-adrenergic signaling was associated with chondrocyte apoptosis and catabolic activity, whereas β-adrenergic signaling was linked to chondrocyte proliferation and dedifferentiation toward a fibroblast-like phenotype in inflammatory osteoarthritic conditions. As such, adrenergic signaling can be either protective or detrimental depending on the cell environment and disease state. α2-adrenergic signaling was associated with pro-catabolic, mitochondrogenic, and anti-anabolic effects in chondrocytes and this suggested that α2-adrenergic receptor activation may promote cartilage degeneration by altering matrix metabolism and mitochondrial activity [33]. Whether adrenergic signaling pathways play a major role in healthy, inflamed, and OA human chondrocytes, remains unexplored.

The reviewed literature also suggested a potential link between chronic psychosocial stress and impaired cartilage or growth plate function. Chronic stress was associated with altered growth plate chondrocyte metabolism, reduced chondrocyte proliferation, and disrupted endochondral ossification [31]. These findings suggest that stress-related catecholaminergic signaling may influence skeletal growth and cartilage physiology. However, the evidence remains limited to few animal models, and further mechanistic studies are needed to define the neural, endocrine, and molecular pathways connecting chronic stress to chondrocyte dysfunction. Finally, dopamine was less extensively studied than norepinephrine but was reported to reduce cartilage degradation and promote cartilage stability in one included study [44]. This finding suggests that catecholaminergic regulation of cartilage may involve multiple neurotransmitters, not only norepinephrine, but dopamine’s exact role in cartilage remodeling and OA also requires further investigation.

4.2. Glutamate/GABA

The studies examining glutamate and GABA also implicates them in several aspects of chondrocyte biology, including differentiation, maturation, mechanotransduction, chondrogenesis, endochondral ossification, inflammation, and osmotic adaptation [18,19,20,21,22,23,26,28,29,36,38,43,46,50,52,54]. Glutamatergic signaling was demonstrated in chondrocytes through their expression of glutamate receptors, transporters, and endogenous glutamate release [54]. In OA mouse models, increased glutamate levels were associated with disease induction, suggesting a potential role for glutamate in OA-associated cartilage pathology [20]. Glutamate transporters also showed differentiation stage–specific expression, supporting a possible role in chondrocyte maturation and endochondral ossification [19]. Further studies would potentially benefit from investigating the translation of glutamate’s role in developmental mineralization to articular cartilage repair.

Among glutamate receptors, N-methyl-D-aspartate receptors (NMDARs) have received the greatest attention and were implicated in chondrocyte mechanotransduction, with normal and OA chondrocytes showing different electrophysiologic responses to mechanical stimulation [21]. This suggests that NMDAR-dependent mechanotransduction may be altered in OA. NMDAR signaling was also associated with pro-inflammatory and pro-catabolic responses, including inflammatory gene expression and potential interaction with other signaling [43]. In addition, NMDAR activity was linked to chondrocyte circadian regulation through downstream CREB and NF-κB pathways, which influenced circadian clock genes, PER2 and BMAL1 expression and may contribute to altered chondrocyte phenotype in OA [26]. In developmental models, NMDAR-mediated calcium signaling was required for chondroprogenitor differentiation into chondroblasts, suggesting that NMDAR activity may have distinct roles in cartilage development versus inflammatory cartilage degeneration [22]. Overall, research supports a strong relationship between NMDAR signaling and chondrocyte biology, but further studies are needed to clarify its relevance in human clinical samples and in human chondrogenic differentiation.

Although NMDARs were the most frequently studied glutamate receptors, AMPA and metabotropic glutamate receptors were also implicated in chondrocyte function. Rat chondrocytes expressed AMPA-related glutamatergic machinery similar to neurons, and AMPA receptor activation appeared to prolong endogenous glutamate release, supporting a potential autocrine or paracrine signaling mechanism [54]. Metabotropic glutamate receptor pathways were also linked to chondrocyte differentiation and endochondral ossification in animal models [23]. Together, these findings suggest that glutamatergic regulation of chondrocytes extends beyond NMDAR signaling. However, the roles of AMPA and metabotropic glutamate receptors in human cartilage and disease pathology remain insufficiently defined and because the evidence supporting these glutamate-chondrocyte findings depends heavily on the type of experimental model used, future research must validate the data ideally in human cartilage models. Although considerably fewer studies have investigated GABA compared with glutamate, the available evidence suggests that GABA may also contribute to chondrocyte differentiation, growth plate physiology, calcium-dependent signaling, and osmotic adaptation [18,28,29,52]. GABAA and B receptor expression was reported in growth plate chondrocytes and chondrogenic cell models, supporting a possible autocrine or paracrine role for GABA in cartilage development [18]. GABAB-R1 expression in prehypertrophic and hypertrophic cartilage regions further suggests involvement in chondrogenesis and endochondral ossification. Mechanistically, GABAB-R1 signaling was linked to enhanced ATF4 nuclear translocation and activation of the ATF4-dependent Ihh promoter, a pathway associated with chondrocyte proliferation and differentiation [52]. Additional studies suggested that GABAB-R1 may regulate growth plate chondrocyte maturation through interactions with calcium-sensing receptor signaling [28].

GABA-related transport was also implicated in chondrocyte osmotic adaptation. Hypertonic conditions induced a time-dependent shift in transport activity, with delayed induction of betaine–GABA transport that required gene expression [29]. Because BGT-1 mRNA was not detected, these findings suggest that chondrocytes may rely on an unidentified osmolyte transport mechanism. Regardless, further studies are needed to define the broader relevance of GABAergic signaling as well as transport pathways and determine whether it is altered in cartilage disease states.

4.3. Acetylcholine

Acetylcholine represents another important neurotransmitter whose functions extend beyond neuronal communication. The identification of transport proteins required for acetylcholine synthesis and release within human cartilage supports the existence of a local non-neuronal cholinergic signaling system. Specifically, acetylcholine-related signaling was implicated in chondrocyte survival, inflammatory regulation, and OA pathology [16,27,40,41,56,69]. Evidence for a potential cholinergic role in cartilage was supported by studies of human hip OA and rheumatoid arthritis cartilage, which demonstrated the presence of acetylcholine receptors and transporters involved in choline import and acetylcholine release [16]. These components appeared to be present across healthy and pathological cartilage states, suggesting that cartilage possesses cholinergic signaling machinery, although its expression may not necessarily differ by disease status. But much of the acetylcholine literature focused on the α7 nicotinic acetylcholine receptor and its role in OA-related cartilage degeneration. In human and murine chondrocytes, α7-nAChR stimulation appeared to exert protective effects by promoting anti-inflammatory and anti-catabolic responses [69]. Moreover, human OA cartilage showed reduced α7-nAChR expression as compared with control cartilage, which was associated with dysregulated autophagy and apoptosis [41]. In α7-nAChR-deficient chondrocytes, reduced LC3 expression and increased sensitivity to chemically induced arthritis further suggested that α7-nAChR may help maintain chondrocyte homeostasis by regulating the balance between autophagy and apoptosis.

However, findings from inflammatory arthritis models suggested that α7-nAChR signaling may be disease-context dependent. In a model of induced arthritis, α7-nAChR-deficient mice demonstrated reduced arthritis severity and less cartilage degradation, in contrast with studies suggesting a protective role for α7-nAChR in OA [56]. These opposing findings suggest that α7-nAChR may have different roles in degenerative OA compared with immune-mediated inflammatory arthritis. Additional studies are needed to clarify whether these differences are due to disease mechanisms, arthritis induction models, immune-cell involvement, receptor-specific signaling, or differences within chondrocyte-intrinsic cholinergic effects.

Although α7-nAChR was the primary focus of the reviewed studies, muscarinic acetylcholine receptors were also implicated in cartilage biology and appeared to have a protective or anti-inflammatory role in collagen antibody-induced arthritis, as M3R deficiency was associated with greater disease sensitivity, increased inflammatory response, and enhanced type II collagen degradation in articular cartilage [27]. Further, M1 receptor signaling was also suggested to influence ECM metabolism in chick scleral chondrocytes, as muscarinic receptor antagonism reduced glycosaminoglycan synthesis [40]. However, the relevance of this finding to articular cartilage remains uncertain because the study used scleral rather than joint chondrocytes. Together, these findings suggest that acetylcholine signaling contributes to maintaining cartilage homeostasis while also modulating inflammatory responses under pathological conditions. But future studies using human articular cartilage, disease-specific models, and receptor-targeted approaches are needed to clearly define the roles of nicotinic and muscarinic acetylcholine receptors in cartilage pathology.

4.4. Serotonin

Compared with the aforementioned neurotransmitters, serotonin remains the least well characterized in cartilage biology. However, the available evidence suggests that serotonin may contribute to chondrocyte inflammatory responses, calcium signaling, catabolic activity, and OA-related pathology. In primary equine chondrocytes, inflammatory stimulation altered the cellular response to serotonin, shifting calcium transients from a single-peak pattern to an oscillatory pattern as a result of overstimulation of Gq-coupled receptors, including 5-HT2A, and suggesting that inflammatory conditions may modify serotonin-dependent calcium signaling in chondrocytes [50].

Additional studies with primary equine chondrocytes demonstrated that OA chondrocytes exhibited greater serotonin-induced intracellular calcium release compared with healthy controls, suggesting that serotonin signaling may be altered in OA chondrocytes and may contribute to calcium-dependent mechanisms relevant to OA-associated pain processing [49]. However, the results warrant further investigation to test whether this calcium response directly contributes to pain signaling or reflects broader inflammatory changes in OA chondrocytes through improved nociceptive outcome measurements.

Research also implicates serotonin receptors in inflammatory and catabolic chondrocyte responses. 5-HT3 receptor signaling may contribute to inflammation, as antagonism of this receptor was associated with reduced production of inflammatory and oxidative mediators, including IL-6, PGE2, and free radicals [24]. Serotonin signaling was also linked to catabolic activity in human OA chondrocytes, where serotonergic receptor activation was associated with phospholipase A2 activation, increased PGE2 synthesis, and enhanced collagenolytic activity, suggesting that serotonin may promote inflammatory and ECM-degrading pathways in chondrocytes under OA or inflammatory conditions [25]. However, further studies are needed to define serotonin receptor subtype-specific mechanisms, determine whether these pathways operate in vivo, and clarify how serotonin signaling contributes to OA progression, pain-related mechanisms, and other aspects of chondrocyte biology.

5. Conclusions

The available data demonstrate that classical neurotransmitters are involved in the regulation of cartilage biology despite the tissue being avascular and aneural. Norepinephrine, glutamate, GABA, acetylcholine, and serotonin each are demonstrated to have processes involved in chondrocyte differentiation, mechanotransduction, inflammation, ECM homeostasis, and cartilage degeneration and repair through distinct receptor-mediated pathways. Although many findings are supported by animal models or in vitro systems, translation to human cartilage biology remains limited. In addition, the manuscripts reviewed used a vast array of experimental models ranging from primary human articular chondrocytes to ATDC5 cells. Therefore, these results must be interpreted with caution: human cartilage findings carry greater clinical relevance, while other models suggest avenues for further investigation using higher-order experimental systems. We suggest that future studies using human chondrocytes and cartilage, receptor-specific approaches, and clinically relevant disease models will be essential to clarify neurotransmitter-specific mechanisms and determine whether these signaling pathways can be harnessed as novel therapeutic targets for OA and other cartilage-related disorders.