The CB₂ Receptor in Immune Regulation and Disease: Genetic Architecture, Epigenetic Control, and Emerging Therapeutic Strategies

Abstract

The cannabinoid receptor type 2 (CB₂) is increasingly recognized as a crucial regulator of neuroimmune balance in the brain. In addition to its well-established role in immunity, the CB₂ receptor has been identified in specific populations of neurons and glial cells throughout various brain regions, and its expression is dynamically increased during inflammatory and neuropathological conditions, positioning it as a potential non-psychoactive target for modifying neurological diseases. The expression of the CB₂ gene (CNR2) is finely tuned by epigenetic processes, including promoter CpG methylation, histone modifications, and non-coding RNAs, which regulate receptor availability and signaling preferences in response to stress, inflammation, and environmental factors. CB₂ signaling interacts with TRP channels (such as TRPV1), nuclear receptors (PPARγ), and orphan G Protein-Coupled Receptors (GPCRs, including GPR55 and GPR18) within the endocannabinoidome (eCBome), influencing microglial characteristics, cytokine production, and synaptic activity. We review how these interconnected mechanisms affect neurodegenerative and neuropsychiatric disorders, underscore the species- and cell-type-specificities that pose challenges for translation, and explore emerging strategies, including selective agonists, positive allosteric modulators, and biased ligands, that leverage the signaling adaptability of the CB₂ receptor while reducing central effects mediated by the CB₁ receptor. This focus on the neuro-centric perspective repositions the CB₂ receptor as an epigenetically informed, context-dependent hub within the eCBome, making it a promising candidate for precision therapies in conditions featuring neuroinflammation.

1. Introduction

It is now accepted that the endocannabinoid system (ECS) is a sophisticated and evolutionarily preserved lipid signaling network that regulates various physiological and pathological processes, encompassing immune surveillance, inflammation, energy balance, emotional regulation, pain perception, and synaptic transmission. Throughout history, cannabis has been utilized by ancient civilizations for its pain-relieving and therapeutic benefits. However, it was only in the mid-20th century that the compounds derived from cannabis were identified structurally, leading to the structure elucidation of Δ9-tetrahydrocannabinol (THC) and cannabidiol (CBD) as the primary active components of Cannabis sativa [1,2]. A pivotal advancement occurred with the uncovering of endogenous cannabinoids anandamide (N-arachidonoyl-ethanolamine; AEA) and 2-arachidonoyl-glycerol (2-AG), alongside their corresponding receptors, CB₁ receptor and CB₂ receptor, as well as the enzymes responsible for their synthesis and degradation, such as N-acyl phosphatidylethanolamine-specific phospholipase D (NAPE-PLD), diacylglycerol lipase (DAGLα/β), fatty acid amide hydrolase (FAAH), and monoacylglycerol lipase (MAGL) (reviewed in [3,4]). CB₁ receptors are highly represented in the central nervous system (CNS), primarily mediating the psychoactive effects of THC and regulating neurotransmitter release [5,6]. Conversely, the CB₂ receptor was initially cloned in the myelocytic cell line HL60 and identified as a peripheral receptor in immune tissues, such as the spleen, tonsils, and circulating leukocytes [5,7,8]. This system functions as a precisely regulated homeostatic network, activated as needed in response to stress, injury, or immune challenges [3]. The CB₂ receptor has attracted significant interest due to its non-psychoactive characteristics and diverse immunomodulatory roles (reviewed in [9,10]). In basic and translational studies, researchers are actively investigating its functions in inflammation, immune cell movement, mood regulation, neurodegeneration, and cancer immunity.

As the understanding of CB₂ receptor dynamics deepens, including its inducible expression, cell-type specificity, and distinct signaling capabilities, there is growing interest in leveraging its potential for therapeutic intervention. Although several excellent reviews cited above have addressed the CB₂ receptor in the context of inflammation or cancer, none have systematically synthesized how epigenetic and transcriptional regulation intersect with disease-specific immune contexts. Accordingly, this review outlines the biology and regulation of the CB₂ receptor, then synthesizes its roles in immune modulation, disease contexts (including autoimmune, neuropsychiatric, and cancer), and emerging checkpoint-like functions, before discussing therapeutic strategies. It uniquely emphasizes how genetic, epigenetic, and environmental cues converge to shape CB₂ receptor expression and function, positioning it as a context-sensitive hub. By consolidating findings from transcriptomic studies, knockout models, and emerging pharmacological tools, this article aims to reframe the CB₂ receptor as a precision-modulated therapeutic target within the broader endocannabinoidome (eCBome) network.

2. Molecular Biology of the CB₂ Receptor

The CB₂ receptor is encoded by the CNR2 gene in humans, which is located on chromosome 1p36 [8,11]. The typical isoform consists of a protein composed of 360 amino acids, featuring seven transmembrane α-helices that are characteristic of class A GPCRs, with an extracellular N-terminus that plays a role in ligand binding and an intracellular C-terminal domain essential for signaling and receptor trafficking. The CB₂ receptor shares around 44% sequence homology with the CB₁ receptor but displays expression patterns and pharmacological characteristics that vary by tissue and species [8]. Notably, the CB₂ receptor exhibits 82% sequence similarity between humans and rodents; however, there are species-specific differences in expression and function that must be considered when interpreting preclinical results [11,12,13]. Recently, a comprehensive transcriptomic analysis of human peripheral leukocytes revealed that CB2 receptor mRNA is significantly expressed in granulocytes (neutrophils and eosinophils) and PBMCs (monocytes, B and T lymphocytes) [5,7]. This receptor can also be found in stem cells derived from bone marrow and various epithelial and endothelial tissues. In the brain, the CB₂ receptor has been identified in particular regions, including the hippocampus, substantia nigra, cortex, amygdala, and prefrontal cortex [14,15,16].

3. Epigenetic and Transcriptional Regulation of the CB₂ Receptor

The regulation of the CNR2 gene through epigenetic processes is crucial for modulating CB₂ receptor expression in response to cellular stimuli. Different layers of epigenetic control, including DNA methylation, histone modifications, and non-coding RNAs, regulate the accessibility and transcriptional activity of the CNR2 region. These processes not only calibrate receptor expression but also enable the CB₂ receptor to adjust its roles in line with physiological demands [17,18,19].

A key mechanism influencing CB₂ receptor expression is the methylation of DNA at CpG-rich regions within the CNR2 promoter. In the absence of inflammation, DNA methyltransferases (DNMT1, DNMT3A, and DNMT3B) add methyl groups to cytosines in these CpG islands [20,21,22,23]. This methylation inhibits gene transcription primarily through two means: (1) it directly obstructs transcription factor binding, and (2) it recruits methyl-CpG-binding proteins (MeCP) like methyl-CpG-binding protein 2(MeCP2). MeCP2 then acts as a platform to attract histone deacetylases (HDACs) and chromatin remodeling complexes, which compact the chromatin and maintain a transcriptionally inactive state [24,25,26,27,28,29]. Environmental factors, such as inflammation, infection, psychological stress, or exposure to cannabinoids, can also trigger ten-eleven translocation (TET) enzymes (TET1, TET2, and TET3), which convert 5-methylcytosine to 5-hydroxymethylcytosine. This modification promotes a shift toward a more open chromatin state, creating a permissive environment for gene transcription, which may include reactivation of the CNR2 gene under appropriate cellular conditions [23,30].

Enhancing histone marks, such as acetylation of histone H3 at lysine 9 (H3K9ac) and trimethylation of histone H3 at lysine 4 (H3K4me3), are closely linked to open chromatin and active gene transcription. These activating changes are facilitated by enzymes such as CREB-binding protein/E1A binding protein p300 (CBP/p300), a group of transcriptional coactivators crucial for regulating gene expression, and mixed lineage leukemia (MLL) family methyltransferases, also known as the lysine methyltransferase 2 (KMT2) family [31,32]. Their activity is frequently stimulated by pro-inflammatory signals, immune activation, or engagement of the CB₂ receptor, which kick-starts downstream signaling pathways such as mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) and phosphoinositide 3-kinase (PI3K)/protein kinase B (PKB, also known as AKT) [20]. These pathways further enhance the recruitment of co-activators and transcription factors to the CNR2 promoter, boosting gene expression [33,34]. Conversely, repressive histone marks, such as Histone H3 lysine 9 dimethylation (H3K9me2) and Histone H3 lysine 27 trimethylation (H3K27me3), are introduced by histone methyltransferases, such as histone H3 Lys-9 methyltransferase (G9a) and Enhancer of zeste homolog 2 (EZH2), respectively, the primary components of the Polycomb Repressive Complex 2 (PRC2) [35]. Non-coding RNAs add a layer of regulatory complexity to the expression of the CB₂ receptor. MicroRNAs (miRNAs), such as miR-139 and miR-665, have been identified as binders to the 3′ untranslated region (3′ UTR) of CNR2 mRNA, inhibiting translation under resting conditions (Figure 1) [36].

In cases of chronic heart failure, there is a notable increase in CB₂ receptor expression within human heart muscle tissue, indicating a possible adaptive mechanism involving the endocannabinoid system. Weis et al. (2010) found that levels of CB₂ receptor mRNA were significantly elevated in the myocardium of left ventricular failure, while CB₁ receptor expression showed a slight decline [37]. This shift in receptor expression was correlated with a considerable increase in circulating endocannabinoids, indicating an overall activation of the endocannabinoid system. Similarly, Möhnle et al. (2014) demonstrated that miR-665 functions as a post-transcriptional regulator of the CB₂ receptor [38]. Their research indicated that miR-665 was notably downregulated in heart tissue samples from patients with heart failure and predicted to target the 3′ UTR of CNR2 mRNA. The negative correlation observed between miR-665 and CB₂ receptor expression suggests a regulatory mechanism in which lower levels of miR-665 may alleviate the translational inhibition of CB₂ receptor, thereby promoting increased receptor expression. Collectively, these results highlight a coordinated mechanism in which modulation of miRNA and heightened endocannabinoid activity may facilitate CB₂ receptor-mediated responses as a compensatory adaptation in the failing heart.

4. CB₂ Receptor Pharmacology

The pharmacological and signaling heterogeneity associated with cannabinoid receptors, particularly the CB₂ subtype, underscores the complexity of predicting biological outcomes based solely on receptor binding affinity. Selective CB₂ agonists, such as dimethylbutyl-deoxy-delta-8-THC (JWH-133), JWH-015, Onternabez (HU-308), and WIN55,212-2, exemplify this diversity, as do mixed cannabinoid agonists and endogenous mediators. Variations in potency and efficacy observed across different experimental assays and cellular contexts may be attributed to the inherent flexibility of the CNR2 locus, alongside the capacity of downstream transcriptional machinery to activate CB₂-receptor-dependent signaling pathways effectively. This intricacy underscores the necessity for a nuanced understanding of cannabinoid pharmacology, particularly in relation to various tissues and pathological states (

Table 1. Representative CB₁ and CB₂ receptor ligands with binding affinities and pharmacological profiles. This table highlights commonly used synthetic and endogenous ligands at CB₁ and CB₂ receptors. Selective agonists such as JWH-133, JWH-015, and HU-308 are key tools for CB₂-receptor focused studies, while WIN55,212-2 (WIN) serves as a non-selective reference agonist. Endogenous ligands AEA and 2-AG are included as physiological benchmarks, despite their rapid enzymatic breakdown.

Compound	Chemical Class	Ki CB1 (nM)	Ki CB2 (nM)	Key Notes	Ref.
JWH-133	Dimethylbutyl-deoxy-Δ8-THC	677	3.4	Widely used CB2 agonist; strong in vivo efficacy; off-target at high doses.	[39]
JWH-015	1-Propyl-2-methyl-3-(1-naphthoyl) indole	383	13.8	CB2-preferring; retains CB1 activity in neural systems.	[40]
HU-308	bicyclo [3.1.1]hept-2-ene	>10,000	22.7	Peripherally restricted CB2 agonist; strong immune/inflammation applications.	[41]
WIN55,212-2	aminoalkylindole analog	1.89–123	0.28–16.2	Potent dual cannabinoid receptor agonist.	[42]
AEA	N-acylethanolamine	61–543	279–1940	Endogenous ligand; partial agonist; rapidly hydrolyzed by FAAH.	[43]
2-AG	monoacyl-glycerol	58.3–472	145–1400	Full agonist; hydrolyzed by MAGL; critical in physiology.	[43]

).

When activated by endogenous ligands (AEA, 2-AG) or synthetic compounds such as JWH-133, WIN, and/or HU-308, CB₂ receptors associate with G protein alpha subunit (Gαi/o) proteins, leading to the inhibition of adenylyl cyclase, decreased cyclic adenosine monophosphate (cAMP) synthesis, and subsequent suppression of Protein Kinase A (PKA) [14]. Beyond traditional G protein signaling, CB₂ receptors also interact with β-arrestin pathways, which are responsible for receptor internalization, desensitization, and alternative signaling routes, including ERK1/2, p38 MAPK, and PI3K/Akt, and activate transcription of cAMP response element-binding protein (CREB) and nuclear factor kappa-light-chain-enhancer of activated B cells (NF-KB) genes to control cellular functions (Figure 2 and [9]). Evidence of ligand-biased signaling has shown that different ligands can preferentially engage either G protein or β-arrestin pathways, allowing for pharmacological fine-tuning. Allosteric modulation is another mechanism modulating CB₂ receptor activity selectively [44]. PAMs, such as the 2-oxopyridine-3-carboxamide derivative (EC21a) and endogenous peptides like RVD-hemopressin (Pepcan-12), increase the sensitivity of CB₂ receptors to endogenous cannabinoids without acting as direct agonists, thereby preserving physiological signaling dynamics [44,45,46]. Conversely, negative allosteric modulators (NAMs) can diminish receptor activity and may provide therapeutic advantages in conditions involving excessive CB₂ receptor activation [47]. CB₂ receptors also undergo post-translational modifications, including glycosylation and phosphorylation, which influence receptor transport, localization, and the accuracy of signaling. G protein-coupled receptor kinases (GRKs) and β-arrestins regulate receptor desensitization and recycling. In contrast, receptor dimerization with the CB₁ receptor or other GPCRs introduces another layer of functional complexity.

5. The Role of CB₂ Receptor in Immune Regulation

The CB₂ receptor serves as a crucial modulator of both innate and adaptive immune responses. The activation of the CB₂ receptor results in the inhibition of pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-α), interleukin-1β (IL-1β), interleukin-6 (IL-6), and interferon-gamma (IFN-γ), while it encourages the production of anti-inflammatory cytokines, including IL-10 and transforming growth factor-beta (TGF-β) [6]. Additionally, the CB₂ receptor influences inflammasome activation and the generation of reactive oxygen species (ROS), which contributes to its documented anti-inflammatory characteristics [9,10]. CB₂ receptor signaling promotes the macrophage phenotype 2 (M2) in monocytes and macrophages over the macrophage phenotype 1 (M1) [48]. M2 macrophages that CB₂ receptor agonists stimulate display increased phagocytic ability and diminished antigen presentation, both of which are vital for resolving inflammation and conducting tissue repair [49]. In dendritic cells (DCs), activation of the CB₂ receptor decreases the expression of major histocompatibility complex class II (MHC-II) and co-stimulatory molecules, cluster of differentiation 80 and 86 (CD80, CD86), which hinders maturation and reduces their capacity to activate naïve T cells [50]. These consequences produce a tolerogenic DC phenotype that lessens T-cell activation and promotes immune resolution. The ability of CB₂ receptors to inhibit the migration of dendritic cells to lymph nodes through the downregulation of CC chemokine receptor type 7 (CCR7) and changes in cytoskeletal structures further limits immune amplification. T lymphocytes exhibit low baseline expression of the CB₂ receptor, but this expression can increase upon activation. In the latter study, it has been demonstrated that CB₂ receptor signaling in CD4 + T helper cells (CD4 + T cells) appears to restrict proliferation and differentiation into T helper 1 cells (Th1) and T helper 17 cells (Th17) subsets while encouraging the development of regulatory T cells (Tregs) [51]. This immunosuppressive action is partially facilitated by reducing the secretion of interleukin-2 (IL-2), IFN-γ, and interleukin-17 (IL-17), as well as modulating transcription factors like T-box expressed in T cells (T-bet) and RAR-related orphan receptor gamma t (RORγt) [52,53,54]. In conclusion, the CB₂ receptor orchestrates a complex immunoregulatory program, and precisely targeting the CB₂ receptor with selective modulators, PAMs, or biased ligands presents a novel strategy for adjusting immune function without broadly suppressing host immunity.

6. CB₂ Receptor and Microglia in Neuroinflammation

Microglia, the brain’s resident macrophages, are crucial for preserving CNS homeostasis and managing immune responses during injury and disease. In a healthy brain, microglia exhibit a surveying state characterized by branching shapes and minimal expression of inflammatory proteins [55]. When faced with pathological triggers such as infection, trauma, or neurodegeneration, microglia quickly transform morphologically and functionally into reactive states, such as M1 or M2. Although this M1/M2 classification is somewhat simplistic, it serves as a helpful conceptual framework for understanding immune polarization in the CNS. Notably, the CB₂ receptor participates in regulating this polarization process [56]. Mechanistically, activating the CB₂ receptor can prevent the nuclear translocation of NF-κB, diminish MAPK pathway activation, and modulate the production of ROS and reactive nitrogen species (RNS), thereby dampening the pro-inflammatory signaling cascade at several levels [57].

Recent studies conducted both in vivo and in vitro support the role of the CB₂ receptor in modulating the microglial phenotype, for instance, activation of the CB₂ receptor, as evidenced by decreased expression of inducible nitric oxide synthase (iNOS) and increased levels of arginase-1 (Arg-1) (Figure 3) [58,59,60,61]. In models of Alzheimer’s disease (AD), traumatic brain injuries, and stroke, pharmacological stimulation of CB₂ receptors has lessened neuroinflammation, reduced neuronal death, improved cognitive functioning, and minimized gliosis [62,63,64,65,66,67,68,69,70]. In conditions like neuropathic pain and multiple sclerosis (MS), CB₂ receptor signaling has been found to reduce neurotoxicity caused by microglia and promote remyelination. Consistent with a microglia-centric view of CB₂ in CNS inflammation, two reporter/knockout studies in Alzheimer’s models localize the inducible CB₂ pool to plaque-associated microglia and probe its functional impact. Using CB2^EGFP/f/f reporter mice (Enhanced Green Fluorescent Protein (EGFP) under the CNR2 promoter) and 5xFAD mice co-expressing five familial Alzheimer’s Disease mutations, crossing (CB2^EGFP/f/f/5xFAD), López et al. (2018) showed that the CB₂ signal is low in the healthy brain but robustly upregulated by intense inflammation and amyloid deposition in microglia clustered around amyloid plaques; significantly, global CB₂ deletion in CB2^−/−/5xFAD mice reduced plaque burden and glial reactivity, indicating that disease-induced CB₂ is not uniformly protective in vivo [71].

Extending this, Esteban S. et al. in 2022 combined CB2^EGFP/f/f, CB2^−/−, and 5xFAD lines (CB2^EGFP/f/f/5xFAD and CB2^−/−/5xFAD) using microglia, and demonstrated that loss of CB₂ blunts microglial phagocytosis and p38-MAPK activation in vitro, alters plaque-proximal microglial phenotypes in vivo, and, consistent with López et al., is accompanied by fewer methoxy-X04⁺ plaques in 5xFAD/CB2^−/− brains [72]. The authors highlighted a crucial function of cannabinoid CB₂ receptors in microglial activities and the metabolism of β-amyloid peptide (Aβ) within an animal model of AD (5xFAD). They discovered that the lack of CB₂ receptors reduces both the overall number of microglial cells and their capacity to engulf Aβ, as well as having a regulatory effect on the build-up of the insoluble form of this harmful peptide. Their findings imply that microglial CB₂ receptors might be continually activated in the context of Alzheimer’s Disease, as evidenced by the phosphorylation state of p38 MAPK.

In Parkinson’s disease, the CB₂ receptor has been repeatedly implicated in neuroinflammation and dopaminergic neuron protection, and elevated levels of brain iron [73,74,75]. Experimental models demonstrate that the CB₂ receptor is markedly upregulated in activated microglia of the substantia nigra, and its stimulation reduces neurotoxin-induced damage [76]. For example, in MPTP-treated mice, selective CB₂ receptor agonists decreased microglial activation and preserved dopaminergic neurons, whereas CB₂ receptor deletion exacerbated neuronal loss and motor impairment [77]. Similar neuroprotection has been observed with CB₂ receptor activation in 6-OHDA and Lipopolysaccharide (LPS) models of Parkinson’s disease, where treatment suppressed pro-inflammatory cytokines (TNF-α, IL-1β) and enhanced anti-inflammatory mediators, such as IL-10 [78]. Positron emission tomography studies using CB₂ receptor ligands have further confirmed the dynamic upregulation of CB₂ receptors in microglial populations during Parkinson’s disease progression, reinforcing its translational relevance [79]. Although direct evidence linking CB₂ receptor signaling to epigenetic regulation in Parkinson’s disease is still scarce, emerging data from related neuroinflammatory contexts point to potential mechanisms. Together, these findings suggest that CB₂ receptor contributes to Parkinson’s disease pathology not only via receptor signaling but also potentially through epigenetic feedback loops that govern inflammatory and neuroprotective gene programs (Figure 4).

7. CB₂ Receptor in Depression and Psychiatric Disorders

Depression is increasingly acknowledged as a complex disorder involving the neuroimmune system, marked by a subtle interaction between chronic inflammation, improper stress response regulation, reduced neuroplasticity, and altered microglial cell activity. Elevated concentrations of pro-inflammatory cytokines, such as TNF-α, IL-1β, and IL-6, are frequently found in the serum and cerebrospinal fluid of individuals with depression [80]. These cytokines interfere with neurotransmission by decreasing serotonin production through the indoleamine 2,3-dioxygenase (IDO-mediated metabolism of tryptophan), hindering neurogenesis in the hippocampus, and enhancing excitotoxicity [81]. Furthermore, chronic stress triggers the activation of microglia and immune cells, leading to a systemic inflammatory response that worsens neurobiological impairments. Research has shown increased levels of CB₂ receptor expression in brain tissues and immune cells of animal models and humans with major depressive disorder [82]. Activation of the CB₂ receptor has been found to alleviate excessive hypothalamic–pituitary–adrenal axis activity, likely by reducing both central and peripheral inflammation. CB₂ receptor signaling diminishes the production of pro-inflammatory cytokines that typically contribute to hypothalamic–pituitary–adrenal axis hyperactivity [83]. Importantly, CB₂ receptors may serve a dual role by regulating cytokine release from immune cells and inhibiting neuronal hyperexcitability in stress-sensitive regions such as the hippocampus and prefrontal cortex [84]. Pharmacological stimulation of CB₂ receptors has been shown to normalize serum corticosterone levels, diminish glial reactivity, and restore expression of brain-derived neurotrophic factor (BDNF), all of which are crucial for mood regulation and synaptic plasticity [85].

Transcription factors such as nuclear erythroid 2-related factor 2 (NRF-2), NF-κB, and Activator Protein-1 (AP-1) have been associated with the regulation of the CB₂ receptor gene. NRF-2 connects oxidative stress responses with the activation of the CB₂ receptor [86]. When exposed to stress, the translocation of these transcription factors to the nucleus can trigger the transcription of CNR2, suggesting a dynamic interaction among redox status, inflammation, and cannabinoid signaling [29]. MeCP2 plays a key role in maintaining the CNR2 gene in a transcriptionally repressed state under homeostatic conditions [87]. This binding is crucial for the action of inhibitors (SNRIs), which modulate the inflammatory environment and thereby diminish the effectiveness of monoamines [88]. Overall, the CB₂ receptor acts as a link between the immune system and CNS functions by modulating inflammation, releasing stress hormones, synaptic adaptability, and glial responsiveness.

8. CB₂ Receptor in Chronic Pain and Neuropathy

Chronic pain is a complex condition affecting millions around the world, often resulting from prolonged tissue damage, nerve injury, or disrupted neuroimmune signaling. It includes various types, such as inflammatory pain, pain related to cancer, and neuropathic pain [89]. Neuropathic pain occurs due to injuries or dysfunctions in the somatosensory system and often proves resistant to traditional pain relievers like nonsteroidal anti-inflammatory drugs (NSAIDs) or opioids [89,90]. After a nerve injury, peripheral immune cells, such as macrophages and neutrophils, move into the site of injury, releasing pro-inflammatory substances like IL-6, TNF-α, and prostaglandins. These mediators make nociceptors more sensitive by altering ion channels and increasing receptor expression at the peripheral ends [91]. At the same time, microglia and astrocytes in the spinal cord and dorsal root ganglia (DRG) become active, and this interaction between glial and neuronal cells maintains central sensitization and allodynia. Significantly, following peripheral nerve injury, CB₂ receptor expression rises in DRG macrophages, spinal microglia, and astrocytes [92,93,94,95,96,97,98]. This increase makes the CB₂ receptor a potential molecular regulator that can help control excessive neuroimmune activation, as previously suggested [99]. Studies conducted in preclinical settings using CB₂ receptor agonists, such as JWH-133, AM1241, and BCP, have exhibited significant reductions in mechanical and thermal hyperalgesia [39,90,100,101,102]. In both spinal cord slices and in vivo models, the administration of CB₂ receptor agonists or the overexpression of the CB₂ receptor has been shown to decrease excitatory neurotransmission by inhibiting glutamate release and restricting synaptic facilitation at dorsal horn neurons [83]. Concurrently, CB₂ receptor activation boosts inhibitory GABAergic and glycinergic signaling, thereby restoring the excitatory-inhibitory balance disrupted in chronic pain conditions. Notably, inhibiting MAGL not only increases the levels of 2-AG but also reduces the production of prostaglandins, thereby further enhancing pain relief [103]. Strategies that combine CB₂ receptor agonists with FAAH inhibitors have shown enhanced analgesic effects, highlighting the significance of endogenous ligand support for the control mediated by CB₂ receptor. Phytocannabinoids, such as CBD and cannabigerol (CBG), exhibit anti-inflammatory and pain-relieving effects, which are often partially mediated through CB₂ receptor signaling [104]. CBG functions as a weak antagonist of the CB₁ receptor and a partial agonist of the CB₂ receptor while also interacting with TRP channels and α2-adrenergic receptors [105,106,107]. In models of neuropathic pain, CBG alleviates hypersensitivity and mitigates the effects of microglial activation, which are reversed by CB₂ receptor antagonists, indicating its dependence on CB₂ receptors. The combination of CBD and CBG exhibits synergistic effects, likely resulting from their joint action on CB₂ receptors and TRP channels, as well as the suppression of cytokine release. Clinical investigations of balanced THC:CBD formulations such as nabiximols (Sativex^®) have demonstrated effectiveness in treating neuropathic pain associated with multiple sclerosis, although the roles of the CB₂ receptor compared to the CB₁ receptor continue to be explored [108,109,110,111,112,113,114,115,116]. There is a scarcity of clinical trials targeting the CB₂ receptor for pain management; however, selective ligands are beginning to undergo early-phase assessments. The limited expression of the CB₂ receptor in disease-activated cells and the absence of CNS side effects underscore its potential as an appealing target for next-generation analgesics [117].

9. CB₂ Receptor in Schizophrenia and Dopaminergic Circuits

Schizophrenia is a multifaceted neuropsychiatric condition marked by positive symptoms, negative symptoms, and cognitive impairments [118]. Studies in rodents have demonstrated that the CB₂ receptor is present in both the tyrosine hydroxylase-positive neurons of the ventral tegmental area and medium spiny neurons expressing dopamine receptor 1 (D1) or dopamine receptor 2 (D2) in the nucleus accumbens [119,120,121,122]. The activation of CB₂ receptors in these areas leads to decreased neuronal excitability and reduced dopamine release, indicating a role in regulating dopaminergic tone. The agonism of the CB₂ receptor results in neuronal hyperpolarization within dopaminergic circuits, which is diminished after depolarization in key regions, such as the striatum and prefrontal cortex. Preclinical research utilizing a selective CB₂ receptor agonist, JWH-133, has demonstrated that the activation of CB₂ receptors inhibits the hyperactivity of ventral tegmental area’s dopamine neurons both in vitro and in vivo [121,122,123,124,125,126]. Microdialysis investigations indicate that the administration of CB₂ receptor agonists into the nucleus accumbens significantly decreases extracellular dopamine levels, while CB₂ receptor antagonist iodopravadoline (AM630) enhances dopaminergic activity [120,125]. These effects do not occur in CB₂ receptor knockout mice, confirming the receptor specificity of the impact. This bidirectional modulation implies that CB₂ receptor signaling likely establishes a negative feedback mechanism within mesolimbic circuits. As hyperdopaminergic activity in the striatum is linked to psychotic symptoms, the attenuation by CB₂ receptor may serve to balance dopaminergic overactivity, presenting a potential mechanism for antipsychotic-like effects without the motor side effects associated with D2 antagonists.

Genetic research has further indicated the role of the CB₂ receptor in schizophrenia [127]. Variations in the CNR2 gene, specifically single-nucleotide polymorphisms (SNPs), have been linked to an increased risk of psychosis, decreased CB₂ receptor expression, and altered inflammatory gene networks [128,129]. For example, the gene variation rs35761398 (Q63R) polymorphism has been associated with reduced functionality of the CB₂ receptor and increased susceptibility to stress-related behavioral traits [128,130,131]. In patients with schizophrenia, notable changes in DNA methylation patterns and histone modifications are observed at the CNR2 gene locus due to epigenetic factors [132,133,134,135,136,137]. Environmental stressors, infections, and toxins can also be recognized as risk factors for schizophrenia, which can trigger epigenetic alterations in CB₂ receptor expression within the CNS. Also, elevated levels of IL-6 and TNF-α and activated microglia have been observed in both postmortem studies and in vivo imaging of individuals with schizophrenia [138,139,140]. In cases where the CB₂ receptor is absent, studies in mice have shown worsened inflammatory reactions and behavioral abnormalities like those seen in schizophrenia, such as reduced prepulse inhibition, altered social behavior, and heightened locomotor activity in response to psychostimulants [141,142,143].

On the other hand, activating the CB₂ receptor pharmacologically has been shown to reverse prepulse inhibition deficits caused by dizocilpine (MK-801) and restore abnormal dopamine signaling in models of schizophrenia [144]. In these models, the levels of 2-AG are frequently found to be increased, and the dysregulation of enzymes such as MAGL and FAAH leads to circuit imbalances [145,146,147]. Using biased CB₂ receptor ligands or PAMs, for instance, EC21a, selectively promotes CB₂ receptor-mediated anti-inflammatory and antinociceptive effects without fully activating the receptor, thereby minimizing potential side effects [148,149,150]. Considering the multifaceted causes of schizophrenia, approaches that target multiple pathways could prove to be effective. The convergence of cannabinoid signaling, immune system regulation, and synaptic plasticity positions the CB₂ receptor as the most promising candidate in the field of psychopharmacology.

10. Crosstalk Within the eCBome

The eCBome, particularly the CB₂ receptor, holds significant promise for developing innovative therapies targeting neuroinflammation and neurodegenerative conditions [66,67,83,122,151,152,153,154,155]. Targeting the CB₂ receptor could help reestablish equilibrium in both neuronal and glial cells, thereby reducing degenerative and inflammatory damage. Consequently, modulating the CB₂ receptor may represent a promising strategy for addressing complex neurodegenerative disorders and their effects in a dynamic epigenetic landscape, where current studies are limited [156]. The activity of CB₂ receptors within the expanding concept of the eCBome does not function independently but collaborates with various signaling mechanisms, including TRP ion channels, nuclear receptors, and GPCRs [157]. This interconnected signaling framework captures the complexity of the roles of CB₂ receptors across neuroimmune disorders.

One of the most significant interactions occurs between the CB₂ receptor and TRPV1, a calcium-permeable ion channel that plays a vital role in pain signaling, oxidative stress, and immune response (reviewed in [158,159,160]). The CB₂ receptor and TRPV1 are co-expressed in microglia, peripheral macrophages, and sensory neurons [77,161,162]. Evidence suggests that CB₂ receptor activation can mitigate TRPV1-driven neuroinflammation by reducing calcium influx, ROS generation, and the production of pro-inflammatory cytokines, such as IL-6 and TNF-α, in the progression of neurodegenerative diseases, including Alzheimer’s Disease and Parkinson’s disease [77,163,164,165]. Additionally, phytocannabinoids such as CBD and CBG, which exhibit weak binding to the CB₂ receptor, also act on TRP channels, displaying agonistic effects at TRPV1 and antagonistic effects at transient receptor potential melastatin subtype 8 (TRPM8), thereby enhancing the CB₂ receptor’s contribution to an overarching anti-nociceptive and anti-inflammatory pathway [166,167]. This collaborative action underscores the potential for dual targeting of CB₂ receptor and TRPV1 in developing innovative pain management strategies and neuroprotective treatments.

The CB₂ receptor also exhibits a synergistic regulatory relationship with PPARγ, which regulates the expression of anti-inflammatory genes and mediates metabolic change [168]. Both endogenous and plant-derived cannabinoids, including AEA, palmitoylethanolamide (PEA), and oleoylethanolamide (OEA), interact with the CB₂ receptor and PPARγ to shift macrophages toward the M2 phenotype, increase IL-10 levels, and inhibit NF-κB signaling [169,170,171,172,173]. In microglia, the interaction between the CB₂ receptor and PPARγ supports neuroprotection by limiting excitotoxicity and facilitating the clearance of phagocytic debris [174,175].

Moreover, the CB₂ receptor signaling interacts with orphan GPCRs, such as GPR55 and GPR18, which share endogenous ligands with the CB₂ receptor [176]. GPR55 is a cannabinoid-responsive GPCR incorporated into the eCBome and engaged most robustly by the lysophospholipid lysophosphatidylinositol (LPI) [177]. In the brain, GPR55 is detected in neurons and microglia across the striatum, hippocampus, forebrain, cortex, and cerebellum [178,179]. In Alzheimer’s Disease tissues and models, GPR55 immunostaining is strongest in neurons, and exposure to Aβ42 increases neuronal GPR55 expression, providing evidence that the receptor is dynamically regulated in neurodegeneration [178]. At the mechanistic level, LPI-GPR55 drives RhoA-dependent Ca²⁺/NFAT signaling, providing a well-described route to transcriptional changes relevant to neuroinflammation and plasticity [180]. On the other hand, GPR18 has emerged as a microglia-facing GPCR within the broader eCBome, as it is expressed in microglia and other CNS cells [181]. It can be engaged by endogenous lipids derived from the endocannabinoid pathway, positioning it to couple immune tone with neuronal viability [182]. In organotypic hippocampal slice cultures subjected to NMDA excitotoxicity, the endogenous anandamide derivative N-arachidonoyl-glycine reduced neuronal damage, and this neuroprotection was prevented by pharmacological blockade of GPR18; the same study detected GPR18 mRNA/protein in microglia, astrocytes, and neurons and reported that N-arachidonoyl-glycine modulated glial activation states, indicating a glia-neuron mechanism of action [183]. In cultured microglia and heterologous GPR18 systems, N-arachidonoyl-glycine also drives directed migration and MAPK signaling at low nanomolar concentrations, responses consistent with a role in microglial motility [181]. Collectively, the CB₂ receptor stands out as a crucial signaling hub within the eCBome, integrating immune, metabolic, and neuronal signals through various receptor interactions. Its actions in regulating immunity are amplified and refined through interactions with TRP channels, PPARs, and orphan GPCRs, creating a dynamic network that can finely adjust inflammation and immune resolution in the brain [184,185].

11. Conclusions

This review provides a detailed and integrative examination of the CB₂ receptor, emphasizing its crucial role within the eCBome and its substantial contributions to neuroimmune regulation. Moving beyond the conventional perception of the CB₂ receptor as merely a peripheral immune receptor, we emphasize its dynamic and inducible expression across microglia and its context-dependent upregulation within neuronal cells, underscoring its adaptability across both immune and non-immune landscapes. The CB₂ receptor is notable for its inducible characteristics and the intricate regulatory frameworks that govern its expression, especially in the settings of immune activation, inflammation, and neuroimmune signaling. Its transcriptional control is tuned by a multilayered network of DNA- and RNA-based mechanisms, including promoter methylation, histone modifications, and the activity of non-coding RNAs. Understanding the specific epigenetic pathways involved could be crucial for grasping how endocannabinoids and cannabinoids affect inflammation in both neurological diseases. By identifying the exact epigenetic mechanisms that cannabinoids utilize to reduce inflammatory responses, scientists may discover new targets for preventing and treating a range of inflammatory and neurological disorders. This investigation not only enhances our understanding of the effects of cannabinoids on epigenetics but also has the potential to transform therapeutic approaches in addressing these complex conditions. The discussed epigenetic processes ensure CB₂ receptor expression is precisely modulated in response to physiological demands or pathological stress, enabling it to shape immune cell behavior and inflammatory outcomes with high specificity. Beyond transcriptional control, CB₂ receptor signaling exhibits functional versatility through mechanisms such as biased agonism, receptor dimerization, and interaction with complementary pathways. This signaling plasticity positions the CB₂ receptor as a regulatory hub within the broader eCBome network. Framed within multiple pathological contexts, including neuroinflammation, depression, chronic pain, schizophrenia, Alzheimer’s disease, and Parkinson’s disease, this review highlights how the CB₂ receptor modulates the immune landscape by promoting regulatory over effector responses, reprogramming macrophage and microglial phenotypes, altering cytokine profiles, and influencing cell trafficking.

While preclinical studies provide strong evidence for the therapeutic potential of CB₂ receptors, clinical translation faces several challenges, including species-specific differences in CB₂ receptor biology, limited pharmacological selectivity, and the absence of predictive biomarkers for patient stratification. Nonetheless, emerging strategies such as allosteric modulation, biased ligand design, and polypharmacological approaches, particularly the co-targeting of CB₂ receptor-PPARγ and CB₂ receptor-TRPV1, are paving the way for more precise and effective therapeutic interventions.

In conclusion, the CB₂ receptor functions as an inducible, epigenetically regulated node within the eCBome, serving as a precise and adaptable mechanism to adjust immune-neuronal interactions in the diseased brain. Significantly, the expression and coupling of CNR2 are influenced by chromatin status (including DNA methylation and histone modifications) and by the surrounding network, emphasizing that effective treatments need to consider both epigenetic factors and the eCBome. Collectively, these findings enhance our understanding of the CB₂ receptor as a target not only for peripheral inflammatory modulation but also as a promising candidate for innovative therapies targeting immune-mediated and neurological disorders.