Search Results (10)

Tefluthrin (Tef) is categorized as a type-I pyrethroid insecticide, telmisartan (Tel) functions as an angiotensin II receptor blocker, and KB-R7943 has been identified as an inhibitor of the Na⁺-Ca²⁺ exchange process. However, the influence of these compounds on the amplitude and gating properties of voltage-gated Na⁺ current (I_Na) in neurons associated with pain signaling remains unclear. In cultured dorsal root ganglion (DRG) neurons, whole-cell current recordings revealed that Tef or Tel increased the peak amplitude of I_Na, concomitant with an elevation in the time constant of I_Na inactivation, particularly in the slow component. Conversely, exposure to KB-R7943 resulted in a depression in I_Na, coupled with a decrease in the slow component of the inactivation time constant of I_Na. Theoretical simulations and bifurcation analyses were performed on a modeled interneuron in the spinal dorsal horn. The occurrence of I_Na inactivation accentuated the subthreshold oscillations (SO) in the membrane potential. With an increase in applied current, SO became more pronounced, accompanied by the emergence of high-frequency spiking (HS) with a frequency of approximately 150 Hz. Moreover, an elevation in I_Na conductance further intensified both SO and HF. Consequently, through experimental and in silico studies, this work reflects that Tef, Tel, or KB-R7943 significantly impacts the magnitude and gating properties of I_Na in neurons associated with pain signaling. The alterations in I_Na magnitude and gating in these neurons suggest a close relationship with pain transmission. Full article

A great deal of evidence supports the inevitable importance of spinal glycinergic inhibition in the development of chronic pain conditions. However, it remains unclear how glycinergic neurons contribute to the formation of spinal neural circuits underlying pain-related information processing. Thus, we intended to explore the synaptic targets of spinal glycinergic neurons in the pain processing region (laminae I–III) of the spinal dorsal horn by combining transgenic technology with immunocytochemistry and in situ hybridization accompanied by light and electron microscopy. First, our results suggest that, in addition to neurons in laminae I–III, glycinergic neurons with cell bodies in lamina IV may contribute substantially to spinal pain processing. On the one hand, we show that glycine transporter 2 immunostained glycinergic axon terminals target almost all types of excitatory and inhibitory interneurons identified by their neuronal markers in laminae I–III. Thus, glycinergic postsynaptic inhibition, including glycinergic inhibition of inhibitory interneurons, must be a common functional mechanism of spinal pain processing. On the other hand, our results demonstrate that glycine transporter 2 containing axon terminals target only specific subsets of axon terminals in laminae I–III, including nonpeptidergic nociceptive C fibers binding IB4 and nonnociceptive myelinated A fibers immunoreactive for type 1 vesicular glutamate transporter, indicating that glycinergic presynaptic inhibition may be important for targeting functionally specific subpopulations of primary afferent inputs. Full article

Neuropathic pain is a chronic pain condition that occurs after nerve damage; allodynia, which refers to pain caused by generally innocuous stimuli, is a hallmark symptom. Although allodynia is often resistant to analgesics, the antidepressant duloxetine has been used as an effective therapeutic option. Duloxetine increases spinal noradrenaline (NA) levels by inhibiting its transporter at NAergic terminals in the spinal dorsal horn (SDH), which has been proposed to contribute to its pain-relieving effect. However, the mechanism through which duloxetine suppresses neuropathic allodynia remains unclear. Here, we identified an SDH inhibitory interneuron subset (captured by adeno-associated viral (AAV) vectors incorporating a rat neuropeptide Y promoter; AAV-NpyP⁺ neurons) that is mostly depolarized by NA. Furthermore, this excitatory effect was suppressed by pharmacological blockade or genetic knockdown of α_1B-adrenoceptors (ARs) in AAV-NpyP⁺ SDH neurons. We found that duloxetine suppressed Aβ fiber-mediated allodynia-like behavioral responses after nerve injury and that this effect was not observed in AAV-NpyP⁺ SDH neuron-selective α_1B-AR-knockdown. These results indicate that α_1B-AR and AAV-NpyP⁺ neurons are critical targets for spinal NA and are necessary for the therapeutic effect of duloxetine on neuropathic pain, which can support the development of novel analgesics. Full article

The anterior cingulate cortex (ACC) is particularly critical for pain information processing. Peripheral nerve injury triggers neuronal hyper-excitability in the ACC and mediates descending facilitation to the spinal dorsal horn. The mechanically gated ion channel Piezo1 is involved in the transmission of pain information in the peripheral nervous system. However, the pain-processing role of Piezo1 in the brain is unknown. In this work, we found that spared (sciatic) nerve injury (SNI) increased Piezo1 protein levels in inhibitory parvalbumin (PV)-expressing interneurons (PV-INs) but not in glutaminergic CaMKⅡ⁺ neurons, in the bilateral ACC. A reduction in the number of PV-INs but not in the number of CaMKⅡ⁺ neurons and a significant reduction in inhibitory synaptic terminals was observed in the SNI chronic pain model. Further, observation of morphological changes in the microglia in the ACC showed their activated amoeba-like transformation, with a reduction in process length and an increase in cell body area. Combined with the encapsulation of Piezo1-positive neurons by Iba1⁺ microglia, the loss of PV-INs after SNI might result from phagocytosis by the microglia. In cellular experiments, administration of recombinant rat TNF-α (rrTNF) to the BV2 cell culture or ACC neuron primary culture elevated the protein levels of Piezo1 and NOD-like receptor (NLR) family pyrin domain containing 3 (NLRP3). The administration of the NLRP3 inhibitor MCC950 in these cells blocked the rrTNF-induced expression of caspase-1 and interleukin-1β (key downstream factors of the activated NLRP3 inflammasome) in vitro and reversed the SNI-induced Piezo1 overexpression in the ACC and alleviated SNI-induced allodynia in vivo. These results suggest that NLRP3 may be the key factor in causing Piezo1 upregulation in SNI, promoting an imbalance between ACC excitation and inhibition by inducing the microglial phagocytosis of PV-INs and, thereby, facilitating spinal pain transmission. Full article

Mechanical allodynia (pain to normally innocuous tactile stimuli) is a widespread symptom of inflammatory and neuropathic pain. Spinal or medullary dorsal horn (SDH or MDH) circuits mediating tactile sensation and pain need to interact in order to evoke mechanical allodynia. PKCγ-expressing (PKCγ⁺) interneurons and inhibitory controls within SDH/MDH inner lamina II (II_i) are pivotal in connecting touch and pain circuits. However, the relative contribution of GABA and glycine to PKCγ⁺ interneuron inhibition remains unknown. We characterized inhibitory inputs onto PKCγ⁺ interneurons by combining electrophysiology to record spontaneous and miniature IPSCs (sIPSCs, mIPSCs) and immunohistochemical detection of GABA_ARα2 and GlyRα1 subunits in adult rat MDH. While GlyR-only- and GABA_AR-only-mediated mIPSCs/sIPSCs are predominantly recorded from PKCγ⁺ interneurons, immunohistochemistry reveals that ~80% of their inhibitory synapses possess both GABA_ARα2 and GlyRα1. Moreover, nearly all inhibitory boutons at gephyrin-expressing synapses on these cells contain glutamate decarboxylase and are therefore GABAergic, with around half possessing the neuronal glycine transporter (GlyT2) and therefore being glycinergic. Thus, while GABA and glycine are presumably co-released and GABA_ARs and GlyRs are present at most inhibitory synapses on PKCγ⁺ interneurons, these interneurons exhibit almost exclusively GABA_AR-only and GlyR-only quantal postsynaptic inhibitory currents, suggesting a pharmacological specialization of their inhibitory synapses. Full article

Since the inception of spinal cord stimulation (SCS) in 1967, the technology has evolved dramatically with important advancements in waveforms and frequencies. One such advancement is Nevro’s Senza^® SCS System for HF10, which received Food and Drug and Administration (FDA) approval in 2015. Low-frequency SCS works by activating large-diameter Aβ fibers in the lateral discriminatory pathway (pain location, intensity, quality) at the dorsal column (DC), creating paresthesia-based stimulation at lower-frequencies (30–120 Hz), high-amplitude (3.5–8.5 mA), and longer-duration/pulse-width (100–500 μs). In contrast, high-frequency 10 kHz SCS works with a proposed different mechanism of action that is paresthesia-free with programming at a frequency of 10,000 Hz, low amplitude (1–5 mA), and short-duration/pulse-width (30 μS). This stimulation pattern selectively activates inhibitory interneurons in the dorsal horn (DH) at low stimulation intensities, which do not activate the dorsal column fibers. This ostensibly leads to suppression of hyperexcitable wide dynamic range neurons (WDR), which are sensitized and hyperactive in chronic pain states. It has also been reported to act on the medial pathway (drives attention and pain perception), in addition to the lateral pathways. Other theories include a reversible depolarization blockade, desynchronization of neural signals, membrane integration, glial–neuronal interaction, and induced temporal summation. The body of clinical evidence regarding 10 kHz SCS treatment for chronic back pain and neuropathic pain continues to grow. There is high-quality evidence supporting its use in patients with persistent back and radicular pain, particularly after spinal surgery. High-frequency 10 kHz SCS studies have demonstrated robust statistically and clinically significant superiority in pain control, compared to paresthesia-based SCS, supported by level I clinical evidence. Yet, as the field continues to grow with the technological advancements of multiple waveforms and programming stimulation algorithms, we encourage further research to focus on the ability to modulate pain with precision and efficacy, as the field of neuromodulation continues to adapt to the modern healthcare era. Full article

New strategies for spinal cord stimulation (SCS) for chronic pain have emerged in recent years, which may work better via different analgesic mechanisms than traditional low-frequency (e.g., 50 Hz) paresthesia-based SCS. To determine if 10 kHz and burst SCS waveforms might have a similar mechanistic basis, we examined whether these SCS strategies at intensities ostensibly below sensory thresholds would modulate spinal dorsal horn (DH) neuronal function in a neuron type-dependent manner. By using an in vivo electrophysiological approach in rodents, we found that low-intensity 10 kHz SCS, but not burst SCS, selectively activates inhibitory interneurons in the spinal DH. This study suggests that low-intensity 10 kHz SCS may inhibit pain-sensory processing in the spinal DH by activating inhibitory interneurons without activating DC fibers, resulting in paresthesia-free pain relief, whereas burst SCS likely operates via other mechanisms. Full article

Craniofacial neuropathic pain affects millions of people worldwide and is often difficult to treat. Two key mechanisms underlying this condition are a loss of the negative control exerted by inhibitory interneurons and an early microglial reaction. Basic features of these mechanisms, however, are still poorly understood. Using the chronic constriction injury of the infraorbital nerve (CCI-IoN) model of neuropathic pain in mice, we have examined the changes in the expression of GAD, the synthetic enzyme of GABA, and GlyT2, the membrane transporter of glycine, as well as the microgliosis that occur at early (5 days) and late (21 days) stages post-CCI in the medullary and upper spinal dorsal horn. Our results show that CCI-IoN induces a down-regulation of GAD at both postinjury survival times, uniformly across the superficial laminae. The expression of GlyT2 showed a more discrete and heterogeneous reduction due to the basal presence in lamina III of ‘patches’ of higher expression, interspersed within a less immunoreactive ‘matrix’, which showed a more substantial reduction in the expression of GlyT2. These patches coincided with foci lacking any perceptible microglial reaction, which stood out against a more diffuse area of strong microgliosis. These findings may provide clues to better understand the neural mechanisms underlying allodynia in neuropathic pain syndromes. Full article

The phosphorylation of serine 10 in histone 3 (p-S10H3) has recently been demonstrated to participate in spinal nociceptive processing. However, superficial dorsal horn (SDH) neurons involved in p-S10H3-mediated nociception have not been fully characterized. In the present work, we combined immunohistochemistry, in situ hybridization with the retrograde labeling of projection neurons to reveal the subset of dorsal horn neurons presenting an elevated level of p-S10H3 in response to noxious heat (60 °C), causing burn injury. Projection neurons only represented a small percentage (5%) of p-S10H3-positive cells, while the greater part of them belonged to excitatory SDH interneurons. The combined immunolabeling of p-S10H3 with markers of already established interneuronal classes of the SDH revealed that the largest subset of neurons with burn injury-induced p-S10H3 expression was dynorphin immunopositive in mice. Furthermore, the majority of p-S10H3-expressing dynorphinergic neurons proved to be excitatory, as they lacked Pax-2 and showed Lmx1b-immunopositivity. Thus, we showed that neurochemically heterogeneous SDH neurons exhibit the upregulation of p-S10H3 shortly after noxious heat-induced burn injury and consequential tissue damage, and that a dedicated subset of excitatory dynorphinergic neurons is likely a key player in the development of central sensitization via the p-S10H3 mediated pathway. Full article

The molecular mechanisms regulating neurogenesis involve the control of gene expression by transcription factors. Gbx1 and Gbx2, two members of the Gbx family of homeodomain-containing transcription factors, are known for their essential roles in central nervous system development. The expression domains of mouse Gbx1 and Gbx2 include regions of the forebrain, anterior hindbrain, and spinal cord. In the spinal cord, Gbx1 and Gbx2 are expressed in PAX2⁺ interneurons of the dorsal horn and ventral motor neuron progenitors. Based on their shared domains of expression and instances of overlap, we investigated the functional relationship between Gbx family members in the developing spinal cord using Gbx1^−/−, Gbx2^−/−, and Gbx1^−/−/Gbx2^−/− embryos. In situ hybridization analyses of embryonic spinal cords show upregulation of Gbx2 expression in Gbx1^−/− embryos and upregulation of Gbx1 expression in Gbx2^−/− embryos. Additionally, our data demonstrate that Gbx genes regulate development of a subset of PAX2⁺ dorsal inhibitory interneurons. While we observe no difference in overall proliferative status of the developing ependymal layer, expansion of proliferative cells into the anatomically defined mantle zone occurs in Gbx mutants. Lastly, our data shows a marked increase in apoptotic cell death in the ventral spinal cord of Gbx mutants during mid-embryonic stages. While our studies reveal that both members of the Gbx gene family are involved in development of subsets of PAX2⁺ dorsal interneurons and survival of ventral motor neurons, Gbx1 and Gbx2 are not sufficient to genetically compensate for the loss of one another. Thus, our studies provide novel insight to the relationship harbored between Gbx1 and Gbx2 in spinal cord development. Full article