The α5 subunit containing GABAA receptors contribute to chronic pain

Rate-Dependent Depression of the Hoffmann Reflex: Practical Applications in Painful Diabetic Neuropathy

Measurement of the rate-dependent depression (RDD) of the Hoffmann (H) reflex, a technique developed over half a century ago, is founded on repeated stimulation of the H-reflex with tracking of sequentially evoked H-wave amplitudes in the resulting electromyogram. RDD offers insight into the integrity of spinal reflex pathways and spinal inhibitory regulation. Initially, RDD was predominantly utilized in the mechanistic exploration and evaluation of movement disorders characterized by spasticity symptoms, as may occur following spinal cord injury. However, there is increasing recognition that sensory input from the periphery is modified at the spinal level before ascending to the higher central nervous system and that some pain states can arise from, or be exaggerated by, disruption of spinal processing via a mechanism termed spinal disinhibition. This, along with the urgent clinical need to identify biological markers of pain generator and/or amplifier sites to facilitate targeted pain therapies, has prompted interest in RDD as a biomarker for the contribution of spinal disinhibition to neuropathic pain states. Current research in animals and humans with diabetes has revealed specific disorders of spinal GABAergic function associated with impaired RDD. Future investigations on RDD aim to further elucidate its underlying pathways and enhance its clinical applications.

Neuroinflammation signatures in dorsal root ganglia following chronic constriction injury

Neuropathic pain (NP) is a common debilitating chronic pain condition with limited effective therapeutics. Further investigating mechanisms underlying NP is therefore of great importance for discovering more promising therapeutic targets. In the current study, we employed high-throughput RNA sequencing to explore transcriptome profiles of mRNAs and microRNAs in the dorsal root ganglia (DRG) following chronic constriction injury (CCI) and also integrated published datasets for comprehensive analysis. First, we established CCI rat model confirmed by behavioral testings, and excavated 467 differentially expressed mRNAs (DEGs) and 16 differentially expressed microRNAs (DEmiRNAs) in the ipsilateral lumbar 4-6 DRG of CCI rats 11 days after surgery. Functional enrichment analysis of 337 upregulated DEGs showed that most of the DEGs were enriched in inflammation- and immune-associated biological processes and signaling pathways. The protein-protein interaction networks were constructed and hub DEGs were screened. Besides hub DEGs, we also identified 113 overlapped DEGs by intersecting our dataset with dataset GSE100122. Subsequently, we predicted potential miRNA-mRNA regulatory pairs using DEmiRNAs and a given set of key DEGs (including hub and overlapped DEGs). By integrative analysis, we found commonly differentially expressed mRNAs and miRNAs following CCI of different time points and different nerve injury types. Highlighted mRNAs include Atf3, Vip, Gal, Npy, Adcyap1, Reg3b, Jun, Cd74, Gadd45a, Tgm1, Csrp3, Sprr1a, Serpina3n, Gap43, Serpinb2 and Vtcn1, while miRNAs include miR-21-5p, miR-34a-5p, miR-200a-3p, miR-130a-5p, miR-216b-5p, miR-217-5p, and miR-541-5p. Additionally, 15 DEGs, including macrophages-specific (Cx3cr1, Arg1, Cd68, Csf1r) and the ones related to macrophages' involvement in NP (Ccl2, Fcgr3a, Bdnf, Ctss, Tyrobp) were verified by qRT-PCR. By functional experiments in future studies, promising therapeutic targets for NP treatment may be identified among these mRNAs and miRNAs.

Current status of GABA receptor subtypes in analgesia

Gamma-aminobutyric acid (GABA), a non-protein-producing amino acid synthesized from the excitatory amino acid glutamate via the enzyme glutamic acid decarboxylase, is extensively found in microorganisms, plants and vertebrates, and is abundantly expressed in the spinal cord and brain. It is the major inhibitory neurotransmitter in the mammalian nervous system. GABA plays crucial roles in the regulation of synaptic transmission, the promotion of neuronal development and relaxation, and the prevention of insomnia and depression. As the major inhibitory neurotransmitter, GABA plays pivotal roles in the regulation of pain sensation, which is initiated by the activation of peripheral nociceptors and transmitted to the spinal cord and brain along nerves. GABA exerts these roles by directly acting on three types of receptors: ionotropic GABAA and GABAC receptors and G protein-coupled GABAB receptor. The chloride-permeable ion channel receptors GABAA and GABAC mediate fast neurotransmission, while the metabotropic GABAB receptor mediates slow effect. Different GABA receptors regulate pain sensation via different signaling pathways. Here we highlight recent updates on the involvement of specific GABA receptors and their subtypes in the process of pain sensation. Further understanding of different GABA receptors and signaling pathways in pain sensation will benefit the development of novel analgesics for pain management by targeting specific GABA receptor subtypes and signaling pathways.

Involvement of Kir4.1 in pain insensitivity of the BTBR mouse model of autism spectrum disorder

Autism spectrum disorder (ASD) is a severe neurodevelopmental disorder. Abnormal pain sensation is a common clinical symptom of ASD that seriously affects the quality of life of patients with ASD and their families. However, the underlying mechanism is unclear. It is believed to be related to the excitability of neurons and the expression of ion channels. Herein, we confirmed that baseline pain and Complete Freund's adjuvant (CFA)-induced chronic inflammatory pain were impaired in the BTBR T+ Itpr3tf/J (BTBR) mouse model of ASD. RNA sequencing (RNA-seq) analyses of the dorsal root ganglia (DRG), which are closely related to pain in ASD model mice, revealed that high expression of KCNJ10 (encoding Kir4.1) might be an important factor in ASD pain sensation abnormalities. The levels of Kir4.1 were further verified by western blotting, RT-qPCR, and immunofluorescence. By inhibiting Kir4.1, the pain insensitivity of BTBR mice improved, confirming that a high expression level of Kir4.1 was highly correlated with decreased pain sensitivity in ASD. Meanwhile, we found that the anxiety behaviours and the social novelty recognition were changed after CFA induced inflammatory pain. And after inhibiting Kir4.1, the stereotyped behaviours and social novelty recognition of BTBR mice were also improved. Further, we found that the expression levels of glutamate transporters, excitatory amino acid transporter 1 (EAAT1), and excitatory amino acid transporter 2 (EAAT2) were increased in the DRG of BTBR mice but decreased after inhibiting Kir4.1. This suggests that Kir4.1 may play a key role in the improvement of pain insensitivity in ASD by regulating glutamate transporters. In conclusion, our findings revealed the possible mechanism and role of Kir4.1 in the pain insensitivity in ASD, using bioinformatics analyses and animal experiments, and provided a theoretical basis for clinically targeted intervention in ASD.

14-3-3ζ Mediates GABAAR Activation by Interacting with BIG1

Most fast synaptic inhibitions in the mammalian brain are mediated by GABA_A receptors (GABA_ARs). An appropriate level of GABA_AR expression at the cell surface is essential for neurodevelopment and the efficacy of GABAergic synaptic transmission. We previously reported that brefeldin A-inhibited GDP/GTP exchange factor 1 (BIG1), a binding partner of GABA_ARs, plays an important role in trafficking GABA_ARs to the cell surface. However, its regulatory mechanisms remain unknown. In the present study, we identified a new cellular protein, 14-3-3ζ, which can interact with the β subunit of GABA_ARs and BIG1 both in vitro and in vivo and colocalizes in the soma, dendrites, and axons of hippocampal neurons. Overexpression of 14-3-3ζ-WT increased the surface expression of BIG1 in dendrites and axons, as well as the binding of BIG1 with GABA_AR. Depleted 14-3-3ζ with efficacious siRNA attenuated the interaction between BIG1 and GABA_ARs and resulted in significant decreases in the surface expression levels of BIG1 and GABA_AR. GABA_AR agonist treatment increased the expression levels of BIG1 and 14-3-3ζ on the surface, indicating that 14-3-3ζ is involved in regulating BIG1-mediated GABA_AR surface expression. Depletion of BIG1 or 14-3-3ζ significantly decreased GABA_AR expression at the cell surface and suppressed the GABA-gated influx of chloride ions. These data indicate that the combination of 14-3-3ζ and BIG1 is required for GABA_AR membrane expression. Our results provide a potential promising therapeutic target for neurological disorders involving GABAergic synaptic transmission.

Intrathecal liproxstatin-1 delivery inhibits ferroptosis and attenuates mechanical and thermal hypersensitivities in rats with complete Freund’s adjuvant-induced inflammatory pain

Previous studies have confirmed the relationship between iron-dependent ferroptosis and a peripheral nerve injury-induced neuropathic pain model. However, the role of ferroptosis in inflammatory pain remains inconclusive. Therefore, we aimed to explore whether ferroptosis in the spinal cord and dorsal root ganglion contributes to complete Freund’s adjuvant (CFA)-induced painful behaviors in rats. Our results revealed that various biochemical and morphological changes were associated with ferroptosis in the spinal cord and dorsal root ganglion tissues of CFA rats. These changes included iron overload, enhanced lipid peroxidation, disorders of anti-acyl-coenzyme A synthetase long-chain family member 4 and glutathione peroxidase 4 levels, and abnormal morphological changes in mitochondria. Intrathecal treatment of liproxstatin-1 (a ferroptosis inhibitor) reversed these ferroptosis-related changes and alleviated mechanical and thermal hypersensitivities in CFA rats. Our study demonstrated the occurrence of ferroptosis in the spinal cord and dorsal root ganglion tissues in a rodent model of inflammatory pain and indicated that intrathecal administration of ferroptosis inhibitors, such as liproxstatin-1, is a potential therapeutic strategy for treating inflammatory pain.

GABA facilitates spike propagation through branch points of sensory axons in the spinal cord

Movement and posture depend on sensory feedback that is regulated by specialized GABAergic neurons (GAD2+) that form axo-axonic contacts onto myelinated proprioceptive sensory axons and are thought to be inhibitory. However, we report here that activating GAD2+ neurons directly with optogenetics or indirectly by cutaneous stimulation actually facilitates sensory feedback to motor neurons in rodents and humans. GABAA receptors located at or near nodes of Ranvier of sensory axons cause this facilitation by preventing spike propagation failure at the many axon branch points, which is otherwise common without GABA. In contrast, GABAA receptors are generally lacking from axon terminals and so cannot inhibit transmitter release onto motor neurons, unlike GABAB receptors that cause presynaptic inhibition. GABAergic innervation near nodes and branch points allows individual branches to function autonomously, with GAD2+ neurons regulating which branches conduct, adding a computational layer to the neuronal networks generating movement and likely generalizing to other central nervous system axons.

Differential synaptic mechanism underlying the neuronal modulation of prefrontal cortex, amygdala, and hippocampus in response to chronic postsurgical pain with or without cognitive deficits in rats

Chronic Postsurgical Pain (CPSP) is well recognized to impair cognition, particularly memory. Mounting evidence suggests anatomic and mechanistic overlap between pain and cognition on several levels. Interestingly, the drugs currently used for treating chronic pain, including opioids, gabapentin, and NMDAR (N-methyl-D-aspartate receptor) antagonists, are also known to impair cognition. So whether pain-related cognitive deficits have different synaptic mechanisms as those underlying pain remains to be elucidated. In this context, the synaptic transmission in the unsusceptible group (cognitively normal pain rats) was isolated from that in the susceptible group (cognitively compromised pain rats). It was revealed that nearly two-thirds of the CPSP rats suffered cognitive impairment. The whole-cell voltage-clamp recordings revealed that the neuronal excitability and synaptic transmission in the prefrontal cortex and amygdala neurons were enhanced in the unsusceptible group, while these parameters remained the same in the susceptible group. Moreover, the neuronal excitability and synaptic transmission in hippocampus neurons demonstrated the opposite trend. Correspondingly, the levels of synaptic transmission-related proteins demonstrated a tendency similar to that of the excitatory and inhibitory synaptic transmission. Furthermore, morphologically, the synapse ultrastructure varied in the postsynaptic density (PSD) between the CPSP rats with and without cognitive deficits. Together, these observations indicated that basal excitatory and inhibitory synaptic transmission changes were strikingly different between the CPSP rats with and without cognitive deficits.

SCFAs Ameliorate Chronic Postsurgical Pain-Related Cognition Dysfunction via the ACSS2-HDAC2 Axis in Rats

Patients with chronic postsurgical pain (CPSP) frequently exhibit comorbid cognitive deficits. Recent observations have emphasized the critical effects of gut microbial metabolites, like short-chain fatty acids (SCFAs), in regulating cognitive function. However, the underlying mechanisms and effective interventions remain unclear. According to hierarchical clustering and 16S rRNA analysis, over two-thirds of the CPSP rats had cognitive impairment, and the CPSP rats with cognitive impairment had an aberrant composition of gut SCFA-producing bacteria. Then, using feces microbiota transplantation, researchers identified a causal relationship between cognitive-behavioral and microbic changes. Similarly, the number of genera that generated SCFAs was decreased in the feces from recipients of cognitive impairment microbiota. Moreover, treatment with the SCFAs alleviated the cognitive-behavioral deficits in the cognitively compromised pain rats. Finally, we observed that SCFA supplementation improved histone acetylation and abnormal synaptic transmission in the medial prefrontal cortex (mPFC), hippocampal CA1, and central amygdala (CeA) area via the ACSS2 (acetyl-CoA synthetase2)-HDAC2 (histone deacetylase 2) axis. These findings link pain-related cognition dysfunction, gut microbiota, and short-chain fatty acids, shedding fresh insight into the pathogenesis and therapy of pain-associated cognition dysfunction.

Hippocampal Inhibitory Synapsis Deficits Induced by α5-Containing GABAA Receptors Mediate Chronic Neuropathic Pain-Related Cognitive Impairment

Chronic neuropathic pain often leads to cognitive impairment, but the exact mechanism remains unclear. Gamma-aminobutyric acid A receptors (GABA_ARs) are the major inhibitory receptors in the brain, of which the α5-containing GABA_ARs (GABA_ARs-α5) are implicated in a range of neuropsychiatric disorders with cognitive deficits. However, whether GABA_ARs-α5 are involved in chronic neuropathic pain-related cognitive impairment remains unknown. In this study, the rats with chronic neuropathic pain induced by right sciatic nerve ligation injury (SNI) exhibited cognitive impairment with declined spontaneous alternation in Y-maze test and discrimination index in novel object recognition test. The GABA_ARs-α5 expressing on parvalbumin and somatostatin interneurons increased remarkably in hippocampus, resulting in decreased mean frequency of spontaneous inhibitory postsynaptic currents in hippocampal pyramidal neurons. Significantly, antagonizing the GABA_ARs-α5 by L655708 rescued weakened inhibitory synaptic transmission and cognitive impairment induced by chronic neuropathic pain. Taken together, these data suggest that the GABA_ARs-α5 play a crucial role in chronic neuropathic pain-induced cognitive impairment by weakening inhibitory synaptic transmission, which may provide insights into the pharmacologic treatment of chronic neuropathic pain-related cognitive impairment.

MD2 contributes to the pathogenesis of perioperative neurocognitive disorder via the regulation of α5GABAA receptors in aged mice

BACKGROUND

Perioperative neurocognitive disorder (PND) is a long-term postoperative complication in elderly surgical patients. The underlying mechanism of PND is unclear, and no effective therapies are currently available. It is believed that neuroinflammation plays an important role in triggering PND. The secreted glycoprotein myeloid differentiation factor 2 (MD2) functions as an activator of the Toll-like receptor 4 (TLR4) inflammatory pathway, and α5GABA_A receptors (α5GABA_ARs) are known to play a key role in regulating inflammation-induced cognitive deficits. Thus, in this study, we aimed to investigate the role of MD2 in PND and determine whether α5GABA_ARs are involved in the function of MD2.

METHODS

Eighteen-month-old C57BL/6J mice were subjected to laparotomy under isoflurane anesthesia to induce PND. The Barnes maze was used to assess spatial reference learning and memory, and the expression of hippocampal MD2 was assayed by western blotting. MD2 expression was downregulated by bilateral injection of AAV-shMD2 into the hippocampus or tail vein injection of the synthetic MD2 degrading peptide Tat-CIRP-CMA (TCM) to evaluate the effect of MD2. Primary cultured neurons from brain tissue block containing cortices and hippocampus were treated with Tat-CIRP-CMA to investigate whether downregulating MD2 expression affected the expression of α5GABA_ARs. Electrophysiology was employed to measure tonic currents. For α5GABA_ARs intervention experiments, L-655,708 and L-838,417 were used to inhibit or activate α5GABA_ARs, respectively.

RESULTS

Surgery under inhaled isoflurane anesthesia induced cognitive impairments and elevated the expression of MD2 in the hippocampus. Downregulation of MD2 expression by AAV-shMD2 or Tat-CIRP-CMA improved the spatial reference learning and memory in animals subjected to anesthesia and surgery. Furthermore, Tat-CIRP-CMA treatment decreased the expression of membrane α5GABA_ARs and tonic currents in CA1 pyramidal neurons in the hippocampus. Inhibition of α5GABA_ARs by L-655,708 alleviated cognitive impairments after anesthesia and surgery. More importantly, activation of α5GABA_ARs by L-838,417 abrogated the protective effects of Tat-CIRP-CMA against anesthesia and surgery-induced spatial reference learning and memory deficits.

CONCLUSIONS

MD2 contributes to the occurrence of PND by regulating α5GABA_ARs in aged mice, and Tat-CIRP-CMA is a promising neuroprotectant against PND.

The role of spinal cord extrasynaptic α5 GABAA receptors in chronic pain

Chronic pain is an incapacitating condition that affects a large population worldwide. Until now, there is no drug treatment to relieve it. The impairment of GABAergic inhibition mediated by GABAA receptors (GABAA R) is considered a relevant factor in mediating chronic pain. Even though both synaptic and extrasynaptic GABAA inhibition are present in neurons that process nociceptive information, the latter is not considered relevant as a target for the development of pain treatments. In particular, the extrasynaptic α5 GABAA Rs are expressed in laminae I-II of the spinal cord neurons, sensory neurons, and motoneurons. In this review, we discuss evidence showing that blockade of the extrasynaptic α5 GABAA Rs reduces mechanical allodynia in various models of chronic pain and restores the associated loss of rate-dependent depression of the Hoffmann reflex. Furthermore, in healthy animals, extrasynaptic α5 GABAA R blockade induces both allodynia and hyperalgesia. These results indicate that this receptor may have an antinociceptive and pronociceptive role in healthy and chronic pain-affected animals, respectively. We propose a hypothesis to explain the relevant role of the extrasynaptic α5 GABAA Rs in the processing of nociceptive information. The data discussed here strongly suggest that this receptor could be a valid pharmacological target to treat chronic pain states.

Neuropathic Injury-Induced Plasticity of GABAergic System in Peripheral Sensory Ganglia

GABA is a major inhibitory neurotransmitter in the mammalian central nervous system (CNS). Inhibitory GABA_A channel circuits in the dorsal spinal cord are the gatekeepers of the nociceptive input from the periphery to the CNS. Weakening of these spinal inhibitory mechanisms is a hallmark of chronic pain. Yet, recent studies have suggested the existence of an earlier GABAergic “gate” within the peripheral sensory ganglia. In this study, we performed systematic investigation of plastic changes of the GABA-related proteins in the dorsal root ganglion (DRG) in the process of neuropathic pain development. We found that chronic constriction injury (CCI) induced general downregulation of most GABA_A channel subunits and the GABA-producing enzyme, glutamate decarboxylase, consistent with the weakening of the GABAergic inhibition at the periphery. Strikingly, the α5 GABA_A subunit was consistently upregulated. Knock-down of the α5 subunit in vivo moderately alleviated neuropathic hyperalgesia. Our findings suggest that while the development of neuropathic pain is generally accompanied by weakening of the peripheral GABAergic system, the α5 GABA_A subunit may have a unique pro-algesic role and, hence, might represent a new therapeutic target.

Nociception induces a differential presynaptic modulation of the synaptic efficacy of nociceptive and proprioceptive joint afferents

A previous study has indicated that during the state of central sensitization induced by the intradermic injection of capsaicin, there is a gradual facilitation of the dorsal horn neuronal responses produced by stimulation of the high-threshold articular afferents that is counteracted by a concurrent increase of descending inhibitory actions. Since these changes occurred without significantly affecting the responses produced by stimulation of the low-threshold articular afferents, it was suggested that the capsaicin-induced descending inhibition included a preferential presynaptic modulation of the synaptic efficacy of the slow conducting nociceptive joint afferents (Ramírez-Morales et al., Exp Brain Res 237:1629-1641, 2019). The present study was aimed to investigate more directly the contribution of presynaptic mechanisms in this descending control. We found that in the barbiturate anesthetized cat, stimulation of the high-threshold myelinated afferents in the posterior articular nerve (PAN) produces primary afferent hyperpolarization (PAH) in the slow conducting (25-35 m/s) and primary afferent depolarization (PAD) in the fast conducting (40-50 m/s) articular fibers. During the state of central sensitization induced by capsaicin, there is a supraspinally mediated shift of the autogenic PAH to PAD that takes place in the slow conducting fibers, basically without affecting the autogenic PAD generated in the fast conducting afferents. It is suggested that the change of presynaptic facilitation to presynaptic inhibition induced by capsaicin on the slow articular afferents is part of an homeostatic process aimed to keep the nociceptive-induced neuronal activity within manageable limits while preserving the proprioceptive information required for proper control of movement.

Interaction of NHE1 and TRPA1 Activity in DRG Neurons Isolated from Adult Rats and its Role in Inflammatory Nociception

Transient receptor potential ankyrin 1 (TRPA1) channel is expressed in a subset of nociceptive neurons. This channel integrates several nociceptive signals. Particularly, it is modulated by intracellular pH (pH_i). Na⁺/H⁺ exchanger 1 (NHE1) contributes to the maintenance of pH_i in nociceptors. However, it is currently unknown whether the interaction between TRPA1 and NHE1 contributes to the nociceptive processing. Thus, the purpose of this study was to assess the functional interaction between NHE1 and TRPA1 in small dorsal root ganglion (DRG) neurons from primary culture obtained from adult rats. Moreover, we also evaluated their possible interaction in acute and inflammatory pain. Zoniporide (selective NHE1 inhibitor) reduced pH_i and increased intracellular calcium in a concentration-dependent fashion in DRG neurons. Zoniporide and allyl isothiocyanate (AITC, TRPA1 agonist) increased calcium transients in the same DRG neuron, whereas that A-967079 (TRPA1 antagonist) prevented the effect of zoniporide in DRG neurons. Repeated AITC induced TRPA1 desensitization and this effect was prevented by zoniporide. Both NHE1 and TRPA1 were localized at the membrane surface of DRG neurons in culture. Local peripheral zoniporide enhanced AITC-induced pronociception and this effect was prevented by A-967079. Likewise, zoniporide potentiated Complete Freund's Adjuvant (CFA)-induced hypersensitivity, effect which was prevented by A-967079 in vivo. CFA paw injection increased TRPA1 and decresed NHE1 protein expression in DRG. These results suggest a functional interaction between NHE1 and TRPA1 in DRG neurons in vitro. Moreover, data suggest that this interaction participates in acute and inflamatory pain conditions in vivo.

Pharmacological Modulation of Rate-Dependent Depression of the Spinal H-Reflex Predicts Therapeutic Efficacy against Painful Diabetic Neuropathy

Impaired rate-dependent depression (RDD) of the spinal H-reflex occurs in diabetic rodents and a sub-set of patients with painful diabetic neuropathy. RDD is unaffected in animal models of painful neuropathy associated with peripheral pain mechanisms and diabetic patients with painless neuropathy, suggesting RDD could serve as a biomarker for individuals in whom spinal disinhibition contributes to painful neuropathy and help identify therapies that target impaired spinal inhibitory function. The spinal pharmacology of RDD was investigated in normal rats and rats after 4 and 8 weeks of streptozotocin-induced diabetes. In normal rats, dependence of RDD on spinal GABAergic inhibitory function encompassed both GABA_A and GABA_B receptor sub-types. The time-dependent emergence of impaired RDD in diabetic rats was preceded by depletion of potassium-chloride co-transporter 2 (KCC2) protein in the dorsal, but not ventral, spinal cord and by dysfunction of GABA_A receptor-mediated inhibition. GABA_B receptor-mediated spinal inhibition remained functional and initially compensated for loss of GABA_A receptor-mediated inhibition. Administration of the GABA_B receptor agonist baclofen restored RDD and alleviated indices of neuropathic pain in diabetic rats, as did spinal delivery of the carbonic anhydrase inhibitor acetazolamide. Pharmacological manipulation of RDD can be used to identify potential therapies that act against neuropathic pain arising from spinal disinhibition.

Upregulation of cortical GABAA receptor concentration in fibromyalgia

An imbalance between excitatory and inhibitory neurotransmission has been linked to fibromyalgia (FM). Magnetic resonance spectroscopy has shown increased levels of glutamate in the insula and posterior cingulate cortex in FM as well as reduced insular levels of gamma-aminobutyric acid (GABA). Both of these changes have been associated with increased pain sensitivity. However, it is not clear whether excitatory and/or inhibitory neurotransmission is altered across the brain. Therefore, the aim of this study was to quantify GABAA receptor concentration on the whole brain level in FM to investigate a potential dysregulation of the GABAergic system. Fifty-one postmenopausal women (26 FM, 25 matched controls) underwent assessments of pain sensitivity, attention and memory, psychological status and function, as well as positron emission tomography imaging using a tracer for GABAA receptors, [F]flumazenil. Patients showed increased pain sensitivity, impaired immediate memory, and increased cortical GABAA receptor concentration in the attention and default-mode networks. No decrease of GABAA receptor concentration was observed. Across the 2 groups, GABAA receptor concentration correlated positively with functional scores and current pain in areas overlapping with regions of increased GABAA receptor concentration. This study shows increased GABAA receptor concentration in FM, associated with pain symptoms and impaired function. The changes were widespread and not restricted to pain-processing regions. These findings suggest that the GABAergic system is altered, possibly indicating an imbalance between excitatory and inhibitory neurotransmission. Future studies should try to understand the nature of the dysregulation of the GABAergic system in FM and in other pain syndromes.

γ-Aminobutyric acid (GABA) from satellite glial cells tonically depresses the excitability of primary afferent fibers

Primary afferent fibers express extrasynaptic GABA_A and GABA_B receptors in the axons and soma. However, whether these receptors are tonically activated by ambient GABA and the source of the neurotransmitter is presently unknown. Here, we show that GABA release from dorsal root ganglia (DRG) does not depend on extracellular calcium, but depends upon calcium released from intracellular stores, and is mediated by Best1 channels. Using a preparation consisting of the spinal nerve in continuity with the DRG and the dorsal root, we found that endogenous GABA tonically activates GABA receptors, depressing the excitability of the primary afferents. In addition, using HPLC we found that GABA is released in the DRG, and by immunofluorescence microscopy we show the presence of GABA, the Best1 channel, and some enzymes of the putrescine pathway of GABA biosynthesis, in glutamine synthase- and GFAP-positive satellite glial cells. Last, we found that the blockade of the Best1 channel activity reduced the excitability of primary afferents and prevented the activation of the GABA receptors. These results suggest that satellite glial cells may be the source of endogenous GABA released in the DRG via Best1 channels, which tonically activates extrasynaptic GABA receptors.

Sex-dependent pronociceptive role of spinal α5 -GABAA receptor and its epigenetic regulation in neuropathic rodents

Extrasynaptic α₅ -subunit containing GABA_A (α₅ -GABA_A ) receptors participate in chronic pain. Previously, we reported a sex difference in the action of α₅ -GABA_A receptors in dysfunctional pain. However, the underlying mechanisms remain unknown. The aim of this study was to examine this sexual dimorphism in neuropathic rodents and the mechanisms involved. Female and male Wistar rats or ICR mice were subjected to nerve injury followed by α₅ -GABA_A receptor inverse agonist intrathecal administration, L-655,708. The drug produced an antiallodynic effect in nerve-injured female rats and mice, and a lower effect in males. We hypothesized that changes in α₅ -GABA_A receptor, probably influenced by hormonal and epigenetic status, might underlie this sex difference. Thus, we performed qPCR and western blot. Nerve injury increased α₅ -GABA_A mRNA and protein in female dorsal root ganglia (DRG) and decreased them in DRG and spinal cord of males. To investigate the hormonal influence over α₅ -GABA_A receptor actions, we performed nerve injury to ovariectomized rats and reconstituted them with 17β-estradiol (E2). Ovariectomy abrogated L-655,708 antiallodynic effect and E2 restored it. Ovariectomy decreased α₅ -GABA_A receptor and estrogen receptor α protein in DRG of neuropathic female rats, while E2 enhanced them. Since DNA methylation might contribute to α₅ -GABA_A receptor down-regulation in males, we examined CpG island DNA methylation of α₅ -GABA_A receptor coding gene through pyrosequencing. Nerve injury increased methylation in male, but not female rats. Pharmacological inhibition of DNA methyltransferases increased α₅ -GABA_A receptor and enabled L-655,708 antinociceptive effect in male rats. These results suggest that α₅ -GABA_A receptor is a suitable target to treat chronic pain in females.

Machine-learned analysis of the association of next-generation sequencing-based genotypes with persistent pain after breast cancer surgery

Cancer and its surgical treatment are among the most important triggering events for persistent pain, but additional factors need to be present for the clinical manifestation, such as variants in pain-relevant genes. In a cohort of 140 women undergoing breast cancer surgery, assigned based on a 3-year follow-up to either a persistent or nonpersistent pain phenotype, next-generation sequencing was performed for 77 genes selected for known functional involvement in persistent pain. Applying machine-learning and item categorization techniques, 21 variants in 13 different genes were found to be relevant to the assignment of a patient to either the persistent pain or the nonpersistent pain phenotype group. In descending order of importance for correct group assignment, the relevant genes comprised DRD1, FAAH, GCH1, GPR132, OPRM1, DRD3, RELN, GABRA5, NF1, COMT, TRPA1, ABHD6, and DRD4, of which one in the DRD4 gene was a novel discovery. Particularly relevant variants were found in the DRD1 and GPR132 genes, or in a cis-eCTL position of the OPRM1 gene. Supervised machine-learning-based classifiers, trained with 2/3 of the data, identified the correct pain phenotype group in the remaining 1/3 of the patients at accuracies and areas under the receiver operator characteristic curves of 65% to 72%. When using conservative classical statistical approaches, none of the variants passed α-corrected testing. The present data analysis approach, using machine learning and training artificial intelligences, provided biologically plausible results and outperformed classical approaches to genotype-phenotype association.

Chloride - The Underrated Ion in Nociceptors

In contrast to pain processing neurons in the spinal cord, where the importance of chloride conductances is already well established, chloride homeostasis in primary afferent neurons has received less attention. Sensory neurons maintain high intracellular chloride concentrations through balanced activity of Na+-K+-2Cl- cotransporter 1 (NKCC1) and K+-Cl- cotransporter 2 (KCC2). Whereas in other cell types activation of chloride conductances causes hyperpolarization, activation of the same conductances in primary afferent neurons may lead to inhibitory or excitatory depolarization depending on the actual chloride reversal potential and the total amount of chloride efflux during channel or transporter activation. Dorsal root ganglion (DRG) neurons express a multitude of chloride channel types belonging to different channel families, such as ligand-gated, ionotropic γ-aminobutyric acid (GABA) or glycine receptors, Ca2+-activated chloride channels of the anoctamin/TMEM16, bestrophin or tweety-homolog family, CLC chloride channels and transporters, cystic fibrosis transmembrane conductance regulator (CFTR) as well as volume-regulated anion channels (VRACs). Specific chloride conductances are involved in signal transduction and amplification at the peripheral nerve terminal, contribute to excitability and action potential generation of sensory neurons, or crucially shape synaptic transmission in the spinal dorsal horn. In addition, chloride channels can be modified by a plethora of inflammatory mediators affecting them directly, via protein-protein interaction, or through signaling cascades. Since chloride channels as well as mediators that modulate chloride fluxes are regulated in pain disorders and contribute to nociceptor excitation and sensitization it is timely and important to emphasize their critical role in nociceptive primary afferents in this review.

NeuroHeal Treatment Alleviates Neuropathic Pain and Enhances Sensory Axon Regeneration

Peripheral nerve injury (PNI) leads to the loss of motor, sensory, and autonomic functions, and often triggers neuropathic pain. During the last years, many efforts have focused on finding new therapies to increase axonal regeneration or to alleviate painful conditions. Still only a few of them have targeted both phenomena. Incipient or aberrant sensory axon regeneration is related to abnormal unpleasant sensations, such as hyperalgesia or allodynia. We recently have discovered NeuroHeal, a combination of two repurposed drugs; Acamprosate and Ribavirin. NeuroHeal is a neuroprotective agent that also enhances motor axon regeneration after PNI. In this work, we investigated its effect on sensory fiber regeneration and PNI-induced painful sensations in a rat model of spare nerve injury and nerve crush. The follow up of the animals showed that NeuroHeal treatment reduced the signs of neuropathic pain in both models. Besides, the treatment favored sensory axon regeneration, as observed in dorsal root ganglion explants. Mechanistically, the effects observed in vivo may improve the resolution of cell-protective autophagy. Additionally, NeuroHeal treatment modulated the P2X4-BDNF-KCC2 axis, which is an essential driver of neuropathic pain. These data open a new therapeutic avenue based on autophagic modulation to foster endogenous regenerative mechanisms and reduce the appearance of neuropathic pain in PNI.

Blockade of spinal α5-GABAA receptors differentially reduces reserpine-induced fibromyalgia-type pain in female rats

The role of spinal α5 subunit-containing GABAA (α5-GABAA) receptors in chronic pain is controversial. The purpose of this study was to investigate the participation of spinal α5-GABAA receptors in the reserpine-induced pain model. Reserpine administration induced tactile allodynia and muscle hyperalgesia in female and male rats. Intrathecal injection of L-655,708 and TB 21007 (7 days after the last reserpine injection) decreased tactile allodynia and, at a lesser extent, muscle hyperalgesia in female rats. The effects of these drugs produced a lower antiallodynic and antihyperalgesic effect in male than in female rats. Contrariwise, these drugs produced tactile allodynia and muscle hyperalgesia in naïve rats and these effects were lower in naïve male than female rats. Intrathecal L-838,417 prevented or reversed L-655,708-induced antiallodynia in reserpine-treated female rats. Repeated treatment with α5-GABAA receptor small interfering RNA (siRNA), but not scramble siRNA, reduced reserpine-induced allodynia in female rats. Accordingly, α5-GABAA receptor siRNA induced nociceptive hypersensitivity in naïve female rats. Reserpine enhanced α5-GABAA receptors expression in spinal cord and dorsal root ganglia (DRG), while it increased CD11b (OX-42) and glial fibrillary acidic protein (GFAP) fluorescence intensity in the lumbar spinal cord. In contrast, reserpine diminished K+-Cl- co-transporter 2 (KCC2) protein in the lumbar spinal cord. Data suggest that spinal α5-GABAA receptors play a sex-dependent proallodynic effect in reserpine-treated rats. In contrast, these receptors have a sex-dependent antiallodynic role in naïve rats.

Human Hyperekplexic Mutations in Glycine Receptors Disinhibit the Brainstem by Hijacking GABAA Receptors

Hyperekplexia disease is usually caused by naturally occurring point mutations in glycine receptors (GlyRs). However, the γ-aminobutyric acid type A receptor (GABA_AR) seems to be also involved regarding the therapeutic basis for hyperekplexia using benzodiazepines, which target GABA_ARs but not GlyRs. Here, we show that the function of GABA_ARs was significantly impaired in the hypoglossal nucleus of hyperekplexic transgenic mice. Such impairment appeared to be mediated by interaction between GABA_AR and mutant GlyR. The GABA_AR dysfunction was caused only by mutant GlyR consisting of homomeric α₁ subunits, which locate primarily at pre- and extra-synaptic sites. In addition, the rescue effects of diazepam were attenuated by Xli-093, which specifically blocked diazepam-induced potentiation on α₅-containing GABA_AR, a major form of pre- and extra-synaptic GABA_AR in the brainstem. Thus, our results suggest that the pre- and extra-synaptic GABA_ARs could be a potential therapeutic target for hyperekplexia disease caused by GlyR mutations.

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Hyperekplexic mutant GlyRs interact with GABA_ARs and disrupt the GABA_AR function

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Pre- and extra-synaptic GABA_ARs are deficient in the hyperekplexia disease

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α₅-Containing GABA_AR is a potential therapeutic target for the hyperekplexia disease

Trigeminal neuropathic pain development and maintenance in rats are suppressed by a positive modulator of α6 GABAA receptors

γ-Aminobutyric acid type A (GABAA ) receptors containing the α6 subunit are located in trigeminal ganglia, and their reduction by small interfering RNA increases inflammatory temporomandibular and myofascial pain in rats. We thus hypothesized that enhancing their activity may help in neuropathic syndromes originating from the trigeminal system. Here, we performed a detailed electrophysiological and pharmacokinetic analysis of two recently developed deuterated structurally similar pyrazoloquinolinone compounds. DK-I-56-1 at concentrations below 1 µM enhanced γ-aminobutyric acid (GABA) currents at recombinant rat α6β3γ2, α6β3δ and α6β3 receptors, whereas it was inactive at most GABAA receptor subtypes containing other α subunits. DK-I-87-1 at concentrations below 1 µM was inactive at α6-containing receptors and only weakly modulated other GABAA receptors investigated. Both plasma and brain tissue kinetics of DK-I-56-1 were relatively slow, with half-lives of 6 and 13 hr, respectively, enabling the persistence of estimated free brain concentrations in the range 10-300 nM throughout a 24-hr period. Results obtained in two protocols of chronic constriction injury of the infraorbital nerve in rats dosed intraperitoneally with DK-I-56-1 during 14 days after surgery or with DK-I-56-1 or DK-I-87-1 during 14 days after trigeminal neuropathy were already established, demonstrated that DK-I-56-1 but not DK-I-87-1 significantly reduced the hypersensitivity response to von Frey filaments. SIGNIFICANCE: Neuropathic pain induced by trigeminal nerve damage is poorly controlled by current treatments. DK-I-56-1 that positively modulates α6 GABAA receptors is appropriate for repeated administration and thus may represent a novel treatment option against the development and maintenance of trigeminal neuropathic pain.

Contribution of the α5 GABAA receptor to the discriminative stimulus effects of propofol in rat

Propofol as an agonist of GABA_A receptor has a rewarding and discriminative stimulus effect. However, which subtype of the GABA_A receptor is involved in the discriminative stimulus effects of propofol is still not clear. We observed the effects of an agonist or an antagonist of the subtype-selective GABA_A receptor on discriminative stimulus effects of propofol. Male Sprague-Dawley rats were trained to discriminate 10 mg/kg (intraperitoneal) propofol from intralipid under a fixed-ratio 10 schedule of food reinforcement. We found that propofol produced dose-dependent substitution for propofol at 10 mg/kg, with response rate reduction only at a dose above those producing the complete substitution. CL218,872 (1–3 mg/kg, intraperitoneal), an α1 subunit-selective GABA_A receptor agonist, and SL651,498 (0.3–3 mg/kg, intraperitoneal), an α2/3 GABA_A receptor selective agonist, could partially substitute for the discriminative stimulus effects of propofol (40–80% propofol-appropriate responding). Meanwhile, L838,417 (0.2–0.6 mg/kg, intravenous), a α2/3/5 GABA_A receptor selective agonist, could produce near 100% propofol-appropriate responding and completely substitute for propofol effects. Moreover, the administration of L655,708, the α5 GABA_A receptor inverse agonist, could dose dependently attenuate the discriminative stimulus of propofol. In contrast, the α1 GABA_A receptor antagonist β-CCt (1–3 mg/kg) combined with propofol (10 mg/kg) failed to block the propofol effect. The data showed that propofol produces discriminative stimulus effects in a dose-dependent manner and acts mainly on the α5 GABA_A to produce the discriminative stimulus effect.

Analgesic potential of PF-06372865, an α2/α3/α5 subtype-selective GABAA partial agonist, in humans

BACKGROUND

This study investigated the analgesic effects of two doses (15 and 65 mg) of PF-06372865, a novel α2/α3/α5 gamma-aminobutyric acid A (GABA_A) subunit selective partial positive allosteric modulator (PAM), compared with placebo and pregabalin (300 mg) as a positive control.

METHODS

We performed a randomised placebo-controlled crossover study (NCT02238717) in 20 healthy subjects, using a battery of pain tasks (electrical, pressure, heat, cold and inflammatory pain, including a paradigm of conditioned pain modulation). Pharmacodynamic measurements were performed at baseline and up to 10 h after dose.

RESULTS

A dose of 15 mg PF-06372865 increased pain tolerance thresholds (PTTs) for pressure pain at a ratio of 1.11 (90% confidence interval [CI]: 1.02, 1.22) compared with placebo. A dose of 65 mg PF-06372865 led to an increase in PTT for the cold pressor at a ratio of 1.17 (90% CI: 1.03, 1.32), and pressure pain task: 1.11 (90% CI: 1.01, 1.21). Pregabalin showed an increase in PTT for pressure pain at a ratio of 1.15 (95% CI: 1.06, 1.26) and cold pressor task: 1.31 (90% CI: 1.16, 1.48).

CONCLUSION

We conclude that PF-06372865 has analgesic potential at doses that do not induce significant sedation or other intolerable adverse events limiting its clinical use. In addition, the present study established the potential role for this battery of pain tasks as a tool in the development of analgesics with a novel mechanism of action, for the treatment of various pain states including neuropathic pain and to establish proof-of-concept.

CLINICAL TRIALS REGISTRATION

NCT0223871.

Development of an AmpliSeqTM Panel for Next-Generation Sequencing of a Set of Genetic Predictors of Persisting Pain

Background: Many gene variants modulate the individual perception of pain and possibly also its persistence. The limited selection of single functional variants is increasingly being replaced by analyses of the full coding and regulatory sequences of pain-relevant genes accessible by means of next generation sequencing (NGS).

Methods: An NGS panel was created for a set of 77 human genes selected following different lines of evidence supporting their role in persisting pain. To address the role of these candidate genes, we established a sequencing assay based on a custom AmpliSeq^TM panel to assess the exomic sequences in 72 subjects of Caucasian ethnicity. To identify the systems biology of the genes, the biological functions associated with these genes were assessed by means of a computational over-representation analysis.

Results: Sequencing generated a median of 2.85 ⋅ 10⁶ reads per run with a mean depth close to 200 reads, mean read length of 205 called bases and an average chip loading of 71%. A total of 3,185 genetic variants were called. A computational functional genomics analysis indicated that the proposed NGS gene panel covers biological processes identified previously as characterizing the functional genomics of persisting pain.

Conclusion: Results of the NGS assay suggested that the produced nucleotide sequences are comparable to those earned with the classical Sanger sequencing technique. The assay is applicable for small to large-scale experimental setups to target the accessing of information about any nucleotide within the addressed genes in a study cohort.

An effective and concise device for detecting cold allodynia in mice

Detection of cold allodynia is a very important aspect in the study of pain behavior. An effective and concise device for detecting cold pain has always been the hope of many researchers. Here, an easily produced and operated cold plate device is presented for the assessment of cold allodynia in mice. The device used to detect cold allodynia has two components: a chamber consists of a cylinder for animal experiment and a cube box around the chamber for holding ice to keep temperature stable. In the testing chamber, a mouse was placed on the circular plexiglass plate steady at 4 °C above ice for five minutes. The tested mouse will lift its paw when exposed to the cold plate. The number of lifts will present animal's response to the degree of cold stimulation. To evaluate this approach, three commonly used pain models of mice were tested: formalin test, bone cancer pain (BCP), and chronic constriction injury (CCI). As is reported in other literatures, these three pain mice models showed increased sensitivity to cold stimulation. The new device is indeed suitable for detecting cold allodynia behavior in mice. Comparisons with existing devices of detecting cold allodynia, such as the cold plate in the market (UGO, Panlab, Columbus, etc.), the new device has the advantages of low cost, simple operation and easy popularization and can detect cold allodynia behavior of mice very well. This is a very practical and economical device to detect cold allodynia behavior.

Mirogabalin and emerging therapies for diabetic neuropathy

There are currently no approved disease-modifying therapies for diabetic neuropathy, and there are only 3 US Food and Drug Administration-approved therapies (pregabalin, duloxetine, and tapentadol) for painful diabetic neuropathy. They each have moderate efficacy with adverse effects limiting optimal dose titration. There is a considerable need for new therapies for the management of painful diabetic neuropathy. We reviewed the potential role of mirogabalin, which like gabapentin and pregabalin modulates the alpha-2/delta-1 subunit of the voltage-gated calcium channel, allowing the influx of calcium and release of neurotransmitters at the synaptic cleft in the central nervous system and spinal cord. It has shown efficacy and good tolerability in a Phase II study in diabetic painful neuropathy and based on the results of two Phase III clinical trials in diabetic painful neuropathy and post-herpetic neuralgia, Daiichi Sankyo submitted a marketing application for neuropathic pain in Japan in February 2018. We have also reviewed potential new therapies, currently in Phase II clinical trials that may modify disease and/or relieve neuropathic pain through novel modes of action.

ATF2, but not ATF3, participates in the maintenance of nerve injury-induced tactile allodynia and thermal hyperalgesia

Transcription factors are proteins that modulate the transcriptional rate of target genes in the nucleus in response to extracellular or cytoplasmic signals. Activating transcription factors 2 (ATF2) and 3 (ATF3) respond to environmental signals and maintain cellular homeostasis. There is evidence that inflammation and nerve injury modulate ATF2 and ATF3 expression. However, the function of these transcription factors in pain is unknown. The purpose of this study was to investigate the contribution of ATF2 and ATF3 to nerve injury-induced neuropathic pain. L5/6 spinal nerve ligation induced tactile allodynia and thermal hyperalgesia. Moreover, nerve damage enhanced ATF2 and ATF3 protein expression in injured L5/6 dorsal root ganglia and spinal cord but not in uninjured L4 dorsal root ganglia. Nerve damage also enhanced ATF2 immunoreactivity in dorsal root ganglia and spinal cord 7 to 21 days post-injury. Repeated intrathecal post-treatment with a small-interfering RNA targeted against ATF2 (ATF2 siRNA) or anti-ATF2 antibody partially reversed tactile allodynia and thermal hyperalgesia. In contrast, ATF3 siRNA or anti-ATF3 antibody did not modify nociceptive behaviors. ATF2 immunoreactivity was found in dorsal root ganglia and spinal cord co-labeling with NeuN mainly in non-peptidergic (IB4⁺) but also in peptidergic (CGRP⁺) neurons. ATF2 was found mainly in small- and medium-sized neurons. These results suggest that ATF2, but not ATF3, is found in strategic sites related to spinal nociceptive processing and participates in the maintenance of neuropathic pain in rats.

Heteromeric α/β glycine receptors regulate excitability in parvalbumin-expressing dorsal horn neurons through phasic and tonic glycinergic inhibition

Key points

Spinal parvalbumin‐expressing interneurons have been identified as a critical source of inhibition to regulate sensory thresholds by gating mechanical inputs in the dorsal horn.

This study assessed the inhibitory regulation of the parvalbumin‐expressing interneurons, showing that synaptic and tonic glycinergic currents dominate, blocking neuronal or glial glycine transporters enhances tonic glycinergic currents, and these manipulations reduce excitability.

Synaptically released glycine also enhanced tonic glycinergic currents and resulted in decreased parvalbumin‐expressing interneuron excitability.

Analysis of the glycine receptor properties mediating inhibition of parvalbumin neurons, as well as single channel recordings, indicates that heteromeric α/β subunit‐containing receptors underlie both synaptic and tonic glycinergic currents.

Our findings indicate that glycinergic inhibition provides critical control of excitability in parvalbumin‐expressing interneurons in the dorsal horn and represents a pharmacological target to manipulate spinal sensory processing.

Abstract

The dorsal horn (DH) of the spinal cord is an important site for modality‐specific processing of sensory information and is essential for contextually relevant sensory experience. Parvalbumin‐expressing inhibitory interneurons (PV+ INs) have functional properties and connectivity that enables them to segregate tactile and nociceptive information. Here we examine inhibitory drive to PV+ INs using targeted patch‐clamp recording in spinal cord slices from adult transgenic mice that express enhanced green fluorescent protein in PV+ INs. Analysis of inhibitory synaptic currents showed glycinergic transmission is the dominant form of phasic inhibition to PV+ INs. In addition, PV+ INs expressed robust glycine‐mediated tonic currents; however, we found no evidence for tonic GABAergic currents. Manipulation of extracellular glycine by blocking either, or both, the glial and neuronal glycine transporters markedly decreased PV+ IN excitability, as assessed by action potential discharge. This decreased excitability was replicated when tonic glycinergic currents were increased by electrically activating glycinergic synapses. Finally, we show that both phasic and tonic forms of glycinergic inhibition are mediated by heteromeric α/β glycine receptors. This differs from GABA_A receptors in the dorsal horn, where different receptor stoichiometries underlie phasic and tonic inhibition. Together these data suggest both phasic and tonic glycinergic inhibition regulate the output of PV+ INs and contribute to the processing and segregation of tactile and nociceptive information. The shared stoichiometry for phasic and tonic glycine receptors suggests pharmacology is unlikely to be able to selectively target each form of inhibition in PV+ INs.

Spinal parvalbumin‐expressing interneurons have been identified as a critical source of inhibition to regulate sensory thresholds by gating mechanical inputs in the dorsal horn.

This study assessed the inhibitory regulation of the parvalbumin‐expressing interneurons, showing that synaptic and tonic glycinergic currents dominate, blocking neuronal or glial glycine transporters enhances tonic glycinergic currents, and these manipulations reduce excitability.

Synaptically released glycine also enhanced tonic glycinergic currents and resulted in decreased parvalbumin‐expressing interneuron excitability.

Analysis of the glycine receptor properties mediating inhibition of parvalbumin neurons, as well as single channel recordings, indicates that heteromeric α/β subunit‐containing receptors underlie both synaptic and tonic glycinergic currents.

Our findings indicate that glycinergic inhibition provides critical control of excitability in parvalbumin‐expressing interneurons in the dorsal horn and represents a pharmacological target to manipulate spinal sensory processing.

Tonically Active α5GABAA Receptors Reduce Motoneuron Excitability and Decrease the Monosynaptic Reflex

Motoneurons, the final common path of the Central Nervous System (CNS), are under a complex control of its excitability in order to precisely translate the interneuronal pattern of activity into skeletal muscle contraction and relaxation. To fulfill this relevant function, motoneurons are provided with a vast repertoire of receptors and channels, including the extrasynaptic GABA_A receptors which have been poorly investigated. Here, we confirmed that extrasynaptic α5 subunit-containing GABA_A receptors localize with choline acetyltransferase (ChAT) positive cells, suggesting that these receptors are expressed in turtle motoneurons as previously reported in rodents. In these cells, α₅GABA_A receptors are activated by ambient GABA, producing a tonic shunt that reduces motoneurons’ membrane resistance and affects their action potential firing properties. In addition, α₅GABA_A receptors shunted the synaptic excitatory inputs depressing the monosynaptic reflex (MSR) induced by activation of primary afferents. Therefore, our results suggest that α₅GABA_A receptors may play a relevant physiological role in motor control.

Effects of niflumic acid on γ-aminobutyric acid-induced currents in isolated dorsal root ganglion neurons of neuropathic pain rats

Niflumic acid (NFA) is a type of non-steroidal anti-inflammatory drug. Neuropathic pain is caused by a decrease in presynaptic inhibition mediated by γ-aminobutyric acid (GABA). In the present study, a whole-cell patch-clamp technique and intracellular recording were used to assess the effect of NFA on GABA-induced inward current in dorsal root ganglion (DRG) neurons of a chronic constriction injury (CCI) model. It was observed that 1–1,000 µmol/l GABA induced a concentration-dependent inward current in DRG neurons. Compared with pseudo-operated rats, the thermal withdrawal latency (TWL) of CCI rats significantly decreased (P<0.01); however, the TWLs of each NFA group (50 and 300 µmol/l) were significantly longer than that of the CCI group (P<0.01). In the CCI group, the response evoked by GABA (10⁻⁶-10⁻³ mol/l) was reduced in a concentration dependent manner compared with a normal control group (P<0.01), and the current amplitudes of CCI rats activated by the same concentrations of GABA (10⁻⁶-10⁻³ mol/l) were significantly decreased compared with the control group (P<0.05). The inward currents activated by 100 µmol/l GABA were suppressed by treatment with 1, 10 and 100 µmol/l NFA (5.32±3.51, 33.8±5.20, and 52.2±6.32%, respectively; P<0.05). The inverse potentials of GABA-induced currents were 9.87±1.32 and 9.64±1.24 mV with and without NFA, respectively (P<0.05). Pre-treatment with NFA exerted a strong inhibitory effect on the peak value of GABA-induced current, and the GABA-induced response was inhibited by the same concentrations of NFA (1, 10 and 100 µmol/l) in the control and CCI groups (P<0.05). The results suggest that NFA reduced the primary afferent depolarization (PAD) associated with neuropathic pain and mediated by the GABA_A receptor. NFA may regulate neuropathic pain by inhibiting dorsal root reflexes, which are triggered PAD.