Loss of glycinergic and GABAergic inhibition in chronic pain--contributions of inflammation and microglia

Effects of swimming exercise on neuropathic pain in a rat model: role of glutamate

OBJECTIVE

The pain-reducing effects of the exercise were exerted through different mechanisms. Knowing more clear mechanisms helps to find more approach that is therapeutic. The objective of the present study is the evaluation of cerebrospinal fluid (CSF) glutamate level alteration in neuropathic pain rats and whether physical activity could modulate it.

METHODS

In the present study 104 male Wistar rats weighing 180-220 g were randomly divided into 4 groups (Sham, Sham + Exe, Neuropathy, and Neuropathy + Exe) which in turn each group subdivided into 4 groups according to time points for behavioral testing and CSF sampling (Baseline, 2 weeks, 3 weeks, and 4 weeks). To induction of neuropathy (by chronic constriction injury,), after anesthetizing with a mixture of ketamine (80 mg/kg) and xylazine (10 mg/kg), the animal's right sciatic nerve was exposed and was ligated using four movable catgut chromic suture 4/0. The exercise protocol included 25 min of daily swimming, 5 days a week for 4 weeks. Thermal hyperalgesia and mechanical tactile threshold were detected using the plantar test and Von Frey filaments, respectively. CSF glutamate level was determined using high-performance liquid chromatography.

RESULTS

Findings indicated that mechanical and thermal thresholds significantly (p < 0.01, p < 0.05 respectively) decreased in the neuropathy group against that in sham groups. On the other hand, exercise significantly increased mechanical tactile threshold (p < 0.0012) and thermal threshold (p < 0.05) compared to the neuropathy group. Moreover, CSF glutamate level prominently (p < 0.01) was increased in the neuropathy group compared to the sham group, and swimming exercise significantly (p < 0.001) reduced it.

IN CONCLUSION

The present findings provide new evidence showing that medium-intensity swimming exercise attenuates pain-like behaviors in neuropathic pain animals, which is possibly due to decreasing CSF glutamate level and its neurotransmission.

Role of autophagy in the pathogenesis and regulation of pain

Pain is a ubiquitous and highly concerned clinical symptom, usually caused by peripheral or central nervous injury, tissue damage, or other diseases. The long-term existence of pain can seriously affect daily physical function and quality of life and produce great torture on the physiological and psychological levels. However, the complex pathogenesis of pain involving molecular mechanisms and signaling pathways has not been fully elucidated, and managing pain remains highly challenging. As a result, finding new targets to pursue effective and long-term pain treatment strategies is required and urgent. Autophagy is an intracellular degradation and recycling process that maintains tissue homeostasis and energy supply, which can be cytoprotective and is vital in maintaining neural plasticity and proper nervous system function. Much evidence has shown that autophagy dysregulation is linked to the emergence of neuropathic pain, such as postherpetic neuralgia and cancer-related pain. Autophagy has also been connected to pain caused by osteoarthritis and lumbar disc degeneration. It is worth noting that in recent years, studies on traditional Chinese medicine have also proved that several traditional Chinese medicine monomers involve autophagy in the mechanism of pain relief. Therefore, autophagy can serve as a potential regulatory target to provide new ideas and inspiration for pain management.

Adenosine signaling mediate pain transmission in the central nervous system

Pain is a common clinical symptom that seriously affects the quality of life in a variety of patient populations. In recent years, research on the role of adenosine signaling in pain modulation has made great progress. Adenosine is a purine nucleoside and a neuromodulator, and regulates multiple physiological and pathophysiological functions through the activation of four G protein-coupled receptors, which are classified as A₁, A_2A, A_2B, and A₃ adenosine receptors (ARs). Adenosine and its receptors that are widespread in the central nervous system (CNS) play an important role in the processing of nociceptive sensory signals in different pain models. A₁Rs have the highest affinity to adenosine, and the role in analgesia has been well investigated. The roles of A_2ARs and A_2BRs in the modulation of pain are controversial because they have both analgesic and pronociceptive effects. The analgesic effects of A₃Rs are primarily manifested in neuropathic pain. In this article, we have reviewed the recent studies on ARs in the modulation of neuropathic pain, inflammatory pain, postoperative pain, and visceral pain in the CNS. Furthermore, we have outlined the pathways through which ARs contribute to pain regulation, thereby shedding light on how this mechanism can be targeted to provide effective pain relief.

The Emerging Role of Quercetin in the Treatment of Chronic Pain

Despite much research efforts being devoted to designing alternative pharmacological interventions, chronic pain remains to be an unresolved clinical problem. Quercetin, a compound that belongs to the flavonoids family, is abundantly found in fruits and vegetables. Emerging evidence indicates that quercetin possesses anti-nociceptive effects in different rodent models of chronic pain, including inflammatory pain, neuropathic pain and cancer pain. In this review, we summarize the mechanisms underlying the analgesic effect of quercetin in preclinical studies. These studies showed that quercetin exerts potent analgesic effects against chronic pain via suppressing neuroinflammation and oxidative stress as well as modulation of synaptic plasticity, GABAergic system, and opioidergic system. Considering that the safety of quercetin is well established, it has great potential for clinical use in pain treatment.

Indicators of central sensitization in chronic neuropathic pain after spinal cord injury

BACKGROUND

Central sensitization is considered a key mechanism underlying neuropathic pain (NP) after spinal cord injury (SCI).

METHODS

Two novel proxies for central sensitization were investigated in thoracic SCI subjects with (SCI-NP) and without NP (SCI-nonNP) compared to healthy controls (HC). Specifically, temporal summation of pain (TSP) was investigated by examining pain ratings during a 2-min tonic heat application to the volar forearm. Additionally, palmar heat-induced sympathetic skin responses (SSR) were recorded in order to reveal changes in pain-autonomic interaction above the lesion level. Pain extent was assessed as the percentage of the body area and the number of body regions being affected by NP.

RESULTS

Enhanced TSP was observed in SCI-NP (+66%) compared to SCI-nonNP (-75%, p = 0.009) and HC (-59%, p = 0.021). In contrast, no group differences were found (p = 0.685) for SSR habituation. However, pain extent in SCI-NP was positively correlated with deficient SSR habituation (body area: r = 0.561, p = 0.024; body regions: r = 0.564, p = 0.023).

CONCLUSIONS

These results support the value of TSP and heat-induced SSRs as proxies for central sensitization in widespread neuropathic pain syndromes after SCI. Measures of pain-autonomic interaction emerged as a promising tool for the objective investigation of sensitized neuronal states in chronic pain conditions.

SIGNIFICANCE

We present two surrogate readouts for central sensitization in neuropathic pain following SCI. On the one hand, temporal summation of tonic heat pain is enhanced in subjects with neuropathic pain. On the other hand, pain-autonomic interaction reveals potential advanced measures in chronic pain, as subjects with a high extent of neuropathic pain showed diminished habituation of pain-induced sympathetic measures. A possible implication for clinical practice is constituted by an improved assessment of neuronal hyperexcitability potentially enabling mechanism-based treatment.

Spinal Cord Injury and Loss of Cortical Inhibition

After spinal cord injury (SCI), the destruction of spinal parenchyma causes permanent deficits in motor functions, which correlates with the severity and location of the lesion. Despite being disconnected from their targets, most cortical motor neurons survive the acute phase of SCI, and these neurons can therefore be a resource for functional recovery, provided that they are properly reconnected and retuned to a physiological state. However, inappropriate re-integration of cortical neurons or aberrant activity of corticospinal networks may worsen the long-term outcomes of SCI. In this review, we revisit recent studies addressing the relation between cortical disinhibition and functional recovery after SCI. Evidence suggests that cortical disinhibition can be either beneficial or detrimental in a context-dependent manner. A careful examination of clinical data helps to resolve apparent paradoxes and explain the heterogeneity of treatment outcomes. Additionally, evidence gained from SCI animal models indicates probable mechanisms mediating cortical disinhibition. Understanding the mechanisms and dynamics of cortical disinhibition is a prerequisite to improve current interventions through targeted pharmacological and/or rehabilitative interventions following SCI.

Involvement of selective GABA-A receptor subtypes in amelioration of cisplatin-induced neuropathic pain by 2'-chloro-6-methyl flavone (2'-Cl-6MF)

Cisplatin-induced peripheral neuropathic pain is a common adverse effect of chemotherapy. The present study evaluated the effects of 2'-chloro-6-methylflavone (2'-Cl-6MF) at recombinant α1β2γ2L, α2β1-3γ2L, and α3β1-3γ2L GABA-A receptor subtypes expressed in Xenopus oocytes and subsequently evaluated its effectiveness in cisplatin-induced neuropathic pain. The results showed that 2'-Cl-6MF potentiated GABA-elicited currents at α2β2/3γ2L and α3β2/3γ2L GABA-A receptor subtypes. The potentiation was blocked by the co-application of flumazenil (a benzodiazepine (BDZs) site antagonist). In behavioral studies, mechanical allodynia was induced by intraplantar injection of cisplatin (40 μg/paw) in Sprague Dawley rats, and behavioral assessments were made 24 h after injection. 2'-Cl-6MF (1, 10, 30, and 100 mg/kg, i.p.), was administered 1 h before behavioral evaluation. Administration of 2'-Cl-6MF (30 and 100 mg/kg, i.p) significantly enhanced the paw withdrawal threshold and decreased mechanical allodynia. The standard drugs, gabapentin (GBP) at the dose of 70 mg/kg, and HZ 166 (16 mg/kg), i.p. also significantly enhanced the paw withdrawal threshold in mechanical allodynia. Pretreatment with pentylenetetrazole (PTZ) (15 mg/kg, i.p.) and flumazenil reversed the antinociceptive effect of 2'-Cl-6MF in mechanical allodynia indicating GABAergic mechanisms. Moreover, the binding mechanism of 2'-Cl-6MF was rationalized by in silico modeling tools. The 3D-coordinates of α2β2γ2L and α2β3γ2L were generated after homology modeling of the α2 subtype and 2'-Cl-6MF was at predicted binding sites of the developed models. The α2 model was compared with the α1 and α3 subunits via structural and sequence alignment. Molecular docking depicted that the compound binds efficiently at the neuromodulator binding site of the receptors. The findings of this study revealed that 2'-Cl-6MF ameliorated the manifestations of cisplatin-induced neuropathic pain in rats. Furthermore, we also conclude that GABAergic mechanisms may contribute to the antinociceptive effect of 2'-Cl-6MF. The molecular docking studies also confirm the involvement of the BDZs site of GABA-A receptors. It was observed that Ile230 of α2 stabilize the chlorophenyl ring of 2'-Cl-6MF through hydrophobic interactions, which is replaced by Val203 in α1 subunit. However, the smaller side chain of Val203 does not provide hydrophobic interaction to the compound due to high conformational flexibility of α1 subunit.

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.

Exploring the Promise of Flavonoids to Combat Neuropathic Pain: From Molecular Mechanisms to Therapeutic Implications

Neuropathic pain (NP) is the result of irregular processing in the central or peripheral nervous system, which is generally caused by neuronal injury. The management of NP represents a great challenge owing to its heterogeneous profile and the significant undesirable side effects of the frequently prescribed psychoactive agents, including benzodiazepines (BDZ). Currently, several established drugs including antidepressants, anticonvulsants, topical lidocaine, and opioids are used to treat NP, but they exert a wide range of adverse effects. To reduce the burden of adverse effects, we need to investigate alternative therapeutics for the management of NP. Flavonoids are the most common secondary metabolites of plants used in folkloric medicine as tranquilizers, and have been claimed to have a selective affinity to the BDZ binding site. Several studies in animal models have reported that flavonoids can reduce NP. In this paper, we emphasize the potentiality of flavonoids for the management of NP.

GABAergic modulation of secondary hyperalgesia: A randomized controlled 4-way crossover trial with the α2-subunit preferring GABA positive allosteric modulator, N-desmethyl-clobazam in healthy volunteers

The antihyperalgesic and sedative effects of the α2-subunit preferring GABA_A positive allosteric modulator (GAM), N-desmethyl-clobazam (NDMC), 20 and 60 mg, were assessed in a randomized, placebo and active-controlled (clonazepam 1,5 mg), 4-way crossover study, in healthy volunteers, using the ultraviolet B-induced experimental pain model. Single (20, 40, 60 mg) and repeated doses (20 mg over 15 days) of NDMC pharmacokinetics were evaluated. Thirty-two subjects participated in the study. Primary outcome parameter was maximal change in the area of cutaneous UVB irradiation-induced secondary hyperalgesia (ASH). ASH decreased under all treatments. Mean (SD) relative change was 79 (22)%, 83 (24)%, 77 (30)% and 92 (16)% for placebo, NDMC20, NDMC60 and clonazepam, respectively. Neither absolute change nor relative change in ASH was significantly different between NDMC60 and placebo (mean difference = 2.3 cm² [95% CI 4.0-8.5], p = .462 and 0.4% [-11.9 to 12.6], p = .952, respectively). An overall treatment effect was found on level of sedation. Compared to placebo, sedation was higher under clonazepam (mean difference = 39 mm [30-49] on a visual analogue scale, p < .001) while NDMC was free of sedative effect. NDMC pharmacokinetics after single doses showed poor absorption, but was linear. Steady-state plasma concentrations of NDMC20 were attained within 14 days, with low between-subjects variability. Mean steady-state concentration (C_S-S , SD) reached 209 (22) ng/ml. NDMC absence of sedative effect and its overall well-characterized safety coming from years of utilization as a metabolite from clobazam, raise the prospect of dose escalating trials in patients to quantify its clinical utility. SIGNIFICANCE: This article, presenting the Phase I data of the new antihyperalgesic compound, α2-subunit GABA_A positive allosteric modulator, N-desmethyl-clobazam (NDMC) is exploring the modulation of a new target in the treatment of neuropathic pain. Based on these results and on its preclinical properties NDMC would qualify as a good tool compound to seek confirmation of the clinical utility of selective GABA allosteric modulators in neuropathic pain patients.

NMDA receptor activation induces long-term potentiation of glycine synapses

Of the fast ionotropic synapses, glycinergic synapses are the least well understood, but are vital for the maintenance of inhibitory signaling in the brain and spinal cord. Glycinergic signaling comprises half of the inhibitory signaling in the spinal cord, and glycinergic synapses are likely to regulate local nociceptive processing as well as the transmission to the brain of peripheral nociceptive information. Here we have investigated the rapid and prolonged potentiation of glycinergic synapses in the superficial dorsal horn of young male and female mice after brief activation of NMDA receptors (NMDARs). Glycinergic inhibitory postsynaptic currents (IPSCs) evoked with lamina II-III stimulation in identified GABAergic neurons in lamina II were potentiated by bath-applied Zn2+ and were depressed by the prostaglandin PGE2, consistent with the presence of both GlyRα1- and GlyRα3-containing receptors. NMDA application rapidly potentiated synaptic glycinergic currents. Whole-cell currents evoked by exogenous glycine were also readily potentiated by NMDA, indicating that the potentiation results from altered numbers or conductance of postsynaptic glycine receptors. Repetitive depolarization alone of the postsynaptic GABAergic neuron also potentiated glycinergic synapses, and intracellular EGTA prevented both NMDA-induced and depolarization-induced potentiation of glycinergic IPSCs. Optogenetic activation of trpv1 lineage afferents also triggered NMDAR-dependent potentiation of glycinergic synapses. Our results suggest that during peripheral injury or inflammation, nociceptor firing during injury is likely to potentiate glycinergic synapses on GABAergic neurons. This disinhibition mechanism may be engaged rapidly, altering dorsal horn circuitry to promote the transmission of nociceptive information to the brain.

Sensory Modulation Disorder (SMD) and Pain: A New Perspective

Sensory modulation disorder (SMD) affects sensory processing across single or multiple sensory systems. The sensory over-responsivity (SOR) subtype of SMD is manifested clinically as a condition in which non-painful stimuli are perceived as abnormally irritating, unpleasant, or even painful. Moreover, SOR interferes with participation in daily routines and activities (Dunn, 2007; Bar-Shalita et al., 2008; Chien et al., 2016), co-occurs with daily pain hyper-sensitivity, and reduces quality of life due to bodily pain. Laboratory behavioral studies have confirmed abnormal pain perception, as demonstrated by hyperalgesia and an enhanced lingering painful sensation, in children and adults with SMD. Advanced quantitative sensory testing (QST) has revealed the mechanisms of altered pain processing in SOR whereby despite the existence of normal peripheral sensory processing, there is enhanced facilitation of pain-transmitting pathways along with preserved but delayed inhibitory pain modulation. These findings point to central nervous system (CNS) involvement as the underlying mechanism of pain hypersensitivity in SOR. Based on the mutual central processing of both non-painful and painful sensory stimuli, we suggest shared mechanisms such as cortical hyper-excitation, an excitatory-inhibitory neuronal imbalance, and sensory modulation alterations. This is supported by novel findings indicating that SOR is a risk factor and comorbidity of chronic non-neuropathic pain disorders. This is the first review to summarize current empirical knowledge investigating SMD and pain, a sensory modality not yet part of the official SMD realm. We propose a neurophysiological mechanism-based model for the interrelation between pain and SMD. Embracing the pain domain could significantly contribute to the understanding of this condition’s pathogenesis and how it manifests in daily life, as well as suggesting the basis for future potential mechanism-based therapies.

Mutations affecting glycinergic neurotransmission in hyperekplexia increase pain sensitivity

See Dickenson (doi:10.1093/brain/awx334) for a scientific commentary on this article.Inhibitory interneurons in the spinal cord use glycine and GABA for fast inhibitory neurotransmission. While there is abundant research on these inhibitory pain pathways in animal models, their relevance in humans remains unclear, largely due to the limited possibility to manipulate selectively these pathways in humans. Hyperekplexia is a rare human disease that is caused by loss-of-function mutations in genes encoding for glycine receptors and glycine transporters. In the present study, we tested whether hyperekplexia patients display altered pain perception or central pain modulation compared with healthy subjects. Seven patients with genetically and clinically confirmed hyperekplexia were compared to 14 healthy age- and sex-matched controls. The following quantitative sensory tests were performed: pressure pain detection threshold (primary outcome), ice water tolerance, single and repeated electrical pain detection thresholds, nociceptive withdrawal reflex threshold, and conditioned pain modulation. Statistical analysis was performed using linear mixed models. Hyperekplexia patients displayed lower pain thresholds than healthy controls for all of the quantitative sensory tests [mean (standard deviation)]: pressure pain detection threshold [273 (170) versus 475 (115) kPa, P = 0.003], ice water tolerance [49.2 (36.5) versus 85.7 (35.0) s, P = 0.015], electrical single pain detection threshold [5.42 (2.64) versus 7.47 (2.62) mA, P = 0.012], electrical repeated pain detection threshold [3.76 (1.41) versus 5.8 (1.73) mA, P = 0.003], and nociceptive withdrawal reflex [7.42 (3.63) versus 14.1 (6.9) mA, P = 0.015]. Conditioned pain modulation was significantly reduced in hyperekplexia [increase to baseline: 53.2 (63.7) versus 105 (57) kPa, P = 0.030]. Our data demonstrate increased pain sensitivity and impaired central pain modulation in hyperekplexia patients, supporting the importance of glycinergic neurotransmission for central pain modulation in humans.

Gelsemine and koumine, principal active ingredients of Gelsemium, exhibit mechanical antiallodynia via spinal glycine receptor activation-induced allopregnanolone biosynthesis

Gelsemine, the principal active alkaloid from Gelsemium sempervirens Ait., and koumine, the most dominant alkaloids from Gelsemium elegans Benth., produced antinociception in a variety of rodent models of painful hypersensitivity. The present study explored the molecular mechanisms underlying gelsemine- and koumine-induced mechanical antiallodynia in neuropathic pain. The radioligand binding and displacement assays indicated that gelsemine and koumine, like glycine, were reversible and orthosteric agonists of glycine receptors with full efficacy and probably acted on same binding site as the glycine receptor antagonist strychnine. Treatment with gelsemine, koumine and glycine in primary cultures of spinal neurons (but not microglia or astrocytes) concentration dependently increased 3α-hydroxysteroid oxidoreductase (3α-HSOR) mRNA expression, which was inhibited by pretreatment with strychnine but not the glial inhibitor minocycline. Intrathecal injection of gelsemine, koumine and glycine stimulated 3α-HSOR mRNA expression in the spinal cords of neuropathic rats and produced mechanical antiallodynia. Their spinal mechanical antiallodynia was completely blocked by strychnine, the selective 3α-HSOR inhibitor medroxyprogesterone acetate (MPA), 3α-HSOR gene silencer siRNA/3α-HSOR and specific GABA_A receptor antagonist isoallopregnanolone, but not minocycline. All the results taken together uncovered that gelsemine and koumine are orthosteric agonists of glycine receptors, and produce mechanical antiallodynia through neuronal glycine receptor/3α-HSOR/allopregnanolone/GABA_A receptor pathway.

A single GABA neuron receives contacts from myelinated primary afferents of two adjacent peripheral nerves. A possible role in neuropathic pain

GAD67-EGFP mice were used in a series of experiments to provide anatomical evidence for the role of the reduction in myelinated primary afferent input to GABA spinal neurons in the production of neuropathic pain following peripheral L5 nerve injury. First, we confirmed that L5 injury in these mice produced mechanical and thermal hyperalgesia in the ipsilateral foot. Second, we injected a mixture of cholera toxin subunit-B (CTb) and isolectin B4 (IB4) in the sciatic nerve to selectively label its myelinated and unmyelinated primary afferents. Results showed that primary afferents of sciatic nerve extend from L2-L6 spinal segments. Third, we determined the central terminations of myelinated primary afferents of L4 and L5 spinal nerves following CTb injection in either nerve. The myelinated primary afferents of both nerves terminated in the corresponding and two to three rostral spinal segments with some fibers descending to terminate in the segment caudal to the level at which they entered indicating an intermingling of their terminals at the dorsal horn of the spinal cord. Fourthly, we injected CTb in L5 nerve and CTb HRP-conjugate in L4 nerve. Confocal microscopy and subsequent image analyses showed that individual EGFP-labeled neurons in L4 segment receive myelinated primary afferent contacts from both L4 and L5 nerves. Eliminating inputs from L5 nerve following its injury would result in less involvement of spinal GABA neurons which would very likely initiate neuronal sensitization in L4 segment. This could lead to the development of hyperalgesia in response to the stimulation of the adjacent uninjured L4 nerve.

Riluzole induces LTD of spinal nociceptive signaling via postsynaptic GluR2 receptors

Purpose

Riluzole - a major therapeutic medicine for patients with amyotrophic lateral sclerosis - reportedly has anti-nociceptive and anti-allodynic efficacies in neuropathic pain models. However, little is known about its effect on neurotransmission in the spinal superficial dorsal horn (SDH). The present study aims to investigate the effects of riluzole on the synaptic transmission of SDH nociceptive pathways in both physiological and pathological conditions.

Materials and methods

Spinal nerve ligation was used to produce a neuropathic pain model. Mechanical allodynia behavior was assessed with Von Frey filaments. Riluzole's effects on nociceptive synaptic transmission under both physiological and pathological conditions were examined by patch-clamp recordings in rat SDH neurons.

Results

The principal findings of the present study are three-fold. First, we affirm that riluzole has a remarkable long-lasting analgesic effect on both in vitro and in vivo pathological pain models. Second, the prolonged inhibitory effects of riluzole on spinal nociceptive signaling are mediated by both presynaptic and postsynaptic mechanisms. Finally, endocytosis of post-synaptic GluR2 contributes to the riluzole-induced long-term depression (LTD) of the spinal nociceptive pathway.

Conclusion

The present study finds that riluzole induces LTD of nociceptive signaling in the SDH and produces long-lasting anti-allodynia effects in nerve injury-induced neuropathic pain conditions via postsynaptic AMPA receptors associated with the endocytosis of GluR2.

Bidirectional Microglia-Neuron Communication in Health and Disease

Microglia are ramified cells that exhibit highly motile processes, which continuously survey the brain parenchyma and react to any insult to the CNS homeostasis. Although microglia have long been recognized as a crucial player in generating and maintaining inflammatory responses in the CNS, now it has become clear, that their function are much more diverse, particularly in the healthy brain. The innate immune response and phagocytosis represent only a little segment of microglia functional repertoire that also includes maintenance of biochemical homeostasis, neuronal circuit maturation during development and experience-dependent remodeling of neuronal circuits in the adult brain. Being equipped by numerous receptors and cell surface molecules microglia can perform bidirectional interactions with other cell types in the CNS. There is accumulating evidence showing that neurons inform microglia about their status and thus are capable of controlling microglial activation and motility while microglia also modulate neuronal activities. This review addresses the topic: how microglia communicate with other cell types in the brain, including fractalkine signaling, secreted soluble factors and extracellular vesicles. We summarize the current state of knowledge of physiological role and function of microglia during brain development and in the mature brain and further highlight microglial contribution to brain pathologies such as Alzheimer’s and Parkinson’s disease, brain ischemia, traumatic brain injury, brain tumor as well as neuropsychiatric diseases (depression, bipolar disorder, and schizophrenia).

The GlyT1 Inhibitor Bitopertin Ameliorates Allodynia and Hyperalgesia in Animal Models of Neuropathic and Inflammatory Pain

Background: Chronic pain conditions are difficult to treat and the therapeutic outcome is frequently unsatisfactory. Changes in excitation/inhibition balance within the dorsal horn contribute to the establishment and persistence of chronic pain. Thus, facilitation of inhibitory neurotransmission is a promising approach to treat chronic pain pharmacologically. Glycine transporter 1 (GlyT1) plays an important role in regulating extracellular glycine concentrations. Aim of the present study therefore was to investigate whether the specific GlyT1 inhibitor bitopertin (RG1678; RO4917838) might constitute a novel treatment for chronic pain by facilitating glycinergic inhibition.

Methods: Mechanical allodynia and thermal hyperalgesia were induced by chronic constriction injury of the sciatic nerve or carrageenan injections into the plantar surface of the hind paw in rodents. The effect of acute and long-term bitopertin application on the reaction threshold to mechanical and thermal stimuli was determined. General activity was determined in open field experiments. The glycine concentration in cerebrospinal fluid and blood was measured by HPLC.

Results: Systemic application of bitopertin in chronic pain conditions lead to a significant increase of the reaction thresholds to mechanical and thermal stimuli in a time and dose-dependent manner. Long-term application of bitopertin effectuated stable beneficial effects over 4 weeks. Bitopertin did not alter reaction thresholds to stimuli in control animals and had no effect on general locomotor activity and anxiety but lead to an increased glycine concentration in cerebrospinal fluid.

Conclusion: These findings suggest that inhibition of the GlyT1 by bitopertin represents a promising new approach for the treatment of chronic pain.

Basic/Translational Development of Forthcoming Opioid- and Nonopioid-Targeted Pain Therapeutics

Opioids represent an efficacious therapeutic modality for some, but not all pain states. Singular reliance on opioid therapy for pain management has limitations, and abuse potential has deleterious consequences for patient and society. Our understanding of pain biology has yielded insights and opportunities for alternatives to conventional opioid agonists. The aim is to have efficacious therapies, with acceptable side effect profiles and minimal abuse potential, which is to say an absence of reinforcing activity in the absence of a pain state. The present work provides a nonexclusive overview of current drug targets and potential future directions of research and development. We discuss channel activators and blockers, including sodium channel blockers, potassium channel activators, and calcium channel blockers; glutamate receptor-targeted agents, including N-methyl-D-aspartate, α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, and metabotropic receptors. Furthermore, we discuss therapeutics targeted at γ-aminobutyric acid, α2-adrenergic, and opioid receptors. We also considered antagonists of angiotensin 2 and Toll receptors and agonists/antagonists of adenosine, purine receptors, and cannabinoids. Novel targets considered are those focusing on lipid mediators and anti-inflammatory cytokines. Of interest is development of novel targeting strategies, which produce long-term alterations in pain signaling, including viral transfection and toxins. We consider issues in the development of druggable molecules, including preclinical screening. While there are examples of successful translation, mechanistically promising preclinical candidates may unexpectedly fail during clinical trials because the preclinical models may not recapitulate the particular human pain condition being addressed. Molecular target characterization can diminish the disconnect between preclinical and humans' targets, which should assist in developing nonaddictive analgesics.

Functional Synaptic Integration of Forebrain GABAergic Precursors into the Adult Spinal Cord

Spinal cord transplants of embryonic cortical GABAergic progenitor cells derived from the medial ganglionic eminence (MGE) can reverse mechanical hypersensitivity in the mouse models of peripheral nerve injury- and paclitaxel-induced neuropathic pain. Here, we used electrophysiology, immunohistochemistry, and electron microscopy to examine the extent to which MGE cells integrate into host circuitry and recapitulate endogenous inhibitory circuits. Whether the transplants were performed before or after nerve injury, the MGE cells developed into mature neurons and exhibited firing patterns characteristic of subpopulations of cortical and spinal cord inhibitory interneurons. Conversely, the transplanted cells preserved cortical morphological and neurochemical properties. We also observed a robust anatomical and functional synaptic integration of the transplanted cells into host circuitry in both injured and uninjured animals. The MGE cells were activated by primary afferents, including TRPV1-expressing nociceptors, and formed GABAergic, bicuculline-sensitive, synapses onto host neurons. Unexpectedly, MGE cells transplanted before injury prevented the development of mechanical hypersensitivity. Together, our findings provide direct confirmation of an extensive, functional synaptic integration of MGE cells into host spinal cord circuits. This integration underlies normalization of the dorsal horn inhibitory tone after injury and may be responsible for the prophylactic effect of preinjury transplants.

SIGNIFICANCE STATEMENT

Spinal cord transplants of embryonic cortical GABAergic interneuron progenitors from the medial ganglionic eminence (MGE), can overcome the mechanical hypersensitivity produced in different neuropathic pain models in adult mice. Here, we examined the properties of transplanted MGE cells and the extent to which they integrate into spinal cord circuitry. Using electrophysiology, immunohistochemistry, and electron microscopy, we demonstrate that MGE cells, whether transplanted before or after nerve injury, develop into inhibitory neurons, are activated by nociceptive primary afferents, and form GABA-A-mediated inhibitory synapses with the host. Unexpectedly, cells transplanted into naive spinal cord prevented the development of nerve-injury-induced mechanical hypersensitivity. These results illustrate the remarkable plasticity of adult spinal cord and the potential of cell-based therapies against neuropathic pain.

Pharmacological and antihyperalgesic properties of the novel α2/3 preferring GABAA receptor ligand MP-III-024

γ-Aminobutyric acid type A (GABAA) receptors are located in spinal nociceptive circuits where they modulate the transmission of pain sensory signals from the periphery to higher centers. Benzodiazepine-type drugs bind to GABAA receptors containing α1, α2, α3, and α5 subunits (α1GABAA, α2GABAA, α3GABAA and α5GABAA receptors, respectively) through which they inhibit the transmission of these signals. In the present study we describe the novel benzodiazepine site positive allosteric modulator modulator methyl 8-ethynyl-6-(pyridin-2-yl)-4H-benzo[f]imidazo[1,5-a][1,4]diazepine-3-carboxylate (MP-III-024). MP-III-024 displayed preference for α2GABAA and α3GABAA receptors relative to α1GABAA and α5GABAA receptors as well as an improved metabolic profile relative to subtype-selective positive modulators that are available currently. Administration of MP-III-024 resulted in a dose- and time-dependent reversal of mechanical hyperalgesia. On locomotor activity and schedule-controlled responding, MP-III-024 was ineffective across the doses tested. These data provide further evidence that α2GABAA and α3GABAA receptors play an important role in the antihyperalgesic effects and may not be involved in some of the undesired effects of benzodiazepine-like drugs. Further, these findings suggest that MP-III-024 is a suitable research tool for investigating the role of α2GABAA and α3GABAA receptors in the behavioral properties of benzodiazepine-like drugs in mice.

Current and Future Issues in the Development of Spinal Agents for the Management of Pain

Targeting analgesic drugs for spinal delivery reflects the fact that while the conscious experience of pain is mediated supraspinally, input initiated by high intensity stimuli, tissue injury and/or nerve injury is encoded at the level of the spinal dorsal horn and this output informs the brain as to the peripheral environment. This encoding process is subject to strong upregulation resulting in hyperesthetic states and downregulation reducing the ongoing processing of nociceptive stimuli reversing the hyperesthesia and pain processing. The present review addresses the biology of spinal nociceptive processing as relevant to the effects of intrathecally-delivered drugs in altering pain processing following acute stimulation, tissue inflammation/injury and nerve injury. The review covers i) the major classes of spinal agents currently employed as intrathecal analgesics (opioid agonists, alpha 2 agonists; sodium channel blockers; calcium channel blockers; NMDA blockers; GABA A/B agonists; COX inhibitors; ii) ongoing developments in the pharmacology of spinal therapeutics focusing on less studied agents/targets (cholinesterase inhibition; Adenosine agonists; iii) novel intrathecal targeting methodologies including gene-based approaches (viral vectors, plasmids, interfering RNAs); antisense, and toxins (botulinum toxins; resniferatoxin, substance P Saporin); and iv) issues relevant to intrathecal drug delivery (neuraxial drug distribution), infusate delivery profile, drug dosing, formulation and principals involved in the preclinical evaluation of intrathecal drug safety.

The Emerging Role of Spinal Dynorphin in Chronic Pain: A Therapeutic Perspective

Notable findings point to the significance of the dynorphin peptide neurotransmitter in chronic pain. Spinal dynorphin neuropeptide levels are elevated during development of chronic pain and sustained during persistent chronic pain. Importantly, knockout of the dynorphin gene prevents development of chronic pain in mice, but acute nociception is unaffected. Intrathecal (IT) administration of opioid and nonopioid dynorphin peptides initiates allodynia through a nonopioid receptor mechanism; furthermore, antidynorphin antibodies administered by the IT route attenuate chronic pain. Thus, this review presents the compelling evidence in the field that supports the role of dynorphin in facilitating the development of a persistent pain state. These observations illustrate the importance of elucidating the control mechanisms responsible for the upregulation of spinal dynorphin in chronic pain. Also, spinal dynorphin regulation of downstream signaling molecules may be implicated in hyperpathic states. Therapeutic strategies to block the upregulation of spinal dynorphin may provide a nonaddictive approach to improve the devastating condition of chronic pain that occurs in numerous human diseases.

Neurosteroids are reduced in diabetic neuropathy and may be associated with the development of neuropathic pain

Introduction: Peripheral and central sensitisation are implicated in the development of neuropathic pain. Hypersensitivity of pain pathway neurons has been described in animal models of diabetic neuropathy, which is postulated to be related to an imbalance between inhibitory and excitatory signals within the spinal cord. GABAergic neurons within the pain pathway are vital for the transmission of painful stimuli to higher centres. A developmental change in the rate of exponential decay of GABAergic synaptic events has been observed in other types of neurons and this may be associated with fluctuations in endogenous neurosteroid tone. 

Methods: The whole-cell patch-clamp technique was used on slices of neural tissue. Electrophysiological recordings were obtained from wild type mice between the ages of 6 and 80 days in the spinal cord, the nucleus reticularis of the thalamus and the cerebral cortex. Recordings were also obtained from mice with diabetic neuropathy (ob/ob and db/db) between the ages of 60 and 80 days. Behavioural experiments were performed to examine mechanical and thermal nociception.

Results: Electrophysiological recordings from cortical pain pathway neurons from mature type-2 diabetic mice revealed that the endogenous neurosteroid tone is reduced compared to control. However, selected neurosteroid compounds had a more pronounced effect on the GABA _A receptors of these diabetic mice. ob/ob mice exhibit mechanical hyperalgesia and allodynia, which was reduced by neurosteroids applied exogenously.

Conclusions: The reduced endogenous neurosteroid tone in ob/ob mice may be linked to their hypersensitivity. Neurosteroids may exert analgesic effects in pathological pain states by attempting to restore the physiological GABAergic inhibitory tone.

Better Antiretroviral Central Nervous System Penetration is Not Associated with Reduced Chronic Pain in People Living with Human Immunodeficiency Virus

OBJECTIVE

To determine if better antiretroviral (ARV) central nervous system (CNS) penetration is associated with reduced rates of chronic pain in people living with HIV (PLWH).

BACKGROUND

Chronic pain remains prevalent in PLWH despite widespread ARV use. Mechanisms underlying this prevalence remain unknown, though neuroinflammation from persistent CNS HIV infection and maladaptive plastic changes in the CNS have been implicated. Here we hypothesize that better CNS ARV penetration, measured using the CNS Penetration-Effectiveness (CPE) score, would decrease rates of chronic pain.

METHODS

We interviewed 254 consecutive adults from an HIV clinic in Chiang Mai, Thailand. We collected data on demographics, HIV history, ARV use, and pain characteristics. Patients were evaluated for depression using a Thai two question Patient Health Questionnaire (PHQ-2). Modified CPE score was calculated using established methods and grouped a priori into "low CPE" (≤7, poor penetration) and "high CPE" (≥8, good penetration). CPE score was compared with chronic pain scores in SPSS using appropriate statistical tests. A relationship between CPE score and a positive depression screen was tested further using multivariable binary logistic models.

RESULTS

245 of 254 subjects were on ARVs. Complete ARV data was available for 235 patients. 137 of these 235 patients (58.3%) had a CPE score ≤7, and 98 (41.7%) had a score ≥8. 49 patients had chronic pain, and 9 had neuropathic pain. Low CPE score was not associated with chronic pain (p=0.64), neuropathic pain (p=0.56), or frequent pain (p=0.80), nor was it associated with the severity of reported "worst pain" or "average pain" in the last 24 hours (p=0.18 and 0.48, respectively). Post-hoc analysis revealed that higher CPE score was a significant independent risk factor for depression measured by a positive PHQ-2 screen [OR (95% CI) = 1.29 (1.04-1.61), p=0.02]. This relationship was mediated primarily by exposure to zidovudine.

CONCLUSIONS

CPE score is not associated with chronic pain in PLWH. Post-hoc analysis demonstrated that CPE score, and zidovudine exposure in particular, predicts a positive depression screen. Given the substantial morbidity associated with chronic pain and mood disorders in PLWH, additional studies to determine preventable and treatable factors are imperative.

Strong Manual Acupuncture Stimulation of "Huantiao" (GB 30) Reduces Pain-Induced Anxiety and p-ERK in the Anterior Cingulate Cortex in a Rat Model of Neuropathic Pain

Persistent neuropathic pain is associated with anxiety. The phosphorylation of extracellular signal-regulated kinase (p-ERK) in the anterior cingulate cortex (ACC) plays an important role in pain-induced anxiety. Acupuncture is widely used for pain and anxiety. However, little is known about which acupuncture technique is optimal on pain-induced anxiety and the relationship between acupuncture effect and p-ERK. The rat model was induced by L5 spinal nerve ligation (SNL). Male adult SD rats were randomly divided into control, SNL, strong manual acupuncture (sMA), mild manual acupuncture (mMA), and electroacupuncture (EA) group. Bilateral "Huantiao" (GB 30) were stimulated by sMA, mMA, and EA, respectively. The pain withdrawal thresholds (PWTs) and anxiety behavior were measured, and p-ERK protein expression and immunoreactivity cells in ACC were detected. PWTs increased significantly in both sMA and EA groups. Meanwhile, anxiety-like behavior was improved significantly in the sMA and mMA groups. Furthermore, the overexpression of p-ERK induced by SNL was downregulated by strong and mild manual acupuncture. Therefore, strong manual acupuncture on bilateral "Huantiao" (GB 30) could be a proper therapy relieving both pain and pain-induced anxiety. The effect of different acupuncture techniques on pain-induced anxiety may arise from the regulation of p-ERK in ACC.

GABAergic modulation in central sensitization in humans: a randomized placebo-controlled pharmacokinetic-pharmacodynamic study comparing clobazam with clonazepam in healthy volunteers

Positive allosteric modulators of GABAA receptors (GAMs) acting at specific subtypes of GABAA receptors effectively restore compromised spinal pain control in rodents. Studies addressing a similar antihyperalgesic effect in humans are sparse and are hampered by sedative effects of nonselective GAMs available for use in humans. We present results from a randomized controlled double-blind crossover study in 25 healthy volunteers, which addressed potential antihyperalgesic actions of clobazam (CBZ) and clonazepam (CLN) at mildly sedating equianticonvulsive doses. Clobazam was chosen because of its relatively low sedative properties and CLN because of its use in neuropathic pain. Tolterodine (TLT) was used as an active placebo. The primary outcome parameter was a change in the area of cutaneous UVB irradiation-induced secondary hyperalgesia (ASH), which was monitored for 8 hours after drug application. Sedative effects were assessed in parallel to antihyperalgesia. Compared with TLT, recovery from hyperalgesia was significantly faster in the CBZ and CLN groups (P = 0.009). At the time point of maximum effect, the rate of recovery from hyperalgesia was accelerated by CBZ and CLN, relative to placebo by 15.7% (95% confidence interval [CI] 0.8-30.5), P = 0.040, and 28.6% (95% CI 4.5-52.6), P = 0.022, respectively. Active compounds induced stronger sedation than placebo, but these differences disappeared 8 hours after drug application. We demonstrate here that GAMs effectively reduce central sensitization in healthy volunteers. These results provide proof-of-principle evidence supporting efficacy of GAMs as antihyperalgesic agents in humans and should stimulate further research on compounds with improved subtype specificity.

The search for novel analgesics: targets and mechanisms

The management of the pain state is of great therapeutic relevance to virtually every medical specialty. Failure to manage its expression has deleterious consequence to the well-being of the organism. An understanding of the complex biology of the mechanisms underlying the processing of nociceptive information provides an important pathway towards development of novel and robust therapeutics. Importantly, preclinical models have been of considerable use in determining the linkage between mechanism and the associated behaviorally defined pain state. This review seeks to provide an overview of current thinking targeting pain biology, the use of preclinical models and the development of novel pain therapeutics. Issues pertinent to the strengths and weaknesses of current development strategies for analgesics are considered.

Therapy using implanted organic bioelectronics

Many drugs provide their therapeutic action only at specific sites in the body, but are administered in ways that cause the drug's spread throughout the organism. This can lead to serious side effects. Local delivery from an implanted device may avoid these issues, especially if the delivery rate can be tuned according to the need of the patient. We turned to electronically and ionically conducting polymers to design a device that could be implanted and used for local electrically controlled delivery of therapeutics. The conducting polymers in our device allow electronic pulses to be transduced into biological signals, in the form of ionic and molecular fluxes, which provide a way of interfacing biology with electronics. Devices based on conducting polymers and polyelectrolytes have been demonstrated in controlled substance delivery to neural tissue, biosensing, and neural recording and stimulation. While providing proof of principle of bioelectronic integration, such demonstrations have been performed in vitro or in anesthetized animals. Here, we demonstrate the efficacy of an implantable organic electronic delivery device for the treatment of neuropathic pain in an animal model. Devices were implanted onto the spinal cord of rats, and 2 days after implantation, local delivery of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) was initiated. Highly localized delivery resulted in a significant decrease in pain response with low dosage and no observable side effects. This demonstration of organic bioelectronics-based therapy in awake animals illustrates a viable alternative to existing pain treatments, paving the way for future implantable bioelectronic therapeutics.

The legacy of the benzodiazepine receptor: from flumazenil to enhancing cognition in Down syndrome and social interaction in autism

The study of the psychopharmacology of benzodiazepines continues to provide new insights into diverse brain functions related to vigilance, anxiety, mood, epileptiform activity, schizophrenia, cognitive performance, and autism-related social behavior. In this endeavor, the discovery of the benzodiazepine receptor was a key event, as it supplied the primary benzodiazepine drug-target site, provided the molecular link to the allosteric modulation of GABAA receptors and, following the recognition of GABAA receptor subtypes, furnished the platform for future, more selective drug actions. This review has two parts. In a retrospective first part, it acknowledges the contributions to the field made by my collaborators over the years, initially at Hoffmann-La Roche in Basle and later, in academia, at the University and the ETH of Zurich. In the second part, the new frontier of GABA pharmacology, targeting GABAA receptor subtypes, is reviewed with special focus on nonsedative anxiolytics, antidepressants, analgesics, as well as enhancers of cognition in Down syndrome and attenuators of symptoms of autism spectrum disorders. It is encouraging that a clinical trial has been initiated with a partial inverse agonist acting on α5 GABAA receptors in an attempt to alleviate the cognitive deficits in Down syndrome.

Dendritic spine dysgenesis in neuropathic pain

Neuropathic pain is a significant unmet medical need in patients with variety of injury or disease insults to the nervous system. Neuropathic pain often presents as a painful sensation described as electrical, burning, or tingling. Currently available treatments have limited effectiveness and narrow therapeutic windows for safety. More powerful analgesics, e.g., opioids, carry a high risk for chemical dependence. Thus, a major challenge for pain research is the elucidation of the mechanisms that underlie neuropathic pain and developing targeted strategies to alleviate pathological pain. The mechanistic link between dendritic spine structure and circuit function could explain why neuropathic pain is difficult to treat, since nociceptive processing pathways are adversely "hard-wired" through the reorganization of dendritic spines. Several studies in animal models of neuropathic pain have begun to reveal the functional contribution of dendritic spine dysgenesis in neuropathic pain. Previous reports have demonstrated three primary changes in dendritic spine structure on nociceptive dorsal horn neurons following injury or disease, which accompany chronic intractable pain: (I) increased density of dendritic spines, particularly mature mushroom-spine spines, (II) redistribution of spines toward dendritic branch locations close to the cell body, and (III) enlargement of the spine head diameter, which generally presents as a mushroom-shaped spine. Given the important functional implications of spine distribution, density, and shape for synaptic and neuronal function, the study of dendritic spine abnormality may provide a new perspective for investigating pain, and the identification of specific molecular players that regulate spine morphology may guide the development of more effective and long-lasting therapies.

Antihyperalgesic effect of the GABA(A) ligand clobazam in a neuropathic pain model in mice: a pharmacokinetic-pharmacodynamic study

Facilitation of spinal GABAergic inhibition with benzodiazepines (BZDs) reverses pain sensitization in animals; however, the use of BZDs in man is limited by their sedative effect. The antihyperalgesic effects of GABA(A) agonists are mediated by GABA(A) receptors containing α2 subunits, whereas sedation is linked to α1 subunit-containing receptors. α2 and α3 selective GABA(A) receptor modulators have been tested in animals but are not yet available for use in human beings. Clobazam is a 1,5-BZD, which exhibits less cognitive side effects than other benzodiazepines. Here, we studied its antihyperalgesic effects in a mouse model of neuropathic pain. Clobazam showed a dose-dependent antihyperalgesic effect in the chronic constriction injury (CCI) model of neuropathic pain, peaking at 1 hr after administration and lasting for 4 hr with no relevant sedation at a dose of 3 mg/kg. At higher doses, the antihyperalgesic effect was stronger, but sedation became significant. The blood and brain kinetics of clobazam were linear over the range of doses tested with a short half-life of the parent compound and a ready penetration of the blood-brain barrier. Clobazam blood concentrations decreased rapidly, falling below the limit of detection at 120 min. after drug application. Its main metabolite, N-desmethyl-clobazam, showed more delayed and prolonged pharmacokinetics, partly explaining why antihyperalgesia persisted when clobazam was no longer detectable in the blood. Considering its therapeutic margin and its pharmacokinetic properties, clobazam would be a valuable compound to assess the role of the GABAergic pathway in pain transmission in human beings.

Tumor suppressor menin mediates peripheral nerve injury-induced neuropathic pain through potentiating synaptic plasticity

Synaptic plasticity is a crucial step in the development of central sensitization in the pathogenesis of neuropathic hyperalgesia. Menin, the product of the multiple endocrine neoplasia type 1 (MEN1) gene, possesses the property of synaptogenesis which plays an essential role in neuronal activity. We tested the contributing role of spinal menin in peripheral nerve injury-induced neuropathic hypersensitivity through modulating neuronal synaptic plasticity. After approval by the Institutional Animal Care and Use Committee, nociceptive responses were detected with von Frey filaments and thermal plate after spared nerve injury in C57BL/6 mice who were treated with either intrathecal antisense oligonucleotide of MEN1 (ASO) or vehicle. Extracellular spontaneous discharge frequency, field excitatory postsynaptic potential (fEPSP), and monosynaptic excitatory postsynaptic currents (EPSCs) were measured electrophysiologically. Intrathecal ASO alleviated nerve injury-induced mechanical and thermal hypersensitivity. Upregulated spinal menin after nerve injury colocalized with NeuN in the superficial laminae; genetic knockdown of spinal menin reduced nerve injury induced in vivo spontaneous activity and instantaneous frequency and in vitro field potentials; ASO decreased the frequency and amplitude of monosynaptic EPSCs, and reduced synaptic strength and total charge. Collectively, these findings highlight the role of upregulated neuronal menin in the spinal cord in potentiating spinal synaptic plasticity in peripheral nerve injury-induced neuropathic hypersensitivity.

Long-term actions of BDNF on inhibitory synaptic transmission in identified neurons of the rat substantia gelatinosa

Peripheral nerve injury promotes the release of brain-derived neurotrophic factor (BDNF) from spinal microglial cells and primary afferent terminals. This induces an increase in dorsal horn excitability that contributes to "central sensitization" and to the onset of neuropathic pain. Although it is accepted that impairment of GABAergic and/or glycinergic inhibition contributes to this process, certain lines of evidence suggest that GABA release in the dorsal horn may increase after nerve injury. To resolve these contradictory findings, we exposed rat spinal cord neurons in defined-medium organotypic culture to 200 ng/ml BDNF for 6 days to mimic the change in spinal BDNF levels that accompanies peripheral nerve injury. Morphological and electrophysiological criteria and glutamic acid decarboxylase (GAD) immunohistochemistry were used to distinguish putative inhibitory tonic-islet-central neurons from putative excitatory delay-radial neurons. Whole cell recording in the presence of 1 μM tetrodotoxin showed that BDNF increased the amplitude of GABAergic and glycinergic miniature inhibitory postsynaptic currents (mIPSCs) in both cell types. It also increased the amplitude and frequency of spontaneous, action potential-dependent IPSCs (sIPSCs) in putative excitatory neurons. By contrast, BDNF reduced sIPSC amplitude in inhibitory neurons but frequency was unchanged. This increase in inhibitory drive to excitatory neurons and decreased inhibitory drive to inhibitory neurons seems inconsistent with the observation that BDNF increases overall dorsal horn excitability. One of several explanations for this discrepancy is that the action of BDNF in the substantia gelatinosa is dominated by previously documented increases in excitatory synaptic transmission rather than by impediment of inhibitory transmission.

Receptor and channel heteromers as pain targets

Recent discoveries indicate that many G-protein coupled receptors (GPCRs) and channels involved in pain modulation are able to form receptor heteromers. Receptor and channel heteromers often display distinct signaling characteristics, pharmacological properties and physiological function in comparison to monomer/homomer receptor or ion channel counterparts. It may be possible to capitalize on such unique properties to augment therapeutic efficacy while minimizing side effects. For example, drugs specifically targeting heteromers may have greater tissue specificity and analgesic efficacy. This review will focus on current progress in our understanding of roles of heteromeric GPCRs and channels in pain pathways as well as strategies for controlling pain pathways via targeting heteromeric receptors and channels. This approach may be instrumental in the discovery of novel classes of drugs and expand our repertoire of targets for pain pharmacotherapy.

Mechanisms of neuropathic pain

Neuropathic pain is a disease of global burden. Its symptoms include spontaneous and stimulus-evoked painful sensations. Several maladaptive mechanisms underlying these symptoms have been elucidated in recent years: peripheral sensitization of nociception, abnormal excitability of afferent neurons, central sensitization comprising pronociceptive facilitation, disinhibition of nociception and central reorganization processes, and sympathetically maintained pain. This review aims to illustrate these pathophysiological principles, focussing on molecular and neurophysiological findings. Finally therapeutic options based on these findings are discussed.

Prolonged zymosan-induced inflammatory pain hypersensitivity in mice lacking glycine receptor alpha2

Glycinergic synapses play a major role in shaping the activity of spinal cord neurons under normal conditions and during persistent pain. However, the role of different glycine receptor (GlyR) subtypes in pain processing has only begun to be unraveled. Here, we analysed whether the GlyR alpha2 subunit might be involved in the processing of acute or persistent pain. Real-time RT-PCR and in situ hybridization analyses revealed that GlyR alpha2 mRNA is enriched in the dorsal horn of the mouse spinal cord. Mice lacking GlyR alpha2 (Glra2(-/-) mice) demonstrated a normal nociceptive behavior in models of acute pain and after peripheral nerve injury. However, mechanical hyperalgesia induced by peripheral injection of zymosan was significantly prolonged in Glra2(-/-) mice as compared to wild-type littermates. We conclude that spinal GlyRs containing the alpha2 subunit exert a previously unrecognized role in the resolution of inflammatory pain.

The mechanisms of microgliosis and pain following peripheral nerve injury

Microglia are the resident macrophages in the central nervous system (CNS). Any insult to the CNS homeostasis will induce a rapid change in microglia morphology, gene expression profile and functional behaviour. These responses of microglia have been collectively known as 'microgliosis'. Interestingly, damage to the nervous system outside the CNS, such as axotomy of a peripheral nerve, can lead to microgliosis in the spinal cord. There is a variation in the degree of microgliosis depending on the model of nerve injury employed for instance this response is more marked following traumatic nerve injury than in models of chemotherapy induced neuropathy. Following peripheral nerve injury nociceptive inputs from sensory neurons appear to be critical in triggering the development of spinal microgliosis. A number of signalling pathways including growth factors such as Neuregulin-1, matrix metalloproteases such as MMP-9 and multiple chemokines enable direct communication between injured primary afferents and microglia. In addition, we describe a group of mediators which although not demonstrably shown to be released from neurons are known to modulate microglial phenotype. There is a great functional diversity of the microglial response to peripheral nerve injury which includes: Cellular migration, proliferation, cytokine release, phagocytosis, antigen presentation and recruitment of T cells. It should also be noted that in certain contexts microglia may have a role in the resolution of neuro-inflammation. Although there is still no direct evidence demonstrating that spinal microglia have a role in neuropathic pain in humans, these patients present a pro-inflammatory cytokine profile and it is a reasonable hypothesis that these cells may contribute to this inflammatory response. Modulating microglial functions offers a novel therapeutic opportunity following nerve injury which ideally would involve reducing the pro-inflammatory nature of these cells whilst retaining their potential beneficial functions.

Presynaptic alpha2-GABAA receptors in primary afferent depolarization and spinal pain control

Spinal dorsal horn GABA(A) receptors are found both postsynaptically on central neurons and presynaptically on axons and/or terminals of primary sensory neurons, where they mediate primary afferent depolarization (PAD) and presynaptic inhibition. Both phenomena have been studied extensively on a cellular level, but their role in sensory processing in vivo has remained elusive, due to inherent difficulties to selectively interfere with presynaptic receptors. Here, we address the contribution of a major subpopulation of GABA(A) receptors (those containing the α2 subunit) to spinal pain control in mice lacking α2-GABA(A) receptors specifically in primary nociceptors (sns-α2(-/-) mice). sns-α2(-/-) mice exhibited GABA(A) receptor currents and dorsal root potentials of normal amplitude in vitro, and normal response thresholds to thermal and mechanical stimulation in vivo, and developed normal inflammatory and neuropathic pain sensitization. However, the positive allosteric GABA(A) receptor modulator diazepam (DZP) had almost completely lost its potentiating effect on PAD and presynaptic inhibition in vitro and a major part of its spinal antihyperalgesic action against inflammatory hyperalgesia in vivo. Our results thus show that part of the antihyperalgesic action of spinally applied DZP occurs through facilitated activation of GABA(A) receptors residing on primary nociceptors.

Physiology of microglia

Microglial cells are the resident macrophages in the central nervous system. These cells of mesodermal/mesenchymal origin migrate into all regions of the central nervous system, disseminate through the brain parenchyma, and acquire a specific ramified morphological phenotype termed "resting microglia." Recent studies indicate that even in the normal brain, microglia have highly motile processes by which they scan their territorial domains. By a large number of signaling pathways they can communicate with macroglial cells and neurons and with cells of the immune system. Likewise, microglial cells express receptors classically described for brain-specific communication such as neurotransmitter receptors and those first discovered as immune cell-specific such as for cytokines. Microglial cells are considered the most susceptible sensors of brain pathology. Upon any detection of signs for brain lesions or nervous system dysfunction, microglial cells undergo a complex, multistage activation process that converts them into the "activated microglial cell." This cell form has the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells. Activated microglial cells can migrate to the site of injury, proliferate, and phagocytose cells and cellular compartments.