Glycinergic and GABAergic tonic inhibition fine tune inhibitory control in regionally distinct subpopulations of dorsal horn neurons

Anti-hyperalgesic effects of a novel TRPM8 agonist in neuropathic rats: a comparison with topical menthol

Menthol has historically been used topically to alleviate various pain conditions. At low concentrations, this non-selective TRPM8 agonist elicits a cooling sensation, however higher concentrations result in cold hyperalgesia in normal subjects and paradoxically analgesia in neuropathic patients. Through behavioural and electrophysiological means, we examined whether this back-translated into a pre-clinical rodent model. Menthol was applied topically to the hind paws of naive and spinal nerve-ligated (SNL) rats. In behavioural assays, menthol did not affect withdrawal thresholds to mechanical stimulation and 10% and 40% menthol rarely sensitised withdrawals to innocuous cooling in naïve rats. However, in SNL rats, 10% and 40% menthol alleviated cold hypersensitivity. This was partly corroborated by in vivo electrophysiological recordings of dorsal horn lamina V/VI neurones. As several studies have implicated TRPM8 in analgesia, we examined whether a novel systemically available TRPM8 agonist, M8-Ag, had more potent anti-hyperalgesic effects than menthol in neuropathic rats. In vitro, M8-Ag activates TRPM8, expressed in HEK293 cells, with an EC50 of 44.97 nM. In vivo, M8-Ag inhibited neuronal responses to innocuous and noxious cooling in SNL rats with no effect in sham-operated rats. This effect was modality selective; M8-Ag did not alter neuronal responses to mechanical, heat or brush stimulation. In addition, M8-Ag attenuated behavioural hypersensitivity to innocuous cooling but not mechanical stimulation. These data suggest that menthol induced hyperalgesia is not consistently replicable in the rat and that the analgesic properties are revealed by injury. Systemic TRPM8 agonists might be beneficial in neuropathy without affecting normal cold sensitivity.

Glycine transport inhibitors for the treatment of pain

Opioids, local anesthetics, anticonvulsant drugs, antidepressants, and non-steroidal anti-inflammatory drugs (NSAIDs) are used to provide pain relief but they do not provide adequate pain relief in a large proportion of chronic pain patients and are often associated with unacceptable side effects. Inhibitory glycinergic neurotransmission is impaired in chronic pain states, and this provides a novel target for drug development. Inhibitors of the glycine transporter 2 (GlyT2) enhance inhibitory neurotransmission and show particular promise for the treatment of neuropathic pain. N-arachidonyl-glycine (NAGly) is an endogenous lipid that inhibits glycine transport by GlyT2 and also shows potential as an analgesic, which may be further exploited in drug development. In this review we discuss the role of glycine neurotransmission in chronic pain and future prospects for the use of glycine transport inhibitors in the treatment of pain.

Long-term potentiation of glycinergic synapses triggered by interleukin 1β

Long-term potentiation (LTP) is a persistent increase in synaptic strength required for many behavioral adaptations, including learning and memory, visual and somatosensory system functional development, and drug addiction. Recent work has suggested a role for LTP-like phenomena in the processing of nociceptive information in the dorsal horn and in the generation of central sensitization during chronic pain states. Whereas LTP of glutamatergic and GABAergic synapses has been characterized throughout the central nervous system, to our knowledge there have been no reports of LTP at mammalian glycinergic synapses. Glycine receptors (GlyRs) are structurally related to GABAA receptors and have a similar inhibitory role. Here we report that in the superficial dorsal horn of the spinal cord, glycinergic synapses on inhibitory GABAergic neurons exhibit LTP, occurring rapidly after exposure to the inflammatory cytokine interleukin-1 beta. This form of LTP (GlyR LTP) results from an increase in the number and/or change in biophysical properties of postsynaptic glycine receptors. Notably, formalin-induced peripheral inflammation in vivo potentiates glycinergic synapses on dorsal horn neurons, suggesting that GlyR LTP is triggered during inflammatory peripheral injury. Our results define a previously unidentified mechanism that could disinhibit neurons transmitting nociceptive information and may represent a useful therapeutic target for the treatment of pain.

Diabetic neuropathic pain: a role for testosterone metabolites

Diabetic neuropathy is associated with neuropathic pain in about 50% of diabetic subjects. Clinical management of neuropathic pain is complex and so far unsatisfactory. In this study, we analyzed the effects of the testosterone metabolites, dihydrotestosterone (DHT), and 3α-diol, on nociceptive and allodynia thresholds and on molecular and functional parameters related to pain modulation in the dorsal horns of the spinal cord and in the dorsal root ganglia of rats rendered diabetic by streptozotocin injection. Furthermore, the levels of DHT and 3α-diol were analyzed in the spinal cord. Diabetes resulted in a significant decrease in DHT levels in the spinal cord that was reverted by DHT or 3α-diol treatments. In addition, 3α-diol treatment resulted in a significant increase in 3α-diol in the spinal cord compared with control values. Both steroids showed analgesic properties on diabetic neuropathic pain, affecting different pain parameters and possibly by different mechanisms of action. Indeed, DHT counteracted the effect of diabetes on the mechanical nociceptive threshold, pre- and post-synaptic components, glutamate release, astrocyte immunoreactivity, and expression of interleukin-1β (IL1β), while 3α-diol was effective on tactile allodynia threshold, glutamate release, astrocyte immunoreactivity and the expression of substance P, toll-like receptor 4, tumor necrosis factor-α, transforming growth factor β-1, IL1β, and translocator protein. These results indicate that testosterone metabolites are potential agents for the treatment of diabetic neuropathic pain.

The impact of tonic GABAA receptor-mediated inhibition on neuronal excitability varies across brain region and cell type

The diversity of GABA_A receptor (GABA_AR) subunits and the numerous configurations during subunit assembly give rise to a variety of receptors with different functional properties. This heterogeneity results in variations in GABAergic conductances across numerous brain regions and cell types. Phasic inhibition is mediated by synaptically-localized receptors with a low affinity for GABA and results in a transient, rapidly desensitizing GABAergic conductance; whereas, tonic inhibition is mediated by extrasynaptic receptors with a high affinity for GABA and results in a persistent GABAergic conductance. The specific functions of tonic versus phasic GABAergic inhibition in different cell types and the impact on specific neural circuits are only beginning to be unraveled. Here we review the diversity in the magnitude of tonic GABAergic inhibition in various brain regions and cell types, and highlight the impact on neuronal excitability in different neuronal circuits. Further, we discuss the relevance of tonic inhibition in various physiological and pathological contexts as well as the potential of targeting these receptor subtypes for treatment of diseases, such as epilepsy.

Normal and abnormal coding of somatosensory stimuli causing pain

Noxious stimuli usually cause pain and pain usually arises from noxious stimuli, but exceptions to these apparent truisms are the basis for clinically important problems and provide valuable insight into the neural code for pain. In this Review, we discuss how painful sensations arise. We argue that, although primary somatosensory afferents are tuned to specific stimulus features, natural stimuli often activate more than one type of afferent. Manipulating coactivation patterns can alter perception in ways that argue against each type of afferent acting independently (as expected for strictly labeled lines), suggesting instead that signals conveyed by different types of afferents interact. Deciphering the central circuits that mediate those interactions is critical for explaining the generation and modulation of neural signals that ultimately elicit pain. The advent of genetic and optical dissection techniques promise to dramatically accelerate progress toward this goal, which will facilitate the rational design of future pain therapeutics.

Restoring ionotropic inhibition as an analgesic strategy

Neuronal inhibition in nociceptive relays of the spinal cord is essential for the proper processing of nociceptive information. In the spinal cord dorsal horn, the activity of synaptic and extrasynaptic GABAA and glycine receptors generates rapid, Cl(-)-dependent neuronal inhibition. A loss of this ionotropic inhibition, particularly through the collapse of the inhibitory Cl(-)-gradient, is a key mechanism by which pathological pain conditions develop. This review summarizes the roles of ionotropic inhibition in the regulation of nociception, and explores recent evidence that the potentiation of GABAA or glycine receptor activity or the enhancement of inhibitory drive can reverse pathological pain.

Activation of glycine and extrasynaptic GABA(A) receptors by taurine on the substantia gelatinosa neurons of the trigeminal subnucleus caudalis

The substantia gelatinosa (SG) of the trigeminal subnucleus caudalis (Vc) has been known for the processing and transmission of orofacial nociceptive information. Taurine, one of the most plentiful free amino-acids in humans, has proved to be involved in pain modulation. In this study, using whole-cell patch clamp technique, we investigated the direct membrane effects of taurine and the action mechanism behind taurine-mediated responses on the SG neurons of the Vc. Taurine showed non-desensitizing and repeatable membrane depolarizations and inward currents which remained in the presence of amino-acid receptors blocking cocktail (AARBC) with tetrodotoxin, indicating that taurine acts directly on the postsynaptic SG neurons. Further, application of taurine at different doses (10 μM to 3 mM) showed a concentration dependent depolarizations and inward currents with the EC₅₀ of 84.3 μM and 723 μM, respectively. Taurine-mediated responses were partially blocked by picrotoxin (50 μM) and almost completely blocked by strychnine (2 μM), suggesting that taurine-mediated responses are via glycine receptor (GlyR) activation. In addition, taurine (1 mM) activated extrasynaptic GABA_A receptor (GABA_AR)-mediated currents. Taken together, our results indicate that taurine can be a target molecule for orofacial pain modulation through the activation of GlyRs and/or extrasynaptic GABA_ARs on the SG neurons.

A feed-forward spinal cord glycinergic neural circuit gates mechanical allodynia

Neuropathic pain is characterized by mechanical allodynia induced by low-threshold myelinated Aβ-fiber activation. The original gate theory of pain proposes that inhibitory interneurons in the lamina II of the spinal dorsal horn (DH) act as "gate control" units for preventing the interaction between innocuous and nociceptive signals. However, our understanding of the neuronal circuits underlying pain signaling and modulation in the spinal DH is incomplete. Using a rat model, we have shown that the convergence of glycinergic inhibitory and excitatory Aβ-fiber inputs onto PKCγ+ neurons in the superficial DH forms a feed-forward inhibitory circuit that prevents Aβ input from activating the nociceptive pathway. This feed-forward inhibition was suppressed following peripheral nerve injury or glycine blockage, leading to inappropriate induction of action potential outputs in the nociceptive pathway by Aβ-fiber stimulation. Furthermore, spinal blockage of glycinergic synaptic transmission in vivo induced marked mechanical allodynia. Our findings identify a glycinergic feed-forward inhibitory circuit that functions as a gate control to separate the innocuous mechanoreceptive pathway and the nociceptive pathway in the spinal DH. Disruption of this glycinergic inhibitory circuit after peripheral nerve injury has the potential to elicit mechanical allodynia, a cardinal symptom of neuropathic pain.

Dynamic effects of TNF-α on synaptic transmission in mice over time following sciatic nerve chronic constriction injury

Nerve injury-induced central sensitization can manifest as an increase in excitatory synaptic transmission and/or as a decrease in inhibitory synaptic transmission in spinal dorsal horn neurons. Cytokines such as tumor necrosis factor-α (TNF-α) are induced in the spinal cord under various injury conditions and contribute to neuropathic pain. In this study we examined the effect of TNF-α in modulating excitatory and inhibitory synaptic input to spinal substantia gelatinosa (SG) neurons over time in mice following chronic constriction injury (CCI) of the sciatic nerve. Whole cell patch-clamp studies from SG neurons showed that TNF-α enhanced overall excitability of the spinal cord early in time following nerve injury 3 days after CCI compared with that in sham control mice. In contrast, the effects of TNF were blunted 14 days after CCI in nerve-injured mice compared with sham surgery mice. Immunohistochemical staining showed that the expression of TNF-α receptor 1 (TNFR1) was increased at 3 days but decreased at 14 days following CCI in the ipsilateral vs. the contralateral spinal cord dorsal horn. These results suggest that TNF-α acting at TNFR1 is important in the development of neuropathic pain by facilitating excitatory synaptic signaling in the acute phases after nerve injury but has a reduced effect on spinal neuron signaling in the later phases of nerve injury-induced pain. Failure of the facilatory effects of TNF-α on excitatory synaptic signaling in the dorsal horn to resolve following nerve injury may be an important component in the transition between acute and chronic pain conditions.

Pre- and postsynaptic inhibitory control in the spinal cord dorsal horn

Sensory information transmitted to the spinal cord dorsal horn is modulated by a complex network of excitatory and inhibitory interneurons. The two main inhibitory transmitters, GABA and glycine, control the flow of sensory information mainly by regulating the excitability of dorsal horn neurons. A presynaptic action of GABA has also been proposed as an important modulatory mechanism of transmitter release from sensory primary afferent terminals. By inhibiting the release of glutamate from primary afferent terminals, activation of presynaptic GABA receptors could play an important role in nociceptive and tactile sensory coding, while changes in their expression or function could be involved in pathological pain conditions, such as allodynia.

Deficits in glycinergic inhibition within adult spinal nociceptive circuits after neonatal tissue damage

Tissue injury during a critical period of early postnatal development can alter pain sensitivity throughout life. However, the degree to which neonatal tissue damage exerts prolonged effects on synaptic signaling within adult spinal nociceptive circuits remains unknown. Here we provide evidence that a transient surgical injury of the hind paw during the neonatal period compromises inhibitory transmission within the adult mouse superficial dorsal horn (SDH), while the same incision occurring during the third week of life failed to evoke these long-term modifications of the SDH synaptic network. The decrease in phasic inhibitory signaling after early tissue damage reflected a selective reduction in glycine receptor (GlyR)-mediated input onto both GABAergic and presumed glutamatergic neurons within lamina II of the adult SDH. Meanwhile, neonatal incision significantly decreased the density of tonic GlyR-mediated current only in the presumed glutamatergic population during adulthood. These persistent changes in synaptic function following early injury occurred in the absence of significant alterations in the transcription of genes known to be important for glycinergic transmission. These findings suggest that aberrant sensory input during early life has permanent consequences for the functional organization of nociceptive synaptic circuits within the adult spinal cord.

Genes, molecules and patients--emerging topics to guide clinical pain research

This review selectively explores some areas of pain research that, until recently, have been poorly understood. We have chosen four topics that relate to clinical pain and we discuss the underlying mechanisms and related pathophysiologies contributing to these pain states. A key issue in pain medicine involves crucial events and mediators that contribute to normal and abnormal pain signaling, but remain unseen without genetic, biomarker or imaging analysis. Here we consider how the altered genetic make-up of familial pains reveals the human importance of channels discovered by preclinical research, followed by the contribution of receptors as stimulus transducers in cold sensing and cold pain. Finally we review recent data on the neuro-immune interactions in chronic pain and the potential targets for treatment in cancer-induced bone pain.

Properties of spinal lamina III GABAergic neurons in naïve and in neuropathic mice

BACKGROUND

Nerve injury leads to Aβ-fibre-mediated mechanical allodynia that is in part due to an impaired GABAergic inhibition in the spinal cord dorsal horn. The properties and function of GABAergic neurons in spinal cord lamina III, an area where low-threshold mechanosensitive Aβ-fibres terminate are, however, largely unknown.

METHODS

We used transgenic mice, which express enhanced green fluorescent protein (EGFP) under control of the promoter GAD67. The morphology and neurochemical characteristics of GABAergic, EGFP-expressing neurons were characterized. We assessed active and passive membrane properties of spinal lamina III GABAergic neurons in naïve animals and animals with a chronic constriction injury (CCI) of the sciatic nerve.

RESULTS

EGFP-expressing neurons in lamina III were predominantly islet cells (47%), whereas non-EGFP-expressing neurons were largely inverted stalked cells (40%). EGFP-expressing neurons accounted for about 25% of GABAergic neurons in lamina III. Forty-four percent co-expressed glycine, 10% neuronal nitric oxide synthase and 3% co-expressed parvalbumin. We found costaining with protein kinase CβII in 42% of EGFP-expressing neurons but no expression of protein kinase Cγ. Membrane properties and excitability of EGFP-and non-EGFP-expressing neurons from naïve and neuropathic animals were indistinguishable. The most frequent firing pattern was tonic firing (naïve: 35%, neuropathic: 37%) followed by gap firing (naïve: 33%, neuropathic: 25%). Delayed, initial burst and single-spike firing patterns made up the remainder in both groups.

CONCLUSION

A change in membrane excitability or discharge pattern of this group of lamina III GABAergic neurons is unlikely the cause for mechanical allodynia in animals with CCI.

Tranexamic acid concentrations associated with human seizures inhibit glycine receptors

Antifibrinolytic drugs are widely used to reduce blood loss during surgery. One serious adverse effect of these drugs is convulsive seizures; however, the mechanisms underlying such seizures remain poorly understood. The antifibrinolytic drugs tranexamic acid (TXA) and ε-aminocaproic acid (EACA) are structurally similar to the inhibitory neurotransmitter glycine. Since reduced function of glycine receptors causes seizures, we hypothesized that TXA and EACA inhibit the activity of glycine receptors. Here we demonstrate that TXA and EACA are competitive antagonists of glycine receptors in mice. We also showed that the general anesthetic isoflurane, and to a lesser extent propofol, reverses TXA inhibition of glycine receptor-mediated current, suggesting that these drugs could potentially be used to treat TXA-induced seizures. Finally, we measured the concentration of TXA in the cerebrospinal fluid (CSF) of patients undergoing major cardiovascular surgery. Surprisingly, peak TXA concentration in the CSF occurred after termination of drug infusion and in one patient coincided with the onset of seizures. Collectively, these results show that concentrations of TXA equivalent to those measured in the CSF of patients inhibited glycine receptors. Furthermore, isoflurane or propofol may prevent or reverse TXA-induced seizures.

Sensitivity of spinal neurons to GABA and glycine during voluntary movement in behaving monkeys

GABAergic and glycinergic inhibition play key roles in the function of spinal motor pathways. However, there is little direct information on the extent to which inhibition controls the activity of spinal neurons during behavior or the relative effectiveness of GABA and glycine on cell activity under normal conditions. These issues were investigated in three macaque monkeys trained to perform voluntary ramp-and-hold wrist movements and grip. Pipettes with an extracellular recording electrode and iontophoresis barrels were used to eject GABA, glycine, and/or their respective antagonists, bicuculline and strychnine, as the activity of single neurons was recorded in the C6-T1 spinal segments during hand movements. The firing rate of the vast majority of neurons decreased when an inhibitory neurotransmitter was ejected from the electrode, suggesting that most movement-related spinal neurons are sensitive to both GABA and glycine. Most movement-related neurons exhibited increased activity during iontophoresis of an antagonist, suggesting that both GABAergic and glycinergic inhibition actively regulate the majority of spinal neurons during movement. These conclusions were supported by the responses of neurons tested with both agonists or both antagonists. Bicuculline and strychnine produced the largest increases in firing rate during dynamic movements (ramp phase), smaller increases during maintained torque/force (hold phase), and the smallest increase during the rest period. Since excitatory inputs also tend to increase progressively from rest to static to dynamic muscle contractions, this result is consistent with coupled excitatory and inhibitory inputs to spinal neurons during movement.

Spinal cord stimulation: neurophysiological and neurochemical mechanisms of action

Chronic neuropathic pain can significantly reduce quality of life and place an economic burden on individuals and society. Spinal cord stimulation (SCS) is an alternative approach to the treatment of neuropathic pain when standard pharmacological agents have failed. However, an improved understanding of the mechanisms by which SCS inhibits pain is needed to enhance its clinical utility. This review summarizes important findings from recent studies of SCS in animal models of neuropathic pain, highlights current understanding of the spinal neurophysiological and neurochemical mechanisms by which SCS produces an analgesic effect, and discusses the potential clinical applicability of these findings and future directions for research.

Behavioral characterization of knockin mice with mutations M287L and Q266I in the glycine receptor α1 subunit

We used behavioral pharmacology to characterize heterozygous knockin mice with mutations (Q266I or M287L) in the α1 subunit of the glycine receptor (GlyR) (J Pharmacol Exp Ther 340:304-316, 2012). These mutations were designed to reduce (M287L) or eliminate (Q266I) ethanol potentiation of GlyR function. We asked which behavioral effects of ethanol would be reduced more in the Q266I mutant than the M287L and found rotarod ataxia to be the behavior that fulfilled this criterion. Compared with controls, the mutant mice also differed in ethanol consumption, ethanol-stimulated startle response, signs of acute physical dependence, and duration of loss of righting response produced by ethanol, butanol, ketamine, pentobarbital, and flurazepam. Some of these behavioral changes were mimicked in wild-type mice by acute injections of low, subconvulsive doses of strychnine. Both mutants showed increased acoustic startle response and increased sensitivity to strychnine seizures. Thus, in addition to reducing ethanol action on the GlyRs, these mutations reduced glycinergic inhibition, which may also alter sensitivity to GABAergic drugs.

GABAA receptors mediate motoneuron tonic inhibition in the turtle spinal cord

GABA(A) receptors mediating tonic inhibitory currents are present in neurons from hippocampus, cerebellum, sensory cortex and thalamus. These receptors located at peri- and extra-synaptic sites are constituted mainly by α(4/6) and α(5) subunits which confer them high affinity for GABA and low desensitization. Immunohistochemical and in vitro hybridization studies have shown the expression of these subunits, while functional studies have reported the presence of GABAergic tonic currents in spinal dorsal horn neurons. However, the presence of this inhibitory current has not been documented in motoneurons. In addition, we previously reported that the monosynaptic reflex is facilitated by furosemide, an antagonist of the α(4/6) GABA(A) receptors, without affecting the dorsal root potential, which suggests the presence of a GABAergic tonic inhibitory current in motoneurons. The aim of this work was to investigate the presence of high affinity GABA(A) receptors in motoneurons. By intracellular recordings made with sharp electrodes and the whole-cell patch clamp recording technique we show here that the membrane input resistance and the monosynaptic excitatory post-synaptic potential (EPSPs) are significantly increased by bicuculline. Likewise, the depression of the EPSPs and the input membrane resistance normally induced by muscimol was partially reverted by 20 μM bicuculline and abolished when the concentration of the antagonist was raised to 100 μM. Last, bicuculline at low concentration did not affect the holding current as occur with the high concentration that block the tonic inhibitory GABAergic current. Together these results suggest that the excitability in motoneurons may be tonically inhibited by high affinity GABA(A) receptors.

Central sensitization: implications for the diagnosis and treatment of pain

Nociceptor inputs can trigger a prolonged but reversible increase in the excitability and synaptic efficacy of neurons in central nociceptive pathways, the phenomenon of central sensitization. Central sensitization manifests as pain hypersensitivity, particularly dynamic tactile allodynia, secondary punctate or pressure hyperalgesia, aftersensations, and enhanced temporal summation. It can be readily and rapidly elicited in human volunteers by diverse experimental noxious conditioning stimuli to skin, muscles or viscera, and in addition to producing pain hypersensitivity, results in secondary changes in brain activity that can be detected by electrophysiological or imaging techniques. Studies in clinical cohorts reveal changes in pain sensitivity that have been interpreted as revealing an important contribution of central sensitization to the pain phenotype in patients with fibromyalgia, osteoarthritis, musculoskeletal disorders with generalized pain hypersensitivity, headache, temporomandibular joint disorders, dental pain, neuropathic pain, visceral pain hypersensitivity disorders and post-surgical pain. The comorbidity of those pain hypersensitivity syndromes that present in the absence of inflammation or a neural lesion, their similar pattern of clinical presentation and response to centrally acting analgesics, may reflect a commonality of central sensitization to their pathophysiology. An important question that still needs to be determined is whether there are individuals with a higher inherited propensity for developing central sensitization than others, and if so, whether this conveys an increased risk in both developing conditions with pain hypersensitivity, and their chronification. Diagnostic criteria to establish the presence of central sensitization in patients will greatly assist the phenotyping of patients for choosing treatments that produce analgesia by normalizing hyperexcitable central neural activity. We have certainly come a long way since the first discovery of activity-dependent synaptic plasticity in the spinal cord and the revelation that it occurs and produces pain hypersensitivity in patients. Nevertheless, discovering the genetic and environmental contributors to and objective biomarkers of central sensitization will be highly beneficial, as will additional treatment options to prevent or reduce this prevalent and promiscuous form of pain plasticity.