Spinal strychnine alters response properties of nociceptive-specific neurons in rat medial thalamus

Streptozotocin-Induced Diabetic Rats Showed a Differential Glycine Receptor Expression in the Spinal Cord: A GlyR Role in Diabetic Neuropathy

In the spinal cord, attenuation of the inhibitory action of glycine is related to an increase in both inflammatory and diabetic neuropathic pain; however, the glycine receptor involvement in diabetic neuropathy has not been reported. We determined the expression of the glycine receptor subunits (α1-α3 and β) in streptozotocin-induced diabetic Long-Evans rats by qPCR and Western blot. The total mRNA and protein expression (whole spinal cord homogenate) of the α1, α3, and β subunits did not change during diabetes; however, the α2 subunit mRNA, but not the protein, was overexpressed 45 days after diabetes induction. By contrast, the synaptic expression of the α1 and α2 subunits decreased in all the studied stages of diabetes, but that of the α3 subunit increased on day 45 after diabetes induction. Intradermal capsaicin produced higher paw-licking behavior in the streptozotocin-induced diabetic rats than in the control animals. In addition, the nocifensive response was higher at 45 days than at 20 days. During diabetes, the expression of the glycine receptor was altered in the spinal cord, which strongly suggests its involvement in diabetic neuropathy.

Nociception in the Glycine Receptor Deficient Mutant Mouse Spastic

Glycine receptors (GlyRs) are the primary mediators of fast inhibitory transmission in the mammalian spinal cord, where they modulate sensory and motor signaling. Mutations in GlyR genes as well as some other genes underlie the hereditary disorder hyperekplexia, characterized by episodic muscle stiffness and exaggerated startle responses. Here, we have investigated pain-related behavior and GlyR expression in the spinal cord of the GlyR deficient mutant mouse spastic (spa). In spastic mice, the GlyR number is reduced due to a β subunit gene (Glrb) mutation resulting in aberrant splicing of GlyRβ transcripts. Via direct physical interaction with the GlyR anchoring protein gephyrin, this subunit is crucially involved in the postsynaptic clustering of heteromeric GlyRs. We show that the mutation differentially affects aspects of the pain-related behavior of homozygous Glrb^spa/Glrb^spa mice. While response latencies to noxious heat were unchanged, chemically induced pain-related behavior revealed a reduction of the licking time and an increase in flinching in spastic homozygotes during both phases of the formalin test. Mechanically induced nocifensive behavior was reduced in spastic mice, although hind paw inflammation (by zymosan) resulted in allodynia comparable to wild-type mice. Immunohistochemical staining of the spinal cord revealed a massive reduction of dotted GlyRα subunit immunoreactivity in both ventral and dorsal horns, suggesting a reduction of clustered receptors at synaptic sites. Transcripts for all GlyRα subunit variants, however, were not reduced throughout the dorsal horn of spastic mice. These findings suggest that the loss of functional GlyRβ subunits and hence synaptically localized GlyRs compromises sensory processing differentially, depending on stimulus modality.

Inhibition of Glycine Re-Uptake: A Potential Approach for Treating Pain by Augmenting Glycine-Mediated Spinal Neurotransmission and Blunting Central Nociceptive Signaling

Among the myriad of cellular and molecular processes identified as contributing to pathological pain, disinhibition of spinal cord nociceptive signaling to higher cortical centers plays a critical role. Importantly, evidence suggests that impaired glycinergic neurotransmission develops in the dorsal horn of the spinal cord in inflammatory and neuropathic pain models and is a key maladaptive mechanism causing mechanical hyperalgesia and allodynia. Thus, it has been hypothesized that pharmacological agents capable of augmenting glycinergic tone within the dorsal horn may be able to blunt or block aberrant nociceptor signaling to the brain and serve as a novel class of analgesics for various pathological pain states. Indeed, drugs that enhance dysfunctional glycinergic transmission, and in particular inhibitors of the glycine transporters (GlyT1 and GlyT2), are generating widespread interest as a potential class of novel analgesics. The GlyTs are Na⁺/Cl⁻-dependent transporters of the solute carrier 6 (SLC6) family and it has been proposed that the inhibition of them presents a possible mechanism by which to increase spinal extracellular glycine concentrations and enhance GlyR-mediated inhibitory neurotransmission in the dorsal horn. Various inhibitors of both GlyT1 and GlyT2 have demonstrated broad analgesic efficacy in several preclinical models of acute and chronic pain, providing promise for the approach to deliver a first-in-class non-opioid analgesic with a mechanism of action differentiated from current standard of care. This review will highlight the therapeutic potential of GlyT inhibitors as a novel class of analgesics, present recent advances reported for the field, and discuss the key challenges associated with the development of a GlyT inhibitor into a safe and effective agent to treat pain.

Modulation of Glycine-Mediated Spinal Neurotransmission for the Treatment of Chronic Pain

Chronic pain constitutes a significant and expanding worldwide health crisis. Currently available analgesics poorly serve individuals suffering from chronic pain, and new therapeutic agents that are more effective, safer, and devoid of abuse liabilities are desperately needed. Among the myriad of cellular and molecular processes contributing to chronic pain, spinal disinhibition of pain signaling to higher cortical centers plays a critical role. Accumulating evidence shows that glycinergic inhibitory neurotransmission in the spinal cord dorsal horn gates nociceptive signaling, is essential in maintaining physiological pain sensitivity, and is diminished in pathological pain states. Thus, it is hypothesized that agents capable of enhancing glycinergic tone within the dorsal horn could obtund nociceptor signaling to the brain and serve as analgesics for persistent pain. This Perspective highlights the potential that pharmacotherapies capable of increasing inhibitory spinal glycinergic neurotransmission hold in providing new and transformative analgesic therapies for the treatment of chronic pain.

Reduction of spinal glycine receptor-mediated miniature inhibitory postsynaptic currents in streptozotocin-induced diabetic neuropathic pain

Diabetic neuropathic pain (DNP) is a common clinical problem, and the mechanisms underlying the onset and progression of this complication are poorly understood. The present study examined the glycine receptors (GlyR) in the control of synaptic input to dorsal horn neurons in diabetes. Male Sprague-Dawley rats with or without streptozotocin (STZ) intraperitoneal injections were used. Tactile sensitivities were assessed by measuring paw withdrawal thresholds to von Frey filaments for four weeks. The extent of GlyR-mediated inhibition controlling primary afferent-evoked excitation in dorsal horn neurons was examined by using the whole cell patch clamp recording technique in isolated adult rat spinal cord slices. The content of the spinal dorsal horn glycine levels was measured by microdialysis. An intrathecal glycine agonist injection was used to test whether mimicking endogenous glycine-receptor-mediated inhibition reduces DNP. We found that persistent hyperglycemia induced by the administration of STZ caused a decrease in the paw withdrawal latency to mechanical stimuli. The miniature inhibitory post-synaptic current (mIPSC) rise, decay kinetics and mean GlyR-mediated mIPSC amplitude were not affected in DNP. The mean frequency of GlyR-mediated mIPSC of lamina I neurons from DNP rats was, however, significantly reduced when compared with neurons from control rats. Principal passive and active membrane properties and the firing patterns of spinal lamina I neurons were not changed in DNP rats. Spinal microdialysis rats had a significantly decreased glycine level following its initial elevation. The intrathecal administration of glycine diminished tactile pain hypersensitivity in DNP rats. In conclusion, these results indicate that long-lasting hyperglycemia induced by STZ injections leads to a reduced glycinergic inhibitory control of spinal lamina I neurons through a presynaptic mechanism.

Analgesic effect of GT-0198, a structurally novel glycine transporter 2 inhibitor, in a mouse model of neuropathic pain

This study was conducted to identify the characteristic pharmacological features of GT-0198 that is phenoxymethylbenzamide derivatives. GT-0198 inhibited the function of glycine transporter 2 (GlyT2) in human GlyT2-expressing HEK293 cells and did not bind various major transporters or receptors of neurotransmitters in a competitive manner. Thus, GT-0198 is considered to be a comparatively selective GlyT2 inhibitor. Intravenous, oral, and intrathecal injections of GT-0198 decreased the pain-related response in a model of neuropathic pain with partial sciatic nerve ligation. This result suggests that GT-0198 has an analgesic effect. The analgesic effect of GT-0198 was abolished by the intrathecal injection of strychnine, a glycine receptor antagonist. Therefore, GT-0198 is considered to exhibit its analgesic effect via the activation of a glycine receptor by glycine following presynaptic GlyT2 inhibition in the spinal cord. In summary, GT-0198 is a structurally novel GlyT2 inhibitor bearing a phenoxymethylbenzamide moiety with in vivo efficacy in behavioral models of neuropathic pain.

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.

Blockade of glycine transporter (GlyT) 2, but not GlyT1, ameliorates dynamic and static mechanical allodynia in mice with herpetic or postherpetic pain

Glycine is an inhibitory neurotransmitter in the spinal dorsal horn and its extracellular concentration is regulated by glial glycine transporter (GlyT) 1 and neuronal GlyT2. This study was conducted to elucidate the effects of intrathecal injections of GlyT1 and GlyT2 inhibitors on two distinct types of mechanical allodynia, dynamic and static allodynia, in mice with herpetic or postherpetic pain. The GlyT2 inhibitor ALX1393, but not the GlyT1 inhibitor sarcosine, suppressed dynamic and static allodynia at the herpetic and postherpetic stages. Intrathecal ALX1393 suppressed dynamic allodynia induced by intrathecal strychnine and N-methyl-D-aspartate (NMDA). Intrathecal sarcosine suppressed dynamic allodynia induced by intrathecal strychnine, but not NMDA. Expression level of GlyT1, but not GlyT2, mRNA in the lumbar dorsal horn was decreased at the herpetic and postherpetic stages. Glycine receptor alpha1-subunit mRNA was decreased in the lumbar dorsal horn at the herpetic, but not postherpetic stage, without alteration in alpha3-subunit mRNA. The results suggest that GlyT2 is a potential target for treatment of dynamic and static allodynia in patients with herpes zoster and postherpetic neuralgia. The lack of efficacy of GlyT1 inhibitor may be explained by activation of NMDA receptors and the down-regulation of GlyT1 in the lumbar dorsal horn.

The antinociceptive effect of intrathecal administration of glycine transporter-2 inhibitor ALX1393 in a rat acute pain model

BACKGROUND

Glycinergic neurons in the spinal dorsal horn have been implicated in the inhibition of spinal pain processing in peripheral inflammation and chronic pain states. Neuronal isoform glycine transporter-2 (GlyT2) reuptakes presynaptically released glycine and regulates the glycinergic neurotransmission. In this study, we examined whether a selective GlyT2 inhibitor, ALX1393, elicits an antinociceptive effect in a rat acute pain model.

METHODS

Male Sprague-Dawley rats were implanted with a catheter intrathecally. The effects of intrathecal administration of ALX1393 (4, 20, or 40 microg) on thermal, mechanical, and chemical nociception were evaluated by tail flick, hot plate, paw pressure, and formalin tests. Furthermore, to explore whether ALX1393 affects motor function, a rotarod test was performed.

RESULTS

ALX1393 exhibited antinociceptive effects on the thermal and mechanical stimulations in a dose-dependent manner. The maximal effect of ALX1393 was observed at 15 min after administration, and a significant effect lasted for about 60 min. These antinociceptive effects were reversed completely by strychnine injected immediately after the administration of ALX1393. In the formalin test, ALX1393 inhibited pain behaviors in a dose-dependent manner, both in the early and late phases, although the influence was greater in the late phase. In contrast to antinociceptive action, ALX1393 did not affect motor function up to 40 microg.

CONCLUSIONS

This study demonstrates the antinociceptive action of ALX1393 on acute pain. These findings suggest that the inhibitory neurotransmitter transporters are promising targets for the treatment of acute pain and that the selective inhibitor of GlyT2 could be a novel therapeutic drug.

Spinal antiallodynia action of glycine transporter inhibitors in neuropathic pain models in mice

Neuropathic pain is refractory against conventional analgesics, and thus novel medicaments are desired for the treatment. Glycinergic neurons are localized in specific brain regions, including the spinal cord, where they play an important role in the regulation of pain signal transduction. Glycine transporter (GlyT)1, present in glial cells, and GlyT2, located in neurons, play roles in modulating glycinergic neurotransmission by clearing synaptically released glycine or supplying glycine to the neurons and thus could modify pain signal transmission in the spinal cord. In this study, we demonstrated that i.v. or intrathecal administration of GlyT1 inhibitors, cis-N-methyl-N-(6-methoxy-1-phenyl-1,2,3,4-tetrahydronaphthalen-2-yl methyl)amino methylcarboxylic acid (ORG25935) or sarcosine, and GlyT2 inhibitors, 4-benzyloxy-3,5-dimethoxy-N-[1-(dimethylaminocyclopently)-methyl]benzamide (ORG25543) and (O-[(2-benzyloxyphenyl-3-fluorophenyl)methyl]-L-serine) (ALX1393), or knockdown of spinal GlyTs by small interfering RNA of GlyTs mRNA produced a profound antiallodynia effect in a partial peripheral nerve ligation model and other neuropathic pain models in mice. The antiallodynia effect is mediated through spinal glycine receptor alpha3. These results established GlyTs as the target molecules for the development of medicaments for neuropathic pain. However, these manipulations to stimulate glycinergic neuronal activity were without effect during the 4 days after nerve injury, whereas manipulations to inhibit glycinergic neuronal activity protected against the development of allodynia in this phase. The results implied that the timing of medication with their inhibitors should be considered, because glycinergic control of pain was reversed in the critical period of 3 to 4 days after surgery. This may also provide important information for understanding the underlying molecular mechanisms of the development of neuropathic pain.

Actions of N-arachidonyl-glycine in a rat inflammatory pain model

Background

While cannabinoid receptor agonists have analgesic activity in inflammatory pain states they produce a range of side effects. Recently, it has been demonstrated that the arachidonic acid-amino acid conjugate, N-arachidonyl-glycine (NA-glycine) is effective in acute pain models.

Results

In the present study we examined the effect of NA-glycine in a rat model of inflammatory pain. Intrathecal administration of NA-glycine (70 – 700 nmol) and the pan-cannabinoid receptor agonist HU-210 (10 nmol) reduced the mechanical allodynia and thermal hyperalgesia induced by intraplantar injection of Freund's complete adjuvant (FCA). The actions of HU-210, but not NA-glycine were reduced by the cannabinoid CB₁ receptor antagonist AM251. The cannabinoid CB₂ receptor antagonist SR144528 also had no effect on the actions of NA-glycine. In contrast, N-arachidonyl-GABA (NA-GABA, 700 nmol) and N-arachidonyl-alanine (NA-alanine, 700 nmol) had no effect on allodynia and hyperalgesia. HU-210, but not NA-glycine produced a reduction in rotarod latency.

Conclusion

These findings suggest that NA-glycine may provide a novel non-cannabinoid receptor mediated approach to alleviate inflammatory pain.

Actions of N-arachidonyl-glycine in a rat neuropathic pain model

While cannabinoid receptor agonists reduce the abnormal pain sensations associated with animal models of neuropathic pain states they also produce CB(1) receptor mediated side effects. Recently, a number of arachidonic acid-amino acid conjugates, including N-arachidonyl-glycine (NAGly), have been identified which are structurally related to the endocannabinoid arachidonyl ethanolamide (anandamide). In the present study we examined the effect of NAGly in a rat model of neuropathic pain. Intrathecal administration of NAGly (700 nmol) and the pan-cannabinoid receptor agonist HU-210 (30 nmol) reduced the mechanical allodynia induced by partial ligation of the sciatic nerve. The NAGly induced anti-allodynia was dose dependent and, unlike HU-210, was unaffected by the cannabinoid CB(1) and CB(2) receptor antagonists, AM251 and SR144528 (30 nmol). The NAGly degradation products, arachidonic acid and glycine (700 nmol), did not reduce allodynia. HU-210, but not NAGly produced a reduction in rotarod latency. These findings suggest that NAGly may provide a novel analgesic approach to alleviate neuropathic pain.

Structural features of phenol derivatives determining potency for activation of chloride currents via alpha(1) homomeric and alpha(1)beta heteromeric glycine receptors

Phenol derivatives constitute a family of neuroactive compounds. The aim of our study was to identify structural features that determine their modulatory effects at glycine receptors. We investigated the effects of four methylated phenol derivatives and two halogenated analogues on chloride inward currents via rat alpha(1) and alpha(1)beta glycine receptors, heterologously expressed in HEK 293. All compounds potentiated the effect of a submaximal glycine concentration in both alpha(1) homomeric and alpha(1)beta glycine receptors. While the degree of maximum potentiation of the glycine 10 microM effect in alpha(1)beta receptors was not different between the compounds, the halogenated compounds achieved half-maximum potentiating effects in the low microM range -- at more than 20-fold lower concentrations compared with their nonhalogenated analogues (P<0.0001). The coactivating effect was over-ridden by inhibitory effects at concentrations >300 microM in the halogenated compounds. Neither the number nor the position of the methyl groups significantly affected the EC(50) for coactivation. Only the bimethylated compounds 2,6 and 3,5 dimethylphenol (at concentrations >1000 microM) directly activated both alpha(1) and alpha(1)beta receptors up to 30% of the maximum response evoked by 1000 microM glycine. These results show that halogenation in the para position is a crucial structural feature for the potency of a phenolic compound to positively modulate glycine receptor function, while direct activation is only seen with high concentrations of compounds that carry at least two methyl groups. The presence of the beta subunit is not required for both effects.

Intracisternal administration of mitogen-activated protein kinase inhibitors reduced IL-1β-induced mirror-image mechanical allodynia in the orofacial area of rats

The present study investigated the role of central mitogen-activated protein kinases (MAPKs) in interleukin-1beta (IL-1beta)-induced mirror-image mechanical allodynia in the orofacial area. Experiments were carried out on Sprague-Dawley rats. Under pentobarbital sodium anesthesia, a polyethylene tube was implanted in the subcutaneous area of one vibrissa pad, which enabled us to inject IL-1beta. For an intracisternal injection, each anesthetized rat was mounted on a stereotaxic frame and a polyethylene tube was implanted. Animals were given a recovery time of at least 72 h from surgery. After a subcutaneous administration of 0.01, 0.1, 1, or 10 pg of IL-1beta, we examined the face withdrawal behavioral responses produced by 10 successive trials of air puffs ipsilateral or contralateral to the IL-1beta injection site. Normal animals did not respond to pressure less than 40 psi. The thresholds of air puffs ipsilateral and contralateral to the IL-1beta injection site were significantly lower in the IL-1beta-treated group, compared with the vehicle-treated group. The decrease in the threshold of air puffs appeared 10 min after an IL-1beta injection and persisted for over 3h. Intracisternal pretreatment with PD98059, a p44/42 MAPK inhibitor, or SB203580, a p38 MAPK inhibitor, significantly reduced the decrease in the threshold of air puffs ipsilateral to the IL-1beta injection site produced by 10 pg of IL-1beta. IL-1beta-induced mirror-image mechanical allodynia was also reduced significantly by intracisternal pretreatment with both PD98059 and SB203580. These results indicate that central MAPK pathways mediate IL-1beta-induced mirror-image mechanical allodynia in the orofacial area.

Role of peripheral group I and II metabotropic glutamate receptors in IL-1beta-induced mechanical allodynia in the orofacial area of conscious rats

The present study investigated the role of peripheral group I and II metabotropic glutamate receptors (mGluRs) in interleukin-1beta (IL-1beta)-induced mechanical allodynia in the orofacial area. Experiments were carried out on Sprague-Dawley rats weighing between 230 and 280 g. After subcutaneous administration of 0.01, 0.1, 1, or 10 pg of IL-1beta, we examined withdrawal behavioral responses produced by 10 successive trials of a ramp of air-puffs pressure applied ipsilaterally or contralaterally to the IL-1beta injection site. The thresholds of air puffs were measured 10, 30, 60, 120, or 180 min after 25 microl of IL-1beta was administered through an implanted tube. Subcutaneous injection of IL-1beta produced bilateral mechanical allodynia. While the IL-1beta-induced mechanical allodynia was blocked by pretreatment with an IL-1 receptor antagonist, the IL-1beta-induced mirror-image mechanical allodynia was not blocked by an IL-1 receptor antagonist injected into the contralateral side. Subcutaneous administration of CPCCOEt or LY367385, an mGluR1 antagonist, or MPEP or SIB1893, an mGluR5 antagonist, 10 min prior to injection of IL-1beta abolished IL-1beta-induced mechanical allodynia. Pretreatment with APDC or DCG4, a group II mGluR agonist, blocked the IL-1beta-induced mechanical allodynia. The anti-allodynic effect induced by APDC was inhibited by pretreatment with LY341495, a group II mGluR antagonist. These results suggest that peripheral group I and II mGluRs participate in IL-1beta-induced mechanical allodynia in the orofacial area. Peripheral group I mGluR antagonists blocked the IL-1beta-induced mechanical allodynia, while peripheral group II mGluR agonists produced anti-allodynic effects on IL-1beta-induced mechanical allodynia in the orofacial area of rats.

Comparison of responses of ventral posterolateral and posterior complex thalamic neurons in naive rats and rats with hindpaw inflammation: mu-opioid receptor mediated inhibitions

The aim of the present study was to compare the effects of morphine on thalamic neuronal responses in naive rats and rats with carrageenan-induced hindpaw inflammation. Multiple single unit ventral posterolateral (VPL) and posterior complex (Po) activity was recorded and mechanically- (7 g, 14 g, 21 g, 60 g and 80 g) evoked responses of VPL and Po neurones were measured in naive rats and rats with carrageenan (100 microl, 2%)-induced hindpaw inflammation. Effects of systemic (0.5 mg kg(-1)) and intra-thalamic (66 microM, 250 nL) morphine on neuronal responses were determined. Mechanically-evoked (60 g) nociceptive responses of VPL neurones were significantly larger in inflamed rats (29 +/- 4 spikes s(-1)) compared to naive rats (19 +/- 2 spikes s(-1), P < 0.05). Systemic morphine inhibited 7 g-evoked responses of VPL neurones in inflamed (24 +/- 8% control, P < 0.01), but not in naive rats (123 +/- 3% control). Frank noxious-evoked responses of VPL neurones in inflamed rats were less sensitive to the effects of systemic and intra-thalamic morphine, compared to naive rats (P < 0.05 for both). These data provide evidence for altered evoked responses of neurones at the level of VPL, but not at Po, during hindpaw inflammation and suggest that thalamic sites of action contribute to the effects of systemic morphine.

Role of cation-chloride-cotransporters (CCC) in pain and hyperalgesia

The importance of the GABAergic system in spinal nociceptive processing has long been appreciated but we have only recently begun to understand how this system is modulated by the regulation of anion gradients. In neuronal tissues, cation-chloride cotransporters regulate Cl- homeostasis and the activity and/or expression of these transporters has important implications for the direction and magnitude of anion flow through GABA-A channels. Here we review recent evidence that two cation-chloride cotransporters, NKCC1 and KCC2 are involved in pain and enhanced nociception. On the one hand, NKCC1 activity is upregulated in primary afferents following an inflammatory insult and this produces excessive GABAergic depolarization in primary afferents leading to cross excitation between low and high threshold afferents. On the other hand, KCC2 expression is reduced in dorsal horn neurons following peripheral nerve injury resulting in a loss of GABA-/glycinergic inhibitory tone and, in some cases, inverting its action into net excitation. Pharmacological targeting of these cation chloride cotransporters to restore normal GABA-/glycinergic transmission in the spinal cord represents an entirely novel approach to the development of analgesics.

Laminar distribution of GABAA- and glycine-receptor mediated tonic inhibition in the dorsal horn of the rat lumbar spinal cord: effects of picrotoxin and strychnine on expression of Fos-like immunoreactivity

Inhibitory mechanisms are essential in suppressing the development of allodynia and hyperalgesia in the normal animal and there is evidence that loss of inhibition can lead to the development of neuropathic pain. We used Fos expression to map the distribution of tonically inhibited cells in the healthy rat lumbar spinal cord. In a control group, Fos-like immunoreactive (Fos-LI) cells were rare, averaging 7.5+/-2.2 cells (mean+/-SEM; N=13 sections) per 20 microm thick section of dorsal horn. This rose to 103+/-11 (mean+/-SEM; N=20) in picrotoxin-treated rats and to 88+/-11 (mean+/-SEM; N=18) in strychnine-treated rats. These changes were significant (ANOVA; P<0.001). There were marked regional variations in the distribution of Fos-LI cells between picrotoxin- and strychnine-treated animals. Picrotoxin induced a significant increase in the number of Fos-LI cells throughout the dorsal horn (lamina I-VI) while strychnine significantly elevated Fos-like immunoreactivity only in deep laminae (III-VI). For both picrotoxin and strychnine, the increase in Fos-like immunoreactivity peaked in lamina V (at 3579+/-319 and 3649+/-375% of control, respectively; mean+/-SEM) but for picrotoxin an additional peak was observed in the outer part of lamina II (1959+/-196%). Intrathecal administration of both GABAA and glycine receptor antagonists has been shown elsewhere to induce tactile allodynia. The present data suggest that this allodynia could arise due to blockade of tonic GABAA and glycine-receptor mediated inhibition in the deep dorsal horn. GABAA antagonists also induce hypersensitivity to noxious inputs. The blockade of tonic inhibition in the superficial dorsal horn shown here may underlie this hyperalgesia.

Peripheral glutamate receptors participate in interleukin-1β-induced mechanical allodynia in the orofacial area of rats

The present study was performed to examine peripheral cytokine-induced mechanical allodynia in the orofacial area and to investigate whether peripheral excitatory amino acids participate in the cytokine-induced mechanical allodynia. Experiments were carried out on male Sprague-Dawley rats. After interleukin-1beta (IL-1beta) was applied subcutaneously to the orofacial area, we examined withdrawal responses produced by air puffs applied to the IL-1beta injection site. The threshold of air puffs that produced withdrawal behavioral responses decreased significantly in a dose-dependent manner after injection of IL-1beta. Pretreatment with an IL-1 receptor antagonist abolished the decrease in the threshold of air puffs. Pretreatment with dl-2-amino-5-phosphonvaleric acid, an N-methyl-d-aspartic acid (NMDA) receptor antagonist, did not affect IL-1beta-induced mechanical allodynia. However, pretreatment with 6,7-dinitroquinoxaline-2,3-dione, a non-NMDA receptor antagonist, abolished the decrease in the threshold of air puffs. These results suggest that peripheral cytokine can produce mechanical allodynia in the orofacial area and that excitatory amino acids can modulate IL-1beta-induced mechanical allodynia via non-NMDA receptors.

Functional imaging of the human trigeminal system: Opportunities for new insights into pain processing in health and disease

Peripheral inflammation or nerve damage result in changes in nervous system function, and may be a source of chronic pain. A number of animal studies have indicated that central neural plasticity, including sensitization of neurons within the spinal cord and brain, is part of the response to nervous system insult, and can result in the appearance of altered sensation, including pain. It cannot be assumed, however, that data obtained from animal models unambiguously reflects CNS changes that occur in humans. Currently, the only noninvasive approach to determining objective changes in neural processing and responsiveness within the CNS in humans is the use of functional imaging techniques. It is now possible to use functional magnetic resonance imaging (fMRI) to measure CNS activation in the trigeminal ganglion, spinal trigeminal nucleus, the thalamus, and the somatosensory cortex in healthy volunteers, in a surrogate model of hyperalgesia, and in patients with trigeminal pain. By offering a window into the temporal and functional changes that occur in the damaged nervous system in humans, fMRI can provide both insight into the mechanisms of normal and pathological pain and, potentially, an objective method for measuring altered sensation. These advances are likely to contribute greatly to the diagnosis and treatment of clinical pain conditions affecting the trigeminal system (e.g., neuropathic pain, migraine).

Convergence of cervical and trigeminal sensory afferents

Cranial nociceptive perception shows a distinct topographic distribution, with the trigeminal nerve receiving sensory information from the anterior portions of the head, the greater occipital nerve, and branches of the upper cervical roots in the posterior regions. However, this distribution is not respected during headache attacks, even if the etiology of the headache is specific for only one nerve. Nociceptive information from the trigeminal and cervical territories activates the neurons in the trigeminal nucleus caudalis that extend to the C2 spinal segment and lateral cervical nucleus in the dorsolateral cervical area. These neurons are classified as multimodal because they receive sensory information from more than one afferent type. Clinically, trigeminal activation produces symptoms in the trigeminal and cervical territory and cervical activation produces symptoms in the cervical and trigeminal territory. The overlap between the trigeminal nerve and cervical is known as a convergence mechanism. For some time, convergence mechanisms were thought to be secondary to clinical observations. However, animal studies and clinical evidence have expanded our knowledge of convergence mechanisms. In this paper, the role of convergence mechanisms in nociceptive physiology, physiopathology of the headaches, clinical diagnosis, and therapeutic conduct are reviewed.

Reduction of glycine receptor-mediated miniature inhibitory postsynaptic currents in rat spinal lamina I neurons after peripheral inflammation

Peripheral inflammation may induce long-lasting sensitization in the central nociceptive system. Neurons in lamina I of the spinal dorsal horn play a pivotal role in the integration and relay of pain-related information. In rats we studied whether changes in passive and active membrane properties and/or alteration of glycine receptor-mediated inhibitory control of spinal lamina I neurons may contribute to central sensitization in a model of peripheral long-lasting inflammation (complete Freund's adjuvant, hindpaw). Spontaneously occurring glycine receptor-mediated miniature inhibitory postsynaptic currents (GlyR-mediated mIPSCs) were recorded in lumbar spinal lamina I neurons. Miniature IPSC rise, decay kinetics and mean GlyR-mediated mIPSC amplitude were not affected by peripheral inflammation. The mean frequency of GlyR-mediated mIPSCs of lamina I neurons ipsilateral to the inflamed hindpaw was, however, significantly reduced by peripheral inflammation when compared with neurons from noninflamed animals. Principal passive and active membrane properties and firing patterns of spinal lamina I neurons were not changed by inflammation. These results indicate that long-lasting peripheral inflammation leads to a reduced glycinergic inhibitory control of spinal lamina I neurons by a presynaptic mechanism.

Type I and II metabotropic glutamate receptors modulate periaqueductal grey glycine release: interaction between mGlu2/3 and A1 adenosine receptors

In this study we investigated the effects of type I and II mGlu receptors ligands in glycine extracellular concentrations at the periaqueductal gray (PAG) level by using in vivo microdialysis, in conscious rats. An agonist of type I mGlu receptors, (S)-3,5-DHPG (1 and 5 mM), but not a selective agonist for mGlu5 receptors, CHPG (3 and 5 mM), was noticed to increase the dialysate glycine levels in a concentration-dependent manner (60+/-15% and 136+/-13%, respectively). CPCCOEt (1mM), a selective mGlu1 receptor antagonist, perfused in combination with (S)-3,5-DHPG, counteracted the effect induced by (S)-3,5-DHPG, but did not change per se the extracellular PAG glycine values, even at the highest dosage used (2 mM). MPEP (1 and 2 mM), a selective antagonist of mGlu5 receptor, did not modify extracellular glycine level. An agonist of type II mGlu receptors, 2R,4R-APDC (25 and 50 microM), decreased the dialysate glycine in a concentration-dependent manner (-26+/-4% and -54+/-6%, respectively). The 2R,4R-APDC-induced decrease in extracellular glycine was prevented by EGlu (0.5 mM), a selective type II mGlu receptors antagonist. EGlu (0.5 and 1 mM), per se, led to a significant decrease (-56+/-7% and -57+/-2%, respectively) in extracellular PAG glycine too. This effect was prevented by DPCPX (100 microM), a selective antagonist for A1 adenosine receptors, but was not affected by CPA (1 mM), a selective A1 adenosine receptors agonist. Intra-PAG perfusion of CPA (0.1-1 mM) decreased the extracellular PAG glycine values (-47+/-13%) with 1 mM concentration. The CPA-induced effect was prevented by DPCPX (100 microM), and resulted to be additive with the 2R,4R-APDC-induced decrease in glycine values. DPCPX (1 mM) increased per se extracellular glycine (48+/-7%) at the highest dose used. Dipyridamole (100 microM), an inhibitor of both adenosine reuptake and phosphodiesterases, decreased extracellular glycine (-28+/-7%). Extracellular concentrations of glutamine never changed throughout this study. These data show opposing effects of type I and II mGlu receptors in the regulation of PAG glycine values. Moreover, functional interaction between type II mGlu and adenosine A1 receptors, which possibly operate through a common transductional pathway, may be relevant in the physiological control of glycine release in awake, freely moving rats at the periaqueductal gray matter.

Somatotopic activation in the human trigeminal pain pathway

Functional magnetic resonance imaging was used to image pain-associated activity in three levels of the neuraxis: the medullary dorsal horn, thalamus, and primary somatosensory cortex. In nine subjects, noxious thermal stimuli (46 degrees C) were applied to the facial skin at sites within the three divisions of the trigeminal nerve (V1, V2, and V3) and also to the ipsilateral thumb. Anatomical and functional data were acquired to capture activation across the spinothalamocortical pathway in each individual. Significant activation was observed in the ipsilateral spinal trigeminal nucleus within the medulla and lower pons in response to at least one of the three facial stimuli in all applicable data sets. Activation from the three facial stimulation sites exhibited a somatotopic organization along the longitudinal (rostrocaudal) axis of the brain stem that was consistent with the classically described "onion skin" pattern of sensory deficits observed in patients after trigeminal tractotomy. In the thalamus, activation was observed in the contralateral side involving the ventroposteromedial and dorsomedial nuclei after stimulation of the face and in the ventroposterolateral and dorsomedial nuclei after stimulation of the thumb. Activation in the primary somatosensory cortex displayed a laminar sequence that resembled the trigeminal nucleus, with V2 more rostral, V1 caudal, and V3 medial, abutting the region of cortical activation observed for the thumb. These results represent the first simultaneous imaging of pain-associated activation at three levels of the neuraxis in individual subjects. This approach will be useful for exploring central correlates of plasticity in models of experimental and clinical pain.

Peripheral inflammation increases the functional coherency of spinal responses to tactile but not nociceptive stimulation

Reorganization of central networks and plasticity of neuronal representations have been implicated in recent years in the dynamic expression of somatosensory responses. The functional properties of spinal cells were shown to change in the scale of minutes after peripheral high-intensity stimulations and to undergo profound alterations in their responses in experimental models of chronic pain. These observations, however, are restricted to recordings from individual cells, and no information exists on how these changes may be reflected on the activity of somatosensory neuronal networks involved in pain processing. To understand how spinal cord networks may be altered after the onset of hyperalgesia, we extracellularly recorded from groups of five to nine neighboring neurons in the hindlimb representation area of the dorsal horn. The multineuronal activity evoked by cutaneous innocuous and noxious stimulation was compared before and for 3 h after the subcutaneous injection of diluted formalin. Formalin caused immediate changes in response properties and mechanical threshold of activation for the majority of the neurons and induced the incorporation of previously unresponsive neighboring neurons to the functional network. Analysis of the temporal correlation within the neuronal population revealed that formalin-induced inflammation increased the functional coherence of the network to the nonnociceptive stimulation but not to the painful stimuli. This increase in the tactile acuity of populations of nociceptive neurons may be a basis for the emergence of touch-evoked pain.

Muscarinic mediation of the urethro genital reflex in spinal cord-transected rats

The effects of muscarinic receptor stimulation were tested on the urethro genital reflex (UGR) in anesthetized and spinal cord-transected rats. Drugs were applied directly to the spinal cord. The electromyographic activity (EMG) of the bulbospongiosus (BS) muscle was used for recording UGR. In six animals BS as well as soleus, posterior biceps or peroneus tertius muscle EMG was recorded simultaneously. Muscarine (5, 10, 20, 50 and 100 microg) was applied in 22 animals after cutting L6-S1 dorsal roots. Some observations were made on another six animals, to which an extensive bilateral dorsal rhizotomy (L3-S2) was performed. Rhythmic bursts of similar frequency and size to those seen during UGR were found in BS muscle a few minutes after muscarine application. No rhythmic bursting was found on the hindlimb muscles, but exclusively on BS muscles. The effects of homatropine (25, 50, 100 and 200 microg), an acetylcholine muscarinic receptor antagonist, were tested in 21 rats after UGR was elicited three times at low stimulation intensity (7 mm Hg). Homatropine produced two effects: (i) A significant increase in the latency of UGR. (ii) A facilitation of UGR inhibition. In view of these results it can be speculated that muscarinic receptor stimulation is involved in the elicitation of UGR.

Effects of persistent nociception on periaqueductal gray glycine release

Glycine is a candidate nociception inhibitory transmitter in specific brain regions, like for example the spinal cord, the thalamic nuclei and the periaqueductal gray matter. However, quantitative changes in glycine released in these brain regions during peripheral inflammation episodes have not been characterized in awake animals. To address this issue, an in vivo microdialysis study was carried out in freely moving rats in order to analyse periaqueductal gray matter extracellular glycine concentration following unilateral formalin injection into the dorsal skin of the right hind-paw. The extracellular concentration of glutamine was also evaluated in order to analyse whether or not a non-neurotransmitter amino acid was equally modified. Intra-periaqueductal gray matter tetrodotoxin perfusion reduced extracellular glycine concentration (-44+/-5%), but did not change the glutamine dialysate values. Peripheral injection of formalin reduced the glycine release during the early phase (-62+/-8%) and the late phase (-36+/-6%) of hyperalgesia, although not during the analgesic period. Perfusion with naloxone (300microM) neither prevented the formalin-induced decreases in extacellular glycine concentration, nor modified the perfusate basal values of glycine and glutamine. These results show that, contrary to what has been recognized on the interactive role of opioids and GABA into the periaqueductal gray matter (i.e. opioid disinhibition), endogenous opioids seem not to modulate the activity of glycinergic neurons in the same midbrain area. In the light of these preliminary data, it is reasonable to suppose that GABA and glycine are probably not co-released at the level of periaqueductal gray matter of the rat.

Junctional versus extrajunctional glycine and GABA(A) receptor-mediated IPSCs in identified lamina I neurons of the adult rat spinal cord

Colocalization of GABA and glycine in synaptic terminals of the superficial dorsal horn raises the question of their relative contribution to inhibition of different classes of neurons in this area. To address this issue, miniature IPSCs (mIPSCs) mediated via GABA(A) receptors (GABA(A)Rs) and glycine receptors (GlyRs) were recorded from identified laminae I-II neurons in adult rat spinal cord slices. GABA(A)R-mediated mIPSCs had similar amplitude and rise times, but significantly slower decay kinetics than GlyR-mediated mIPSCs. Lamina I neurons appeared to receive almost exclusively GlyR-mediated mIPSCs, even after application of hypertonic solutions. Yet, all neurons responded to exogenous applications of both GABA and glycine, indicating that they expressed both GABA(A)Rs and GlyRs. Given that virtually all glycinergic interneurons also contain GABA, the possibility was examined that GABA(A)Rs may be located extrasynaptically in lamina I neurons. A slow GABA(A)R-mediated component was revealed in large, but not minimally evoked monosynaptic IPSCs. Administration of the benzodiazepine flunitrazepam unmasked a GABA(A)R component to most mIPSCs, suggesting that both transmitters were released from the same vesicle. The isolated GABA(A)R component of these mIPSCs had rising kinetics 10 times slower than that of the GlyR component (or of GABA(A)R mIPSCs in lamina II). The slow GABA(A)R components were prolonged by GABA uptake blockers. It is concluded that, whereas GABA and glycine are likely released from the same vesicle of transmitter in lamina I, GABA(A)Rs appear to be located extrasynaptically. Thus, glycine mediates most of the tonic inhibition at these synapses. This differential distribution of GABA(A)Rs and GlyRs confers distinct functional properties to inhibition mediated by these two transmitters in lamina I.

Tiagabine antinociception in rodents depends on GABA(B) receptor activation: parallel antinociception testing and medial thalamus GABA microdialysis

The effects of a new antiepileptic drug, tiagabine, (R)-N-[4,4-di-(3-methylthien-2-yl)but-3-enyl] nipecotic acid hydrochloride, were studied in mice and rats in antinociceptive tests, using three kinds of noxious stimuli: mechanical (paw pressure), chemical (abdominal constriction) and thermal (hot plate). In vivo microdialysis was performed in parallel in awake, freely moving rats in order to evaluate possible alterations in extracellular gamma-aminobutyric acid (GABA) levels in a pain-modulating region, the medial thalamus. Systemic administration of tiagabine, 30 mg kg(-1) i.p., increased nearly twofold the extracellular GABA levels in rats and increased significantly the rat paw pressure nociceptive threshold in a time-correlated manner. Dose-related significant tiagabine-induced antinociception was also observed at the doses of 1 and 3 mg kg(-1) i.p. in the mouse hot plate and abdominal constriction tests. The tiagabine antinociception was completely antagonised by pretreatment with the selective GABA(B) receptor antagonist, CGP 35348, (3-aminopropyl-diethoxy-methyl-phosphinic acid) (2.5 microg/mouse or 25 microg/rat i.c.v.), but not by naloxone (1 mg kg(-1) s.c.), both administered 15 min before tiagabine. Thus, it is suggested that tiagabine causes antinociception due to raised endogenous GABA levels which in turn activate GABA(B) receptors.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Milacemide, a glycine pro-drug, inhibits strychnine-allodynia without affecting normal nociception in the rat

The blockade of spinal glycine receptors with intrathecal (i.t.) strychnine (STR) produces reversible, segmentally localized allodynia in the rat. The purpose of this study was: (1) to investigate the effect of the anticonvulsant agent, milacemide, a glycine pro-drug on STR-allodynia; (2) to compare this effect with that of milacemide on normal nociception (without STR); and (3) to determine the sensitivity of the anti-allodynic effect of milacemide to pretreatment with selective monoamine oxidase (MAO)-A (clorgyline) and MAO-B (L-deprenyl) inhibitors. Male Sprague-Dawley rats, fitted with chronic i.t. catheters, were lightly anesthetized with urethane. Hair deflection (HD) evoked maximum changes in blood pressure and heart rate were recorded from left carotid artery, and cortical electroencephalographic (EEG) activity was continuously monitored using subdermal needle electrodes before and after i.t. STR (40 microg). Rats were pretreated with a single intravenous (i.v.) injection of milacemide (100-600 mg/kg), 1 h before i.t. STR. To sustain the allodynic state, STR was injected every hour for up to 4 h. HD was applied to the affected dermatomes (2 min duration) using a cotton-tipped applicator at 5-min intervals for the duration of the STR effect. Normally innocuous HD elicited a marked increase in mean arterial blood pressure and heart rate, an immediate motor responses, and desynchronisation of EEG when applied to the cutaneous dermatomes affected by i.t. STR. Milacemide (100-600 mg/kg, i.v.) dose-dependently inhibited the heart rate and pressor responses (ED50 = 398 mg/kg; 95%CI = 196-873) and the motor responses (ED50 = 404 mg/kg; 95%CI = 275-727). Maximum inhibition was observed approximately 2 h after i.v. injection. The duration of action ranged from 3 h (400 mg/kg) to 4 h (600 mg/kg). Milacemide had no effect on the percent synchrony in the EEG. At the time of maximum inhibition of STR-allodynia (2 h post-infusion), responses evoked by noxious pinch were unaffected by milacemide. Pretreatment with L-deprenyl (3 mg/kg, i.p.), but not clorgyline (10 mg/kg, i.p.) significantly blocked the anti-allodynic effect of milacemide (600 mg/kg i.v). These data indicate that i.v. milacemide significantly attenuates the allodynia arising from spinal glycine receptor blockade, and are consistent with: (1) the selective modulation of low threshold afferent input by STR-sensitive, glycine interneurons in the rat spinal cord; and (2) the pharmacological actions of milacemide as a glycine pro-drug.

Altered receptive fields and sensory modalities of rat VPL thalamic neurons during spinal strychnine-induced allodynia

Altered receptive fields and sensory modalities of rat VPL thalamic neurons during spinal strychnine-induced allodynia. J. Neurophysiol. 78: 2296-2308, 1997. Allodynia is an unpleasant sequela of neural injury or neuropathy that is characterized by the inappropriate perception of light tactile stimuli as pain. This condition may be modeled experimentally in animals by the intrathecal (i.t.) administration of strychnine, a glycine receptor antagonist. Thus after i.t. strychnine, otherwise innocuous tactile stimuli evoke behavioral and autonomic responses that normally are elicited only by noxious stimuli. The current study was undertaken to determine how i.t. strychnine alters the spinal processing of somatosensory input by examining the responses of neurons in the ventroposterolateral thalamic nucleus. Extracellular, single-unit recordings were conducted in the lateral thalamus of 19 urethan-anaesthetized, male, Wistar rats (342 +/- 44 g; mean +/- SD). Receptive fields and responses to noxious and innocuous cutaneous stimuli were determined for 19 units (1 per animal) before and immediately after i.t. strychnine (40 microgram). Eighteen of the animals developed allodynia as evidenced by the ability of otherwise innocuous brush or air jet stimuli to evoke cardiovascular and/or motor reflexes. All (3) of the nociceptive-specific units became responsive to brush stimulation after i.t. strychnine, and one became sensitive to brushing over an expanded receptive field. Expansion of the receptive field, as determined by brush stimulation, also was exhibited by all of the low-threshold mechanoreceptive units (14) and wide dynamic range units (2) after i.t. strychnine. The use of air jet stimuli at fixed cutaneous sites also provided evidence of receptive field expansion, because significant unit responses to air jet developed at 13 cutaneous sites (on 7 animals) where an identical stimulus was ineffective in evoking a unit response before i.t. strychnine. However, the magnitude of the unit response to cutaneous air jet stimulation was not changed at sites that already had been sensitive to this stimulus before i.t. strychnine. The onset of allodynia corresponded with the onset of the altered unit responses (i.e., lowered threshold/receptive field expansion) for the majority of animals (9), but the altered unit response either terminated concurrently with symptoms of allodynia (6) or, more frequently, outlasted the symptoms of allodynia (10) as the effects of strychnine declined. The present results demonstrate that the direct, receptor-mediated actions of strychnine on the spinal processing of sensory information are reflected by changes in the receptive fields and response properties of nociceptive and nonnociceptive thalamic neurons. These changes are consistent with the involvement of thalamocortical mechanisms in the expression of strychnine-induced allodynia and, moreover, suggest that i.t. strychnine also produces changes in innocuous tactile sensation.