Thalamic NMDA receptors modulate inflammation-produced hyperalgesia in the rat

Influence of inflammatory processes on thalamocortical activity

It is known that the thalamus plays an important role in pathological brain conditions involved in demyelinating, inflammatory and neurodegenerative diseases such as Multiple Sclerosis (MS). Beside immune cells and cytokines, ion channels were found to be key players in neuroinflammation. MS is a prototypical example of an autoimmune disease of the central nervous system that is classified as a channelopathy where abnormal ion channel function leads to symptoms and clinical signs. Here we review the influence of the cytokine-ion channel interaction in the thalamocortical system in demyelination and inflammation.

Clinical relevance of depressed kynurenine pathway in episodic migraine patients: potential prognostic markers in the peripheral plasma during the interictal period

Background

Altered glutamatergic neurotransmission and neuropeptide levels play a central role in migraine pathomechanism. Previously, we confirmed that kynurenic acid, an endogenous glutamatergic antagonist, was able to decrease the expression of pituitary adenylate cyclase-activating polypeptide 1–38, a neuropeptide with known migraine-inducing properties. Hence, our aim was to reveal the role of the peripheral kynurenine pathway (KP) in episodic migraineurs. We focused on the complete tryptophan (Trp) catabolism, which comprises the serotonin and melatonin routes in addition to kynurenine metabolites. We investigated the relationship between metabolic alterations and clinical characteristics of migraine patients.

Methods

Female migraine patients aged between 25 and 50 years (n = 50) and healthy control subjects (n = 34) participated in this study. Blood samples were collected from the cubital veins of subjects (during both the interictal/ictal periods in migraineurs, n = 47/12, respectively). 12 metabolites of Trp pathway were determined by neurochemical measurements (UHPLC-MS/MS).

Results

Plasma concentrations of the most Trp metabolites were remarkably decreased in the interictal period of migraineurs compared to healthy control subjects, especially in the migraine without aura (MWoA) subgroup: Trp (p < 0.025), L-kynurenine (p < 0.001), kynurenic acid (p < 0.016), anthranilic acid (p < 0.007), picolinic acid (p < 0.03), 5-hydroxy-indoleaceticacid (p < 0.025) and melatonin (p < 0.023). Several metabolites showed a tendency to elevate during the ictal phase, but this was significant only in the cases of anthranilic acid, 5-hydroxy-indoleaceticacid and melatonin in MWoA patients. In the same subgroup, higher interictal kynurenic acid levels were identified in patients whose headache was severe and not related to their menstruation cycle. Negative linear correlation was detected between the interictal levels of xanthurenic acid/melatonin and attack frequency. Positive associations were found between the ictal 3-hydroxykynurenine levels and the beginning of attacks, just as between ictal picolinic acid levels and last attack before ictal sampling.

Conclusions

Our results suggest that there is a widespread metabolic imbalance in migraineurs, which manifests in a completely depressed peripheral Trp catabolism during the interictal period. It might act as trigger for the migraine attack, contributing to glutamate excess induced neurotoxicity and generalised hyperexcitability. This data can draw attention to the clinical relevance of KP in migraine.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10194-021-01239-1.

The effect of orofacial complete Freund's adjuvant treatment on the expression of migraine-related molecules

Background

Migraine is a neurovascular primary headache disorder, which causes significant socioeconomic problems worldwide. The pathomechanism of disease is enigmatic, but activation of the trigeminovascular system (TS) appears to be essential during the attack. Migraine research of recent years has focused on neuropeptides, such as calcitonin gene-related peptide (CGRP) and pituitary adenylate cyclase-activating polypeptide 1–38 (PACAP1–38) as potential pathogenic factors and possible therapeutic offensives. The goal of present study was to investigate the simultaneous expression of CGRP and precursor of PACAP1–38 (preproPACAP) in the central region of the TS in a time-dependent manner following TS activation in rats.

Methods

The right whisker pad of rats was injected with 50 μl Complete Freund’s Adjuvant (CFA) or saline. A mechanical allodynia test was performed with von Frey filaments before and after treatment. Transcardial perfusion of the animals was initiated 24, 48, 72 and 120 h after injection, followed by the dissection of the nucleus trigeminus caudalis (TNC). After preparation, the samples were stored at − 80 °C until further use. The relative optical density of CGRP and preproPACAP was analyzed by Western blot. One-way ANOVA and Kruskal-Wallis followed by Tukey post hoc test were used to evaluate the data. Regression analysis was applied to explore the correlation between neuropeptides expression and hyperalgesia.

Results

Orofacial CFA injection resulted in significant CGRP and preproPACAP release in the TNC 24, 48, 72 and 120 h after the treatment. The level of neuropeptides reached its maximum at 72 h after CFA injection, corresponding to the peak of facial allodynia. Negative, linear correlation was detected between the expression level of neuropeptides and value of mechanonociceptive threshold.

Conclusion

This is the first study which suggests that the expression of CGRP and preproPACAP simultaneously increases in the central region of activated TS and it influences the formation of mechanical hyperalgesia. Our results contribute to a better understanding of migraine pathogenesis and thereby to the development of more effective therapeutic approaches.

Possible Molecular Mediators Involved and Mechanistic Insight into Fibromyalgia and Associated Co-morbidities

Fibromyalgia is a chronic complex syndrome of non-articulate origin characterized by musculoskeletal pain, painful tender points, sleep problems and co-morbidities including depression, migraine. The etiopathogenesis of fibromyalgia is complex, variable and remains inconclusive. The etiological factors that have been defined include stress, genetic predisposition and environmental components. As per the reports of the American College of Rheumatology (ACR) the prevalence of fibromyalgia varies from 2 to 22% among the general population with poor diagnostic features primarily pain. Fibromyalgia encompasses a spectrum of co-morbid conditions with multifarious pathogenesis. The highly prevalent manifestations of fibromyalgia include heterogeneous pain and aches. Biochemical and neurobiological elements of fibromyalgia include neurotransmitters, hypothalamic pituitary adrenal axis (HPA axis), inflammatory cytokines, monoaminergic pathway, opioid peptides, sex hormones, nerve growth factor (NGF) and local free radical insult. An imbalance in the serotonergic system is the major underlying etiological factor that has been explored most widely. Owing to complex interplay of diverse pathophysiological pathways, overlapping co-morbidities such as depression have been clinically observed. Therapeutic management of fibromyalgia involves both non pharmacological and pharmacological measures. The current review presents various dysregulations and their association with symptoms of fibromyalgia along with their underlying neurobiological aspects.

[Physiology of pain]

Pain research is based broadly on physiological disciplines and its development follows the methodological progress of the era, from classical psychophysiology to electrophysiological investigations at peripheral and central nociceptive systems, single cells and ion channels to modern imaging of nociceptive processing. Physiological pain research in Germany has long been part of an interdisciplinary research network extending beyond all political boundaries, and this situation has continued since molecular techniques started to dominate all biomedical research. Current scientific questions, such as intracellular nociceptive signal mechanisms, interactions with other physiological systems including the immune system, or the genetic basis of epidemic and chronic pain diseases can only be solved interdisciplinary and with international collaboration.

Two classes of excitatory synaptic responses in rat thalamic reticular neurons

The thalamic reticular nucleus (nRt), composed of GABAergic cells providing inhibition of relay neurons in the dorsal thalamus, receives excitation from the neocortex and thalamus. The two excitatory pathways promoting feedback or feedforward inhibition of thalamocortical neurons contribute to sensory processing and rhythm generation. While synaptic inhibition within the nRt has been carefully characterized, little is known regarding the biophysics of synaptic excitation. To characterize the functional properties of thalamocortical and corticothalamic connections to the nRt, we recorded minimal electrically evoked excitatory postsynaptic currents from nRt cells in vitro. A hierarchical clustering algorithm distinguished two types of events. Type 1 events had larger amplitudes and faster kinetics, largely mediated by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, whereas type 2 responses had more prominent N-methyl-d-aspartate (NMDA) receptor contribution. Type 1 responses showed subnormal axonal propagation and paired pulse depression, consistent with thalamocortical inputs. Furthermore, responses kinetically similar to type 1 events were evoked by glutamate-mediated activation of thalamic neurons. Type 2 responses, in contrast, likely arise from corticothalamic inputs, with larger NMDA conductance and weak Mg(2+)-dependent block, suggesting that NMDA receptors are critical for the cortical excitation of reticular neurons. The long-lasting action of NMDA receptors would promote reticular cell burst firing and produce powerful inhibitory output to relay neurons proposed to be important in triggering epilepsy. This work provides the first complete voltage-clamp analysis of the kinetics and voltage dependence of AMPA and NMDA responses of thalamocortical and corticothalamic synapses in the nRt and will be critical in optimizing biologically realistic neural network models of thalamocortical circuits relevant to sensory processing and thalamocortical oscillations.

Changes in Activity of the Same Thalamic Neurons to Repeated Nociception in Behaving Mice

The sensory thalamus has been reported to play a key role in central pain sensory modulation and processing, but its response to repeated nociception at thalamic level is not well known. Current study investigated thalamic response to repeated nociception by recording and comparing the activity of the same thalamic neuron during the 1^st and 2^nd formalin injection induced nociception, with a week interval between injections, in awake and behaving mice. Behaviorally, the 2^nd injection induced greater nociceptive responses than the 1^st. Thalamic activity mirrored these behavioral changes with greater firing rate during the 2^nd injection. Analysis of tonic and burst firing, characteristic firing pattern of thalamic neurons, revealed that tonic firing activity was potentiated while burst firing activity was not significantly changed by the 2^nd injection relative to the 1^st. Likewise, burst firing property changes, which has been consistently associated with different phases of nociception, were not induced by the 2^nd injection. Overall, data suggest that repeated nociception potentiated responsiveness of thalamic neurons and confirmed that tonic firing transmits nociceptive signals.

Function of mGlu1 receptors in the modulation of nociceptive processing in the thalamus

As postsynaptic metabotropic subtype 1 (mGlu1) receptors are present in the thalamus, we have investigated the effect of potentiating and antagonising mGlu1 receptors on responses of thalamic neurones to noxious sensory stimulation. Extracellular recordings were made in vivo with multi-barrel iontophoretic electrodes from single neurones in the thalamus of urethane-anaesthetised rats. Responses to iontophoretic applications of the Group I mGlu agonist 3,5-dihydroxy-phenylglycine (DHPG) were selectively potentiated by co-application of the mGlu1 positive allosteric modulator Ro67-4853, whereas they were selectively reduced upon co-application of the mGlu1 receptor orthosteric antagonist LY367385. This indicates that thalamic DHPG responses are mediated primarily via mGlu1 receptors, consistent with the high postsynaptic levels of this receptor in the thalamus. Furthermore, potentiation of DHPG responses by Ro67-4853 were greater when the initial DHPG response was of a low magnitude. Ro67-4853 also potentiated responses of thalamic neurones to noxious thermal stimulation, whilst having little effect on the baseline activity of nociceptive neurones. By contrast, nociceptive responses were reduced by LY367385. In a further series of experiments we found that inactivation of somatosensory cortex by cooling resulted in a reduction of thalamic nociceptive responses. These results underline the importance of mGlu1 receptors in the processing of sensory information in the thalamus, particularly with respect to nociceptive responses. Furthermore, the involvement of mGlu1 receptors may reflect the activity of descending cortico-thalamic afferents.

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Pharmacological potentiation of mGlu1 receptors enhances nociceptive responses of thalamic neurones.

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Antagonism of mGlu1 receptors reduces nociceptive responses of thalamic neurones.

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Inactivation of somatosensory cortex reduces nociceptive responses of thalamic neurones.

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This suggests a role for mGlu1 receptors and cortico-thalamic pathways in nociceptive processing.

Somatosensory abnormalities for painful and innocuous stimuli at the back and at a site distinct from the region of pain in chronic back pain patients

Chronic low back pain (CLBP) was shown to be associated with pathophysiological changes at several levels of the sensorimotor system. Changes in sensory thresholds have been reported but complete profiles of Quantitative Sensory Testing (QST) were only rarely obtained in CLBP patients. The aim of the present study was to investigate comprehensive QST profiles in CLBP at the painful site (back) and at a site distinct from their painful region (hand) and to compare these data with similar data in healthy controls. We found increased detection thresholds in CLBP patients compared to healthy controls for all innocuous stimuli at the back and extraterritorial to the painful region at the hand. Additionally, CLBP patients showed decreased pain thresholds at both sites. Importantly, there was no interaction between the investigated site and group, i.e. thresholds were changed both at the affected body site and for the site distinct from the painful region (hand). Our results demonstrate severe, widespread changes in somatosensory sensitivity in CLBP patients. These widespread changes point to alterations at higher levels of the neuraxis or/and to a vulnerability to nociceptive plasticity in CLBP patients.

Enhanced sensitivity to punctate painful stimuli in female patients with chronic low back pain

Background

Chronic low back pain (CLBP) has been shown to be associated with various pathophysiological changes at several level of the sensorimotor system, pointing to a general hypersensitivity in CLBP patients. The aim of the present study was to investigate signs of generalized mechanical pain hypersensitivity in CLBP patients on the hand and on the painful site of the back.

Methods

Pinprick stimulation according to a validated standardized quantitative sensory testing protocol was used in 14 female CLBP patients and 14 healthy controls (HC) matched for sex and age. Stimulus response functions to pinprick stimulation on the skin were examined at the affected back and reference sites (hand palmar and hand dorsum). Data from CLBP patients were compared with HC and with reference data from the German Research Network on Neuropathic Pain.

Results

We found significant differences in the stimulus response functions between CLBP patients and HC. Pain ratings to the pinpricks were increased for low and moderate pinprick stimuli in CLBP patients. Importantly, this kind of specific pinprick hyperalgesia was found not only for the affected body site (back), but also for the remote reference sites (hand dorsum and hand palmar).

Conclusions

We interpret our results as pointing to changes in the nociceptive processing in CLBP at higher levels of the neuraxis, possibly thalamus and/or attentional control, rather than changes of spinal processing. Alternatively, there might be a higher vulnerability to noxious stimulation in CLBP patients.

Cortical NR2B NMDA subunit antagonism reduces inflammatory pain in male and female rats

Background

Studies have shown that N-methyl-D-aspartate (NMDA) receptors play a critical role in pain processing at different levels of the central nervous system.

Methods

In this study, we used adult Wistar rats to examine gender differences in the effects of NR2B NMDA antagonism at the level of the anterior cingulate cortex in phasic pain, and in the first and second phases of a formalin test. Rats underwent stereotactic surgery for cannula implantation in the anterior cingulate cortex. After recovery, paw withdrawal latency to a noxious thermal stimulus was assessed. Rats were also subjected to a formalin pain test whereby 60 μL of 5% formalin was injected into the right hind paw.

Results

Female and male rats that received Ro 25-6981, an NR2B antagonist, before formalin injection showed significantly reduced pain responses to the formalin test compared with saline-injected control rats (P < 0.05). No gender differences in phasic pain responses were found in rats treated with Ro 25-6981.

Conclusion

These results suggest that cortical antagonism of the NR2B subunit reduces inflammatory pain levels in both genders of rat.

Kynurenine metabolites and migraine: experimental studies and therapeutic perspectives

Migraine is one of the commonest neurological disorders. Despite intensive research, its exact pathomechanism is still not fully understood and effective therapy is not always available. One of the key molecules involved in migraine is glutamate, whose receptors are found on the first-, second- and third-order trigeminal neurones and are also present in the migraine generators, including the dorsal raphe nucleus, nucleus raphe magnus, locus coeruleus and periaqueductal grey matter. Glutamate receptors are important in cortical spreading depression, which may be the electrophysiological correlate of migraine aura. The kynurenine metabolites, endogenous tryptophan metabolites, include kynurenic acid (KYNA), which exerts a blocking effect on ionotropic glutamate and α7-nicotinic acetylcholine receptors. Thus, KYNA and its derivatives may act as modulators at various levels of the pathomechanism of migraine. They can give rise to antinociceptive effects at the periphery, in the trigeminal nucleus caudalis, and may also act on migraine generators and cortical spreading depression. The experimental data suggest that KYNA or its derivatives might offer a novel approach to migraine therapy.

Central effects of a local inflammation in three commonly used mouse strains with a different anxious phenotype

As in humans, genetic background in rodents may influence a peculiar set of behavioural traits such as sensitivity to pain and stressors or anxiety-related behaviours. Therefore, we tested the hypothesis that mice with different genetic backgrounds [outbred (CD1), inbred (C57BL/6J) and hybrid (B6C3F1) adult male mice] display altered reactivity to pain, stress and anxiety related behaviours. We demonstrated that B6C3F1 mice displayed the more anxious phenotype with respect to C57BL/6J or CD1 animals, with the latter being the less anxious strain when tested in an open field and on an elevated plus maze. No difference was observed across strains in thermal sensitivity to a radiant heat source. Mice were then treated with a sub-plantar injection of the inflammatory agent Complete Freund's Adjuvant (CFA), 24h later they were hyperalgesic with respect to saline exposed animals, irrespective of strain. We then measured intra-strain differences and CFA-induced inter-strain effects on the expression of various genes with a recognized role in pain and anxiety: BDNF, IL-6, IL-1β, IL-18 and NMDA receptor subunits in the mouse thalamus, hippocampus and hypothalamus. The more anxious phenotype observed in B6C3F1 hybrid mice displayed lower levels of BDNF mRNA in the hippocampus and hypothalamus when compared to outbred CD1 and C57BL/6J inbred mice. CFA led to a general decrease in central gene expression of the evaluated targets especially in CD1 mice, while BDNF hypothalamic downregulation stands out as a common effect of CFA in all three strains evaluated.

Central mechanisms of pathological pain

Chronic pain is a major challenge to clinical practice and basic science. The peripheral and central neural networks that mediate nociception show extensive plasticity in pathological disease states. Disease-induced plasticity can occur at both structural and functional levels and is manifest as changes in individual molecules, synapses, cellular function and network activity. Recent work has yielded a better understanding of communication within the neural matrix of physiological pain and has also brought important advances in concepts of injury-induced hyperalgesia and tactile allodynia and how these might contribute to the complex, multidimensional state of chronic pain. This review focuses on the molecular determinants of network plasticity in the central nervous system (CNS) and discusses their relevance to the development of new therapeutic approaches.

Contralateral electroacupuncture pretreatment suppresses carrageenan-induced inflammatory pain via the opioid-mu receptor

Acupuncture has been used to treat various clinical diseases in Eastern medicine. To investigate the analgesic effect of electroacupuncture (EA) pretreatment on carrageenan-induced inflammatory pain, we studied on the effect of EA parameters on an animal model of acute arthritic pain. Pretreatment with 1 mA, 10 Hz EA prior to carrageenan injection under halothane anesthesia suppressed carrageenan-induced pain. Interestingly, EA stimulation of the 'Zu-San-Li' (ST36) acupuncture point (1 mA, 10 Hz) contralateral to the site of the carrageenan injection in the rat synovial cavity produced significantly greater improvement of the weight-bearing force compared with EA stimulation of the 'San-Yin-Jiao' acupuncture point. To determine how ST36 EA treatment suppresses carrageenan-induced inflammatory pain, we examined the effect of a mu opioid receptor antagonist on ST36 EA-induced analgesia. The selective antagonist of the mu opioid receptor (OR) significantly suppressed contralateral ST36 EA-induced analgesia against carrageenan-induced inflammation. These results suggested that the analgesic effect mediated by the mu OR during low-frequency contralateral EA pretreatment has an anti-nociceptive action against inflammatory pain and that it may provide a potential strategy to treat inflammatory arthritic pain.

Nociceptive behaviour upon modulation of mu-opioid receptors in the ventrobasal complex of the thalamus of rats

The role of mu-opioid receptors (MORs) in the inflammatory pain processing mechanisms within the ventrobasal complex of the thalamus (VB) is not well understood. This study investigated the effect of modulating MOR activity upon nociception, by stereotaxically injecting specific ligands in the VB. Nociceptive behaviour was evaluated in two established animal models of inflammatory pain, by using the formalin (acute and tonic pain) and the ankle-bend (chronic monoarthritic pain) tests. Control (saline intra-VB injection) formalin-injected rats showed acute and tonic pain-related behaviours. In contrast, intrathalamic administration of [D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin acetate (DAMGO), a MOR-specific agonist, induced a statistically significant decrease of all tonic phase pain-related behaviours assessed until 30-35min after formalin hind paw injection. In the acute phase only the number of paw-jerks was affected. In monoarthritic rats, there was a noticeable antinociceptive effect with approximately 40min of duration, as denoted by the reduced ankle-bend scores observed after DAMGO injection. Intra-VB injection of D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) (CTOP), a specific MOR antagonist, or of CTOP followed, 10min after, by DAMGO had no effects in either formalin or ankle-bend tests. Data show that DAMGO-induced MOR activation in the VB has an antinociceptive effect in the formalin test as well as in chronic pain observed in MA rats, suggesting an important and specific role for MORs in the VB processing of inflammatory pain.

Bilateral hyperexcitability of thalamic VPL neurons following unilateral spinal injury in rats

In the present study, we have examined whether spinal hemisection injury induces changes in the electrophysiological properties of thalamic ventral posteriorlateral (VPL) neurons in rats. Male Sprague-Dawley rats were subjected to unilateral spinal cord injury by transverse hemisection at the T13 spinal segment. Four weeks after the T13 spinal hemisection, the injured rats displayed robust allodynic behaviors on both sides of hindpaws compared to sham controls (P < 0.05). Extracellular recordings taken 4 weeks after the hemisection revealed that wide dynamic range (WDR) neurons had significantly increased spontaneous and brush-, pressure-, and pinch-evoked activities, respectively, on both sides of the thalamic VPL regions (P < 0.05). In contrast, low threshold (LT) neurons showed only an increase in the brush-evoked activity compared to sham controls (P < 0.05). However, afterdischarge activity in both types of neurons showed no changes. In addition, both sides of the thalamic VPL regions showed higher incidences of WDR neurons. In conclusion, our data demonstrate that spinal unilateral injury induces bilaterally increased evoked activity in thalamic VPL neurons.

A Selective Role for alpha3 Subunit Glycine Receptors in Inflammatory Pain

GlyR α3 has previously been found to play a critical role in pain hypersensitivity following spinal PGE₂ injection, complete Freund's adjuvant (CFA) and zymosan induced peripheral inflammation. In this study, although all models displayed typical phenotypic behaviours, no significant differences were observed when comparing the pain behaviours of Glra3^−/− and wild-type littermates following the injection of capsaicin, carrageenan, kaolin/carrageenan or monosodium iodoacetate, models of rheumatoid and osteoarthritis, respectively. However, clear differences were observed following CFA injection (p < 0.01). No significant differences were observed in the pain behaviours of Glra3^−/− and wild-type littermates following experimentally induced neuropathic pain (partial sciatic nerve ligation). Similarly, Glra3^−/− and wild-type littermates displayed indistinguishable visceromotor responses to colorectal distension (a model of visceral pain) and in vivo spinal cord dorsal horn electrophysiology revealed no differences in responses to multimodal suprathreshold stimuli, intensities which equate to higher pain scores such as those reported in the clinic. These data suggest that apart from its clear role in CFA- and zymosan-induced pain sensitisation, hypersensitivity associated with other models of inflammation, neuropathy and visceral disturbances involves mechanisms other than the EP2 receptor – GlyR α3 pathway.

Inhibition of glutamatergic transmission by morphine in the basolateral amygdaloid nucleus reduces pain-induced aversion

We examined the role of glutamatergic transmission within the basolateral amygdaloid nucleus (BLA) in pain-induced aversion using a conditioned place paradigm and an in vivo microdialysis technique in rats. Microinjection of MK-801 (1 or 10 nmol/side) into the bilateral BLA 5 min before intraplantar injection of formalin dose-dependently attenuated formalin-induced conditioned place aversion (F-CPA) without affecting nociceptive behaviors, such as lifting, licking, and biting. On the contrary, microinjection of neither CNQX (30 nmol/side) nor AP-3 (30 nmol/side) showed any significant effect on F-CPA. Microdialysis experiments revealed that intraplantar injection of formalin induced an increase in the extracellular glutamate level within the BLA. This increase in glutamate was suppressed by morphine perfusion (100 microM) via the microdialysis probe. Moreover, intra-BLA injection of morphine (10 microg/side) 5 min before formalin injection attenuated F-CPA without affecting nociceptive behaviors. These results suggest that glutamatergic transmission via NMDA receptors in the BLA plays a crucial role in the pain-induced aversion, and that in addition to the well-characterized effects on the sensory component of pain, morphine also influences the affective component of pain through an inhibitory effect on intra-BLA glutamatergic transmission.

Glutamate receptor ligands

Glutamate acts through a variety of receptors to modulate neurotransmission and neuronal excitability. Glutamate plays a critical role in neuroplasticity as well as in nervous system dysfunctions and disorders. Hyperfunction or dysfunction of glutamatergic neurotransmission also represents a key mechanism of pain-related plastic changes in the central and peripheral nervous system. This chapter will review the classification of glutamate receptors and their role in peripheral and central nociceptive processing. Evidence from preclinical pain models and clinical studies for the therapeutic value of certain glutamate receptor ligands will be discussed.

Vesicular glutamate transporter VGLUT2 expression levels control quantal size and neuropathic pain

Uptake of L-glutamate into synaptic vesicles is mediated by vesicular glutamate transporters (VGLUTs). Three transporters (VGLUT1-VGLUT3) are expressed in the mammalian CNS, with partial overlapping expression patterns, and VGLUT2 is the most abundantly expressed paralog in the thalamus, midbrain, and brainstem. Previous studies have shown that VGLUT1 is necessary for glutamatergic transmission in the hippocampus, but the role of VGLUT2 in excitatory transmission is unexplored in glutamatergic neurons and in vivo. We examined the electrophysiological and behavioral consequences of loss of either one or both alleles of VGLUT2. We show that targeted deletion of VGLUT2 in mice causes perinatal lethality and a 95% reduction in evoked glutamatergic responses in thalamic neurons, although hippocampal synapses function normally. Behavioral analysis of heterozygous VGLUT2 mice showed unchanged motor function, learning and memory, acute nociception, and inflammatory pain, but acquisition of neuropathic pain, maintenance of conditioned taste aversion, and defensive marble burying were all impaired. Reduction or loss of VGLUT2 in heterozygous and homozygous VGLUT2 knock-outs led to a graded reduction in the amplitude of the postsynaptic response to single-vesicle fusion in thalamic neurons, indicating that the vesicular VGLUT content is critically important for quantal size and demonstrating that VGLUT2-mediated reduction of excitatory drive affects specific forms of sensory processing.

Central sensitization in thalamic nociceptive neurons induced by mustard oil application to rat molar tooth pulp

We have recently demonstrated that application of mustard oil (MO), a small-fiber excitant and inflammatory irritant, to the rat maxillary molar tooth pulp induces central sensitization that is reflected in changes in spontaneous activity, mechanoreceptive field (RF) size, mechanical activation threshold, and responses to graded mechanical stimuli applied to the neuronal RF in trigeminal brainstem subnucleus caudalis and subnucleus oralis. The aim of this study was to test whether central sensitization can be induced in nociceptive neurons of the posterior thalamus by MO application to the pulp. Single unit neuronal activity was recorded in the ventroposterior medial nucleus (VPM) or posterior nuclear group (PO) of the thalamus in anesthetized rats, and nociceptive neurons were classified as wide dynamic range (WDR) or nociceptive-specific (NS). MO application to the pulp was studied in 47 thalamic nociceptive neurons and found to excite over 50% of the 35 VPM neurons tested and to produce significant long-lasting (over 40 min) increases in spontaneous activity, cutaneous pinch RF size and responses to graded mechanical stimuli, and a decrease in threshold in the 29 NS neurons tested; a smaller but statistically significant increase in mean spontaneous firing rate and decrease in activation threshold occurred following MO in the six WDR neurons tested. Vehicle application to the pulp did not produce any significant changes in six VPM NS neurons tested. MO application to the pulp produced pronounced increases in spontaneous activity, pinch RF size, and responses to mechanical stimuli, and a decrease in threshold in three of the six PO neurons. In conclusion, application of the inflammatory irritant MO to the tooth pulp results in central sensitization of thalamic nociceptive neurons and this neuronal hyperexcitability likely contributes to the behavioral consequences of peripheral inflammation manifesting as pain referral, hyperalgesia and allodynia.

Early postnatal chronic inflammation produces long-term changes in pain behavior and N-methyl-D-aspartate receptor subtype gene expression in the central nervous system of adult mice

The objective of this study was to test whether postnatal chronic inflammation resulted in altered reactivity to pain later in life when reexposed to the same inflammatory agent and whether this alteration correlated with brain-region-specific patterns of N-methyl-D-aspartate (NMDA) receptor subtype gene expression. Neonatal mouse pups received a single injection of complete Freund's adjuvant (CFA) or saline into the left hind paw on postnatal day 1 or 14. At 12 weeks of age, both neonatal CFA- and saline-treated animals received a unilateral injection of CFA in the left hind paw. Adult behavioral responsiveness of the left paw to a radiant heat source was determined in mice treated neonatally with saline or CFA before and after receiving CFA as adults. Twenty-four hours later, brains were dissected and NMDA receptor subunit gene expression was determined in four different brain areas by using an RNase protection assay. The results indicated that NMDA receptor subtype gene expression in adult mice exposed to persistent neonatal peripheral inflammation was brain region specific and that NMDA gene expression and pain reactivity differed according to the day of neonatal CFA exposure. Similarly, adult behavioral responsiveness to a noxious radiant heat source differed according to the age of neonatal exposure to CFA. The data suggest a possible molecular basis for the hypothesis that chronic persistent inflammation experienced early during development may permanently alter the future behavior and the sensitivity to pain later in life, especially in response to subsequent or recurrent inflammatory events.

Changes in electrophysiological properties and sodium channel Nav1.3 expression in thalamic neurons after spinal cord injury

Spinal cord contusion injury (SCI) is known to induce pain-related behaviour, as well as hyperresponsiveness in lumbar dorsal horn nociceptive neurons associated with the aberrant expression of Na(v)1.3, a rapidly repriming voltage-gated sodium channel. Many of these second-order dorsal horn neurons project to third-order neurons in the ventrobasal complex of the thalamus. In this study we hypothesized that, following SCI, neurons in the thalamus undergo electrophysiological changes linked to aberrant expression of Na(v)1.3. Adult male Sprague-Dawley rats underwent contusion SCI at the T9 thoracic level. Four weeks post-SCI, Na(v)1.3 protein was upregulated within thalamic neurons in ventroposterior lateral (VPL) and ventroposterior medial nuclei, where extracellular unit recordings revealed increased spontaneous discharge, afterdischarge, hyperresponsiveness to innocuous and noxious peripheral stimuli, and expansion of peripheral receptive fields. Altered electrophysiological properties of VPL neurons persisted after interruption of ascending spinal barrage by spinal cord transection above the level of the injury. Lumbar intrathecal administration of specific antisense oligodeoxynucleotides generated against Na(v)1.3 caused a significant reduction in Na(v)1.3 expression in thalamic neurons and reversed electrophysiological alterations. These results show, for the first time, a change in sodium channel expression within neurons in the thalamus after injury to the spinal cord, and suggest that these changes contribute to altered processing of somatosensory information after SCI.

Nitric oxide synthase (NOS)-interacting protein interacts with neuronal NOS and regulates its distribution and activity

Mechanisms governing the activity of neuronal nitric oxide synthase (nNOS), the major source of nitric oxide (NO) in the nervous system, are not completely understood. We report here a protein-protein interaction between nNOS and NOSIP (nitric oxide synthase-interacting protein) in rat brain in vivo. NOSIP and nNOS are concentrated in neuronal synapses and demonstrate significant colocalization in various regions of the central and peripheral nervous systems. NOSIP produces a significant reduction in nNOS activity in a neuroepithelioma cell line stably expressing nNOS. Furthermore, overexpression of NOSIP in cultured primary neurons reduces the availability of nNOS in terminal dendrites. These results thus suggest that the interaction between NOSIP and nNOS is functionally involved in endogenous mechanisms regulating NO synthesis. Furthermore, we found that the subcellular distribution and expression levels of NOSIP are dynamically regulated by neuronal activity in vitro as well as in vivo, suggesting that NOSIP may contribute to a mechanism via which neuronal activity regulates the synaptic availability and activity of nNOS.

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.

Differential roles of neuronal and endothelial nitric oxide synthases during carrageenan-induced inflammatory hyperalgesia

The present study investigated the role of neuronal nitric oxide synthase (nNOS) in carrageenan-induced inflammatory pain by combining genomic and pharmacological strategies. Intrathecal injection of the nNOS inhibitor 7-nitroindazole dose-dependently inhibited carrageenan-induced thermal hyperalgesia in both early and late phases in wild-type mice. However in nNOS knockout mice, carrageenan-induced thermal hyperalgesia remained intact in the early phase but was reduced in the late phase. Spinal Ca2+ -dependent nitric oxide synthase (NOS) activity in nNOS knockout mice was significantly lower than that in wild-type mice. Following carrageenan injection, although the spinal Ca2+ -dependent NOS activity in both wild-type and knockout mice increased, the enzyme activity in nNOS knockout mice reached a level similar to that in wild-type mice. On the other hand, no significant difference in spinal Ca2+ -independent NOS activity was noted between wild-type and nNOS knockout mice before and after carrageenan injection. Furthermore, intrathecal administration of the endothelial NOS (eNOS) inhibitor L-N5-(1-iminoethyl)-ornithinein nNOS knockout mice inhibited the thermal hyperalgesia in both early and late phases, though this inhibitor had no effect in wild-type mice. Meanwhile, Western blot showed that eNOS expression in the spinal cord of nNOS knockout mice was up-regulated compared with wild-type mice; immunohistochemical staining showed that the spinal eNOS was mainly distributed in superficial laminae of the dorsal horn. Finally, double staining with confocal analysis showed that the enhanced spinal eNOS was expressed in astrocytes, but not in neurons. Our current results indicate that nNOS plays different roles in the two phases of carrageenan-induced inflammatory pain. In this model, enhanced spinal eNOS appears to compensate for the role of nNOS in nNOS knockout mice.

The AMPA Receptor Subunits GluR-A and GluR-B Reciprocally Modulate Spinal Synaptic Plasticity and Inflammatory Pain

Ca(2+)-permeable AMPA receptors are densely expressed in the spinal dorsal horn, but their functional significance in pain processing is not understood. By disrupting the genes encoding GluR-A or GluR-B, we generated mice exhibiting increased or decreased numbers of Ca(2+)-permeable AMPA receptors, respectively. Here, we demonstrate that AMPA receptors are critical determinants of nociceptive plasticity and inflammatory pain. A reduction in the number of Ca(2+)-permeable AMPA receptors and density of AMPA channel currents in spinal neurons of GluR-A-deficient mice is accompanied by a loss of nociceptive plasticity in vitro and a reduction in acute inflammatory hyperalgesia in vivo. In contrast, an increase in spinal Ca(2+)-permeable AMPA receptors in GluR-B-deficient mice facilitated nociceptive plasticity and enhanced long-lasting inflammatory hyperalgesia. Thus, AMPA receptors are not mere determinants of fast synaptic transmission underlying basal pain sensitivity as previously thought, but are critically involved in activity-dependent changes in synaptic processing of nociceptive inputs.

Thalamic modulation of visceral nociceptive processing in adult rats with neonatal colon irritation

Visceral pain originates from visceral organs in response to a noxious stimulus which, if prolonged, may lead to chronic changes in the neural network mediating visceral nociception. For instance, colon inflammation enhances the responses of neurons in the thalamus to colorectal distension (CRD), whereas lesion in the dorsal column (DC) reverses this neuronal sensitization, suggesting that the thalamus and the DC play major roles in chronic visceral pain. In this study, we used adult rats sensitized with neonatal painful colon irritation to reveal the contribution of the thalamus and the DC to neuronal hyperexcitability in a model of chronic visceral pain. We recorded the responses of lumbosacral neurons to CRD in control rats and in rats with colon irritation following stimulation or inactivation of the thalamus, and after DC lesion. Our results show that, first, neuronal responses to CRD decreased following thalamic stimulation in control rats, whereas, in rats with colon irritation, responses either decreased or increased; second, DC lesion attenuated or enhanced these effects in the positively or in the negatively modulated group of neurons, respectively; third, lidocaine injection in the thalamus reduced the responses to CRD in some of the neurons recorded in rats with colon irritation, but had no effect on those in control rats. Therefore, it is reasonable to speculate that plasticity in rats with colon irritation that may underlie chronic pain is sustained by feedback loops ascending in the DC and engaging the thalamus.

Conditional gene deletion in primary nociceptive neurons of trigeminal ganglia and dorsal root ganglia

The use of Cre-loxP technology for conditional mutagenesis in pain pathways had been restricted by the unavailability of mice expressing Cre recombinase selectively in functionally distinct components of the nociceptive system. Here we describe the generation of transgenic mouse lines which express Cre recombinase selectively in sensory ganglia using promoter elements of the Na(v)1.8 gene (Scn10a). Cre-mediated recombination was greatly evident in all nociceptive and thermoreceptive neurons of the dorsal root ganglia and trigeminal ganglia, but only in a small proportion of proprioceptive neurons. Cre-mediated recombination was not detectable in the brain, spinal cord, or any nonneural tissues and began perinatally after invasion of primary afferents into the developing spinal cord. Thus, these mice enable selective deletion of genes in subsets of sensory neurons and offer a wide scope for studying potential functions of genes in pain perception, independent of secondary effects arising from developmental defects or global gene ablation.

Immediate reorganization of the rat somatosensory thalamus after partial ligation of sciatic nerve

Nerve injury can result in neuropathic pain, which persists after the injury and may occur after healing is completed. The long-term central reorganization associated with neuropathic pain has been previously studied in animal models. The immediate effects of nerve injury on central representation, however, are poorly understood. We examined the population response properties of closely neighboring neurons located in the hindlimb representation area of the somatosensory thalamus. Changes in the neuronal population properties were characterized before, during, and after (up to 6 hours) partial ligation of the sciatic nerve in the rat. Changes in these properties were observed within minutes after nerve injury. There were changes in neuronal class and receptive field size, emergence of new receptive fields, receptive fields observed before ligation disappeared temporarily after ligation, and changes in number of spikes evoked by the same stimulus. The rates of these changes in central representation were essentially zero before ligation, maximal within minutes after ligation, and decreased to a steady sustained rate of change within 1 to 2 hours. The incidence of functional connectivity, as measured by cross-correlations, remained unchanged. However, the strength of functional connectivity increased after ligation. The results show immediate reorganization of lateral thalamic networks with peripheral nerve damage. When the population response is considered as the underlying code, this reorganization does not reflect the behavioral manifestations of hyperalgesia and allodynia, even though some of the individual neuronal responses do reflect properties consistent with the hyperalgesia and allodynia reported within the same time frame after nerve injury in the rat.

Intact carrageenan-induced thermal hyperalgesia in mice lacking inducible nitric oxide synthase

To date, the exact role of inducible nitric oxide synthase (iNOS) in inflammatory pain remains controversial. In the present study, we combined a pharmacological strategy (using a selective iNOS inhibitor) with a genomic strategy (using mice lacking the iNOS gene) to address the function of iNOS in the central mechanism of carrageenan-induced persistent inflammatory pain. In the wild type mice, intrathecal administration of L-N(6)-(1-iminoethyl)-lysine, a selective iNOS inhibitor, significantly inhibited thermal hyperalgesia in the late phase but not in the early phase of carrageenan inflammation. Moreover, iNOS mRNA expression in the lumbar enlargement segments of the spinal cord was dramatically induced at 24 h (late phase) after injection of carrageenan into a hind paw. Interestingly, targeted disruption of iNOS gene did not affect carrageenan-induced thermal hyperalgesia in either the early (2-6 h) or late phase. In the lumbar enlargement segments of iNOS knockout mice, nitric oxide synthase (NOS) enzyme activity remained at a similar level to that of the wild type mice at 24 h after carrageenan injection. We found that intrathecal administration of 7-nitroindazole (a selective neuronal NOS inhibitor), but not L-N(5)-(1-iminoethyl)-ornithine (a selective endothelial NOS inhibitor), significantly reduced carrageenan-induced thermal hyperalgesia in both the early phase and the late phase in iNOS knockout mice. We also found that expression of neuronal NOS but not endothelial NOS in the lumbar enlargement segments was significantly increased in iNOS knockout mice compared with wild type mice at 24 h after carrageenan injection. Our results indicate that neuronal NOS might compensate for the function of iNOS in the late phase of carrageenan-induced inflammatory pain in iNOS knockout mice. This suggests that iNOS may be sufficient, but not essential, for the late phase of the carrageenan-induced thermal hyperalgesia.

Spinal upregulation of the nitric oxide synthase-interacting protein NOSIP in a rat model of inflammatory pain

Nitric oxide and nitric oxide synthases are key players in synaptic plasticity events in the spinal cord, which underly the development of chronic pain states. To date, little is known about the molecular mechanisms regulating the activity of nitric oxide synthases in nociceptive systems. The present study was aimed at the immunohistochemical determination of the expression of a nitric oxide synthase-interacting protein (NOSIP) in the rat spinal cord and dorsal root ganglia and studying its regulation in states of nociceptive hypersensitivity in a rat model of post-inflammatory pain. NOSIP is predominantly expressed in nociceptive primary neurons and in neurons of the spinal dorsal horn and the number of NOSIP-positive spinal neurons increases significantly following induction of unilateral intraplantar injection of complete Freund's adjuvant. Thus, NOSIP may modulate nitric oxide homeostasis in physiological and pathological pain conditions.

Role of thalamic phospholipase C[beta]4 mediated by metabotropic glutamate receptor type 1 in inflammatory pain

Phospholipase C (PLC) beta4, one of the four isoforms of PLCbetas, is the sole isoform expressed in the mouse ventral posterolateral thalamic nucleus (VPL), a key station in pain processing. The mouse thalamus also has been shown to express a high level of metabotropic glutamate receptor type 1 (mGluR1), which stimulates PLCbetas through activation of Galphaq/11 protein. It is therefore expected that the thalamic mGluR1-PLCbeta4 cascade may play a functional role in nociceptive transmission. To test this hypothesis, we first studied behavioral responses to various nociceptive stimuli in PLCbeta4 knock-out mice. We performed the formalin test and found no difference in the pain behavior in the first phase of the formalin test, which is attributed to acute nociception, between PLCbeta4 knock-out and wild-type mice. Consistent with this result, acute pain responses in the hot plate and tail flick tests were also unaffected in the PLCbeta4 knock-out mice. However, the nociceptive behavior in the second phase of the formalin test, resulting from the tissue inflammation, was attenuated in PLCbeta4 knock-out mice. In the dorsal horn of the spinal cord where PLCbeta1 and PLCbeta4 mRNAs are expressed, no difference was found between the wild-type and knock-out mice in the number of Fos-like immunoreactive neurons, which represent neuronal activity in the second phase in the formalin test. Thus, it is unlikely that spinal PLCbeta4 is involved in the formalin-induced inflammatory pain. Next, we found that pretreatment with PLC inhibitors, mGluR1 antagonists, or both, by either intracerebroventricular or intrathalamic injection, attenuated the formalin-induced pain behavior in the second phase in wild-type mice. Furthermore, activation of mGluR1 at the VPL enhanced pain behavior in the second phase in the wild-type mice. In contrast, PLCbeta4 knock-out mice did not show such enhancement, indicating that mGluR1 is connected to PLCbeta4 in the VPL. Finally, in parallel with the behavioral results, we showed in an electrophysiological study that the time course of firing discharges in VPL corresponds well to that of pain behavior in the formalin test in both wild-type and PLCbeta4 knock-out mice. These findings indicate that the thalamic mGluR1-PLCbeta4 cascade is indispensable for the formalin-induced inflammatory pain by regulating the response of VPL neurons.

The roles of different types of glutamate receptors involved in the mediation of nucleus submedius (Sm) glutamate-evoked antinociception in the rat

Based on our previous findings that glutamate microinjected into the thalamic nucleus submedius (Sm) inhibits dose-dependently the rat tail-flick (TF) reflex, this study investigated which glutamate receptor subtype is involved in mediating this effect. The effects of an NMDA (N-methyl-D-aspartate), non-NMDA or metabotropic glutamate receptor (mGluR) antagonist microinjected into Sm on the TF reflex were examined in untreated or in Sm glutamate treated (microinjection into the Sm) rats. The TF latencies were measured in each of these groups of rats every 5 min. Injection of DNQX [6,7-dinitroquinoxaline-2,3(1H,4H)-dione], a non-NMDA receptor antagonist, or (+/-)-MCPG [(+/-)-alpha-methyl-4-carboxyphenylglycine], a mGluR antagonist, into the Sm blocked the inhibitory effects induced by a subsequent microinjection of glutamate into the same Sm site. The TF latency increased only by 6.6+/-1.6 or 9.0+/-1.1%, respectively, of the baseline value, which was markedly less than that (51.3+/-8.4 or 50.7+/-5.3%) obtained from injection of glutamate only (P<0.001, n=8). However, pre-microinjection of MK-801 [(+)-5-methyl-10,11-dibenzo[a,d]cyclohepten-5,10-imine], an NMDA receptor antagonist, into the Sm had no effect on the Sm glutamate-evoked inhibition of the TF reflex. The TF latency change (40.0+/-11.1%) was not significantly different (P>0.05, n=8) compared with that obtained from glutamate injection alone. These observations suggest that non-NMDA and metabotropic glutamate receptors, but not NMDA receptors, are involved in mediating Sm glutamate-evoked antinociception.

Modulation of formalin-induced behaviors and edema by local and systemic administration of dextromethorphan, memantine and ketamine

The present study examined the effects of local peripheral and systemic administration of three clinically used excitatory amino acid receptor antagonists (dextromethorphan, memantine, ketamine) on pain behaviors and edema produced by formalin (1.5% and 5%) in rats. Peripheral administration of dextromethorphan produced a locally mediated suppression of flinching behaviors induced by 1.5% and 5% formalin, but biting/licking behaviors were not affected. Memantine and ketamine had no effect on either of these behaviors. All three agents augmented edema produced by 1.5% and 5% formalin. When administered alone, dextromethorphan, memantine and ketamine produced an intrinsic paw swelling response, and this was blocked by the biogenic amine receptor antagonists mepyramine, phentolamine, methysergide and ketanserin. Following systemic administration, all three agents suppressed biting/licking behaviors, had no effect on flinching behaviors, and suppressed paw swelling induced by 5% formalin to varying degrees. These results provide evidence for a peripherally mediated antinociceptive action of dextromethorphan in the rat formalin test, but this may not necessarily be due to block of excitatory amino acid receptors as it is not observed with memantine or ketamine. All three agents produce a peripherally mediated paw swelling, which is likely due to blockade of biogenic amine reuptake. Systemic administration of all three agents produces antinociceptive and anti-inflammatory actions that may be due to block of excitatory amino acid receptors in the spinal cord.

Antisense oligonucleotides to N-methyl-D-aspartate receptor subunits attenuate formalin-induced nociception in the rat

Noxious peripheral stimuli increase the sensitivity of central nociceptive neurons to subsequent noxious stimuli. This occurs in part through activation of spinal N-methyl-D-aspartate (NMDA) receptors. These receptors are heteromeric complexes of NMDA-R1 and NMDA-R2 A--D subunits. NMDA-R1 is necessary for the formation of functional NMDA receptors whereas the R2 subunits (A-D) modify the properties of the receptor. However, the role of the various receptor subtypes in nociception has not been established. In this study, we used intrathecally administered phosphodiester antisense oligonucleotides (ODEs) to examine the role of the NMDA-R1, NMDA-R2C and NMDA-R2D subunits in the mediation of formalin-induced nociception in the rat. The antisense ODEs against the NMDA-R1 and NMDAR-2C subunits reduced nociceptive behaviors whereas the corresponding sense ODEs had no effect. In contrast, nociception was unaffected by the antisense ODE to NMDAR-2D. Using an RNase protection assay, we also found that each antisense ODE selectively decreased the level of the corresponding mRNA in the lumbar spinal cord but that the sense ODEs had no such effect. Accordingly, these data provide evidence that the R1 and R2C subunits, but not R2D, of the NMDA receptor participate in the development of formalin-induced nociception.

Extracellular glutamate, aspartate and arginine increase in the ventral posterolateral thalamic nucleus during nociceptive stimulation

Although there is evidence that the thalamus plays a remarkable role in pain processing few in vivo studies on the thalamic neurochemical correlates of pain have been done. In the present experiments a combination of capillary zone electrophoresis with laser-induced fluorescence detection (CZE-LIF) and microdialysis in freely moving rats was used to measure extracellular arginine, glutamate and aspartate in the thalamus during the formalin test. Microdialysis probes were implanted in the left ventral posterolateral (VPL) nucleus of the thalamus in rats. Samples were collected every 30 s, derivatized with fluorescein isothyocyanate and injected into a CZE-LIF instrument. After nine baseline samples, a subcutaneous formalin (5%, 50 microl) injection in the right hind paw caused an increase of arginine, glutamate and aspartate that lasted for about 3 min. These increases were calcium and nerve impulse dependent. These results indicate that the release of arginine, glutamate and aspartate may mediate rapid pain neural transmission in the VPL nucleus of the thalamus.

'Balanced analgesia' in the perioperative period: is there a place for ketamine?

We investigated whether intraoperative 'subanesthetic doses' of ketamine have a postoperative anti-hyperalgesic and an analgesic effect and which is the preferential route of administration, either systemic (intravenous, i.v.) or epidural. One hundred patients scheduled for rectal adenocarcinoma surgery under combined epidural/general anesthesia were included. Before skin incision all the patients received an epidural bolus followed by an infusion of continuous bupivacaine/sufentanil/clonidine mixture. They were randomly assigned to receive no ketamine (group 1), i.v. ketamine at the bolus dose of 0.25 mg/kg followed by an infusion of 0.125 mg/kg per h (group 2), 0.5 mg/kg and 0.25 mg/kg per h (group 3), epidural ketamine 0.25 mg/kg and 0.125 mg/kg per h (group 4), or 0.5 mg/kg and 0.25 mg/kg per h (group 5). All i.v. and epidural analgesics were stopped at the end of surgery and patients were connected to an i.v. morphine patient-controlled analgesia (PCA) device. Short-term postoperative analgesia (72 h) was assessed by pain visual analog scale scores at rest, cough, and movements as well as by PCA requirements. Wound mechanical hyperalgesia was evaluated and residual pain was assessed by asking the patients at 2 weeks, and 1, 6, and 12 months. The area of hyperalgesia and morphine PCA requirements were significantly reduced in group 3. These patients reported significantly less residual pain until the sixth postoperative month. These observations support the theory that subanesthetic doses of i.v. ketamine (0.5 mg/kg bolus followed by 0.25 mg/kg per h) given during anesthesia reduce wound hyperalgesia and are a useful adjuvant in perioperative balanced analgesia. Moreover, they show that the systemic route clearly is the preferential route.

Increased spinal N-methyl-D-aspartate receptor function after 20 h of carrageenan-induced inflammation

Spinal N-methyl-D-aspartate (NMDA) receptors are thought to be important in states of central hyperexcitability induced by e.g. inflammation or painful neuropathies. The carrageenan model of inflammatory pain has been and still is widely used as is the NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (AP5) to investigate NMDA receptor function. Here we present two novel findings using electrophysiological technique: the NMDA receptor function in the spinal cord is increased following 20 h of carrageenan-induced inflammation and further that only the D-isomer of AP5 is active in the spinal cord. Exogenous NMDA (0.5 and 5 nmol) applied onto the dorsal spinal cord produced a significantly greater facilitation and D-AP5 (1.25 micromol) a significantly greater inhibition of the C-fibre evoked response of the wide dynamic range (WDR) neurones studied in carrageenan (20 h after injection) compared to control rats. The present and two recent studies suggest central changes are different and possibly greater in the later (20 h) compared to the earlier (2-6 h) phase of carrageenan-induced inflammation. In conclusion, 20 h of carrageenan-induced inflammation increases the function of spinal NMDA receptor involved in nociceptive transmission and in addition the D-isomer of AP5 should be used when NMDA receptor antagonism is wanted in the spinal cord.

Opioid poorly-responsive cancer pain. Part 2: basic mechanisms that could shift dose response for analgesia

Basic research in experimental pain models may illuminate the phenomenon of cancer pain that is poorly responsive to opioid drugs. Research findings can be valuable in formulating new strategies in clinical practice. This review evaluated experimental observations in terms of the events that occur in cancer patients receiving opioid therapy for pain.

Antinociceptive effects after intrathecal administration of phosphodiester-, 2'-O-allyl-, and C-5-propyne-modified antisense oligodeoxynucleotides targeting the NMDAR1 subunit in mouse

In the present study, we have compared the antinociceptive effect of three different types of antisense oligodeoxynucleotides targeting the N-methyl-D-aspartate (NMDA) R1-subunit in mice. The probes were administrated intrathecally three times during a period of 5 days (1, 5 or 25 microg/injection), followed by evaluation using the formalin test. The antinociceptive effect was correlated to in vitro receptor binding in spinal cord sections. The tissue distribution was studied after a single injection of fluorescein-conjugated probes. The phosphodiester probe showed superficial tissue penetration after 30 min and disappeared within 2 h. The probe did, however, significantly reduce both receptor binding in laminae I and II (by 36-44% compared to saline) as well as pain behavior (32% compared to saline), without apparent side effects. The mismatched probe was ineffective at 25 microg, while some reductions in receptor binding and pain behavior were seen after 5 microg. The C-5-propyne-modified phosphorothioate probe showed pronounced tissue penetration and cellular uptake as soon as 30 min after injection which was still detectable after 24 h. Immediately after injection of the highest dose, long-lasting hind-limb paralysis was observed. Receptor binding was reduced but not in a dose-related manner. Pain behavior was significantly reduced by 40% following 25 microg of antisense probe but not after lower doses or 25 microg of mismatched probe. The 2'-O-allyl-modified probe did not significantly reduce receptor binding or pain behavior. Thus, only the phosphodiester probe showed a significant correlation between reduction in pain behavior and receptor binding. These findings demonstrate that antisense technology is associated with specificity problems, but still could provide a valuable tool to study the role of different target proteins in the drug discovery process.

Glutamate receptors and nociception: implications for the drug treatment of pain

Evidence from the last several decades indicates that the excitatory amino acid glutamate plays a significant role in nociceptive processing. Glutamate and glutamate receptors are located in areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Glutamate acts at several types of receptors, including ionotropic (directly coupled to ion channels) and metabotropic (directly coupled to intracellular second messengers). Ionotropic receptors include those selectively activated by N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate. Metabotropic glutamate receptors are classified into 3 groups based on sequence homology, signal transduction mechanisms and receptor pharmacology. Glutamate also interacts with the opioid system, and intrathecal or systemic coadministration of glutamate receptor antagonists with opioids may enhance analgesia while reducing the development of opioid tolerance and dependence. The actions of glutamate in the brain seem to be more complex. Activation of glutamate receptors in some brain areas seems to be pronociceptive (e.g. thalamus, trigeminal nucleus), although activation of glutamate receptors in other brain areas seems to be antinociceptive (e.g. periaqueductal grey, ventrolateral medulla). Application of glutamate, or agonists selective for one of the several types of glutamate receptor, to the spinal cord or periphery induces nociceptive behaviours. Inhibition of glutamate release, or of glutamate receptors, in the spinal cord or periphery attenuates both acute and chronic pain in animal models. Similar benefits have been seen in studies involving humans (both patients and volunteers); however, results have been inconsistent. More research is needed to clearly define the role of existing treatment options and explore the possibilities for future drug development.

Neurochemical changes after morphine, dizocilpine or riluzole in the ventral posterolateral thalamic nuclei of rats with hyperalgesia

A number of studies suggest the involvement of glutamate in central hyperalgesia through NMDA receptors in animal models of inflammation. Most studies analyze glutamate effects at the spinal cord level. In this work, the effects of morphine, dizocilpine and riluzole on the hyperalgesia induced by carrageenan administration in the rat paw model were investigated. The effects of morphine and riluzole on the release of glutamate and aspartate and on the concentrations of citrulline and arginine in dialysates of the ventral posterolateral nucleus of the thalamus were also examined. All three drugs decreased hyperalgesia when administered prior to carrageenan injection. Morphine decreased the glutamate concentration in dialysates of the ventral posterolateral nucleus but did not affect the concentrations of the other amino acids. The effect of morphine was observed in the absence of painful stimulation and when pressure applied to the rat paw induced a nociceptive reaction. Riluzole decreased the concentrations of glutamate and aspartate and those of citrulline and arginine in the presence or absence of painful stimulation. These experiments suggest that morphine and riluzole attenuate the hyperalgesia induced by injection of carrageenan in the rat hind paw, at least partly, by decreasing glutamate release in the ventral posterolateral thalamic nucleus.

Identifying pain genes: Bottom-up and top-down approaches

A major goal of pain research at the present time is the identification of pain genes. Such genes have been informally defined in a number of ways, including the deletion or transcriptional inhibition of which produces alterations in behavioral responses on nociceptive assays; those the transcription of which is selective to pain-relevant anatomic loci (eg, small-diameter cells of the dorsal root ganglion); those the transcription of which is enhanced in animals experiencing tonic nociception or hypersensitivity states; and, finally, those existing in polymorphic forms relevant to interindividual variability. The purpose of this review is to compare the utility of various bottom-up and top-down approaches in defining, identifying, and studying pain genes. We will focus on 4 major techniques: transgenic knockouts, antisense knockdowns, gene expression assays (including DNA microarray-based expression profiling), and linkage mapping.

Involvement of mGluR(5) on acute nociceptive transmission

The effect of the mGluR(5) antagonist, MPEP (2-Methyl-6-(phenylethynyl)-pyridine), and of the mGluR(1) antagonist, AIDA((RS)-1-Aminoindan-1,5-dicarboxylic acid), were examined on nociceptive neurons in the ventroposterolateral (VPL) nucleus of the thalamus in response to pressure stimuli to the contralateral hindpaw of rats under urethane anesthesia. Intravenous (i.v.) injection of MPEP (0.1, 1, and 10 mg/kg) blocked responses to noxious stimulation in a dose-dependent and reversible manner. AIDA (3 and 15 mg/kg, i.v.), in contrast, had no effect on these cells. MPEP action was selective to noxious stimulation because even when tested at the highest dose (10 mg/kg, i.v.) it did not alter the responses of non-nociceptive neurons to brush stimulation. To investigate the site of action of MPEP, intra-thalamic injections were made during electrophysiological recordings. Using this method, the mGluR(5) antagonist did not affect nociceptive responses, suggesting that thalamic receptors were not involved in this action. On the other hand, the NMDA thalamic receptors seem to be involved because the NMDA receptor antagonist, MK801, successfully blocked responses to noxious pressure stimulation following intra-thalamic injections. In the spinal cord in vitro model, MPEP (30 microM, 60 min) was also able to attenuate ventral root potentials after single shock electrical stimulation of the dorsal root and inhibit wind-up response evoked by repetitive stimulation. Taken together, these findings suggest that blockade of the mGluR(5), but not mGluR(1) decreases nociceptive transmission in the thalamus and that these effects may be mediated by spinal cord receptors.