Injury of primary afferent neurons may contribute to osteoarthritis induced pain: an experimental study using the collagenase model in rats.

OBJECTIVE: Pain exacerbated by movement and loading on the joint is the major symptom of osteoarthritis (OA), but the mechanisms of chronic pain in this pathology are still poorly understood. Using the intra-articular (i.a.) injection of collagenase in the knee of rats as a model of OA, we aimed at evaluating whether injury of sensory neurons may contribute to the development of OA-associated nociception. DESIGN: OA was induced by i.a. injection of collagenase into the left knee joint of adult male Wistar rats. Histopathological changes and movement and loading-induced nociception were assessed for 6 weeks. A time-course analysis of the expression of the neuronal injury markers activating transcription factor-3 (ATF-3) and neuropeptide Y (NPY) and of the neuropeptide SP in the dorsal root ganglion (DRG) was performed. Gabapentin's effect on nociception was evaluated, as well as the expression of the α2δ-1 voltage-gated calcium channel subunit. RESULTS: Collagenase induced the development of OA-like histopathological changes and of movement-induced nociception. Altered expression of ATF-3, NPY and SP was observed in the DRG, correlating with the degree of articular degeneration after 6 weeks of disease progression. Repeated administration of gabapentin reversed the nociceptive responses 6 weeks after the induction of OA. α2δ-1 was upregulated in the DRG. CONCLUSION: By inducing nociceptive behaviours associated with relevant joint structural changes, the i.a. injection of collagenase presents itself as a pertinent model for the study of OA pain. The findings in this study support the hypothesis that injury of sensory neurons innervating OA joints may be a significant element in the mechanisms of OA-associated pain.

Central Pain in Parkinson's Disease: Behavioral and Cognitive Characteristics.

INTRODUCTION: Pain is a major nonmotor symptom of Parkinson's disease (PD), and central parkinsonian pain is the core feature of the putative Park pain subtype of PD. This study aimed to explore the cognitive and behavioral profile of PD patients with central parkinsonian pain. Material and Methods. A structured interview was used to identify and characterize pain in a cohort of 260 consecutive PD patients. The Ford classification of pain was applied. The Dementia Rating Scale-2 (DRS-2) and the Impulse Control Disorders in Parkinson's Disease Short Form (QUIP-S) were administered, and patients' smoking habits were recorded. The Unified Parkinson's Disease Rating Scale (UPDRS) was used to assess motor and nonmotor symptoms in off and on conditions. RESULTS: One hundred and eighty-eight patients (68%) reported pain; and in 41 (22%) of them, the pain was classified as central parkinsonian pain. PD patients with central parkinsonian pain had better cognitive performance in DRS-2 Initiation/Perseveration and Conceptualization subscales but reported more other compulsive behaviors (e.g., hobbyism, punding, and walkabout) and had more current smoking habits than those without pain or with non-central parkinsonian pain. Multiple logistic regression analyses revealed that the DRS-2 Conceptualization subscale, other compulsive behaviors, and smoking habits remained statistically associated with central parkinsonian pain even when other significant covariates were considered. Only patients with pain, regardless of type, had a gambling disorder. Discussion. The study results provide further evidence that pain revealed that patients with central parkinsonian pain are more likely to present compulsive or addictive behaviors, despite having more preserved cognitive performance. Patients with central parkinsonian pain appear to have a distinct phenotype of PD.

Increase in GABAergic Cells and GABA Levels in the Spinal Cord in Unilateral Inflammation of the Hindlimb in the Rat.

The effects of chronic peripheral inflammation on spinal cord gamma-aminobutyric acid (GABA) were examined in the rat. Following the injection of complete Freund's adjuvant in the left hindlimb footpad an increased number of immunoreactive cells occurred in ipsilateral laminae I - III of the dorsal horn from L3 to L5. GABA-immunoreactive cells were more numerous than contralaterally 1 week after the onset of the inflammation, reached maximal numbers after 3 - 4 weeks, and declined thereafter. Differences from control sides were statistically significant except at week 6. GABA levels in homogenates of the ipsilateral lumbar enlargement were increased significantly at 4 weeks. Since increases in GABA occurred in the spinal cord zone of projection of the nerves supplying the inflamed foot, the central response is surmised to result from the increased nociceptive input arriving from the periphery. However, the transmission from primary axons to GABA interneurons is not likely to be monosynaptic since profiles containing glutamate decarboxylase or GABA immunoreactivity are known to be predominantly presynaptic, and rarely postsynaptic, to primary afferent endings in electron micrographs in the rat. The findings support the function attributed to spinal GABA in modulating nociceptive input at segmental level.

Central afferent pathways conveying nociceptive input to the hypothalamic paraventricular nucleus as revealed by a combination of retrograde labeling and c-fos activation.

Previous data have shown that noxious thermal stimulation of the hind leg in the anesthetized rat causes c-fos activation in the paraventricular nucleus of the hypothalamus (PVN); in other brain nuclei, including the cathecholaminergic cell groups of the caudal medulla; and in the adenohypophysis. Stimulation was followed by adrenocorticotropic hormone plasma release but did not provoke cardiovascular changes. In the current study, the afferent central pathways conveying the nociceptive input to the PVN were studied throughout the brain by using double labeling for the Fos-protein and the retrograde tracer cholera toxin subunit B (CTb) injected into the PVN. Although double labeling occurred in several hypothalamic nuclei, the periaqueductal gray, the lateral parabrachial area, and the catecholaminergic medullary groups, high rates of double labeling occurred only in the cells of the A1 region of the ventrolateral medulla ( approximately 83% of CTb-labeled cells expressing c-fos). Further triple labeling with tyrosine hydroxylase (TH) revealed that > 80% of the double-labeled cells were TH-immunoreactive. The spinal cord had the usual strong c-fos expression but showed no retrograde labeling from the PVN. Noxious stimulation caused corticosterone plasma release. To ascertain a possible link of spinofugal neurons with the A1 cells, biotinylated dextran amine was injected into the spinal dorsal horn. Numerous anterogradely labeled fibers with bouton-like structures were observed, with the latter apposed to double- and triple-labeled cells in the A1 region. It is suggested that a dysynaptic route relayed in the A1 region conveys the nociceptive somatic input from the spinal cord to the PVN. Noxious stimulation may act as a systemic stressor, activating the hypothalamic-pituitary-adrenal axis.

Carrageenan-induced inflammation of the hind foot provokes a rise of GABA-immunoreactive cells in the rat spinal cord that is prevented by peripheral neurectomy or neonatal capsaicin treatment.

An increase in the number of gamma-aminobutyric acid (GABA)-immunoreactive cells is reported in the superficial dorsal horn of the rat spinal cord upon unilateral inflammation of the hind foot caused by subcutaneous carrageenan injection. The rise of GABAergic cells was restricted to the ipsilateral dorsal horn, reaching a peak value of 23.4% over the contralateral side 4 days after carrageenan injection. Sciatic neurectomy or neonatal capsaicin treatment prevented this effect. These findings suggest that dorsal horn GABA is up-regulated by the increase of noxious inflow conveyed by unmyelinated C fibers from the inflamed tissues.

Baclofen and midazolam alter c-fos induction by peripheral noxious or innocuous stimulation in the spinal cord of normal and monoarthritic rats.

In order to further clarify the role of gamma-aminobutyric acid (GABA) receptors in spinal sensory processing we have studied the effects of baclofen, a GABA(B) agonist, and midazolam, a benzodiazepine agonist, on the activation of spinal neurones by peripheral innocuous or noxious stimulation, in normal or monoarthritic rats, as signalled by the induction of the proto-oncogene c-fos. Baclofen (10 mg/kg, i.v.) caused a significant reduction in the number of Fos-positive neurones following noxious stimulation of both normal and monoarthritic animals, which was prevented by the GABA(B) antagonist CGP 35348 (200 mg/kg, i.v.). The latter caused an increase of c-fos expression in normal animals subject to noxious stimulation, suggesting an endogenous tonic activation of GABA(B) receptors. This effect was not observed in monoarthritic animals. Baclofen also reduced the number of Fos-positive neurones in monoarthritic animals subject to innocuous stimulation. Midazolam (5 mg/kg, i.v.) had no effect in normal animals, but caused an increase in c-fos expression induced by noxious stimulation in monoarthritic animals. Flumazenil (1 mg/kg, i.v.), a benzodiazepine antagonist, prevented the effect of midazolam, and if given alone evoked a decrease in Fos-positive neurones. It can be concluded that although GABA(B) receptors modulate sensory input at the spinal level, high doses of systemic baclofen are required to inhibit nociceptive-induced c-fos expression. The paradoxical facilitation of c-fos expression by midazolam in monoarthritic animals, may be due to the reported increase in spinal GABA levels found in those animals.

Chemical sensory deafferentation abolishes hypothalamic pituitary activation induced by noxious stimulation or electroacupuncture but only decreases that caused by immobilization stress. A c-fos study.

We have shown in previous c-fos studies that noxious stimulation or electroacupuncture in deeply anaesthetized rats activate the hypothalamic pituitary corticotrope axis in a specific way. C-fos expression was more pronounced in the arcuate than the paraventricular hypothalamic nuclei, and none occurred in the pituitary intermediate lobe. The absence of the usual autonomic responses to psychological stress, such as tachycardia or blood pressure elevation, suggested a specific action of the somatosensory input on the hypothalamic pituitary axis. To prove this hypothesis, c-fos expression was examined in the paraventricular, arcuate and other hypothalamic nuclei, the pituitary gland, and the A1 and A2 medullary catecholaminergic cell groups of animals deprived of nociceptive primary afferent input by neonatal capsaicin. After noxious stimulation or electroacupuncture, no c-fos enhancement occurred in any of those sites in capsaicin-treated animals, and there was no increased plasma release of adrenocorticotropic hormone. In contrast, the hypothalamic pituitary c-fos activation provoked by immobilization stress though markedly decreased, was not abolished by capsaicin, whereas plasma release of adrenocorticotropic hormone remained undiminished. These findings suggest that noxious stimulation or electroacupuncture act on the hypothalamic pituitary corticotrope axis through an exclusively physical effect depending on the noxious signal elicited in the somatosensory pathway. They also demonstrate the occurrence of a minor somatosensory physical component after forced immobilization, acting on the hypothalamic pituitary axis probably together with the prevalent component of emotional arousal elicited by this form of stress.

Metabolic activity changes in the rat spinal cord during adjuvant monoarthritis.

The development of chronic pain is associated with activity-dependent plastic changes in neuronal structures in the peripheral and central nervous system. In order to investigate the time-dependent processing of afferent noxious stimuli in the spinal cord we employed the quantitative autoradiographic 2-deoxyglucose technique in a model of chronic monoarthritic pain in the rat. Spinal metabolic activity was determined at various time-points (two, four and 14 days) after the injection of complete Freund's adjuvant into the left tibiotarsal joint. In addition, the effect of acute noxious mechanical stimulation of the arthritic joint was investigated at 14 days of monoarthritis. Local glucose utilization was determined in lumbar segments L2-L5, ipsi- and contralateral to the inflamed hindpaw, and compared with saline-injected controls. In general, monoarthritic animals had bilaterally increased metabolic activity in all laminae of the spinal cord. Detailing the time-course showed that in rats with two days of monoarthritis metabolic activity was significantly increased to a similar extent on both sides of all spinal laminae. In contrast, at four days, glucose utilization in deep laminae of the dorsal horn (laminae V-VI), the central gray area (laminae X) and the ventral horn (laminae VII-IX) tended to return to control levels. At 14 days of monoarthritis, however, metabolic activity showed a further increase in all laminae of the spinal cord. This increase was more pronounced on the side ipsilateral to inflammation, reaching 65% above corresponding control levels in laminae V, VI. Animals with 14 days of monoarthritis which were subjected to mechanical noxious stimulation of the arthritic joint displayed clear behavioral signs of acute pain. Although in this group metabolic activity was above control levels, it was lower than in animals with 14 days of monoarthritis that were not additionally stimulated. The data show not only a general increase of spinal cord metabolic activity during the time-course of the development of a chronic pain state, but also show a region-specific non-linear time profile. This may reflect the complexity of transducing and suppressive transmitter systems involved in the central processing of ongoing pain.

Supraspinal metabolic activity changes in the rat during adjuvant monoarthritis.

Pain is a multi-dimensional experience including sensory-discriminative and affective-motivational components. The attribution of such components to a corresponding cerebral neuronal substrate in the brain refers to conclusions drawn from electrical brain stimulation, lesion studies, topographic mappings and metabolic imaging. Increases in neuronal metabolic activity in supraspinal brain regions, suggested to be involved in the central processing of pain, have previously been shown in various animal studies. The present investigation is the first to describe supraspinal structures which show increased metabolic activity during ongoing monoarthritic pain at multiple time-points. Experimental chronic monoarthritis of a hindlimb induced by complete Freund's adjuvant is one of the most used models in studies of neuronal plasticity associated with chronic pain. Such animals show typical symptoms of hyperalgesia and allodynia for a prolonged period. Metabolic activity changes in supraspinal brain regions during monoarthritis were assessed using the quantitative [14C]-2deoxyglucose technique at two, four, 14 days of the disease and, furthermore, in a group of 14-day monoarthritic rats which were mechanically stimulated by repeated extensions of the inflamed joint. Local glucose utilization was determined ipsi- and contralateral to the arthritic hindpaw in more than 50 brain regions at various supraspinal levels, and compared with saline-injected controls. At two and 14 days of monoarthritis significant bilateral increases in glucose utilization were seen in many brain structures, including brainstem, thalamic, limbic and cortical regions. Within the brainstem, animals with 14-day monoarthritis showed a higher number of regions with increased metabolic activity compared with two days. No differences between ipsi- and contralateral sides were detected in any of the experimental groups. Average increases ranged from 20 to 40% compared with controls and maximum values were detected in specific brain regions, such as the anterior pretectal nucleus, the anterior cingulate cortex and the nucleus accumbens. Interestingly, at four days of monoarthritis, the glucose utilization values were in the control range in almost all regions studied. Moreover, in monoarthritic rats receiving an additional noxious mechanical stimulation, the rates of glucose utilization were also comparable to controls in all brain areas investigated. Such patterns of brain metabolic activity agreed with concomitant changes in the lumbar spinal cord, described in the accompanying report. The present data show that a large array of supraspinal structures displays elevated metabolic activity during painful monoarthritis, with a non-linear profile for the time-points investigated. This observation most probably reflects mechanisms of transmission and modulation of nociceptive input arising from the monoarthritis and accompanying its development.

Expression of GAD mRNA in spinal cord neurons of normal and monoarthritic rats.

This study was carried out to investigate whether the increase of GABA levels in spinal cord dorsal horn in response to chronic inflammatory lesions results from an enhanced expression of the gene that governs the production of glutamate decarboxylase (GAD), the enzyme responsible for GABA synthesis. In situ hybridization was used to visualize neurons expressing GAD mRNA within the spinal cord, in both intact rats and in animals bearing chronic monoarthritis induced by intraarticular injection of complete Freund's adjuvant. In control normal animals, neuronal labeling by an antisense oligonucleotide probe occurred throughout the spinal gray matter, except in the motoneuronal pool of Rexed's lamina IX. In treated animals 4 days after the induction of monoarthritis, a significant increase in the number of labeled cells occurred in the superficial laminae (25.3%) and the neck (17.2%) of the ipsilateral dorsal horn at segments L4-L5 which contain the projection domain of the ankle joint. At 2 weeks, values were, respectively, 20.2% and 13.9% over contralateral values, and an increase of 12.4% was found in the ventral horn. At 3 weeks, the ipsilateral increase of labeled cells was restricted to the superficial dorsal horn (15.2%). These findings emphasize the role played by the spinal GABAergic system in the modulation of chronic nociceptive input. It is suggested that the response of the spinal GABAergic system depends on the activation of GAD gene transcription in spinal neurons.

Expression of metabotropic glutamate receptors mRNA in the thalamus and brainstem of monoarthritic rats.

Evidence for the involvement of metabotropic glutamate receptors (mGluR) in sensory processing has been emerging. Additionally, the differential distribution of distinct mGluR subtypes mRNA in particular thalamic nuclei of normal rats suggests that they could be involved in the processing of somatosensory information. In the present study, mGluR1, 3, 4 and 7 mRNAs expression was investigated by in situ hybridisation in selected brainstem and thalamic nuclei of adult monoarthritic rats at different time points of the disease (2, 4 and 14 days). Monoarthritic rats displayed behavioural and physical signs of painful arthritis at all time points. At 2 days of monoarthritis the mGluR1 mRNA expression was decreased mainly in the ventrobasal complex (VB) and in the posterior thalamic nuclei (Po) contralateral to the inflamed joint. The mGluR4 mRNA expression was also reduced, but minimum values were found at 4 days of monoarthritis, when no changes could be found in mGluR1 mRNA expression. At 14 days, mGluR4 mRNA expression was similar to controls, while mGluR1 mRNA was again reduced. Similar decreases of mGluR7 mRNA expression in the VB and Po were found at all time points, while mGluR3 mRNA expression was bilaterally increased in the reticular thalamic nucleus (Rt). In the brainstem no changes could be found in the expression of any mGluR subtype mRNA. The reduced expression of mGluR1, 4 and 7 transcripts in VB and Po, and the increases of mGluR3 mRNA in the Rt may contribute to counteract the increased noxious input arising from the periphery.

C-fos expression in the hypothalamo-pituitary system induced by electroacupuncture or noxious stimulation.

In the anaesthetized rat, low frequency electrical stimulation of the Zusanli acupoint (S36) or noxious thermal stimulation caused by immersing the footpad in water at 52 degrees C caused marked expression of c-fos in the anterior lobe of the pituitary gland, as well as in the arcuate and some nearby hypothalamic nuclei. A similar anterior lobe response was caused by immobilization stress in awake rats but in this case Fos-immunoreactive cells extended into the intermediate lobe and were very abundant in the paraventricular nucleus. It is suggested that the anterior pituitary cells that respond to stress are also activated by acupuncture or painful stimulation. However, the mechanisms of pituitary cell activation seem distinct from those occurring in stress, since different hypothalamic nuclei are involved.

Spinal cord projections of the rat main forelimb nerves, studied by transganglionic transport of WGA-HRP and by the disappearance of acid phosphatase.

The spinal cord projections of the 3 main forelimb nerves-median, radial and ulnar, were studied in the rat dorsal horn with transganglionic transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP), or using the disappearance of fluoride resistant acid phosphatase (FRAP) after nerve section. The projection patterns in lamina II were similar following the two procedures. The median and the radial nerve fibers projected to the medial and the intermediate thirds, respectively, of the dorsal horn lamina II in spinal cord segments C4-C8. The ulnar nerve projected to segments C6-C8 between the areas occupied by the other two nerves. The FRAP method also showed that the lateral part of lamina II, which was not filled by radial nerve fibers, received the projections from the dorsal cutaneous branches of cervical spinal nerves. In addition, FRAP disappeared from the medial end of segment T1 after skin incisions extending from the medial brachium to the axilla, which seemed due to severance of the cutaneous branchlets of the lateral anterior thoracic nerve. The FRAP procedure is thus sensitive enough to detect fibers in lamina II arising from small peripheral nerves, and may be used as an alternative to the anterograde tracing methods whenever there are no overlapping projections.

GABA decreases in the spinal cord dorsal horn after peripheral neurectomy.

A significant fall in the number of GABA-immunoreactive cells in laminae I-III of the rat spinal cord occurred in the somatotopic area of projection of the sciatic nerve after nerve transection. The decrease started at 2 weeks post-neurectomy, and at 4 weeks ipsilateral mean cell numbers were approximately 72% of contralateral control values. Similarly, the concentration of GABA in spinal homogenates was significantly reduced 4 weeks post-neurectomy. These data, together with our recent finding of an increase in spinal GABA during chronic inflammation of the hindlimb, suggest that the level of GABA in the dorsal horn is regulated by the amount of primary afferent input.

Distribution of glomeruli with fluoride-resistant acid phosphatase (FRAP)-containing terminals in the substantia gelatinosa of the rat.

In the cervical enlargement of the rat spinal cord, fluoride-resistant acid phosphatase (FRAP) occurs in most of the small dark sinuous primary afferent central terminals (CI-terminals) of type I-synaptic glomeruli of lamina II and is lacking in the large light roundish primary afferent CII-terminals of type II-glomeruli. Reactive CI-terminals are heterogeneously distributed across the dorsoventral thickness of lamina II, with maximal frequency in the zone heavily stained for FRAP in light micrographs, which roughly corresponds to the dorsalmost portion of lamina IIi. In the rest of lamina II many CI-terminals do not contain FRAP. Since all CI-terminals appear to originate from unmyelinated, presumably nociceptive, primary afferents, it is proposed that the FRAP-reactive ones arise from the FRAP-containing subpopulation of DRG small cells, while those lacking FRAP, which have a distinct area of termination in the dorsal horn, belong to the peptide-containing subpopulation.

Differential distribution of metabotropic glutamate receptor subtype mRNAs in the thalamus of the rat.

L-Glutamate (L-Glu) is present in most excitatory synapses of the mammalian brain, acting on several receptor subtypes. Height different genes encoding metabotropic glutamate receptors (mGluRs) subtypes have been described (mGluR1-8), having a distinct distribution in the brain. In the present study, the distribution of mGluR1, 3, 4, 5 and 7 mRNAs was determined in 20 thalamic nuclei of adult rats by performing in situ hybridisation with subtype-specific 35S-labelled oligonucleotide probes. High expression of mGluR1 mRNA mainly occurred in midline nuclei such as the centromedial/centrolateral (CM/CL) nuclei, parafascicular and submedius nuclei, and in the ventroposteromedial (VPM) and posterior (Po) nuclei. In contrast, mGluR5 mRNA was more uniformly distributed at weak to moderate levels, except in the reuniens nucleus where a strong signal was detected. The mGluR3 mRNA was highly expressed in the reticular thalamic nucleus and almost not detectable in any other thalamic region. Additionally, mGluR3 mRNA was found not only in neurones but also in putative glial cells. The mGluR4 mRNA was abundant in most thalamic nuclei, with prominent expression in the CM/CL, Po and ventrobasal complex (VPM and ventroposterolateral, VPL). Finally, mGluR7 transcripts were found evenly distributed throughout the thalamus at moderate levels, the highest signal being detected in the paraventricular thalamic nucleus, VPM, VPL and Po. This differential distribution of mGluR subtypes in the rat thalamus may contribute to the heterogeneity of glutamate effects on thalamic neurones. The mGluR1, mGluR4 and mGluR7 receptors may be involved in the processing of somatosensory information because they are expressed in nuclei that receive direct sensory input.

Complex changes of GABAA and GABAB receptor binding in the spinal cord dorsal horn following peripheral inflammation or neurectomy.

Chronic peripheral inflammation or peripheral neurectomy cause changes in GABA levels and GABA immunoreactivity in the spinal cord dorsal horn. The present study aimed to investigate if such changes are accompanied by alterations in GABA receptor binding. Neurectomy of the sciatic nerve caused an ipsilateral down-regulation of GABAB receptor binding in lamina II of the spinal cord 2-4 weeks after the nerve injury. Since approximately 50% of GABAB receptor binding in that region is located on primary afferent endings, degenerative changes of such endings caused by the nerve lesion can explain the observed reduction. In contrast, GABAA binding was substantially enhanced following neurectomy, which may be due to an up-regulation of the receptors issued by the concomitant decrease of endogenous GABA. In rats bearing unilateral chronic peripheral inflammation induced by intraarticular injection of complete Freund's adjuvant we found a reduction of GABAB binding in the superficial dorsal horn. This effect, which was maximal at 3-4 weeks after adjuvant injection, was attributed to an enhanced release of GABA by spinal interneurons. GABAA receptor binding was not changed in this experimental model. Together, these results suggest that the two receptor types may be located at different loci and are differently affected by variations in sensory input.

Axotomy of the sciatic nerve differentially affects expression of metabotropic glutamate receptor mRNA in adult rat motoneurons.

Previous studies indicated that axotomy exposes motoneurons to glutamatergic excitotoxic stress and protection from glutamatergic overactivation might be crucial for survival. Depending on the experimental model and the subtype involved, activation of metabotropic glutamate receptors (mGluRs) may either enhance excitotoxicity or exert protective effects. To investigate a possible involvement of mGluRs in neuronal rescue mechanisms after axotomy we have monitored the distribution of mGluR mRNA with in situ hybridization in adult rat motoneurons 1, 2, 3, and 4 weeks after sciatic nerve transection. Motoneurons in sham-operated control animals expressed mGluR 1, 4, and 7 mRNA. The mGluR1 mRNA signal was reduced to 49.6+/-6.9% as compared to the contralateral side 2 weeks after axotomy and 31.2+/-8.3% after 4 weeks. The mGluR4 signal declined to 22.1+/-5.1% after 1 week and 10.2+/-1.6% after 2 weeks, remaining stable thereafter. During the entire observation period the mRNA for mGluR7 was not significantly altered. Axotomy did not change the overall number of motoneurons on the ipsi- or contralateral side. The differential regulation of mGluR subtypes may be part of an adaptive cell program that helps to rescue adult motoneurons from excitotoxic cell death during the stress induced by peripheral denervation.

Antinociceptive effect of a group II metabotropic glutamate receptor antagonist in the thalamus of monoarthritic rats.

Adult rats were rendered monoarthritic (MA) by injection of 50 microl of complete Freund's adjuvant (CFA) into the tibiotarsal joint. The ankle-bend (AB) test of nociception was performed in those animals before and during 60 min after the stereotaxic injection of 2 microl of either saline (controls) or (2S)-alpha-ethylglutamic acid (EGLU, 80 nmol in 2 microl), a group II metabotropic glutamate receptors (mGluR) antagonist, in the reticular thalamic nucleus (Rt) contralateral to the arthritic joint. AB scores reached near maximum values before the stereotaxic injections (18.7+/-0.8), and remained constant throughout the entire experimental period in the control group, denoting marked allodynia. In the EGLU-treated group, AB scores gradually decreased after EGLU injection, with minimum values at 10 min (7.7+/-1.6), recovering to scores near maximum at 60 min (19.7+/-0.3). The data point to an activation of group II mGluR by noxious inputs in the Rt of MA rats, suggesting their participation in inhibiting local gamma-aminobutyric acid (GABA)ergic inhibitory neurones.

Opiates modify induction of c-fos proto-oncogene in the spinal cord of the rat following noxious stimulation.

The expression of the proto-oncogene c-fos in neurons of the spinal cord dorsal horn of the rat following noxious thermal stimulation was compared in morphine- and ketamine-treated animals. Intravenous injection of morphine reduced the number of c-fos-positive neurons by up to 85% in laminae III-VI and X. This effect was dose dependent and naloxone reversible. The non-competitive N-methyl-D-aspartate (NMDA) antagonist ketamine had no effect. The present data show that morphine suppresses the induction of c-fos. A block of Ca2+ influx through voltage- and ligand (NMDA)-gated channels does not influence c-fos protein synthesis in the dorsal horn of the spinal cord in vivo.