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

The non-peptide GLP-1 receptor agonist WB4-24 blocks inflammatory nociception by stimulating β-endorphin release from spinal microglia

BACKGROUND AND PURPOSE

Two peptide agonists of the glucagon-like peptide-1 (GLP-1) receptor, exenatide and GLP-1 itself, exert anti-hypersensitive effects in neuropathic, cancer and diabetic pain. In this study, we have assessed the anti-allodynic and anti-hyperalgesic effects of the non-peptide agonist WB4-24 in inflammatory nociception and the possible involvement of microglial β-endorphin and pro-inflammatory cytokines.

EXPERIMENTAL APPROACH

We used rat models of inflammatory nociception induced by formalin, carrageenan or complete Freund's adjuvant (CFA), to test mechanical allodynia and thermal hyperalgesia. Expression of β-endorphin and pro-inflammatory cytokines was measured using real-time quantitative PCR and fluorescent immunoassays.

KEY RESULTS

WB4-24 displaced the specific binding of exendin (9-39) in microglia. Single intrathecal injection of WB4-24 (0.3, 1, 3, 10, 30 and 100 μg) exerted dose-dependent, specific, anti-hypersensitive effects in acute and chronic inflammatory nociception induced by formalin, carrageenan and CFA, with a maximal inhibition of 60-80%. Spinal WB4-24 was not effective in altering nociceptive pain. Subcutaneous injection of WB4-24 was also antinociceptive in CFA-treated rats. WB4-24 evoked β-endorphin release but did not inhibit expression of pro-inflammatory cytokines in either the spinal cord of CFA-treated rats or cultured microglia stimulated by LPS. WB4-24 anti-allodynia was prevented by a microglial inhibitor, β-endorphin antiserum and a μ-opioid receptor antagonist.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that WB4-24 inhibits inflammatory nociception by releasing analgesic β-endorphin rather than inhibiting the expression of proalgesic pro-inflammatory cytokines in spinal microglia, and that the spinal GLP-1 receptor is a potential target molecule for the treatment of pain hypersensitivity including inflammatory nociception.

Changes in synaptic populations in the spinal dorsal horn following a dorsal rhizotomy in the monkey

Studies in monkeys have shown substantial neuronal reorganization and behavioral recovery during the months following a cervical dorsal root lesion (DRL; Darian-Smith [2004] J. Comp. Neurol. 470:134-150; Darian-Smith and Ciferri [2005] J. Comp. Neurol. 491:27-45, [2006] J. Comp. Neurol. 498:552-565). The goal of the present study was to identify ultrastructural synaptic changes post-DRL within the dorsal horn (DH). Two monkeys received a unilateral DRL, as described previously (Darian-Smith and Brown [2000] Nat. Neurosci. 3:476-481), which removed cutaneous and proprioceptive input from the thumb, index finger, and middle finger. Six weeks before terminating the experiment at 4 post-DRL months, hand representation was mapped electrophysiologically within the somatosensory cortex, and anterograde tracers were injected into reactivated cortex to label corticospinal terminals. Sections were collected through the spinal lesion zone. Corticospinal terminals and inhibitory profiles were visualized by using preembedding immunohistochemistry and postembedding gamma-aminobutyric acid (GABA) immunostaining, respectively. Synaptic elements were systematically counted through the superficial DH and included synaptic profiles with round vesicles (R), pleomorphic flattened vesicles (F; presumed inhibitory synapses), similar synapses immunolabeled for GABA (F-GABA), primary afferent synapses (C-type), synapses with dense-cored vesicles (D, mostly primary afferents), and presynaptic dendrites of interneurons (PSD). Synapse types were compared bilaterally via ANOVAs. As expected, we found a significant drop in C-type profiles on the lesioned side ( approximately 16% of contralateral), and R profiles did not differ bilaterally. More surprising was a significant increase in the number of F profiles ( approximately 170% of contralateral) and F-GABA profiles ( approximately 315% of contralateral) on the side of the lesion. Our results demonstrate a striking increase in the inhibitory circuitry within the deafferented DH.

Inflammation-induced shift in the valence of spinal GABA-A receptor-mediated modulation of nociception in the adult rat

The objective of this study was to assess the impact of persistent inflammation on spinal gamma-aminobutyric acid-A (GABA-A) receptor-mediated modulation of evoked nociceptive behavior in the adult rat. Nocifensive threshold was assessed with von Frey filaments applied to the dorsal surface of the hind paw. The GABA-A receptor agonist muscimol, the antagonist gabazine, the benzodiazepine receptor agonist midazolam, and antagonists PK11195 and flumazenil were administered spinally in the presence and absence of complete Freund's adjuvant (CFA)-induced inflammation. In naive rats, muscimol increased and gabazine decreased nociceptive threshold. After CFA, the effects of these compounds were reversed: Low doses of muscimol exacerbated the inflammation-induced decrease in nociceptive threshold and gabazine increased nociceptive threshold. Midazolam increased nociceptive threshold both in the presence and absence of inflammation. Flumazenil but not PK11195 blocked the analgesic effects of midazolam. These findings indicate that inflammation-induced changes in GABA-A signaling are complex and are likely to involve several distinct mechanisms. Rectifying the changes in GABA-A signaling may provide effective relief from hypersensitivity observed in the presence of inflammation.

PERSPECTIVE

An inflammation-induced shift in spinal GABA-A receptor signaling from inhibition to excitation appears to underlie inflammatory pain and hypersensitivity. Use of GABA-A receptor selective general anesthetics in association with therapeutic interventions may be contraindicated. More importantly, rectifying the changes in GABA-A signaling may provide effective relief from inflammatory hypersensitivity.

Increased ability to induce long-term potentiation of spinal dorsal horn neurones in monoarthritic rats

Long-term potentiation (LTP) of transmission of impulses in unmyelinated (C-fibre) primary afferents by prior tetanic conditioning stimulation has been demonstrated in the dorsal horn of the spinal cord. Since this potentiation has been proposed to be relevant to the increased responsiveness of spinal neurones associated with peripheral inflammation (central sensitisation), the present experiments compared the induction of LTP in normal rats and rats with monoarthritis. Monoarthritis was induced by injection of complete Freund's adjuvant (CFA) into the left ankle joint of 12 rats. All animals showed behavioural signs of thermal hyperalgesia and were used for electrophysiological experiments after 4-8 days. In each animal, extracellular recordings were obtained from a single, wide dynamic range (WDR) dorsal horn neurone. High frequency tetanic conditioning stimulation of the sciatic nerve gave varying effects on the C-fibre-evoked responses of neurones in the normal rats, with potentiation in two, no change in five and a depression in five. By contrast, conditioning stimulation in rats with inflammation produced a long-lasting potentiation of C-fibre-evoked responses in 11 out of 12 neurones, with no effect in one. The ease with which LTP was induced in animals with inflammation supports the proposal that the underlying mechanisms of LTP are similar to those of the central sensitisation associated with peripheral inflammation.

Activation of spinal ORL-1 receptors prevents acute cutaneous neurogenic inflammation: role of nociceptin-induced suppression of primary afferent depolarization

Neurogenic inflammation is an inflammatory response of peripheral tissue to vasoactive substances released from sensory afferent terminals. It can be triggered via a local axon reflex and by dorsal root reflex (DRR) activity involving the spinal cord. Nociceptin, an endogenous ligand for the opioid receptor-like (ORL-1) G-protein coupled receptor, has been found to inhibit the local axon reflex-mediated neurogenic inflammation by suppressing the release of vasoactive neuropeptides from sensory afferent terminals. The present study was to explore the role of spinal ORL-1 receptors in the modulation of DRR-induced neurogenic inflammation. We first examined the effect of nociceptin on DRR by recording dorsal root potentials (DRPs) and the associated antidromic discharges, evoked by electrical stimulation of an adjacent dorsal root in an in vitro neonatal rat spinal cord preparation. Nociceptin reversibly inhibited the DRP in a concentration-dependent manner (IC50: approximately 45 nM, maximal inhibition: approximately 50%), an effect that was antagonized by the ORL-1 receptor antagonist, J-113397. Neurochemical studies demonstrated that nociceptin (10 microM) also produced an approximately 40% reduction in gamma amino butyric acid (GABA) release evoked by electrical stimulation of neonatal rat spinal cord slices. On the other hand, nociceptin had no effect on exogenous GABA-evoked DRP. These findings suggest that the nociceptin-induced inhibition of the DRP is most likely due to the suppression of GABA release, the principle transmitter mediating DRP, from GABAergic neurons that are pre-synaptic to primary afferent terminals. Finally, in order to explore the physiological significance of such modulation in a fully integrated system, we evaluated the effect of intrathecally administered nociceptin on capsaicin-induced acute cutaneous neurogenic inflammation in rat hind paw, quantified by examining the degree of paw edema in anesthetized rats. The magnitude of capsaicin-induced increase of paw thickness was reduced by approximately 50% from 31+/-1.34% (n=6) to 15+/-1.63% (n=8; P<0.05) by nociceptin (10 micromol). We conclude that spinal ORL-1 receptors can modulate neurogenic inflammation by suppressing the GABAergic neuronal activity in the dorsal horn that is responsible for generating DRRs.

Antinociceptive effect of clomipramine in monoarthritic rats as revealed by the paw pressure test and the C-fiber-evoked reflex

The antinociceptive effect of clomipramine was studied in monoarthritic rats by using the paw pressure test and the C-fiber-evoked reflex. Monoarthritis was produced by intra-articular injection of complete Freund's adjuvant into the tibio-tarsal joint. Joint circumference as well as vocalization threshold to graded paw pressure were evaluated weekly during a 14-week period after the intra-articular injection. At week 8, monoarthritic and vehicle-injected control rats were given either clomipramine or saline and both the paw pressure threshold and inhibition of the C-fiber-evoked reflex response were evaluated. Results showed that (i) 1.5, 3.0, and 6.0 mg/kg, i.v. of clomipramine induced significantly greater dose-dependent antinociception to paw pressure testing in the monoarthritic group, as compared to the control one; and (ii) 0.75, 1.5, 3.0, and 6.0 mg/kg, i.v. of clomipramine exerted significantly higher dose-dependent inhibition of the C-reflex activity in monoarthritic rats than in controls. Results suggest that the higher sensitivity to clomipramine in monoarthritic rats could be related to adaptive changes occurring in monoamine metabolism or in other neurotransmitter systems during chronic pain.

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.

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.

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.

Dorsal root reflexes and cutaneous neurogenic inflammation after intradermal injection of capsaicin in rats

The role of dorsal root reflexes (DRRs) in acute cutaneous neurogenic inflammation induced by intradermal injection of capsaicin (CAP) was examined in anesthetized rats. Changes in cutaneous blood flow (flare) on the plantar surface of the foot were measured using a laser Doppler flowmeter, and neurogenic edema was examined by measurements of paw thickness. To implicate DRRs in neurogenic inflammation after CAP injection, the ipsilateral sciatic and femoral nerves were sectioned, dorsal rhizotomies were performed at L(3-)-S(1), and antagonists of GABA or excitatory amino acid receptors were administered intrathecally. Intradermal injection of CAP evoked a flare response that was largest at 15-20 mm from the injection site and that spread >30 mm. Acute transection of the sciatic and femoral nerves or dorsal rhizotomies nearly completely abolished the blood flow changes 15-20 mm from the CAP injection site, although there was only a minimal effect on blood flow near the injection site. These procedures also significantly reduced neurogenic edema. Intrathecal bicuculline, 6-cyano-7-nitroquinoxaline-2,3-dione, (CNQX) or D(-)-2-amino-7-phosphonoheptanoic acid (AP7), but not phaclofen, also reduced dramatically the increases in blood flow 15-20 mm from the CAP injection site, but had only a minimal effect on blood flow near the injection site. Neurogenic edema was reduced by the same agents that reduced blood flow. Multiunit DRRs recorded from the central stumps of cut dorsal rootlets in the lumbar spinal cord were enhanced after CAP injection. This enhanced DRR activity could be reduced significantly by posttreatment of the spinal cord with bicuculline, CNQX or AP7, but not phaclofen. It is concluded that peripheral cutaneous inflammation induced by intradermal injection of CAP involves central nervous mechanisms. DRRs play a major role in the development of neurogenic cutaneous inflammation, although a direct action of CAP on peripheral nerve terminals or the generation of axon reflexes also may contribute to changes in the skin near the injection site.

Unilateral joint inflammation induces bilateral and time-dependent changes in neuropeptide FF binding in the superficial dorsal horn of the rat spinal cord: implication of supraspinal descending systems

Using quantitative autoradiography, the effects of acute and chronic inflammation on specific 125I-1DMethyl-FLFQPQRFamide binding were investigated in the rat spinal cord dorsal horn superficial layers, at 6 and 24 h and 2, 4, 6 and 12 weeks after induction of monoarthritis produced by injection of killed Mycobacterium butyricum suspended in Freund adjuvant in one tibio-tarsal joint. Six hours after monoarthritis induction, no modification in specific 125I-1DMethyl-FLFQPQRFamide binding was observed, whereas a significant bilateral increase occurred after 24 h and 2 weeks in L4/L5 dorsal horns, with a return to control values at 4, 6 and 12 weeks. Specific 125I-1DMethyl-FLFQPQRFamide binding was also investigated 24 h after monoarthritis induction in rats submitted 4 days before the induction to spinal cord lesions at the thoracic level (T9-T10). Hemisection of the spinal cord contralateral to the affected ankle prevented the transient bilateral increase in specific 125I-1DMethyl-FLFQPQRFamide binding, whereas total spinal cord section induced a significant bilateral decrease. All of these modifications were restricted to the spinal segments receiving afferent input from the arthritic ankle (L4/L5); no modifications were found at the levels L1 or C6-C8. These data suggest that FLFQPQRFamide is involved in spinal nociceptive processing during sustained peripheral nociceptor activation. The effects of spinal cord lesions in monoarthritic rats indicate that the modifications seen in the FLFQPQRFamide system activity, during sustained peripheral inflammation, depend on afferent fiber activation as well as on supraspinal controls.

Changes in GAD- and GABA- immunoreactivity in the spinal dorsal horn after peripheral nerve injury and promotion of recovery by lumbar transplant of immortalized serotonergic precursors

We have utilized RN46A cells, an immortalized neuronal cell line derived from E13 brainstem raphe, as a model for transplant of bioengineered serotonergic cells. RN46A cells require brain-derived neurotrophic factor (BDNF) for increased survival and serotonin (5HT) synthesis in vitro and in vivo. RN46A cells were transfected with the rat BDNF gene, and the 46A-B14 cell line was subcloned. These cells survive longer than 7 weeks after transplantation into the subarachnoid space of the lumbar spinal cord and synthesize 5HT and BDNF. Chronic constriction injury (CCI) of the sciatic nerve was used to induce chronic neuropathic pain in the affected hindpaw in rats. Transplants of 46A-B14 cells placed 1 week after CCI alleviated chronic neuropathic pain, while transplants of 46A-V1 control cells, negative for 5HT and without the BDNF gene, had no effect on the induction of thermal and tactile nociception. When endogenous cells of the dorsal horn which contain the neurotransmitter gamma-aminobutyric acid (GABA) and its synthetic enzyme glutamate decarboxylase (GAD) were immunohistochemically quantified in the lumbar spinal cord 3 days and 1-8 weeks after CCI, the number of GABA- and GAD-immunoreactive (ir) cells decreased bilateral to the nerve injury as soon as 3 days after CCI. At 1 week after CCI, the number of GABA-ir cells continued to significantly decline bilaterally, returning to near normal numbers on the side contralateral to the nerve injury by 8 weeks after the nerve injury. The number of GAD-ir cells began to increase bilaterally to the nerve injury at 1 week after CCI and continued to significantly increase in numbers over normal values by 8 weeks after the nerve injury. When examined 2 and 8 weeks after CCI plus cell transplants, the transplants of 46A-B14 cells reversed the increase in GAD-ir cell numbers and the decrease in GABA-ir cells by 1 week after transplantation, while 46A-V1 control cell transplants after CCI had no effect on the changes in numbers of GAD-ir or GABA-ir cells. Collectively, these data suggest that altered 5HT levels, and perhaps BDNF secretion, related to the transplants ameliorate chronic pain and reverse the induction and maintenance of an endogenous pain mechanism in the dorsal horn. This induction mechanism is likely dependent on altered GAD regulation and GABA synthesis, initiated by CCI.

GABA and its receptors in the spinal cord

The importance of the inhibitory neurotransmitter, GABA, within higher centres of the mammalian brain is unquestionable. However, its role within the spinal cord is of equal significance. There have been numerous studies over the past two decades that have established GABA as a neurotransmitter at both post- and presynaptic sites in the cord. Here, Marzia Malcangio and Norman Bowery review the current status of GABA in relation to nociception and skeletal muscle tone, and indicate that its contribution to spinal cord function should not be overlooked.

Mechanisms of touch-evoked pain (allodynia): a new model

In this paper we review the current neurophysiological models of touch-evoked pain and present a new proposal that addresses the mechanisms of allodynia. The new model is based on the notion that A-beta mechanoreceptors can gain access to nociceptive neurones by means of a presynaptic link, at central level, between low threshold mechanoreceptors and nociceptors. We propose that the excitation of nociceptors provoked by a peripheral injury activates the spinal interneurones that mediate primary afferent depolarization (PAD) between low threshold mechanoreceptors and nociceptors. As a consequence of the increased and persistent barrage driving these neurones their excitability is increased such that, when activated by low threshold mechanoreceptors from areas surrounding the injury site, they produce a very intense PAD in the nociceptive afferents which is capable of generating spike activity. This activation would be conducted antidromically in the form of dorsal root reflexes (DRRs) but would also be conducted forward activating the second order neurones normally driven by nociceptors. The sensory consequence of this mechanism is pain evoked by the activation of low threshold mechanoreceptors from an area surrounding an injury site (allodynia).

Effect of acute stimulation on Fos expression in spinal neurons in the presence of persisting C-fiber activity

Expression of the inducible transcription factor c-Fos has been examined in the lumbar spinal cord following noxious chemical stimulation (injection of 2% formalin) of the ankles or the ventral skin of the hindpaws of either normal rats, or monoarthritic rats during the chronic phase of the disease. In normal animals the basal expression of c-Fos was low. One day after induction of monoarthritis by an intra-articular injection of killed Mycobacterium butyricum (in complete Freund's adjuvant) there were numerous c-Fos labelled cells in the ipsilateral dorsal horn, and bilaterally in lamina VIII and in other areas of the ventral horn. Four weeks after induction of the arthritis, although marked inflammation of the ankle was still present, all the expression of c-Fos had returned to the basal levels. One hour after formalin stimulation of the ankle or hindpaw skin of normal rats expression of c-Fos was observed throughout the ipsilateral, but not contralateral dorsal horn. Formalin stimulation of the inflamed ankle in four-week arthritic rats induced a 3-to-6 fold increase in c-Fos expression in the ipsilateral dorsal horn compared to formalin stimulation of the ankle in normal rats. In addition, c-Fos expression was induced in the contralateral deep, but not superficial laminae, at a density similar to that produced ipsilaterally by formalin stimulation of the ankle of normal rats. Formalin stimulation of the contralateral ankle in monoarthritic rats (i.e. the non-inflamed ankle) induced an ipsilateral expression of c-Fos which was similar to that observed after stimulation of the arthritic ankle. This stimulation of the normal ankle also resulted in an expression of c-Fos in the contralateral deep, but not superficial laminae, that was similar to that induced ipsilaterally by stimulation of the arthritic ankle. Finally, formalin stimulation of the hindpaw skin (which was not inflamed) of the arthritic limb induced the same number of c-Fos labelled cells in the superficial laminae as did formalin stimulation of the skin of normal rats; but in the deep laminae there was a 1.6-fold increase in the number of labelled cells. These different observations show that the down-regulation of c-Fos expression observed in chronic monoarthritis is in fact associated with a sensitization and an extension of the field of its expression in response to an acute nociceptive stimulation.

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.

Anticonvulsants suppress c-Fos protein expression in spinal cord neurons following noxious thermal stimulation

The expression of the nuclear immediate-early gene-encoded protein c-Fos in spinal cord dorsal horn neurons of the rat following noxious thermal stimulation was compared in carbamazepine-, valproate- and phenytoine-treated animals. Single intraperitoneal injection of carbamazepine (50 mg/kg), valproate (300 mg/kg) or intravenous injection of phenytoine (20 mg/kg) before noxious stimulation reduced the number of c-Fos immunoreactive neurons to 65-80% of control levels in superficial laminae and to 30-60% in deep laminae of the dorsal horn. Pretreatment with carbamazepine or valproate for 4 or 8 days combined with an injection immediately before noxious stimulation further significantly decreased the number of c-Fos neurons in the deep dorsal horn only in animals treated with valproate. The observation that activity-dependent gene expression in the spinal cord is effectively modulated by anticonvulsants discloses a novel therapeutic potential of these compounds. Presumably via an acute suppression of high-frequency repetitive firing and/or altered synaptic transmission of intraspinal or descending neurotransmitter systems these drugs gain access to neuroplastic mechanisms which might be relevant for the restoration of physiological levels of neuronal excitability in the central nervous system.