Morphine insensitive allodynia is produced by intrathecal strychnine in the lightly anesthetized rat

Inhibition of strychnine-allodynia is mediated by spinal adenosine A1- but not A2-receptors in the rat

Intrathecal (i.t.) strychnine produces localized allodynia in the rat without peripheral or central nerve injury. Intrathecal CPA (A1-selective agonist) and CGS-21680 (A2-selective agonist) dose-dependently inhibited strychnine-allodynia but with a 50-fold difference in potency (0.02-0.07 vs. 2.7-3.1 microgram, respectively). The anti-allodynic effect of CPA and CGS was completely blocked by pretreatment with the A1-selective antagonist, DPCPX (10 microgram i.t. ), but unaffected by the A2-selective antagonist, CSC (2 microgram i.t. ). The results indicate that spinal A1-, but not A2-, receptors modulate abnormal somatosensory input in the strychnine model, and suggest a difference in spinal purinergic modulation in injury vs. non-injury models of allodynia.

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

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

A case of 'pure' dynamic mechano-allodynia due to a lesion of the spinal cord: pathophysiological considerations

We report the unusual observation of a patient who presented with the single symptom of a very intense, brush-induced allodynia (dynamic mechanical allodynia) which was strictly confined to the left C2 and C3 dermatomes. All investigations, including a cervical spinal MRI, were initially normal. The clinical picture remained stable for several months until the appearance of spontaneous pain and sensory deficits suggestive of a spinal lesion. A second MRI revealed an intraspinal lesion involving the C2-C5 segments. In accordance with other clinical and animal studies, such an observation of a 'pure' dynamic mechano-allodynia suggests that specific mechanisms underlie each component of neuropathic pain. Possible pathophysiological mechanisms are discussed in the light of recent experimental results obtained in animals.

The cold plate as a test of nociceptive behaviors: description and application to the study of chronic neuropathic and inflammatory pain models

A cold plate apparatus was designed to test the responses of unrestrained rats to low temperature stimulation of the plantar aspect of the paw. At plate temperatures of 10 degrees C and 5 degrees C, rats with either chronic constriction injury (CCI) of the sciatic nerve or complete Freund's adjuvant (CFA) induced inflammation of the hindpaw displayed a stereotyped behavior. Brisk lifts of the treated hindpaw were recorded, while no evidence of other nociceptive behaviors could be discerned. The most consistent responses were obtained with a plate temperature of 5 degrees C in three 5-min testing periods, separated by 10-min intervals during which the animals were returned to a normal environment. Concomitantly to cold testing, the rats were evaluated for their response to heat (plantar test) and mechanical (von Frey hairs) stimuli. In both injury models, while responses to heat stimuli had normalized at 60 days post-injury, a clear lateralization of responses to cold was observed throughout the entire study period. Systemic lidocaine, clonidine, and morphine suppressed responses to cold in a dose-related fashion. At doses that did not affect motor or sensory behavior, both lidocaine and its quaternary derivative QX-314 similarly reduced paw lifts, suggesting that cold hyperalgesia is in part due to peripheral altered nociceptive processing. Clonidine was more potent in CCI then in CFA rats in reducing the response to cold. Paradoxically, clonidine increased the withdrawal latencies to heat in the CCI hindpaw at 40 days and thereafter, at a time when both hindpaws had the same withdrawal latencies in control animals. Morphine was also more potent on CCI than CFA cold responses, indicating that, chronically, CFA-induced hyperalgesia might be opiate resistant. Evidence for tonic endogenous inhibition of cold hyperalgesia was obtained for CFA rats, when systemic naltrexone significantly increased the number of paw lifts; this was not found in rats with CCI. At 60 days, neither morphine nor naltrexone affected cold-induced paw lifting in CFA rats, suggesting that the neuronal circuit mediating cold hyperalgesia in these animals had become opiate insensitive. In conclusion, the cold plate was found to be a reliable method for detecting abnormal nociceptive behavior even at long intervals after nerve or inflammatory injuries, when responses to other nociceptive stimuli have returned to near normal. The results of pharmacological studies suggest that cold hyperalgesia is in part a consequence of altered sensory processing in the periphery, and that it can be independently modulated by opiate and adrenergic systems.

Spinal effects of bicuculline: modulation of an allodynia-like state by an A1-receptor agonist, morphine, and an NMDA-receptor antagonist

Single-unit recordings were made in the intact anesthetized rat of the responses of dorsal horn neurons to C-, Adelta-, and Abeta-fiber stimulation. The postdischarge and windup responses of the same cells along with responses to innocuous stimuli, prod and brush, also were measured. The effects of (-)-bicuculline-methobromide (0.5, 5, 50, and 250 microg) were observed on these neuronal responses. The C- and Adelta-fiber-evoked responses were facilitated significantly in a dose-dependent manner. The input was facilitated, but as the final overall response was not increased by the same factor, windup appeared to be reduced. However, postdischarge, resulting from the increase in the excitability produced by windup, tended to be facilitated. After doses of >/=5 microg bicuculline, stimulation at suprathreshold Abeta-fiber-evoked activity caused enhanced firing, mainly at later latencies corresponding to Adelta-fiber-evoked activity in normal animals. Few cells responded consistently to brush and so no significant change was observed. Responses evoked by innocuous pressure (prod) always were observed in cells that concurrently responded to electrical stimulation with a C-fiber response. This tactile response was facilitated significantly by bicuculline. The effects of N6-cyclopentyladenosine (N6-CPA), an adenosine A1-receptor agonist, was observed after pretreatment with 50 microg bicuculline, as were the effects of morphine and 7-chlorokynurenate (7-CK). N6-CPA inhibited prod, C- and Adelta-fiber-evoked responses as well as the initial and overall final response to the train of C-fiber strength stimuli. Inhibitions were reversed with 8(p-sulphophenyl) theophylline. Morphine, the mu-receptor agonist, also inhibited the postbicuculline responses to prod, C-, and Adelta-fiber responses and initial and final responses to a train of stimuli. Inhibitory effects of morphine were reversed partly by naloxone. 7-CK, an antagonist at the glycine site on the N-methyl-D-aspartate-receptor complex, inhibited the responses to C- and Adelta-fiber-evoked activity as well as prod. The postdischarges were inhibited by this drug. Again both the initial and overall responses of the cell were inhibited. To conclude, bicuculline caused an increase in the responses of deep dorsal horn cells to prod, Adelta-fiber-evoked activity, increased C-fiber input onto these cells along with the appearance of responses at latencies normally associated with Adelta fibers, but evoked by suprathreshold Abeta-fiber stimulation. These alterations may be responsible for some aspects of the clinical phenomenon of allodynia and hyperalgesia. These altered and enhanced responses were modulated by the three separate classes of drugs, the order of effectiveness being 7-CK, N6-CPA, and then morphine.

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

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

Use of differential normal pulse voltammetry for the measurement of locus coeruleus catecholaminergic metabolism in an acute anaesthetized rodent model of allodynia: effect of mexiletine

Neuropathic pain can be triggered by non-painful stimuli (e.g., light touch), a sensory abnormality termed allodynia. The acute blockade of spinal glycine receptors with intrathecal strychnine induces a reversible allodynia-like state in the rat. We describe the application of in vivo differential normal pulse voltammetry with carbon fibre micro-electrodes for monitoring the catechol oxidation current (CAOC) of the locus coeruleus (LC) in the strychnine model of allodynia. In addition, we tested the effect of mexiletine, a drug useful in the management of clinical neuropathic pain in this model. Our results show that somatosensory processing in the spinal cord of urethane-anaesthetized rats is radically altered during glycine receptor blockade such that the normally innocuous stimulus of hair deflection causes the marked activation of the LC as determined using in vivo differential normal pulse voltammetry. Mexiletine suppressed the LC and cardiovascular responses of strychnine induced allodynia. Results of this study indicate that LC CAOC, an index of LC neuronal activity: (a) is a sensitive biochemical index of strychnine-allodynia; (b) is temporally correlated with the cardiovascular and motor responses evoked by hair deflection during glycine receptor blockade; and (c) can be used to quantitate allodynia in the strychnine model.

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

Experiments in both conscious and anesthetized animals indicate that intrathecal (i.t.) strychnine (STR; glycine receptor antagonist) produces acute, reversible allodynia, as evidenced by inappropriate behavioral and autonomic responses to cutaneous tactile stimuli. Although STR is known to produce disinhibition of afferent input to the spinal cord, changes in spinal reflexes cannot fully explain the complex behaviors observed following i.t. STR. Which supraspinal sites are involved in STR-dependent allodynia and how this abnormal somatosensory message is relayed to these sites remain to be determined. The medial thalamus contains many nociceptive-specific (NS) neurons and is believed to be involved in mediating the affective-motivational aspects of pain. It is thus important to determine whether spinally administered STR elicits changes in the responses of medial thalamic NS neurons. Extracellular single-unit recordings were conducted in urethan-anesthetized rats (290-490 g). A detailed characterization of 20 thalamic NS units (1 per rat; 2 in 1 case) was conducted before and immediately after i.t. STR (40 microg). Initially, all of the units in this study were classified as NS, because they were excited by noxious pinch but not by innocuous tactile stimuli. After i.t. STR, all (formerly NS) units exhibited significant responses to innocuous tactile stimuli (brush and/or air jet) applied to lumbar or sacral dermatomes. This effect of STR on thalamic NS neurons was acute and reversible. The majority of units (11 of 20) also exhibited an increase in spontaneous firing rate. Although the complete pinch receptive field (RF) could not be determined for all units, the available data indicate that the RFs for brush stimulation after i.t. STR were substantially different from the pre-STR pinch RFs for all but three units. The same i.t. STR injection that caused the observed changes in medial thalamus also produced allodynia, in the form of brush-evoked cardiovascular or motor responses, in 18 of the 19 rats. The ability of NS cells in medial thalamus to respond to tactile input after i.t. STR suggests that the STR lowers the threshold of nociceptive neurons that project directly and/or indirectly to medial thalamus. These observations suggest that ascending nociceptive pathways and medial thalamic structures contribute to the expression of STR-dependent allodynia.

Identification and characterization of an endogenous ligand for opioid receptor homologue ROR-C: its involvement in allodynic response to innocuous stimulus

We reported here purification and characterization of a novel heptadecapeptide in bovine brain as an endogenous ligand for ROR-C, an opioid receptor homologue cloned from rat cerebrum. The amino acid sequence of the peptide that we purified is identical to those recently identified as nociceptin in rat brain and orphanin FQ in porcine brain. The peptide inhibited the forskolin-induced cyclic AMP accumulation in ROR-C expressing Chinese hamster ovary cells. Studies on inhibitory activity of cyclic AMP accumulation and Northern blot analysis showed that the peptide and its precursor mRNA are present in a number of brain regions, less abundant in the spina cord, and negligible in the cerebellum. In situ hybridization analysis revealed that hybridization-positive neurons were distributed in the superficial layer (lamina I) of the dorsal horn and were also interspersed between the tract of Lissauer in the spinal cord. Intrathecal administration of the peptide into conscious mice induced allodynia, a pain response to innocuous tactile stimuli, in a beli-shaped manner. These results demonstrate that the peptide exists in the brain and spinal cord and plays an important role in pain transmission.

Neuronal model of tactile allodynia produced by spinal strychnine: effects of excitatory amino acid receptor antagonists and a mu-opiate receptor agonist

Touch evoked agitation (allodynia) can be induced by spinal delivery of strychnine and this effect is antagonized by intrathecal NMDA and non-NMDA receptor antagonists, but not by mu-opiate receptor agonists. In this study, we sought to characterize the effect of focal glycine-receptor inhibition on spontaneous and evoked activity in dorsal horn neurons of the chloralose-anesthetized cat. Strychnine (1 mM) applied near the neurons through a dialysis fiber caused an enhanced response to hair deflection, enlargement of the low threshold receptive fields and in some cells, an increase in afterdischarge. These changes were observed only in cells that were activated by both hair deflection and high intensity mechanical stimulation. Subsequent co-administration of an NMDA receptor antagonist (AP-7, 2.0 mM) preferentially blocked strychnine-associated effects without changing the original receptive field characteristics. Co-administration of a non-NMDA excitatory amino acid receptor antagonist (CNQX, 1 mM) with the strychnine served to block low (brush) and high intensity (pinch) afferent input. In contrast, addition of a mu-opiate receptor agonist (alfentanil 2.4 mM) to the strychnine perfusate selectively reduced responsiveness to high intensity stimulation, while having no effect on the exaggerated response to hair deflection. Given the functional and pharmacological similarity of the effects of spinal strychnine to post-nerve injury states in man, disinhibition due to a loss of glycinergic input may be associated with large myelinated fiber-mediated nociceptive states. Consistent with these data is the contention that under normal circumstances, afferent hair follicle input onto convergent neurons is regulated by a tonic glycinergic circuit. Removal of this regulatory influence leads to a magnification of low threshold tactile throughput in dorsal horn. This model may help to provide pharmacological insights into more efficacious treatments for such pain states that are relatively refractory to opioid therapies.

Strychnine-sensitive modulation is selective for non-noxious somatosensory input in the spinal cord of the rat

Touch-evoked allodynia, an important symptom of clinical neural injury pain, can be modelled acutely and reversibly in the urethane-anesthetized rat using intrathecal (i.t.) strychnine (STR). Allodynia, after i.t. STR (40 micrograms), is manifest as a significant enhancement of cardiovascular and motor responses evoked by normally innocuous brushing of the hair (hair deflection), as compared to responses evoked by either hair deflection after i.t. saline (SAL), or to i.t. STR (40 micrograms) with no tactile stimulus. The present study investigated: (1) the pharmacology of afferent neural inputs involved in STR-dependent allodynia using neonatal capsaicin and the non-NMDA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline (NBQX); and (2) the effect of i.t. STR on responses evoked by peripheral noxious stimulation. Neonatal capsaicin (25 mg/kg, s.c., post-natal day (PND) 1, and 50 mg/kg, s.c., PND 2, 3, 4, 11, 25, 55 and 85) significantly attenuated the responses evoked by noxious mechanical, thermal or chemical stimuli, but had no effect on STR-dependent allodynia. All hair deflection-evoked, STR-dependent responses were dose-dependently inhibited by i.t. NBQX. The ED50 values and 95% confidence intervals were 10.4 micrograms (5.5-19.6) for the motor withdrawal response, 14.4 micrograms (8.6-24.0) for changes in MAP and 12.2 micrograms (6.8-21.8) for changes in HR. Cortical EEG synchrony was unchanged by i.t. NBQX confirming its spinal locus of action. Intrathecal STR neither reduced nor enhanced the responses elicited by noxious stimuli in capsaicin- or vehicle-pretreated rats. These results indicate that STR-dependent allodynia is initiated by primary afferents not normally involved in nociception (possibly A beta-fibers), and that STR-sensitive modulation in the spinal cord is selective for non-noxious sensory input. The sensitivity of STR-dependent allodynia to non-NMDA receptor antagonists, and the failure of i.t. STR to produce hyperalgesia to mechanical, thermal or chemical noxious stimuli, confirm the independence of nociceptive pathways and STR-sensitive afferent inputs in this model.

Innocuous hair deflection evokes a nociceptive-like activation of catechol oxidation in the rat locus coeruleus following intrathecal strychnine: a biochemical index of allodynia using in vivo voltammetry

Blockade of spinal glycinergic inhibition with intrathecal (i.t.) strychnine induces a reversible allodynia-like state in both conscious and lightly-anaesthetized rats. Since the locus coeruleus (LC) is activated by noxious stimuli, we determined the effect of non-noxious hair deflection (HD) on noradrenergic neuronal activity in the LC of rats treated with i.t. strychnine. Differential normal pulse voltammetry was used to measure the catechol oxidation current (CA.OC), an index of LC activity. Rats were maintained in a light plane of anaesthesia with i.v. urethane and i.t. strychnine (40 micrograms) was injected near the L1-L2 segment. HD, applied to the caudal dermatomes affected by i.t. strychnine, evoked a significant increase (max. 141 +/- 7%, n = 5, P < 0.05) in CA.OC and mean arterial pressure as compared to baseline (no strychnine). In contrast, HD had no significant effect on CA.OC or mean arterial pressure in the saline-treated rats (n = 5). Pre-treatment with i.t. MK801 (30 micrograms) significantly blocked the increase in CA.OC and mean arterial pressure evoked by HD in strychnine-treated rats. The results of this study indicated that HD, in the presence of i.t. strychnine but not saline, can evoke noradrenergic activity in the LC of lightly anaesthetized rats. This effect on the LC is: (1) comparable to that observed with noxious stimulation without i.t. strychnine; (2) segmentally localized, corresponding to the spinal site of strychnine injection; and (3) mediated by spinal NMDA receptors, consistent with the role of excitatory amino acids in sensory transmission. These data provide the first neurochemical evidence that HD, in the presence of i.t. strychnine, is a nociceptive event, supporting the use of this preparation as an experimental model of allodynia.

Strychnine-dependent allodynia in the urethane-anesthetized rat is segmentally distributed and prevented by intrathecal glycine and betaine

The blockade of spinal glycine receptors with intrathecal strychnine produces a reversible allodynia-like state in the rat. Thus, hair deflection, in the presence of intrathecal strychnine, induces cardiovascular and motor withdrawal responses comparable with those evoked by noxious thermal, mechanical, or chemical stimulation in the absence of strychnine. In the present study, we mapped the cutaneous sites of abnormal sensitivity to hair deflection throughout the strychnine time course to investigate the segmental distribution of strychnine-induced allodynia. The ability of intrathecal glycine and the glycine derivative betaine to reverse strychnine-induced allodynia was also determined using dose-response analysis. Following intrathecal strychnine (40 micrograms), stroking the legs, flanks, lower back, and tail with a cotton-tipped applicator evoked a pronounced increase in mean arterial pressure, tachycardia, and an abrupt motor withdrawal response in urethane-anesthetized rats. These abnormal responses were only evoked by hair deflection at discrete sites, corresponding to the cutaneous dermatomes innervated by spinal segments near the site of strychnine injection. In rats with intrathecal catheters lying laterally in the subarachnoid space, allodynic sites were observed unilaterally on the ipsilateral side of intrathecal strychnine injection. Recovery from strychnine was complete by 30 min in all affected dermatomes. The cardiovascular and motor withdrawal responses to hair deflection were dose dependently inhibited by intrathecal glycine and intrathecal betaine. The ED50 (95% confidence interval) for intrathecal glycine was 609 (429-865) micrograms for the heart rate response, 694 (548-878) micrograms for the pressor response, and 549 (458-658) micrograms for the motor withdrawal response. The corresponding values for intrathecal betaine were 981 (509-1889), 1045 (740-1476), and 1083 (843-1391) micrograms, respectively. There was no difference in the effect of betaine on sensory-evoked cardiovascular and motor responses. Cortical electroencephalographic activity was not affected by intrathecal glycine or betaine, consistent with a spinal locus of action in reversing strychnine-induced allodynia. These results support the hypothesis that removal of spinal glycinergic modulation from low threshold afferent input with intrathecal strychnine results in segmentally localized, tactile-evoked allodynia.