Hyperalgesia induced by altered glycinergic activity at the spinal cord

Evidence that oxytocin is an endogenous stimulator of autonomic sympathetic preganglionics: the pupillary dilatation response to vaginocervical stimulation in the rat

Vaginocervical mechanostimulation (VS) was shown previously to release oxytocin within the spinal cord and to induce pupillary dilatation. In the present study, (a) injection of oxytocin directly to the spinal cord (10 or 25 microg intrathecally [i.t.] in 5 microl saline) induced pupillary dilatation when observed 1 min after the end of the injection and (b) injection of an oxytocin receptor antagonist ([d(CH2)5-Tyr (Me)2-Orn8]-Vasotocin [OTA]; 25 microg i.t. in 5 microl saline) significantly attenuated the pupillary dilatation response to VS, when VS was applied 3 min after the end of the injection. Since activation of autonomic sympathetic preganglionic neurons in the thoracic spinal cord produces pupillary dilatation, we propose that oxytocin is a central nervous system neurotransmitter that stimulates these neurons directly, or perhaps indirectly, and thus is a mediator of VS-produced pupillary dilatation.

Dorsal root section elicits signs of neuropathic pain rather than reversing them in rats with L5 spinal nerve injury

Mechanical allodynia- and hyperalgesia-like behavior which develops in rats after L5 spinal nerve lesion has been suggested to be due to ectopic activity in the lesioned afferent neurons originating at the lesion site and/or in the dorsal root ganglion because it is eliminated by section of the dorsal root. Here we reevaluated the effect of a dorsal rhizotomy in rats after L5 spinal nerve lesion. Using calibrated von Frey hairs, paw withdrawal threshold to single stimuli and paw withdrawal incidence to repetitive stimulation were tested before and after nerve section. Neuropathic pain behavior of similar time course and magnitude also developed after cutting the L5 dorsal root, and L5 spinal nerve lesion-induced abnormal behavior could not be reversed by dorsal rhizotomy. The neuropathic pain behavior elicited by dorsal root section also developed when impulse conduction in the dorsal root axons was blocked during rhizotomy by a local anesthetic, i.e. when the immediate injury discharge was prevented from reaching the spinal cord. These results challenge the widely accepted idea that neuropathic pain behavior developing after spinal nerve lesion is dependent on ectopic activity in the lesioned afferent neurons. However, the present results do not rule out the possibility that after the two nerve lesions the mechanisms generating neuropathic pain behavior are different. After dorsal rhizotomy neuropathic pain behavior may be related to deafferentation whereas after spinal nerve lesion it may be caused by ectopic activity.

Analgesic synergism between AP5 (an NMDA receptor antagonist) and vaginocervical stimulation in the rat

Vaginocervical stimulation (VS) releases multiple neurotransmitters into superfusates of the spinal cord; these can stimulate both nociceptive (e.g., glutamate, and glycine acting at the NMDA site), and antinociceptive (e.g., GABA, norepinephrine, 5-HT, and glycine acting at the strychnine-sensitive receptor) systems. Although the balance between these two opposing systems can determine the nature, magnitude, and duration of the response to VS, the characteristic prevailing response to VS is analgesia. We hypothesized that by counteracting the nociceptive component of this system, the magnitude and duration of the response to VS would be augmented. In the present study, the NMDA receptor antagonist AP5 [10 microg injected intrathecally (i.t.)] significantly increased the magnitude and duration of the analgesia (measured as tail flick latency to radiant heat) produced by VS (200 g force). At several time points the analgesic effect of AP5 combined with VS was greater than the sum of the effects of AP5 and VS separately, suggesting that they act synergistically. We propose that AP5 potentiates the analgesic effect of VS by two mechanisms: (a) antagonizing the putative pain-producing action of glutamate and glycine acting jointly at the NMDA receptor, and consequently, (b) permitting the unimpeded expression of the analgesic action of inhibitory neurotransmitters released by VS (e.g., glycine at the strychnine-sensitive receptor, and GABA).

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.

Liposphere local anesthetic timed-release for perineural site application

PURPOSE

This investigation determines the drug delivery capacity of Lipospheres, which are drug-containing solid-filled vesicles made of triglyceride with a phospholipid outer covering, to release local anesthetic in vitro and to produce sustained peripheral nerve block in vivo.

METHODS

The local anesthetic, bupivacaine, was loaded into Lipospheres in several dosage forms, characterized, and measured for in vitro release. In rats, Lipospheres were administered into a large space between muscle layers surrounding the sciatic nerve to assess sensory and motor block in vivo.

RESULTS

The particle size of Lipospheres was determined to be between 5 and 15 microm, with over 90% surface phospholopid. Lipospheres released bupivacaine over two days under ideal sink conditions. Liposphere nerve application produced dose-dependent and reversible block. Indeed, sustained local anesthetic block (SLAB) was observed for 1-3 days in various in vivo tests: a) Hind paw withdrawal latency to noxious heat was increased over 50% for 96 hr period after application of 3.6% or 5.6% bupivacaine-Lipospheres. The 3.6% and 5.6% doses were estimated to release bupivacaine at 200 and 311 microg drug/ hr, respectively, based on release spanning 72 hr. Application of 1.6% bupivacaine-Lipospheres increased withdraw latency 25-250% but for only a 24 hr duration; b) Similarly, vocalization threshold to hind paw stimulation was increased 25-50% for 72 hr following application of 3.6% bupivacaine-Lipospheres; c) Finally, sensory blockade outlasted or equaled corresponding motor block duration for all Liposphere drug dosages.

CONCLUSIONS

Liposphere delivery of local anesthetic drugs may be well suited for site-specific pharmacotherapy of neural tissue to produce SLAB. Dose-dependent effects in duration of action may include lipophilic tissue storage.

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.

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.

[Myoclonus resulting from high-dose epidural and intravenous morphine infusion]

BACKGROUND

Myoclonus is a possible side effect of opioid therapy, and have been described following systemic as well as spinal application.

CASE REPORT

We report the case of a patient with metastatic carcinoma of the rectum who developed myoclonus following administration of high-dose epidural combined with iv morphine. This complication occurred with maximum daily doses of 300 mg epidurally and 80 mg intravenously and disappeared completely after dose reduction. Treatment trials are presented, the pathophysiology of the myoclonus is discussed.

CONCLUSION

For treatment of opioid-induced myoclonus a dose reduction or a change of the opioid should be considered as well as symptomatic treatment with benzodiazepines or baclofen.

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.

Effects of spinal cord stimulation on touch-evoked allodynia involve GABAergic mechanisms. An experimental study in the mononeuropathic rat

There is much evidence that tactile allodynia in rat models of mononeuropathy produced by sciatic nerve constriction is linked to disturbance of spinal GABAergic functions. Spinal cord stimulation (SCS) applied to such animals via chronically implanted electrodes may in some of the animals induce a significant increase of the withdrawal threshold in response to innocuous mechanical stimulation with von Frey filaments applied to the paw of the nerve ligated leg. The present study was performed in mononeuropathic animals with definite signs of tactile allodynia, which did not respond to SCS, GABA and the GABAB-agonist baclofen were administered intrathecally, in doses per se insufficient to influence the withdrawal thresholds, together with the previously ineffective SCS. This combination resulted in a marked and long-lasting increase of the thresholds. The GABAA-agonist muscimol given together with SCS also produced a similar, but less prominent threshold increase. The GABAB-antagonist 5-aminovaleric acid (5-AVA) produced a transient suppression of the threshold increase induced by SCS together with either GABA or baclofen. In contrast, the GABAA-antagonist bicuculline had no apparent inhibitory effect on the threshold augmentation produced by SCS combined with GABA or baclofen. It is concluded that SCS may operate by upgrading the spinal GABAergic systems and that its potential for producing pain relief is dependent upon the availability of responsive GABA-containing inhibitory interneurons. Moreover, it seems that the effects of SCS are more linked to the GABAB-than to the GABAA-receptor system.

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.

N-methyl-D-aspartate receptor-mediated changes in thermal nociception: allosteric modulation at glycine and polyamine recognition sites

The effects of allosteric modulators of the N-methyl-D-aspartic acid receptor ion-channel complex on the nociceptive tail-flick reflex were studied in awake rats. Intrathecal administration of D-serine (100 fmol-1 mumol) but not L-serine or glycine to the lumbar spinal cord produced a facilitation of the tail-flick reflex at doses > or = 1 pmol (maximum at 0.5-1 min). Intrathecal pretreatment with the glycine modulatory site antagonist 7-chlorokynurenic acid (3 pmol) blocked both D-serine-produced and N-methyl-D-aspartate-produced facilitation of the tail-flick reflex. D-serine-produced facilitation was also blocked by intrathecal pretreatment with a N-methyl-D-aspartate receptor ion-channel blocker, MK 801 (100 fmol), or with an alternate substrate for nitric oxide synthase, NG-nitro-L-arginine-methyl ester (100 nmol). Intrathecal administration of spermine (0.01 nmol-3 mumol) produced biphasic effects on tail-flick latency accompanied by mechanical hyperesthesia and vocalization at greater doses. Spermine-produced facilitation (maximum with 0.01 nmol to 1 nmol at 1 min) was blocked by intrathecal pretreatment with MK 801 (100 fmol), NG-nitro-L-arginine-methyl ester (100 nmol) or the polyamine modulatory site antagonist, arcaine (10 nmol). Spermine-produced inhibition (maximum with 300 nmol at 2 min) was blocked by intrathecal administration of MK 801 (1 nmol). Intrathecal administration of the N-methyl-D-aspartate receptor antagonist, D-2-amino-5-phosphonopentanoic acid (1 nmol), blocked inhibition and uncovered a facilitation produced by 1 mumol spermine. In addition, spermine produced multi-stage motor effects (immediate- and late-onset). Intrathecal pretreatment with MK 801 (1 nmol) blocked only the immediate-onset motor effects while the late-onset motor effects were selectively blocked by pretreatment with the kappa opioid receptor antagonist, nor-binaltorphamine (200 nmol). Taken together, these data suggest that D-serine and spermine facilitate nociceptive transmission by positive allosteric modulation of the N-methyl-D-aspartate receptor ion-channel. Furthermore, activation of the N-methyl-D-aspartate receptor is also necessary to elicit the immediate-onset motor effects and inhibition of the tail-flick reflex produced by greater doses of spermine. Because kappa opioid receptors appear to be involved, the spermine-produced late-onset motor effects may involve endogenous dynorphin release.

Disappearance of morphine-induced hyperalgesia after discontinuing or substituting morphine with other opioid agonists

Hyperalgesia and allodynia in 4 cancer patients treated with morphine disappeared after discontinuing or substituting morphine with other opioid agonists. The first case describes a young female who developed hyperalgesia and myoclonus during intravenous morphine infusion. The hyperalgesia and myoclonus disappeared when the morphine administration was discontinued and she felt comfortable on small and sporadic oral doses of methadone. The second case describes hyperalgesia occurring after a small dose of sustained-release morphine which disappeared after alternative use of oral ketobemidone. The third case describes hyperalgesia following high doses of intramuscular morphine which disappeared after alternative use of continuous subcutaneous infusion of sufentanil. The fourth case describes a boy developing hyperalgesia after high doses of oral and intramuscular morphine. The hyperalgesia disappeared after discontinuing morphine administration but withdrawal symptoms developed due to too small doses of methadone. Possible mechanisms of morphine-induced hyperalgesia are discussed.

Participation of opiatergic, GABAergic, and serotonergic systems in the expression of copulatory analgesia in male rats

Copulation in the male rat provoked an abrupt and significant rise in the threshold to induce vocalization by electrical shock to the tail (copulatory analgesia, CA). The possible effect on CA of the intrathecal (IT) administration of receptor antagonists to neurotransmitters participating in nociception was ascertained in this study. CA was significantly reduced, though not abolished, by IT injections of either naloxone, picrotoxin, or methysergide, but not by strychnine or yohimbine. This analgesic effect was achieved without significantly altering copulatory behavior. Results suggest that both brain and spinal systems participate in the development of CA. Brain effects would be mediated by descending serotonergic fibers, although intrinsic spinal systems would involve both opiate and GABA interneurons.

Momentary analgesia produced by copulation in female rats

To assess possible changes in nociception during copulation in estrous rats, electric shocks that were 20% suprathreshold for eliciting vocalization in response to tail shock (STS), were applied to the tail before the initiation of copulation and, thereafter, coincident with the onset of mounting bouts by the male (Experiment 1). Females vocalized significantly less during non-intromittive mounts (M; P < 0.001), intromissions (I; P < 0.001), and ejaculation (E; P < 0.01) than before the initiation of copulation. In order to assess the importance of vaginal stimulation (VS) by penile insertion during mating, in Experiment 2 30% STS were applied 300-400 ms after the initiation of mounting to ensure that the stimuli fell within the period of penile insertion occurring during I and E. M failed to significantly inhibit vocalizations to 30% STS. By contrast, both I and E markedly inhibited vocalizations in response to STS. This effect was transitory since subjects (Ss) vocalized to nearly all 30% STS when delivered 15 s after I or E. Copulatory analgesia (CA) was abolished by the bilateral transection of the pelvic and hypogastric nerves but not by the transection of the pudendal nerve (Experiment 3). The magnitude of CA was calibrated by determining the doses of morphine sulfate (MS) required to produce similar decrements in vocalization to STS. The analgesic effects of I and E were equivalent to more than 10 mg/kg and 15 mg/kg, respectively, of MS (Experiment 4). Pelvic-hypogastric neurectomy, but not pudendal neurectomy, also significantly reduced the effect of VS on facilitating lordosis, inducing immobilization and hind leg extension, and blocking the withdrawal reflex to foot pinch (Experiment 5).(ABSTRACT TRUNCATED AT 250 WORDS)

Glycine and GABAA antagonists reduce the inhibition of primate spinothalamic tract neurons produced by stimulation in periaqueductal gray

Amino acids are demonstrated to be important neurotransmitters mediating the inhibitory transmission from nucleus raphe magnus to spinal nociceptive dorsal horn neurons. In this study, the role of glycine and GABA in the inhibitory processes evoked by stimulation in periaqueductal gray (PAG) of responses of primate spinothalamic tract (STT) neurons to cutaneous mechanical and thermal stimuli was investigated by examining the effects of strychnine and bicuculline, antagonists of glycine and GABAA receptors, respectively, introduced into the dorsal horn through a microdialysis fiber. The inhibitory effects of iontophoretic application of glycine and GABAA agonists on STT cell activity evoked by noxious mechanical stimulation of the skin were selectively blocked by their specific antagonist, strychnine or bicuculline, infused into the dorsal horn. Similarly, intra-spinal application of strychnine or bicuculline resulted in a significant reduction in the PAG stimulation-induced inhibition of responses of STT cells to cutaneous stimuli. This reduction was mainly on the PAG-induced inhibition of the responses to noxious mechanical stimuli. Our results suggest that glycinergic and GABAergic inhibitory interneurons in the spinal cord dorsal horn synapsing on STT cells are activated during stimulation in PAG and contribute to descending antinociceptive actions.

Perispinal progestins enhance the antinociceptive effects of muscimol in the rat

The intrathecal (IT) injection of progesterone (PROG) or three of its ring A-reduced metabolites (5 beta,3 alpha-pregnanolone, 5 alpha,3 alpha-pregnanolone, or 5 beta,3 beta-pregnanolone) did not significantly alter any of two pain thresholds (vocalization threshold to tail shock, VTTS, or tail flick latency, TFL) in ovariectomized rats when tested in a wide range of doses (2.5-250 micrograms). When combined with a subanalgesic dose of muscimol (MUSC; 1 microgram IT), PROG and its two 3 alpha-hydroxy derivatives, but not the 3 beta, caused significant analgesia in the VTTS but not in the TFL test. No clear dose-response relationships were noted in the analgesic response to the combination of the progestins and MUSC. The present results indicate that PROG, either directly or through its ring A-reduction, can modulate nociceptive information by enhancing the action of GABA agonists on GABAA receptors.

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

The acute blockade of spinal glycinergic inhibition with intrathecal strychnine (i.t. STR; a glycine antagonist) in rats induces a change in somatosensory processing which is very similar to the sensory dysesthesia of clinical neural injury pain. In the present study, the effects of i.t. STR were examined in urethane-anesthetized rats. Noxious paw pinch (PP) or tail immersion (TI) in 55 degree C water evoked a pronounced pressor response, increased heart rate (HR) and desynchronized the electroencephalogram; a non-noxious, hair deflection (HD) elicited only minor cardiovascular responses. After i.t. STR (40 micrograms), an identical HD stimulus evoked markedly enhanced cardiovascular responses, resembling those evoked by noxious stimuli, and a HD-evoked motor withdrawal was observed. Consistent STR-dependent responses were only observed if a light plane of anesthesia was maintained for the duration of the experiment. The effects of i.t. STR were dose-dependent and reversible, lasting 15-30 min. Spinal morphine (50 micrograms) completely abolished the cardiovascular responses to PP and TI, but the HD-evoked, STR-dependent cardiovascular and motor withdrawal responses remained unchanged. In contrast, the non-selective excitatory amino acid antagonist, gamma-D-glutamylglycine (DGG; 50 micrograms) was effective in suppressing both the STR-dependent cardiovascular and motor withdrawal responses. These data suggest that STR-dependent responses evoked by non-noxious stimuli are mediated by mechanisms distinct from those of conventional noxious stimuli and that i.t. STR may be useful for investigating the spinal pharmacology of somatosensory processing following the loss of spinal glycinergic inhibition.

Hyperalgesia and myoclonus in terminal cancer patients treated with continuous intravenous morphine

Eight cancer patients in the terminal stages of the disease treated with high doses of intravenous morphine developed hyperalgesia. All cases were retrospectively sampled from three different hospitals in Copenhagen. Five patients developed universal hyperalgesia and hyperesthesia which in 2 cases were accompanied by myoclonus. In 3 patients a pre-existing neuralgia increased to excruciating intensity and in 2 of these cases myoclonus occurred simultaneously. Although only few clinical descriptions of the relationship between hyperalgesia/myoclonus and high doses of morphine are available, experimental support from animal studies indicates that morphine, or its metabolites, plays a causative role for the observed behavioural syndrome. The possible mechanisms are discussed and treatment proposals given suggesting the use of more efficacious opioids with less excitatory potency in these situations.

Release of amino acids into regional superfusates of the spinal cord by mechano-stimulation of the reproductive tract

Based on pharmacological evidence that inhibitory amino acids mediate vaginocervical mechano-stimulation produced analgesia (VSPA), we hypothesized that inhibitory amino acids would be released endogenously in the spinal cord in response to vaginocervical mechano-stimulation (VS). This hypothesis was tested by HPLC analysis of the amino acid content of 5-min superfusates of the spinal cord before, during and after VS (400 g force applied against the cervix) in urethane-anesthetized rats. Utilizing an in vivo push-pull superfusion method, artificial cerebrospinal fluid was continuously superfused over the spinal cord through the intrathecal space surrounding the sacral-lower thoracic region. In addition, concentrations of amino acids in the superfusate were measured in response to KCl stimulation (increasing the superfusion medium from 3.4 to 40.0 mM KCl to produce non-specific depolarization), and noxious hind paw mechano-stimulation (pinching the hind paw to produce a sustained flexor response in ipsilateral hind leg). There was a significant increase in the concentration of Gly, Tau, Asp, Glu and Lys in the superfusate in response to VS (n = 8) and to KCl (n = 8), but not to hind paw stimulation (n = 5). Also, GABA concentrations increased in response to KCl, and the concentration of Ala, Ser, Gln, Thr, Arg and Phe increased in response to VS, however, GABA levels were sometimes below the limits of detection. In contrast, there was no significant change in any amino acid concentration in response to hind paw pinch stimulation, and VS did not significantly affect the concentrations of Tyr, His, Ile, Leu, Met, Trp or Val. The present findings support our hypothesis that VS releases inhibitory amino acids in the spinal cord. Moreover, other amino acids, including 'excitatory' amino acids, are released into the superfusate. The profile of amino acid release in response to VS differs from that in response to paw pinch or KCl administration.

Raphe magnus stimulation-induced antinociception in the cat is associated with release of amino acids as well as serotonin in the lumbar dorsal horn

Stimulation in the nucleus raphe magnus (NRM) inhibits transmission of nociceptive information within the spinal cord through activation of bulbospinal pathways. This study used microdialysis in combination with high pressure liquid chromatography to measure the release of serotonin (5HT) and several amino acids, including glutamate, aspartate and glycine, from the lumbar dorsal horn during electrical stimulation within the NRM in the alpha-chloralose anesthetized cat. Observed release of putative neurotransmitters was correlated with inhibition of nociceptive projection neurons recorded from sites within 800 microns rostral or caudal to the dialysis fiber. NRM stimulus parameters considered to preferentially activate myelinated fibers caused inhibition of nociceptive evoked activity, and increased the release of excitatory amino acids and glycine within the spinal cord, with no detectable release of 5HT. When pulse widths were lengthened and unmyelinated fibers were also activated, increases in 5HT in the spinal dialysate were observed as well. Strychnine administered through the dialysis fiber (0.02-1 mM) antagonized NRM-induced inhibition when 5HT release was not detected. Inhibition produced by stimulation that increased 5HT concentrations was relatively strychnine resistant. These results point to a raphe-spinal inhibitory pathway that is not dependent on 5HT, the activation of which results in the spinal release of glycine.

Effects of intrathecal strychnine and bicuculline on nerve compression-induced thermal hyperalgesia and selective antagonism by MK-801

We studied the effects of intrathecally administered strychnine (STR; glycine antagonist; 10 or 30 micrograms) and bicuculline (BIC; GABAA antagonist 1 or 3 micrograms) on the thermal hyperalgesia which occurs following sciatic nerve constriction injury in rats. Following unilateral application of loose ligatures around the sciatic nerve, all rats typically displayed an ipsilateral thermal hyperalgesia on day 7. Intrathecal STR or BIC administered just after the nerve lesion and on days 1 and 2 after the nerve lesion significantly enhanced in a dose-dependent fashion the magnitude of the thermal hyperalgesia normally observed on day 7, as compared to intrathecal saline (for STR: 30 micrograms > 10 micrograms > or = saline; for BIC: 30 micrograms > 10 micrograms > or = saline, p < 0.05). Intrathecal MK-801, an N-methyl-D-aspartate antagonist, was without effect upon the response latency of the normal or sham operated paw, but selectively reversed the hyperalgesia. These results suggest that the loss of a spinal STR- and BIC-sensitive inhibition augments development of the hyperalgesia induced by chronic nerve compression.

Amino acids and serotonin are released into the lumbar spinal cord of the anesthetized cat following intradermal capsaicin injections

Several amino acids including aspartate, glutamate and glycine and the monoamine serotonin were retrieved from the extracellular space of the dorsal horn of the lumbar spinal cord in the alpha-chloralose anesthetized cat in vivo using a transverse microdialysis probe. Neurotransmitter concentrations were determined using high pressure liquid chromatography in combination with fluorescence (amino acid) or electrochemical (serotonin) detection. Intradermal injection of 3% capsaicin into the hindleg either ipsilateral or contralateral to the dialysis probe was used to evoke release. Extracellular concentrations of aspartate, glutamate and serotonin increased significantly following capsaicin injection into the ipsilateral limb. An almost equal increase in serotonin and a less pronounced, but still significant, increase in aspartate accompanied contralateral capsaicin injection. Glutamate concentrations increased in the dialysate during contralateral capsaicin injection in about half of the animals. These data are consistent with the hypothesis that Asp and Glu are both neurotransmitters released from nociceptive primary afferent fibers and/or interneurons activated by these fibers. In addition, Asp is presumed to be released from intrinsic spinal or descending systems following nociceptive stimulation. Bilateral release of 5HT into the dorsal horn most likely results from non-topographic activation of descending endogenous analgesia pathways.

Central effects of systemic lidocaine mediated by glycine spinal receptors: an iontophoretic study in the rat spinal cord

The objective of this investigation was to demonstrate the possible interactions of systemic lidocaine (lido) with inhibitory receptors in the spinal cord. In the lumbar dorsal horn of anesthetized and curarized rats, 60 physiologically identified, wide dynamic range (WDR) neurons, were recorded extracellularly. Glutamate, glycine and its selective antagonist, strychnine, were iontophoretically applied onto the neurons either singularly or concurrently. The effects of systemic lido on the drug-induced frequency changes and the interaction with the glycine receptors, using strychnine as a probe, were studied. It was consistently found that: (i) lido (3-4 mg/kg) inhibited the excitatory responses to iontophoretic glutamate, (ii) this inhibition was significantly antagonized by concurrent iontophoretic strychnine, (iii) iontophoretic glycine induced comparable glutamate inhibition that was reversed by strychnine. In contrast, no effect on glutamate-induced excitations was observed when lido was applied by micropressure or a different local anesthetic was systemically administered. The results suggest that central inhibitory effects of lido could be mediated by spinal strychnine-sensitive glycine receptors, activated by lido itself or possibly by its glycine residue-bearing metabolites.

Administration of AP5, a glutamate antagonist, unmasks glycine analgesic actions in the rat

The effect of intrathecal (IT) injection of glycine alone or in combination with 2-amino-5-phosphonopentanoate (AP5) on two nociceptive tests--the vocalization threshold to tail-shock (VTTS) and the tail-flick latency (TFL)--was studied in ovariectomized Sprague-Dawley rats. IT injection of 400 micrograms glycine induced a nonsignificant decrease, that is, in comparison with saline, in both nociceptive thresholds. IT AP5 (10 micrograms) provoked a slight but significant increase in both nociceptive thresholds within the first 15 min postinjection. Combination of both glycine (400 micrograms) and AP5 (10 micrograms) produced marked and prolonged analgesia in both tests, which was significantly different from that obtained with AP5 alone. The results suggest that IT glycine acting through the strychnine-sensitive Gly1 receptor produces analgesia provided its effect on the Gly2 receptor linked to the NMDA receptor is prevented by an antagonist.

Allodynia evoked by intrathecal administration of prostaglandin F2 alpha to conscious mice

The intrathecal administration of prostaglandin F2 alpha to conscious mice resulted in spontaneous agitation and touch-evoked agitation (allodynia) in the animals. The maximum allodynia induced by prostaglandin F2 alpha was observed at 10-15 min after intrathecal injection, and the response did not disappear by 120 min. Prostaglandin F2 alpha produced allodynia over a wide range of dosage from 0.1 pg to 2.5 micrograms/mouse. Dose dependency of prostaglandin F2 alpha for allodynia showed a skewed bell-shaped pattern, and the maximal allodynic effect was observed at 1.0 microgram. This allodynia was dose-dependently relieved by alpha 1-adrenergic (methoxamine), alpha 2-adrenergic (clonidine), and A1-adenosine (RPIA) agonists. Clonidine was 1.5 orders of magnitude more potent than methoxamine in blocking prostaglandin F2 alpha-induced allodynia. The blockade by clonidine was dose-dependently reversed by the alpha 2-adrenergic antagonist yohimbine but not by the alpha 1-adrenergic antagonist prazosin. These results demonstrate that prostaglandin F2 alpha administered intrathecally induces allodynia in conscious mice and that the allodynia involves the alpha 2-adrenergic and A1-adenosine systems. Because this allodynia has a clear resemblance to the characteristics of chronic pain in patients with causalgia and reflex sympathetic dystrophy, prostaglandin F2 alpha may be involved in allodynia observed with these disorders.

Hyperalgesia and myoclonus with intrathecal infusion of high-dose morphine

We report the case of a patient who developed myoclonus and hyperalgesia following administration of high-dose subarachnoid morphine. This complication occurred with 40-80 mg/day continuous infusion. The pathophysiology of these side effects is discussed.

Vasoactive intestinal polypeptide (VIP) potentiates the behavioral effect of substance P intrathecal administration

Intrathecal (IT) injection of 20 micrograms substance P (SP) induced a behavioral syndrome consisting of scratching and biting the flanks (83% and 57%, respectively, of 48 rats), and distress-like vocalization (42% of 26 rats tested) in response to a previously innocuous tactile stimulus with a von Frey fiber (allodynia). These behavioral events following SP were of short latency (1-2 min) and duration (around 10 min). Injection IT of 5 micrograms, but not 1 microgram, of vasoactive intestinal polypeptide (VIP), concurrently with SP, significantly increased the frequency of both scratching and biting bouts over that produced by SP alone. VIP IT alone (1 or 5 micrograms) did not stimulate scratching-biting, but induced allodynia in a significant proportion of rats.

Allodynia-like effects in rat after ischaemic spinal cord injury photochemically induced by laser irradiation

We report behaviours suggesting the presence of allodynia elicited by non-noxious brushing and mechanical pressure following photochemically induced ischaemic spinal cord injury in the rat. Female rats were intravenously injected with Erythrosin B and the T10 vertebra was irradiated with a laser beam for 1, 5 or 10 min. These procedures initiated an intravascular photochemical reaction, resulting in ischaemic spinal cord injury. After irradiation a clear allodynia was observed in most rats. The animals vocalized intensely to light touch during gentle handling and were clearly agitated to light brushing of the flanks. The vocalization threshold in response to the mechanical pressure measured with von Frey hairs was markedly decreased during this period. In some animals the existence of spontaneous pain was suggested by spontaneous vocalization. The duration of the allodynia varied among animals from several hours to several days. The severity and duration of allodynia seemed not to be related to the duration of irradiation. In sham-operated rats a slight, transient allodynia was also noted around the wound within a few hours after surgery, which was effectively relieved by systemic morphine (2 mg/kg, i.p.). Morphine (2 mg/kg, i.p.) also partially relieved the allodynia in spinally injured rats 4 h after irradiation. However, morphine, even at a higher dose (5 mg/kg, i.p.), failed to alleviate the allodynia in spinally injured rats 24-48 h after the injury. Systemic injection of the GABAB agonist baclofen (0.01-0.1 mg/kg, i.p.), but not the GABAA agonist muscimol (1 mg/kg, i.p.), effectively relieved allodynia during this period. Pretreatment with guanethidine 24 h and just prior to the irradiation (20 mg/kg, s.c.) did not prevent the occurrence of allodynia in spinal cord injured rats. The present observation is the first to show that ischaemic spinal cord injury could result in cutaneous mechanical allodynia. This phenomenon is resistant to morphine and may not involve the sympathetic system. Histological examination of allodynic animals 3 days after spinal cord injury revealed considerable morphological damage in the dorsal spinal cord of a rat irradiated for 5 min. The related dorsal roots were also slightly affected in this animal, while the dorsal root ganglia were normal. However, in rats irradiated for 1 min, despite the existence of strong allodynia, no damage could be found at this time in the spinal cord, dorsal roots or dorsal root ganglia. It is suggested that functional deficits in the GABAB system in the spinal cord may be related to this allodynia-like phenomenon.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of homotaurine in experimental analgesia tests

1. The possible antinociceptive action of GABA A receptor agonist homotaurine, has been studied through a battery of chemical (acetic acid) and thermal (hot plate, tail flick and tail immersion) tests in rats and mice. 2. The aminoacid was used at the following doses 22.25; 55.62 and 111.24 mg/kg i.p. and 50-100 micrograms i.c.v.; and measurements were made at the time of and at 5, 15 and 30 min after drug administration. 3. Homotaurine exhibited a significantly antinociceptive effect in all the above mentioned test except hot plate and when administered i.c.v. in the tail flick test. 4. The antinociceptive effect in the chemical test was dose and time dependent. 5. In the tail immersion test, latency time for withdrawal of the tail was significantly increased with the dose of 55.62 mg/kg at 15 min and 111.24 mg/kg at 30 min. 6. In the tail flick test the antinociceptive effect was dose dependent at 15 and 30 min. 7. From the above results the implication of peripheral and spinal mechanisms in the antinociceptive effect of homotaurine may be concluded.

Opioid inhibition of kainic acid-induced scratching: mediation by mu and sigma but not delta and kappa receptors

Scratching induced by intrathecal (IT) administration of kainic acid (0.5 nmol) to rats was inhibited by IT pretreatment with the selective mu agonists levorphanol (30 and 90 nmol), [D-Ala2,N-Met-Phe4,Gly5-ol]-enkephalin (DAGO, 0.4 and 1.1 nmol), or morphine (90 nmol), the mixed mu-delta agonist [D-Ala2,D-Leu5]-enkephalinamide (DADLE, 10 and 30 nmol), or the sigma/phenycyclidine (PCP) agonists dextrorphan (90 nmol) or (+)-N-allyl-N-normetazocine ([+]-NAM, 90 nmol). The kappa agonists dynorphin (1.1 nmol) and ethylketocyclazocine (EKC, 90 nmol) had no significant effect, nor did the selective delta agonist [D-Pen2,D-Pen5]-enkephalinamide (DPDPE, 90 nmol). The nonopioids (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine ([+]-3-PPP, 90 nmol) and PCP (90 nmol), selective for sigma and PCP sites, respectively, both antagonized kainic-induced scratching. Levorphanol- and DADLE-induced attenuation of scratching was partially antagonized by naltrexone. These findings suggest that opioid inhibition of kainic acid-induced scratching is mediated by classical mu receptors as well as sigma and PCP sites.

An electron microscope study of glycine-like immunoreactivity in laminae I-III of the spinal dorsal horn of the rat

The ultrastructural distribution of glycine-like immunoreactivity in laminae I-III of rat spinal dorsal horn was examined by using pre-embedding immunocytochemistry. Immunoreactive axons, dendrites and cell bodies were observed in all three laminae, but were most common in lamina III. The axons were presynaptic at axodendritic and axosomatic synapses, but also at axo-axonic synapses in laminae II and III, where the postsynaptic boutons frequently resembled the terminals of myelinated primary afferents. Some vesicle-containing dendrites in lamina II also showed glycine-like immunoreactivity. Immunoreactive dendrites in laminae II and III were postsynaptic to the central axons of type II, but not type I glomeruli, which suggests that glycinergic neurons receive a major monosynaptic input from myelinated primary afferents. These results support the suggestion that GABA and glycine co-exist in some neurons in laminae I-III of rat dorsal horn, and confirm that glycine is involved in somatosensory processing involving low threshold myelinated cutaneous primary afferents.

Blockage of substance P-induced scratching behavior in rats by the intrathecal administration of inhibitory amino acid agonists

Intrathecal administration of 20 micrograms of substance P induced scratching behavior in most tested rats (80%). Scratching appeared in bouts of short latency and variable duration, intensity and frequency (range 1-60, mean number of scratching bouts in one hour test: 8.93 +/- 1.86). Intrathecal administration of glycine (400 micrograms but not 66 micrograms) significantly decreased the effect of substance P on this behavior. Taurine, in dosages equimolar to glycine, abolished the response to substance P at the high dose level (700 micrograms), but did not significantly affect it at the lower level (120 micrograms). The GABAA agonist, muscimol, abolished the effect of substance P at the 3 micrograms dose level, but the 0.5 microgram dose did not produce a significant effect. Baclofen, a GABAB agonist, was highly effective in significantly reducing the action of SP at 0.9 and 0.15 microgram; only two of 8 rats receiving the low dose of baclofen (0.15 microgram) exhibited scratching. The results suggest that the spinal inhibitory amino acids modulate nociceptive impulses generated by the action of substance P in dorsal horn neurons of the spinothalamic tract.

Behavioral and autonomic correlates of the tactile evoked allodynia produced by spinal glycine inhibition: effects of modulatory receptor systems and excitatory amino acid antagonists

Intrathecal administration of glycine (strychnine) or GABA (bicuculline) but not opioid (naloxone), adrenergic (phentolamine) or serotonin (methysergide) receptor antagonists resulted in a dose-dependent organized agitation response to light tactile stimulation. This effect was maximally evoked by oscillating but not continuous stimulation applied to a dermatome corresponding to the levels of spinal cord acted upon by the intrathecal antagonist. Similar results were observed in chloralose-urethane anesthetized rats in which tactile stimulation evoked hypertensive responses following local tactile stimuli. The effects were only mildly depressed by even high doses of spinal morphine or DADL and not at all by ST-91 or baclofen. In contrast, intrathecal injections of glutamate receptor antagonists resulted in a dose-dependent depression of the strychnine evoked hyperesthesia with the ordering of activity being MK-801, AP-5, kynurenic acid, SKF10047 and ketamine. At doses below those which produced motor dysfunction, however, these agents had no effects on the hot-plate response latency. These data emphasize that low threshold afferent input is likely subject to an ongoing modulation, the loss of which results in a miscoding of the afferent stimulus yielding a pain relevant message. The lack of effect of agents having a powerful effect on somatic pain stimuli and the converse effects of glutamate receptor antagonists on the strychnine hyperesthesia at doses which do not affect the somatic pain response indicate discriminable processing systems, the characteristics of which resemble the clinical phenomenon observed in patients suffering from sensory dysesthesia following central and peripheral horn injury.

Intrathecal GABA, glycine, taurine or beta-alanine elicits dyskinetic movements in mice

Dyskinetic, writhing-like movements, similar to those produced in mice after an intraperitoneal (IP) injection of acetic acid, were elicited by intrathecal (IT) injection of GABA, glycine, taurine or beta-alanine. Baclofen and muscimol failed to produce this behavior. While acetic acid-induced writhing is inhibited by narcotic and nonnarcotic compounds, GABA-induced writhing was found to be insensitive to pretreatment with either morphine or capsaicin. Moreover, acetic acid-induced writhing does not appear to involve GABAergic transmission as IT injections of nipecotic acid did not alter the intensity of response to IP acetic acid while it enhanced the response to IT GABA. Writhing induced by glycine was not inhibited by strychnine at subconvulsive doses, suggesting that it involves an action at strychnine-insensitive receptors. Together these data suggest that while the dyskinetic movements produced by inhibitory amino acids do not appear to reflect an alteration in nociception, they may mimic either the motor response to abdominal pain or spasticity.

Copulatory pelvic thrusting in the male rat is insensitive to the perispinal administration of glycine and GABA antagonists

The role of the inhibitory neurotransmitters glycine and GABA in the pacing of pelvic thrusting during copulation was assessed in male rats by an accelerometric technique. Either strychnine, an antagonist of glycine (10 micrograms), bicuculline, an antagonist of GABA (1 microgram), or a combination of strychnine (5 micrograms) plus bicuculline (0.3 microgram), and saline as control, were administered intrathecally to sexually active males. Administration of the antagonists either alone or in combination, at these dose levels, produced sensory effects (skin hyperalgesia, scratching or biting the skin) in all rats. Generalized motor seizures occurred in only a few animals. The incidence of ejaculations, but not of mounts, tended to decrease after treatment with the amino acids antagonists. On the other hand, the values of the instantaneous frequency, duration, and rhythmicity of the copulatory thrusting movements performed during mounts, intromissions or ejaculations did not differ significantly from the values obtained under saline treatment. These findings indicate that the duration and rhythmicity of copulatory movements in the male rat are either controlled by synapses that are insensitive or inaccessible to strychnine and bicuculline, or these copulatory components are independent of glycinergic and GABAergic control and are under the control of other neurotransmitter systems.

Is substance P a primary afferent neurotransmitter for nociceptive input? III. Valproic acid and chlordiazepoxide decrease behaviors elicited by intrathecal injection of substance P and excitatory compounds

Intrathecal (i.t.) injections of substance P (SP) and kainic acid in rats produced rostrally directed scratches with the hindlimbs and caudally directed bites or licks. These behaviors, together with myoclonic twitches and vocalization, were also produced by I.T. morphine and strychnine. Intrathecal valproic acid (VA) significantly reduced all behaviors when these occurred spontaneously, and VA and chlordiazepoxide both reduced these behaviors when they were evoked by a light cotton swab tap to the lumbosacral region, in rats treated with the excitatory compounds. Since neither anticonvulsant affected the thermal or mechanical pain threshold at these doses, these results suggest that (a) the behaviors elicited by i.t. injection of the excitatory compounds are not responses to perceived pain, but rather the expression of a spinal convulsive-like state, and (b), since scratching and biting were the only behaviors produced by SP, this peptide is neither necessary nor sufficient for the elicitation of pain at the spinal level. Although our experiments do not rule out other roles for SP in pain processes such as that of a neuromodulator, it is unlikely that this compound is a traditional primary afferent neurotransmitter of pain.

Is substance P a primary afferent neurotransmitter for nociceptive input? II. Spinalization does not reduce and intrathecal morphine potentiates behavioral responses to substance P

Scratching elicited by the intrathecal (i.t.) administration of substance P (SP) into the lumbosacral spinal cord of rats was not reduced by spinalization or i.t. pretreatment with the analgesic morphine. Spinalization also did not affect scratching elicited by i.t. kainic acid and potentiated scratching elicited by i.t. strychnine, picrotoxin, and L-glutamic acid. Intrathecal morphine did, however, reduce scratching elicited by i.t. strychnine and kainic acid. These findings demonstrate that the scratching elicited by i.t. SP and other neuroexcitatory agents is a spinally mediated response. That this response, when elicited by SP, is not inhibited by the analgesic morphine strongly suggests that SP does not elicit scratching by action at the primary afferent synapse and that the response is not indicative of pain.

Is substance P a primary afferent neurotransmitter for nociceptive input? I. Analysis of pain-related behaviors resulting from intrathecal administration of substance P and 6 excitatory compounds

Intrathecal (i.t.) injection of substance P (SP), capsaicin, kainic acid, picrotoxin, strychnine, morphine, and L-glutamic acid in rats induced rhythmic scratching movements with the hindlimbs, biting, and, with some of these compounds, vocalization and myoclonic twitches. Although biting was directed to the dermatome corresponding to the injection site, scratching was aimed at anterior dermatomes. Presumably painful chemical stimulation produced by cutaneous and subcutaneous application of capsaicin or acetic acid never elicited scratching. Vocalization was never elicited by SP. When vocalization occurred following i.t. picrotoxin and morphine, it was correlated with myoclonic twitches rather than with scratching and/or biting. These findings indicate that scratching (a) is not pain-related and, (b) when elicited by the i.t. administration of the compounds listed above, does not result from activation of nociceptive primary afferent synapses.

Possible role of inhibitory glycinergic neurons in the regulation of lordosis behavior in the rat

Strychnine sulfate (3.9 or 27 micrograms in 0.5 microliter saline) was bilaterally infused into the ventromedial hypothalamic nucleus (VMH) of ovariectomized sexually inexperienced rats primed 40 hr earlier with 4 micrograms of estradiol benzoate (EB). This dose of EB induced only weak lordosis behavior in 25% of the subjects (Ss). Strychnine at the 3 and 9 micrograms dosages, but not at 27 micrograms, induced intense lordosis behavior, but no proceptivity, in most estrogen-primed Ss (69% in 3 micrograms, 94% in 9 micrograms). Ovariectomized adrenalectomized EB-primed Ss also displayed significant lordosis behavior (59%) following infusion of 9 micrograms of strychnine into the VMH. Strychnine (9 micrograms) failed to stimulate lordosis in ovariectomized Ss that were not estrogen-primed. Administration of 5 micrograms EB followed 40 hr later by 2 mg of progesterone (P) elicited intense lordosis behavior in most Ss. Bilateral injections into the VMH of glycine (100 micrograms), beta-alanine (100 micrograms) or taurine (50 micrograms) to rats that were already displaying estrous behavior (greater than 80 LQ) in response to the sequential administration of EB and P failed to depress lordosis when tested between 5 min and 60 min postinjection. Similarly, glycine (20 or 100 micrograms) injected into the VMH of estrogen-primed, ovariectomized rats within 15 minutes of a 2 mg SC injection of P failed to interfere with the subsequent response to this steroid when tested 2 and 4 hr after P. The results suggest that strychnine injected into the VMH facilitates lordosis behavior in estrogen-primed rats by removing a tonic inhibitor effect exerted by glycinergic neurons on VMH neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Prevention of the convulsant and hyperalgesic action of strychnine by intrathecal glycine and related amino acids

Intrathecal administration of 25 micrograms strychnine induced consistent sensory and motor behavioral events in rats. Sensory events included scratching and biting the lower half of the body, spontaneous vocalizations and skin hyperalgesia, evidenced by vocalization and reflex scratching in response to stimulation with a 5.5 g von Frey fiber. This mild stimulus failed to elicit vocalizations in the preinjection condition. Strychnine induced two types of motor seizures: (1) falling over with tail whipping and (2) convulsions. The effect of equimolar doses of glycine (G) and some related amino acids: beta-alanine (A), taurine (T) and betaine (B) on the strychnine syndrome was tested by administering them (intrathecal route) along with strychnine. T and G but not B significantly decreased most of the sensory events triggered by strychnine. All amino acids significantly decreased the incidence and duration of convulsions; T and B abolished them. A decreased vocalizations and skin hyperalgesia but synergized with strychnine to facilitate scratching and self biting. These results are consistent with findings that G, A and T displace strychnine from its binding sites in the CNS.

Nociceptive responses to altered GABAergic activity at the spinal cord

GABA agonists and antagonists were injected intrathecally at the spinal cord, to determine their effect on nociceptive thresholds. Tactile stimulation, applied against the flank by a medium diameter von Frey fiber (5.5 g force), elicited distress vocalizations after, but not before injection of the GABA antagonists, bicuculline MI or picrotoxin (0.25 and 1 microgram dosages). Vocalization threshold to tail shock was significantly reduced by bicuculline MI or picrotoxin. Tail flick withdrawal latency from radiant heat was not altered by GABA antagonists. The GABA agonist, muscimol, significantly elevated vocalization threshold to tail shock at a 5 micrograms dose. At a lower dose level (1 microgram), muscimol significantly reduced vocalization threshold to tail shock. Tail flick latency was significantly prolonged by the 5 micrograms dose of muscimol; however, flaccid paralysis of the hind limbs was also evident. Nociceptive thresholds were not altered by GABA or saline injection. These findings indicate that GABAergic activity contributes to the tonic modulation of nociception at the spinal cord.