Different roles of alpha 2-adrenoceptors of the medulla versus the spinal cord in modulation of mustard oil-induced central hyperalgesia in rats

Molecular interaction in the mouse PAG between NMDA and opioid receptors in morphine-induced acute thermal nociception

Previous evidence demonstrates that low dose morphine systemic administration induces acute thermal hyperalgesia in normal mice through microOR stimulation of the inositol signaling pathway. We investigated the site of action of morphine and the mechanism of action of microOR activation by morphine to NMDA receptor as it relates to acute thermal hyperalgesia. Our experiments show that acute thermal hyperalgesia is blocked in periaqueductal gray with the microOR antagonist CTOP, the NMDA antagonist MK801 and the protein kinase C inhibitor chelerythrine. Therefore, a site of action of systemically administered morphine low dose on acute thermal hyperalgesic response appears to be located at the periaqueductal gray. At this supraspinal site, microOR stimulation by systemically morphine low dose administration leads to an increased phosphorylation of specific subunit of NMDA receptor. Our experiments show that the phosphorylation of subunit 1 of NMDA receptor parallels the acute thermal hyperalgesia suggesting a role for this subunit in morphine-induced hyperalgesia. Protein kinase C appears to be the key element that links microOR activation by morphine administration to mice with the recruitment of the NMDA/glutamatergic system involved in the thermal hyperalgesic response.

Characterization of spinal alpha-adrenergic modulation of nociceptive transmission and hyperalgesia throughout postnatal development in rats

BACKGROUND AND PURPOSE

The selective alpha(2)-adrenergic agonist dexmedetomidine is used clinically for analgesia and sedation, but effects in early life are not well characterized. Investigation of age-related effects of dexmedetomidine is important for evaluating responses to exogenously administered analgesics and provides insight into postnatal function of noradrenergic pathways.

EXPERIMENTAL APPROACH

We examined effects of epidural dexmedetomidine in anaesthetized rat pups (3, 10 and 21 postnatal days) using a quantitative model of nociception and C-fibre induced hyperalgesia. Electromyographic recordings of withdrawal responses to hindpaw mechanical stimuli measured effects of dexmedetomidine upon the baseline reflex and the response to mustard oil application on the hindpaw (primary hyperalgesia) or hindlimb (secondary hyperalgesia). In addition, we compared epidural with systemic administration, examined effects of spinal transection and evaluated heart rate changes following dexmedetomidine.

KEY RESULTS

Epidural dexmedetomidine dose-dependently prevented mustard oil-induced hyperalgesia at all ages but dose requirements were lower in the youngest pups. Higher doses also suppressed the baseline nociceptive reflex when given epidurally, but had no effect when given systemically. Analgesic efficacy was the same for primary and secondary hyperalgesia, and was not diminished by spinal cord transection.

CONCLUSIONS AND IMPLICATIONS

Our laboratory studies predict that spinally mediated alpha(2)-agonist analgesia would be effective throughout postnatal development, dose requirements would be lower in early life and selective anti-hyperalgesic effects could be achieved with epidural administration at doses lower than associated with antinociceptive or cardiovascular effects. Clinical trials of alpha(2) agonists in neonates and infants should consider developmentally regulated changes.

Spinal and pontine α2-adrenoceptors have opposite effects on pain-related behavior in the neuropathic rat

Descending noradrenergic pathways contribute to feedback inhibition of pain by releasing norepinephrine in the spinal cord. Noradrenergic nuclei in the pons contain abundant alpha(2)-adrenoceptors. We assessed the contribution of pontine alpha(2)-adrenoceptors to endogenous regulation of pain in nerve-injured rats. Tactile allodynia and mechanical hyperalgesia were assessed in the injured dermatome and heat nociception in an uninjured dermatome. Atipamezole, an alpha(2)-adrenoceptor antagonist, or saline was administered systemically or microinjected into the locus coeruleus, the lateral parabrachial nucleus, the central nucleus of the amygdala, the midbrain periaqueductal gray, and/or through an intrathecal (i.t.) catheter to the spinal cord. Atipamezole administered systemically, into the amygdala or the periaqueductal gray had no significant effects on pain behavior. Atipamezole (0.3-5 microg) microinjected into the pons, the locus coeruleus or the lateral parabrachial nucleus, produced a selective and dose-related antiallodynia, which was reversed by i.t. administration of atipamezole (5 microg). I.t. administration of atipamezole alone (5 microg) produced thermal hypersensitivity in the non-neuropathic segment (tail) of nerve-injured animals. In sham-operated controls, i.t. administration of atipamezole had no effect. Suppression of heat nociception in uninjured dermatomes of nerve-injured but not the control animals following i.t. administration of atipamezole indicates that nerve injury produced a tonic activation of noradrenergic feedback inhibition acting on spinal alpha(2)-adrenoceptors. In parallel, antiallodynia induced by pontine administration of atipamezole indicates that nerve injury induces a tonic activation of pontine alpha(2)-adrenoceptors that promotes neuropathic hypersensitivity by attenuating descending inhibition. Thus, spinal and pontine alpha(2)-adrenoceptors have opposite effects on pain-related behavior in neuropathic animals.

Noradrenergic pain modulation

Norepinephrine is involved in intrinsic control of pain. Main sources of norepinephrine are sympathetic nerves peripherally and noradrenergic brainstem nuclei A1-A7 centrally. Peripheral norepinephrine has little influence on pain in healthy tissues, whereas in injured tissues it has variable effects, including aggravation of pain. Its peripheral pronociceptive effect has been associated with injury-induced expression of novel noradrenergic receptors, sprouting of sympathetic nerve fibers, and pronociceptive changes in the ionic channel properties of primary afferent nociceptors, while an interaction with the immune system may contribute in part to peripheral antinociception induced by norepinephrine. In the spinal cord, norepinephrine released from descending pathways suppresses pain by inhibitory action on alpha-2A-adrenoceptors on central terminals of primary afferent nociceptors (presynaptic inhibition), by direct alpha-2-adrenergic action on pain-relay neurons (postsynaptic inhibition), and by alpha-1-adrenoceptor-mediated activation of inhibitory interneurons. Additionally, alpha-2C-adrenoceptors on axon terminals of excitatory interneurons of the spinal dorsal horn possibly contribute to spinal control of pain. At supraspinal levels, the pain modulatory effect by norepinephrine and noradrenergic receptors has varied depending on many factors such as the supraspinal site, the type of the adrenoceptor, the duration of the pain and pathophysiological condition. While in baseline conditions the noradrenergic system may have little effect, sustained pain induces noradrenergic feedback inhibition of pain. Noradrenergic systems may also contribute to top-down control of pain, such as induced by a change in the behavioral state. Following injury or inflammation, the central as well as peripheral noradrenergic system is subject to various plastic changes that influence its antinociceptive efficacy.

Pharmacological properties, central nervous system effects, and potential therapeutic applications of atipamezole, a selective alpha2-adrenoceptor antagonist

Atipamezole is an alpha2-adrenoceptor antagonist with an imidazole structure. Receptor binding studies indicate that its affinity for alpha2-adrenoceptors and its alpha2/alpha1 selectivity ratio are considerably higher than those of yohimbine, the prototype alpha2-adrenoceptor antagonist. Atipamezole is not selective for subtypes of alpha2-adrenoceptors. Unlike many other alpha2-adrenoceptor antagonists, it has negligible affinity for 5-HT1A and I2 binding sites. Atipamezole is rapidly absorbed and distributed from the periphery to the central nervous system. In humans, atipamezole at doses up to 30 mg/subject produced no cardiovascular or subjective side effects, while at a high dose (100 mg/subject) it produced subjective symptoms, such as motor restlessness, and an increase in blood pressure. Atipamezole rapidly reverses sedation/anesthesia induced by alpha2-adrenoceptor agonists. Due to this property, atipamezole is commonly used by veterinarians to awaken animals from sedation/anesthesia induced by alpha2-adrenoceptor agonists alone or in combination with various anesthetics. Atipamezole increased sexual activity in rats and monkeys. In animals with sustained nociception, atipamezole increased pain-related responses by blocking the noradrenergic feedback inhibition of pain. In tests assessing cognitive functions, atipamezole at low doses has beneficial effects on alertness, selective attention, planning, learning, and recall in experimental animals, but not necessarily on short-term working memory. At higher doses atipamezole impaired performance in tests of cognitive functions, probably due to noradrenergic overactivity. Recent experimental animal studies suggest that atipamezole might have beneficial effects in the recovery from brain damage and might potentiate the anti-Parkinsonian effects of dopaminergic drugs. In phase I studies atipamezole has been well tolerated by human subjects.

Role for Medullary Pain Facilitating Neurons in Secondary Thermal Hyperalgesia

The rostral ventromedial medulla (RVM) has recently received considerable attention in efforts to understand mechanisms of hyperalgesia and persistent pain states. Three classes of neurons can be identified in the RVM based on responses associated with nocifensive reflexes: on cells, off cells, and neutral cells. There is now direct evidence that on cells exert a net facilitating effect on spinal nociception and that off cells depress nociception. These experiments tested whether the secondary hyperalgesia produced by topical application of mustard oil involves an activation of on cells in RVM. Firing of a characterized RVM neuron and the latencies of withdrawal reflexes evoked by noxious heat were recorded in lightly anesthetized rats before and after application of mustard oil to the shaved skin of the leg above the knee. Mineral oil was applied as a control. Mustard oil produced a significant increase in ongoing and reflex-related discharge of on cells, as well as a decrease in the activity of off cells. neutral cell firing was uniformly unchanged after application of mustard oil. The alterations in on and off cell firing were associated with a significant decrease in the latency to withdraw the paw of the treated limb from the heat stimulus, and this hyperalgesia was blocked by microinjection of lidocaine within the RVM. Withdrawals evoked by heating the contralateral hindpaw, forepaw, and tail were unchanged after mustard oil application. These experiments support a pronociceptive role for on cells and suggest that these neurons contribute to secondary hyperalgesia in inflammation.

The influence of chemical sympathectomy on pain responsivity and alpha 2-adrenergic antinociception in neuropathic animals

We studied the effect of chemical sympathectomy by 6-hydroxydopamine (6-OHDA) on pain behavior and alpha(2)-adrenergic antinociception in rats with a spinal nerve ligation-induced neuropathy. For assessment of alpha(2)-adrenergic antinociception, the rats were treated systemically with two alpha(2)-adrenoceptor agonists, one of which only poorly (MPV-2426) and the other very well (dexmedetomidine) penetrates the blood-brain barrier. Moreover, the effect of MPV-2426 on spontaneous activity of dorsal root nerve fibers proximal to the nerve injury was determined. Systemic treatment with 6-OHDA produced a marked decrease in immunocytochemical labeling of sympathetic nerve fibers in the skin but it produced no marked change in basal pain sensitivity to mechanical stimulation either in neuropathic or sham-operated animals. Systemic administration of MPV-2426 and dexmedetomidine produced a dose-dependent tactile antiallodynic effect in neuropathic animals. Intraplantar injection of MPV-2426 had an identical antiallodynic effect independent of whether it was injected into the neuropathic or contralateral hindpaw. In a test of mechanical nociception and hyperalgesia, dexmedetomidine markedly attenuated pain responses in all experimental groups, whereas MPV-2426 had a weak but significant pain attenuating effect only in neuropathic animals. In the tail flick test, both alpha(2)-adrenoceptor agonists had a significant antinociceptive effect. The pain attenuating effect of MPV-2426 was enhanced by pretreatment with 6-OHDA, except in a test of tactile allodynia. MPV-2426-induced modulation of spontaneous activity was not a general property of dorsal root fibers proximal to the injury. The results indicate that a chemical destruction of sympathetic postganglionic nerve fibers innervating the skin does not markedly influence cutaneous pain sensitivity nor is it critical for the alpha(2)-adrenoceptor agonist-induced attenuation of pain behavior in neuropathic or non-neuropathic animals. Chemical sympathectomy, independent of neuropathy, enhanced the pain attenuating effect by MPV-2426, probably due to a peripheral action, whereas in non-sympathectomized control and neuropathic animals peripheral mechanisms have only a minor, if any, role in the alpha(2)-adrenoceptor agonist-induced antinociception.

Descending control of pain

Upon receipt in the dorsal horn (DH) of the spinal cord, nociceptive (pain-signalling) information from the viscera, skin and other organs is subject to extensive processing by a diversity of mechanisms, certain of which enhance, and certain of which inhibit, its transfer to higher centres. In this regard, a network of descending pathways projecting from cerebral structures to the DH plays a complex and crucial role. Specific centrifugal pathways either suppress (descending inhibition) or potentiate (descending facilitation) passage of nociceptive messages to the brain. Engagement of descending inhibition by the opioid analgesic, morphine, fulfils an important role in its pain-relieving properties, while induction of analgesia by the adrenergic agonist, clonidine, reflects actions at alpha(2)-adrenoceptors (alpha(2)-ARs) in the DH normally recruited by descending pathways. However, opioids and adrenergic agents exploit but a tiny fraction of the vast panoply of mechanisms now known to be involved in the induction and/or expression of descending controls. For example, no drug interfering with descending facilitation is currently available for clinical use. The present review focuses on: (1) the organisation of descending pathways and their pathophysiological significance; (2) the role of individual transmitters and specific receptor types in the modulation and expression of mechanisms of descending inhibition and facilitation and (3) the advantages and limitations of established and innovative analgesic strategies which act by manipulation of descending controls. Knowledge of descending pathways has increased exponentially in recent years, so this is an opportune moment to survey their operation and therapeutic relevance to the improved management of pain.

Spinal transection increases the potency of clonidine on the tail-flick and hindlimb flexion reflexes

The effect of intrathecal clonidine on thermal nociception and hindlimb flexion was assessed in acute and chronic spinally transected rats. After an acute, 1-day spinalization, there was no change in the antinociceptive dose-response function to clonidine, relative to intact rats. However, there was a significant increase in potency 31 days after spinalization. Low doses of clonidine (0.25, 1, 4 and 20 microg) did not affect the nonnociceptive flexion reflex of acute spinal rats, but they elicited a dose-dependent response in chronic spinal rats. These data provide behavioral evidence of supersensitivity to alpha-adrenoceptor agonists in chronic spinal rats.

The dissociation of sedative from spinal antinociceptive effects following administration of a novel alpha-2-adrenoceptor agonist, MPV-2426, in the locus coeruleus in the rat

BACKGROUND

MPV-2426 is a novel alpha-2-adrenoceptor agonist developed for spinal pain therapy. In the present study we characterized its sedative and antinociceptive properties following microinjections into the brainstem and intrathecally at the lumbar spinal cord level.

METHODS

Sedative effects of MPV-2426 were assessed in a locomotion measuring device following unilateral microinjection into the locus coeruleus (LC) of the brainstem or 1-2 mm rostral to the LC in rats. Antinociceptive effects induced by MPV-2426 in the brainstem, and for comparison intrathecally at the lumbar spinal cord level, were determined with a tail-flick test. Dexmedetomidine was used as the reference alpha-2-adrenoceptor agonist.

RESULTS

MPV-2426 produced a dose-related hypolocomotive/sedative effect, which was significantly stronger following microinjection into the LC than 1-2 mm rostral to the LC. The sedation induced by MPV-2426 was reversed by atipamezole (1 mg/kg s.c.), an alpha-2-adrenoceptor antagonist. The sedative potency of dexmedetomidine, the reference alpha-2-adrenoceptor agonist, was stronger and less dependent on the exact injection site in the brainstem. Following microinjections at sedative doses in the brainstem, only dexmedetomidine produced a significant antinociceptive effect in the tail-flick test. When microinjected into the lumbar spinal cord, MPV-2426 and dexmedetomidine had an equally strong antinociceptive effect in the tail-flick test.

CONCLUSION

The results indicate that the sedative potency of MPV-2426 is considerably weaker than that of dexmedetomidine. Additionally, the spread of MPV-2426 within the central nervous system is more limited than that of dexmedetomidine. This could explain why MPV-2426 is sedative only when injected into the LC while antinociceptive effect is obtained when it is injected intrathecally at the lumbar spinal cord level.

A neuronal correlate of secondary hyperalgesia in the rat spinal dorsal horn is submodality selective and facilitated by supraspinal influence

Tissue injury produces hyperalgesia not only in the injured area (primary hyperalgesia) but also outside of it (secondary hyperalgesia). In the present investigation, the submodality selectivity and the contribution of supraspinal influence to a neural correlate of the secondary hyperalgesia induced by neurogenic inflammation was studied in the presumed pain relay neurons of the rat spinal dorsal horn. Mechanically and thermally evoked responses to wide-dynamic range (WDR) neurons of the spinal dorsal horn were recorded under sodium pentobarbital anesthesia in rats. Neurogenic inflammation was induced by application of mustard oil outside of the receptive fields of WDR neurons. To study the contribution of supraspinal influence to mustard oil-induced changes in neuronal responses, the spinal cord was transected at a midthoracic level or lidocaine was microinjected into the rostroventromedial medulla (RVM). Furthermore, the antidromically evoked compound volley in the sural nerve was determined to reveal excitability changes in the central terminals of primary afferent A-fibers induced by mustard oil. The results indicate that mustard oil adjacent to the receptive fields of spinal WDR neurons significantly enhanced their responses to mechanical but not to noxious heat stimuli, without a significant influence on their spontaneous activity. Both high- and low-threshold mechanoreceptive input to WDR neurons was equally facilitated, whereas mechanoreceptive input to spinal dorsal horn neurons mediating innocuous messages (low-threshold mechanoreceptive neurons) was not changed. Mustard oil in a remote site (forepaw) did not produce any hyperexcitability to responses evoked by hindpaw stimulation. Spinal transection or lidocaine block of the RVM significantly attenuated the mustard oil-induced mechanical hyperexcitability in spinal dorsal horn neurons. Mustard oil had no significant effect on a compound volley in the sural nerve induced by intraspinal stimulation of sural nerve terminals at a submaximal intensity. The selective mechanical hyperexcitability in spinal WDR neurons, without a change in their spontaneous activity, can be explained by a heterosynaptic facilitatory action on presynaptic terminals mediating mechanical signals to these nociceptive spinal neurons. These findings indicate that brain stem-spinal pathways, involving the RVM, do not only suppress nociception but under some pathophysiological conditions concurrent facilitatory influence may predominate and lead to enhancement of mechanical hyperexcitability. The descending facilitatory feed-back loop to nociceptive spinal neurons may help to protect the wounded tissue and thus promote healing.

Peripherally administered alpha2-adrenoceptor agonist in the modulation of chronic allodynia induced by spinal nerve ligation in the rat

We studied whether a peripherally administered alpha2-adrenoceptor agonist modulates mechanical allodynia caused by unilateral ligation of two spinal nerves in the rat. Medetomidine, an alpha2-adrenoceptor agonist, atipamezole, an alpha2-adrenoceptor antagonist, or saline (control) was administered into the footpad of either the allodynic or the contralateral hindpaw. Medetomidine (1-10 microg/kg in 50 microL) reversed the unilateral allodynia in a dose-dependent fashion independent of the site of administration. At this dose range, medetomidine did not influence the heat-induced tail-flick response. The antiallodynic effect of medetomidine was completely reversed by a dose of atipamezole that alone was ineffective (30 microg/kg). At the largest dose used (100 microg/kg in 50 microL), atipamezole decreased the latency of the heat-induced tail flick and had an ipsilateral allodynic effect in the injected paw when administered into the control side. The atipamezole-induced mechanical allodynia was not attenuated by medetomidine. The results indicate that an alpha2-adrenoceptor agonist at a subantinociceptive dose may significantly attenuate allodynia produced by spinal nerve ligation. This antiallodynic effect is not due to a peripheral action, but rather to action on central (presumably spinal) alpha2-adrenoceptors. The allodynic effect of a high concentration of atipamezole (100 microg/kg in 50 microL) in the injected paw can be explained by peripheral nonadrenergic mechanisms (e.g., local irritation).

IMPLICATIONS

The present behavioral results indicate that a selective alpha2-adrenoceptor agonist at a subantinociceptive dose effectively attenuates mechanical allodynia induced by an experimental model of chronic neuropathy in the rat. This antiallodynic action can be explained by central, rather than peripheral, alpha2-adrenergic mechanisms.

Chronic spinal nerve ligation induces changes in response characteristics of nociceptive spinal dorsal horn neurons and in their descending regulation originating in the periaqueductal gray in the rat

We studied whether a chronic neuropathy induced by unilateral spinal nerve ligation changes the response characteristics of spinal dorsal horn wide-dynamic range (WDR) neurons or their periaqueductal gray (PAG)-induced descending modulation. Experiments were performed in rats with behaviorally demonstrated allodynia induced by spinal nerve ligation and in a group of nonneuropathic control rats. The stimulus-response functions of WDR neurons for mechanical and thermal stimuli and the modulation of their peripherally evoked responses by electrical stimulation of the PAG were determined under pentobarbital anesthesia. The results showed that neuropathy caused a significant leftward shift in stimulus-response functions for mechanical stimuli. In contrast, stimulus-response functions for noxious heat stimuli in the neuropathic limb were, if anything, shifted rightward, although this shift was short of statistical significance. In neuropathic rats, PAG stimulation produced a significantly stronger attenuation of spinal neuronal responses induced by noxious heat in the unoperated than in the operated side. At the intensity that produced attenuation of noxious heat stimuli, PAG stimulation did not produce any significant change in spinal neuronal responses evoked by mechanical stimuli either from the operated or the nonoperated hindlimb of the neuropathic rats. Spontaneous activity of WDR neurons was higher in the operated side of neuropathic rats than in control rats. Afterdischarges evoked by peripheral stimuli were observed in 1/16 of the WDR neurons ipsilateral to spinal nerve ligation and not at all in other experimental groups. The WDR neurons studied were not activated by innocuous or noxious cold stimuli. The results indicate that spinal nerve ligation induces increased spontaneous activity and enhanced responses to mechanical stimuli in the spinal dorsal horn WDR neurons, whereas noxious heat-evoked responses are not significantly changed or if anything, attenuated. Moreover, the inhibition of noxious heat stimuli by PAG stimulation is attenuated in the neuropathic side. It is proposed that the observed changes in the response characteristics of the spinal dorsal horn WDR neurons and in their descending modulation may contribute to the neuropathic symptoms in these animals.

Supraspinal influence on hindlimb withdrawal thresholds and mustard oil-induced secondary allodynia in rats

We examined the role of supraspinal structures in secondary allodynia induced by mustard oil in awake rats. To produce allodynia (=unpleasent sensation evoked by innocuous stimuli), mustard oil (50%) was applied for 2 min to the skin of the ankle of one hindlimb. Mechanical hypersensitivity of the skin was tested by determining the hindlimb withdrawal threshold to a series of monofilaments applied to the glabrous foot pad (=distal to the mustard oil-treated ankle). In intact rats, mustard oil produced a secondary allodynia in the mustard oil-treated hindlimb as indicated by a decreased withdrawal threshold to mechanical test stimuli applied to the glabrous skin (=outside the mustard oil-treated ankle), whereas the withdrawal threshold in the contralateral (=control) hindlimb was not changed. Following spinalization, mustard oil treatment produced no secondary allodynia, but the interpretation of this finding was complicated by a concomitant bilateral elevation of hindlimb withdrawal thresholds to mechanical skin stimulation. However, the spinalized rats had shorter tail-flick latencies to radiant heat than intact rats. Administration of an opioid antagonist, naloxone (1 mg/kg, SC), had no effect on withdrawal thresholds in spinalized animals. Importantly, microinjection of lidocaine (4%) into the nucleus raphe magnus in rats with an intact spinal cord had a selective antiallodynic effect when the injection volume was 1.0 microl but not when it was 0.5 microl. Lidocaine (4%, 0.5 microl) in the lateral reticular nucleus of the medulla also attenuated the spinal hypersensitivity, however, concomitantly with motor side effects, due to which this finding maybe artificial. It is concluded that brain stem spinal pathways, originating adjacent to but not within the raphe magnus, contribute to the behavioral expression of secondary allodynia induced by neurogenic inflammation of the skin. Furthermore, there is a differential tonic control of various spinal reflexes by the brain stem as indicated by the dissociative effects of spinalization on mechanically induced hindlimb withdrawal vs. heat-induced tail-flick reflex.

Lidocaine in the rostroventromedial medulla and the periaqueductal gray attenuates allodynia in neuropathic rats

In the present study we attempted to find out if the rostroventromedial medulla (RVM) or the periaqueductal gray (PAG) might contribute to chronic allodynia induced by unilateral ligation of two spinal nerves in the rat. Lidocaine was microinjected in the RVM or PAG and allodynia was quantitatively determined by measuring the hindlimb withdrawal thresholds to mechanical stimulation of the paw. For comparison, lidocaine was also injected systemically (s.c.). Lidocaine in the RVM produced a dose-related (20 and 40 micrograms) antiallodynic effect. Lidocaine (20 micrograms) in the PAG produced identical antiallodynic effect as in the RVM. With systemic administrations of lidocaine, a considerably higher dose (> > 40 micrograms) was needed to produce a significant antiallodynic effect. Naloxone, an opioid-antagonist (1 mg/kg s.c.), did not attenuate the antiallodynic effect of lidocaine in the RVM. An antiallodynic dose of lidocaine (20 micrograms) in the RVM or the PAG did not influence the withdrawal response in the unoperated hindlimb nor the heat-induced tail-flick reflex. The results indicate that the RVM and the PAG have a facilitatory influence on the spinal segmental mechanisms underlying chronic allodynia. The selective attenuation of allodynia induced by lidocaine in the RVM and the PAG is independent of opiate receptors, and it can not be explained by a systemic spread of the drug.