Enhanced descending modulation of nociception in rats with persistent hindpaw inflammation

Induction of long-term potentiation of single wide dynamic range neurones in the dorsal horn is inhibited by descending pathways

Previous studies have shown that long-term potentiation (LTP) in the dorsal horn may be induced by noxious stimuli. In this study it is investigated whether induction of LTP in the dorsal horn may be affected by the descending pathways. Extracellular recordings of wide dynamic range (WDR) neurones in the lumbar dorsal horn in intact urethane-anaesthetized Sprague--Dawley rats were performed, and the electrically evoked neuronal responses in these neurones were defined as A-fibre and C-fibre responses according to latencies. Using a short-term cold block of the thoracic spinal cord, which produced a completely reversible increase of the A-fibre and C-fibre responses, the influence of the descending inhibitory system on the induction of LTP by electrical high-frequency conditioning applied to the sciatic nerve was examined. As previously shown the A-fibre responses were almost unchanged following the conditioning. In contrast, the C-fibre responses following the same conditioning were strongly increased. Thus, a clear LTP of the nociceptive transmission in the dorsal horn was observed following electrical high-frequency conditioning. Interestingly, we found that the LTP was more powerful when the effects of the descending pathways were temporarily eliminated during conditioning. It is concluded that induction of LTP by electrical high-frequency conditioning stimulation, which may be part of the wider term central sensitization, is inhibited by descending pathways.

The pain of being sick: implications of immune-to-brain communication for understanding pain

This review focuses on the powerful pain facilitatory effects produced by the immune system. Immune cells, activated in response to infection, inflammation, or trauma, release proteins called proinflammatory cytokines. These proinflammatory cytokines signal the central nervous system, thereby creating exaggerated pain as well as an entire constellation of physiological, behavioral, and hormonal changes. These changes are collectively referred to as the sickness response. Release of proinflammatory cytokines by immune cells in the body leads, in turn, to release of proinflammatory cytokines by glia within the brain and spinal cord. Evidence is reviewed supporting the idea that proinflammatory cytokines exert powerful pain facilitatory effects following their release in the body, in the brain, and in the spinal cord. Such exaggerated pain states naturally occur in situations involving infection, inflammation, or trauma of the skin, of peripheral nerves, and of the central nervous system itself. Implications for human pain conditions are discussed.

Stage-dependent changes in the modulation of spinal nociceptive neuronal activity during the course of inflammation

Spinal and supraspinal controls can tonically or phasically modulate the output of spinal nociceptive neurons. Alterations of these modulatory systems have been described during the acute stage of inflammation. In the present study in the rat, tonic descending controls were assessed during acute (24--48 h) and chronic (3--4 weeks) stages of monoarthritis of the ankle. The electrophysiological properties of spinal convergent neurons with ankle input were compared before and after spinalization. In a parallel series of experiments, spinal convergent neurons were recorded from the normal side in order to assess the propriospinal and supraspinal inhibitory controls triggered by nociceptive stimulation of the inflamed ankle. Tonic descending inhibition of convergent neurons with input from the inflamed ankle was enhanced during the acute stage and then decreased during the chronic stage of monoarthritis. Contralateral-induced inhibitions exhibited a similar temporal evolution. Time-dependent changes in the spinal transmission of nociceptive signals were shown by removing descending modulation in animals with monoarthritis; sensitization of spinal neurons with input from the inflamed ankle was demonstrated during the acute stage of monoarthritis, whereas a crossed transmission between inflamed and normal sides was observed during the chronic stage of the disease. These results show that dynamic and stage-dependent modifications of descending controls tend to dampen the central changes associated with inflammation.

The release of serotonin in rat spinal dorsal horn and periaqueductal gray following carrageenan inflammation

The level of serotonin (5-HT) and its major metabolite 5-hydroxyindoleacetic acid (5-HIAA) in dorsal horn of spinal cord and periaqueductal gray (PAG) were measured using in vivo microdialysis coupled with high performance liquid chromatography and electrochemical detection. Intraplantar injection of carrageenan was used to evoke release. Extracellular concentrations of 5-HT and 5-HIAA in spinal dorsal horn and PAG significantly increased following carrageenan inflammation. The peak occurred at 3h and then gradually returned to baseline at 5-7h. The carrageenan-induced release of 5-HT and 5-HIAA in spinal cord rather than in PAG was decreased by intracerebroventricular (i.c.v.) injection of naloxone, and both in spinal dorsal horn and PAG, the release was increased by i.c.v. injection of bicuculine. These results suggest that activity of 5-HT in spinal cord and PAG increases with carrageenan inflammation; the carrageenan-induced release of 5-HT and 5-HIAA in spinal cord may be tonically modulated by supraspinal opioid and GABA systems, whereas the release in PAG may only be tonically modulated by endogenous GABA system in supraspinal level.

Antihyperalgesia effect of BmK IT2, a depressant insect-selective scorpion toxin in rat by peripheral administration

The study was undertaken to assess the antihyperalgesia effect of BmK IT2, a sodium channel-specific ligand purified from the venom of Chinese scorpion Buthus martensi Karsch in rat by peripheral injection. The peripheral inflammation of rat was induced by carrageenan resulted in hyperalgesia to heat stimulus. The heat hyperalgesia was measured by paw withdrawal latency (PWL). PWL was increased to 272 +/- 18%, 217 +/- 19% and 186 +/- 16% of the control by application of 10 microl BmK IT2 at the concentration of 0.1, 0. 01 and 0.001 mg/ml in inflammatory rats, respectively. In contrast, PWL was enhanced to about 177 +/- 16%, 141 +/- 15% and 133 +/- 15% of the control at the same applied concentrations of BmK IT2 in normal rats. The results thus suggest that BmK IT2 can produce peripheral antihyperalgesia and antinociception, which might be attributed a pathway of modulating the sodium channels on nociceptor.

Activity-induced plasticity in brain stem pain modulatory circuitry after inflammation

Brain stem descending pathways modulate spinal nociceptive transmission. In a lightly anesthetized rat preparation, we present evidence that such descending modulation undergoes time-dependent changes following persistent hindpaw inflammation. There was an initial decrease and a subsequent increase in the excitability of neurons in the rostral ventromedial medulla (RVM) involving facilitation and inhibition. These changes were most robust after stimulation of the inflamed paw although similar findings were seen on the non-inflamed paw and tail. The enhanced descending modulation appeared to be mediated by changes in the activation of the NMDA excitatory amino acid receptor. These findings demonstrate the dynamic plasticity of the pain modulating pathways in response to persistent tissue injury.

A behavioral test paradigm to measure the aversive quality of inflammatory and neuropathic pain in rats

The present experiment assessed the aversive quality of neuropathic and inflammatory pain in rats. Compared to sham-treated animals, L5 ligated (neuropathic) and complete Freund's adjuvant (inflammatory)-treated animals displayed an initial period of escape followed by avoidance of a preferred location of the test chamber that was associated with mechanical stimulation of the hyperalgesic paw. The onset of the avoidance behavior occurred during the first 10-15 min of behavioral testing and was maximal at 30 min. It is concluded that animals find mechanical stimulation of the hyperalgesic paw aversive and that this behavioral test paradigm is an additional method that may be used to assess nociception in rat neuropathic and inflammatory models.

Antihyperalgesic effects of delta opioid agonists in a rat model of chronic inflammation

Opioid receptors in the brain activate descending pain pathways to inhibit the nociceptive response to acute noxious stimuli. The aim of the present study was to clarify the role of supraspinal opioid receptors in modulating the nociceptive response to persistent inflammation in rats. Subcutaneous administration of 50 microl of complete Freund's Adjuvant (CFA) into the plantar surface of the hindpaw induced a significant decrease in paw withdrawal latency to thermal stimuli (P<0.01) at 24 h post-injection. Intracerebroventricular (i.c.v.) administration of the mu opioid receptor agonists, DAMGO and morphine, and the delta opioid receptor agonists, deltorphin II and SNC80, significantly reversed the hyperalgesic response associated with peripheral inflammation in a dose-dependent manner (P<0.0001). The mu and delta agonists also significantly attenuated the antinociceptive response to acute thermal stimulation in rats (P<0.001). However, deltorphin II and SNC80 were less potent, and in the case of SNC80 less efficacious, in modulating the response to acute thermal nociception in comparison to hyperalgesia associated with persistent inflammation. These results indicate that mu and delta opioid receptors in the brain modulate descending pain pathways to attenuate the nociceptive response to acute thermal stimuli in both normal and inflamed tissues. The heightened response to delta agonists in the hyperalgesia model suggests that delta opioid receptors in the brain are promising targets for the treatment of pain arising from chronic inflammation.

The analgesic effects of supraspinal mu and delta opioid receptor agonists are potentiated during persistent inflammation

This study examined the antihyperalgesic and antinociceptive effects of opioid receptor agonists microinjected in the rostral ventromedial medulla (RVM) of rats 4 hr, 4 d, and 2 weeks after the induction of an inflammatory injury by injection of complete Freund's adjuvant (CFA) in one hindpaw. Nociceptive sensitivity of the ipsilateral, inflamed and the contralateral, uninflamed hindpaws was determined by the radiant-heat paw withdrawal test. The antihyperalgesic potency of the mu opioid receptor agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO), determined for the inflamed hindpaw, was enhanced 4 d and 2 weeks after injury. The antinociceptive potency of DAMGO, determined for the contralateral, uninflamed hindpaw, was also progressively enhanced 4 hr, 4 d, and 2 weeks after injury. The magnitude of enhancement paralleled the chronicity of the injury. The greatest potentiation occurred 2 weeks after injury when the ED(50) value of DAMGO in CFA-treated rats was one-tenth that in saline-treated rats. The antihyperalgesic and antinociceptive effects of the delta opioid receptor agonist [D-Ala(2),Glu(4)]deltorphin were also increased 2 weeks after injury. These results indicate that peripheral inflammatory injury alters the pharmacology of excitatory and inhibitory inputs that modulate the activity of RVM neurons in such a manner as to enhance the effects of opioid agonists in this region. These changes have ramifications not only for the alleviation of hyperalgesia at the site of injury but also for opioid-induced antinociception at sites remote to the injury as revealed by increases in the potency of opioid agonists to suppress nociceptive responses of the contralateral, uninflamed hindpaw.

Gestational and ovarian sex steroid antinociception: relevance of uterine afferent and spinal alpha(2)-noradrenergic activity

Pregnancy is associated with an antinociception that is multifactorial and results from spinal (kappa/delta) opioid antinociceptive pathways as well as peripheral processes (ovarian sex steroids, uterine afferent neurotransmission). The present results provide the first indication that the full manifestation of pregnancy-induced analgesia also requires a supraspinal component. The analgesia of gestation or its hormonal simulation (via estrogen and progesterone administration; HSP) is substantially attenuated (>/=60%) following blockade of spinal alpha(2) (but not alpha(1)) adrenergic receptors. HSP antinociception is also attenuated by transection of the hypogastric nerve, the magnitude of which is indistinguishable from that produced by spinal alpha(2) receptor blockade. Additionally, hypogastric neurectomy abolishes the component of the antinociception associated with HSP that is mediated by spinal alpha(2) receptors. This suggests that the augmented spinal noradrenergic activity during HSP is not due to activation at the terminal of noradrenergic spinal projection neurons but requires supraspinal activity. It is suggested that enhanced spinal noradrenergic activity amplifies ongoing spinal kappa/delta antinociception as has been observed following the concomitant intrathecal application of alpha(2) and opioid agonists. The current observations underscore the importance of visceral afferent activity as well as its modulation by a female-specific hormonal milieu to the efficacy of endogenous spinal opioid antinociception.

Persistent Fos protein expression after orofacial deep or cutaneous tissue inflammation in rats: implications for persistent orofacial pain

This study was designed to systematically examine the effects of persistent orofacial tissue injury on prolonged neuronal activation in the trigeminal nociceptive pathways by directly comparing the effects of orofacial deep vs. cutaneous tissue inflammation on brainstem Fos protein expression, a marker of neuronal activation. Complete Freund's adjuvant (CFA) was injected unilaterally into the rat temporomandibular joint (TMJ) or perioral (PO) skin to produce inflammation in deep or cutaneous tissues, respectively. Rats were perfused 2 hours, 24 hours, 3 days, or 10 days following CFA injection. The TMJ and PO inflammation-induced Fos expression paralleled the intensity and course of inflammation over the 10-day observation period, suggesting that the increase in intensities and persistence of Fos protein expression may be associated with a maintained increase in peripheral input. Compared to PO CFA injection, the injection of CFA into the TMJ produced a significantly stronger inflammation associated with a greater Fos expression. In TMJ- but not in PO-inflamed rats, Fos-like immunoreactivity (LI) spread from superficial to deep upper cervical dorsal horn as the inflammation persisted and there was a dominant ipsilateral Fos-labeling in the paratrigeminal nucleus. Common to TMJ and PO inflammation, Fos-LI was induced in the trigeminal subnuclei interpolaris and caudalis, C1-2 dorsal horn, and other medullary nuclei. Substantial bilateral Fos-LI was found in the interpolaris-caudalis trigeminal transition zone. Further analysis revealed that Fos-LI in the ventral transition zone was equivalent bilaterally, whereas Fos-LI in the dorsal transition zone was predominantly ipsilateral to the inflammation. The differential induction of Fos expression suggests that an increase in TMJ C-fiber input after inflammation and robust central neuronal hyperexcitability contribute to persistent pain associated with temporomandibular disorders.

Endogenous mechanisms of sensory modulation

We provide evidence supporting the idea that the relationship between tissue damage, or the threat of tissue damage, and the response to such stimuli is variant and dependent on neuronal networks by which attentional, emotional and cognitive components of pain experience activate endogenous descending modulatory systems. Most previous studies have focused on responses to transient noxious stimuli with little information on the influence of descending modulation on behavioral responses to persistent pain and hyperalgesia after tissue or nerve injury. Utilizing correlative behavioral and neuronal studies we have demonstrated that (1) behavioral context modulates neuronal activity in nociceptive and non-nociceptive somatosensory pathways, supporting the hypothesis that responses in these pathways are not immutable; (2) descending modulation influences behavior and neuronal activity at spinal cord levels after inflammation and persistent pain; and (3) there are descending facilitatory as well as inhibitory influences on behavior and spinal cord neuronal activity that may impact on persistent pain particularly of deep muscle and visceral origin. Cortical as well as subcortical pathways are available by which dorsal horn activity can be modulated by attentional, motivational and cognitive factors. It appears that the same neuronal mechanisms in the forebrain and brain stem are available for behavioral modulation in a learned task involving the threat of tissue damage (transient noxious stimuli) as are available in the development and amplification of persistent pain produced by inflammation. These parallel brain mechanisms emphasize the saliency of pain experience as an important learned behavior for the survival of the organism, similar to sequential goal-directed behaviors in an operant task.

Implications of recent advances in the understanding of pain pathophysiology for the assessment of pain in patients

As we approach the new millennium, it is clear that we are on the brink of a major change in clinical pain management. We are poised to move from a treatment paradigm that has been almost entirely empirical to one that will be derived from an understanding of the actual mechanisms involved in the pathogenesis of pain. When this is achieved, pain treatment will at last be rationally based. The implications of this are immense and will necessitate major changes in the way we classify pain, which until now has been based on disease, duration and anatomy, to a mechanism-based classification. In addition, the assessment, diagnosis and treatment of pain will change. The aim in the future will be to identify in individual patients what mechanisms are responsible for their pain and to target treatment specifically at those mechanisms. We present for discussion, a new approach for classifying pain, based on an analysis of mechanisms, and show how this could be used to assess pain clinically. Such kinds of pain assessment, which need to be designed to reveal as much as possible about mechanisms, are necessary for more sophisticated epidemiology and clinical research as well as for providing the outcome measures necessary for the evaluation of the efficacy of new treatments targeted at particular pain mechanisms.

Supraspinal contributions to hyperalgesia

Tissue injury is associated with sensitization of nociceptors and subsequent changes in the excitability of central (spinal) neurons, termed central sensitization. Nociceptor sensitization and central sensitization are considered to underlie, respectively, development of primary hyperalgesia and secondary hyperalgesia. Because central sensitization is considered to reflect plasticity at spinal synapses, the spinal cord has been the principal focus of studies of mechanisms of hyperalgesia. Not surprisingly, glutamate, acting at a spinal N-methyl-D-aspartate (NMDA) receptor, has been implicated in development of secondary hyperalgesia associated with somatic, neural, and visceral structures. Downstream of NMDA receptor activation, spinal nitric oxide (NO.), protein kinase C, and other mediators have been implicated in maintaining such hyperalgesia. Accumulating evidence, however, reveals a significant contribution of supraspinal influences to development and maintenance of hyperalgesia. Spinal cord transection prevents development of secondary, but not primary, mechanical and/or thermal hyperalgesia after topical mustard oil application, carrageenan inflammation, or nerve-root ligation. Similarly, inactivation of the rostral ventromedial medulla (RVM) attenuates hyperalgesia and central sensitization in several models of persistent pain. Inhibition of medullary NMDA receptors or NO. generation attenuates somatic and visceral hyperalgesia. In support, topical mustard oil application or colonic inflammation increases expression of NO. synthase in the RVM. These data suggest a prominent role for the RVM in mediating the sensitization of spinal neurons and development of secondary hyperalgesia. Results to date suggest that peripheral injury and persistent input engage spinobulbospinal mechanisms that may be the prepotent contributors to central sensitization and development of secondary hyperalgesia.

Recording of long-term potentiation in single dorsal horn neurons in vivo in the rat

We have published several reports on long-term potentiation (LTP) in single spinal wide dynamic range (WDR) neurons (responding to both innocuous and noxious stimuli) in urethane-anaesthetised rats. The protocol presented here, with single unit recordings of dorsal horn neurons before and after a nociceptive conditioning stimulation, may be useful in many electrophysiological studies of plastic changes in the spinal cord, such as LTP. We invite others to use this protocol for the study of spinal plasticity. Findings using this technique may be relevant for the understanding of changes in nociceptive transmission, induction of central sensitisation and maybe even in mechanisms of pathological pain and chronic pain states. We describe modified and alternative protocols for the study of LTP mechanisms under different conditions in intact and in spinalised animals, and after natural noxious stimuli. We present a novel method minimising peripheral influence of afferent input induced by antidromic neurogenic inflammation or inflammatory changes following a natural noxious stimulation. This is made possible by dissection of the sciatic nerve at two separate locations and local anaesthetic block distal to the stimulation site.

A role for spinal lamina I neurokinin-1-positive neurons in cold thermoreception in the rat

Lamina I neurons of the spinal cord convey specific nociceptive activity to the brain. A subpopulation of lamina I cells bears substance P receptors (neurokinin-1) and recent studies have shown that these neurons encode for the intensity of noxious peripheral stimulation. Here, we report that cool thermal stimuli, applied to the hindpaw of anaesthetized rats, induce Fos expression in lamina I neurokinin-1 neurons that is graded with respect to the intensity of the thermal stimulus. Thus, as the temperature of the stimulus was reduced, both the total number of neurokinin-l-positive neurons expressing Fos and the proportion of Fos nuclei present within neurokinin-1 cells showed a significant increase. These data show that lamina I neurokinin-1 cells encode the intensity of noxious cooling of the skin. In laminae III and IV, although there was no correlation between neurokinin-1 cell activation and stimulus intensity, the total Fos count in these layers was inversely related to the depth of cooling. Thus, neurons in laminae III and IV may also play a role in thermoreception.

Laminar-selective noradrenergic and serotoninergic modulation includes spinoparabrachial cells after inflammation

We evaluated the effects of chemical lesions on hindpaw inflammation-induced Fos protein expression in spinoparabrachial neurons that were retrogradely labeled by Fluoro-Gold. The descending serotoninergic and noradrenergic pathways were destroyed by the selective neurotoxins, 5,7-DHT and DSP-4, respectively. After 5,7-DHT treatment there was a significant increase in double-labeled neurons only in the lateral reticulated neck of the dorsal horn 24h after inflammation compared with vehicle-injected controls. In contrast, the DSP-4 treatment resulted in a more robust increase in double-labeled neurons in the ipsilateral superficial dorsal horn than in the neck of the dorsal horn. These results indicate that after inflammation the enhanced modulation from descending serotoninergic and noradrenergic pathways targets supraspinally projecting neurons to dampen increased ascending nociceptive input. Further, these pathways differentially suppress the responses of spinoparabrachial neurons in the deep and superficial dorsal horn.

Central mechanisms in pain

Nociceptive input into the central nervous system is not simply passively received but rather is subject to modulation through spinal cord neuroplasticity and descending influences from supraspinal sites activated by a variety of environmental signals, including the acute or persistent nociceptive input itself and behavioral and emotional stimuli. The significant role of NMDA receptors and production of NO. in central sensitization, hyperalgesia, and chronic pain has been demonstrated in numerous models of peripheral injury. It has been shown that persistent nociceptive input is also subject to centrifugal descending modulation through activation of both prominent facilitatory and masked inhibitory influences from supraspinal sites (e.g., RVM) likely involving a spino-bulbar-spinal loop. These descending modulatory influences from the RVM appear to contribute selectively to hyperalgesia observed in uninjured tissue, distant from the site of insult (secondary hyperalgesia), and involve mechanisms similar to those found in the spinal cord (i.e., NMDA receptors and production of NO.). The significant role that modulatory influences in the central nervous system have in the development and maintenance of chronic pain and hyperalgesia clearly supports continued investigation into the physiologic mechanisms contributing to these events.

Alteration of descending modulation of nociception during the course of monoarthritis in the rat

Diffuse noxious inhibitory controls (DNIC), which involve supraspinal structures and modulate the transmission of nociceptive signals, were investigated at different stages during the development of adjuvant-induced monoarthritis in the rat. After behavioral evaluation, recordings of trigeminal convergent neurons were performed in anesthetized animals with acute (24-48 hr) or chronic (3-4 weeks) monoarthritis of the ankle. Inhibitions of C-fiber-evoked neuronal responses during and after the application of noxious conditioning stimuli to the ankle were measured to evaluate DNIC. The conditioning stimuli consisted of mechanical (maximal flexion and graded pressures) and graded thermal stimuli and were applied alternately to normal and arthritic ankles. Behaviorally, the two groups of animals exhibited a similar increased sensitivity to mechanical stimuli applied to the arthritic joint (i.e., an increased ankle-bend score and a decreased vocalization threshold to pressure stimuli). However, they showed different electrophysiological profiles. In the animals with acute monoarthritis, the DNIC-induced inhibitions produced by mechanical or thermal stimulation of the arthritic joint were significantly increased at all intensities compared with the normal joint. In contrast, in the chronic stage of monoarthritis, the DNIC-induced inhibitions triggered by thermal or pressure stimuli were similar for both ankles, except with the most intense mechanical stimuli. This discrepancy between the behavioral and electrophysiological findings suggests that inputs activated during chronic monoarthritis may fail to recruit DNIC and may thus be functionally different from those activated in the acute stage of inflammation.

Substance P receptor (neurokinin-1)-expressing neurons in lamina I of the spinal cord encode for the intensity of noxious stimulation: a c-Fos study in rat

The substance P receptor neurokinin-1 is expressed by a subset of neurons in the rat spinal cord. We have combined immunostaining for Fos, a marker of noxious peripheral stimulation, and neurokinin-1 to examine whether nociceptive signals from particular peripheral tissues (skin, muscle or knee joint) or activity generated by nerve injury or formalin-induced inflammation are preferentially modulated by substance P. Our results indicate that superficial and deep spinal neurokinin-1-positive neurons process nociceptive information in markedly different ways. In lamina I, the number of double-labelled neurons was positively correlated with the intensity of the stimulus (defined by the total Fos count) and was not directly related to any particular peripheral target. However, in the deeper layers of the spinal cord (V-X), there was no such correlation, and stimulation of joint nociceptors and formalin-induced inflammation produced the greatest proportion of Fos/neurokinin-1 co-localization, suggesting a particular role for substance P in the mediation of joint pain and inflammatory hyperalgesia. Thus, lamina I neurokinin-1 receptor-bearing neurons appear to be involved in intensity discriminative aspects of pain, whereas the deep neurokinin-1 cells are involved in spatial localization or the detection of particular nociceptive submodalities.

Dorsolateral funiculus-lesions unmask inhibitory or disfacilitatory mechanisms which modulate the effects of innocuous mechanical stimulation on spinal Fos expression after inflammation

To examine the contribution of low threshold mechanoreceptive afferent input to the development of allodynia and the involvement of descending pathways, we investigated the effects of repeated innocuous brush on inflammation-induced spinal Fos protein expression in dorsolateral funiculus-lesioned (DLFX) rats following hindpaw inflammation. In DLF sham-operated animals, brush stimuli induced a significant increase in the number of Fos-labeled neurons in ipsilateral laminae I-IV, and a slight suppression of Fos expression in ipsilateral laminae V-VI when compared to sham-lesioned rats without brushing. In rats receiving DLFX, the brush-induced increase in Fos expression in laminae I-IV was significantly reduced. The DLFX also unmasked a brush-induced suppression of laminae VII-VIII neurons. These results suggest that innocuous mechanical stimulation of an inflamed hindpaw gives rise to further facilitation of neuronal activity in laminae I-IV and inhibition of neuronal activity in laminae V-VIII. We propose that there is an unmasking of inhibitory mechanisms or a reduction in descending facilitatory effects after DLFX that alter Fos protein expression produced by innocuous mechanical stimulation.

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

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

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

The medullary dorsal reticular nucleus facilitates pain behaviour induced by formalin in the rat

The influence of the dorsal reticular nucleus (DRt) on pain behaviour during the formalin test was studied in the rat by lesioning the nucleus through local application of electrical current or quinolinic acid. Animals in which the DRt was lesioned ipsilaterally to the paw injected with formalin spent less time in focused (licking, biting or scratching the injected paw) and total (focused pain behaviour plus protection of the injected paw during movements) pain behaviour, and showed paw-jerks less frequently than non-lesioned animals in both phases 1 and 2 of the test. Animals in which the DRt was lesioned contralaterally to the injected paw presented a decrease in total pain behaviour and number of paw-jerks only during phase 2. The number of superficial (laminae I-II) and deep (laminae III-VI) spinal dorsal horn cells expressing the c-fos proto-oncogene 2 h after subcutaneous injection of formalin was reduced by 34% and 50%, respectively, in animals with an ipsilateral DRt lesion as compared to non-lesioned rats. No differences in c-fos expression were observed after lesioning the DRt contralateral to the formalin injection. The results indicate that the DRt is involved in the facilitation of nociception during the formalin test by enhancing the response capacity of dorsal horn neurons to noxious stimulation. It is suggested that the pronociceptive action of the DRt is mediated by the reciprocal connections it establishes with the spinal dorsal horn.

Descending modulation of opioid-containing nociceptive neurons in rats with peripheral inflammation and hyperalgesia

Inflammation and hyperalgesia induce a dramatic up-regulation of opioid messenger RNA and peptide levels in nociceptive neurons of the spinal dorsal horn. Descending axons modulate nociceptive transmission at the spinal level during inflammatory pain, and may play a role in the development of persistent pain. The role of descending bulbospinal pathways in opioid-containing nociceptive neurons was examined. Removal of descending inputs to the spinal cord was performed by complete spinal transection at the midthoracic level. Seven days after spinal transection, rats received a unilateral hindpaw injection of complete Freund's adjuvant, a noxious stimulus that produces inflammation and hyperalgesia. Tissues from the L4 and L5 segments of the spinal cord were removed and analysed by northern blotting and immunocytochemistry. Spinal transection resulted in a further increase in both dynorphin and enkephalin messenger RNA content following complete Freund's adjuvant injection. There was a similar distribution and number of dynorphin-immunoreactive cells in transected rats compared to rats which received sham surgery. These data suggest that increased dynorphin messenger RNA ipsilateral to inflammation, in rats without descending axons, was due to increased expression within the same cells and not to recruitment of additional dynorphin-expressing cells. This reflects a greater dynamic response of nociceptive neurons to noxious stimuli in the absence of descending modulation. Therefore, the net effect of descending afferents on spinal nociceptive circuits may be to reduce the response of opioid-containing neurons to noxious stimulation from the periphery.

Relationship between neuronal activity and substance P-immunoreactivity in the rat spinal cord during acute and persistent myositis

The spinal level of substance P (SP) is assumed to be an important determinant of neuronal activity under pathophysiological conditions. In rat dorsal horn neurones, impulse activity was studied during a carrageenan-induced acute (2-8 h) and a Freund's adjuvant-induced persistent (12 days) myositis and compared with the spinal substance P-immunoreactivity (SP-IR) of the same animals. Myositis-induced changes in responsiveness of the neurones reached a maximum within 2-8 h, whereas background activity of the neurones was highest after 12 days of myositis. The area of SP-IR in the superficial dorsal horn decreased during acute and persistent myositis and the integrated density of the staining was largely unchanged. The difference in time-course between neuronal activity and SP-IR suggest that during persistent myositis factors other than SP gain more influence on the behaviour of the neurones.

Morphine microinjected into the nucleus raphe magnus does not block the activity of spinal trigeminal nucleus oralis convergent neurons in the rat

This study investigated the effects of morphine microinjection into the nucleus raphe magnus (RMg) on electrically evoked C-fiber activities of convergent neurons in the spinal trigeminal nucleus oralis (Sp5O), in halothane-anesthetized rats. Although the neurons could be depressed by systemic morphine (6 mg/kg, i.v.) in a naloxone-reversible fashion, morphine microinjected into the RMg (2. 5 microgram or 5 microgram) neither depressed their C-fiber-evoked responses, nor the diffuse noxious inhibitory controls acting on them. It is concluded that the RMg is not involved in reinforcing descending inhibitory controls that are tonic or triggered by noxious stimuli acting on Sp5O convergent neurons.

Spinal serotonergic receptors mediate facilitation of a nociceptive reflex by subcutaneous formalin injection into the hindpaw in rats

It is documented that spinal nociceptive transmission receives descending facilitatory and inhibitory modulation from supraspinal structures. The rostral ventral medulla (RVM), including the nucleus raphe magnus (NRM), nuclei reticularis gigantocellularis (NGC) and gigantocellularis pars alpha (NGCalpha), is the major bulbar relay of descending modulatory influences. Pharmacological studies show that facilitation of a spinal nociceptive tail-flick (TF) reflex induced by stimulation in the NGC and NGCalpha is mediated by spinal serotonergic receptors. The present series of experiments provide evidence that activation of spinal serotonergic systems are critical for both induction and maintenance of secondary hyperalgesia induced by subcutaneous injection of formalin into one hindpaw. Subcutaneous injection of formalin produced facilitation of tail withdrawal (mechanical) and the TF reflex (thermal). Facilitatory effects persisted for at least 30 min. Peripheral blockade of the activity by local injection of a hydrophilic lidocaine derivative (QX-314, 5%) into the injected hindpaw abolished both mechanical and thermal facilitation, indicating that peripheral input is important to maintain long-lasting facilitation. Intrathecal application of a serotonergic receptor antagonist methysergide at a dose (64 nmol) which completely blocked descending facilitation produced by electrical- or chemical-stimulation in the NGC and NGCalpha also significantly attenuated or completely abolished facilitation of tail withdrawal and the TF reflex induced by formalin. Methysergide was effective whether the injection was performed before or after the formalin injection. These results suggest that activation of descending facilitatory serotonergic influences by a prolonged noxious stimulation could contribute to secondary hyperalgesia observed at the tail.

Influence of spinalization on spinal withdrawal reflex responses varies depending on the submodality of the test stimulus and the experimental pathophysiological condition in the rat

The influence of midthoracic spinalization on thermally and mechanically induced spinal withdrawal reflex responses was studied in the rat. There were three experimental groups of rats: healthy controls, rats with a spinal nerve ligation-induced unilateral neuropathy, and rats with a carrageenan-induced inflammation of one hindpaw. Tail flick response was induced by radiant heat. Hindlimb withdrawal was induced by radiant heat, ice water, and innocuous or noxious mechanical stimulation of the paw. Prior to spinalization, spinal nerve ligated and carrageenan-treated animals had a marked unilateral allodynia and hyperalgesia. Spinalization tended to induce a facilitation of noxious heat-evoked reflexes. This spinalization-induced facilitation was stronger on tail than hindlimb withdrawal. Spinalization-induced skin temperature change did not explain the facilitation of noxious heat-evoked reflexes. In contrast, spinal withdrawal responses induced by noxious cold or mechanical stimulation were significantly suppressed following spinalization. The spinalization-induced facilitatory effects as well as inhibitory ones on spinal reflexes were enhanced in inflamed/neuropathic animals. The results indicate that the tonic descending control of spinal nocifensive responses varies depending on the submodality of the test stimulus, the segmental level of the reflex (tail vs. hindlimb), and on the pathophysiological condition.

Force of limb withdrawals elicited by graded noxious heat compared with other behavioral measures of carrageenan-induced hyperalgesia and allodynia

We investigated if carrageenan-induced thermal hyperalgesia and mechanical allodynia are associated with a reduction in threshold and-or enhancement of suprathreshold nocifensive responses, using a method to measure the force of a hind limb wilhdrawal reflex elicited by graded noxious heat stimuli (36-52 degrees C, 5s) delivered by Peltier thermode tethered to the ventral hind paw of conscious rats. Withdrawal reflexes were recorded 2.5 h after intraplantar injection of carrageenan (1 or 0.1%) or saline vehicle in sessions >2 weeks apart: baseline reflexes were assessed the day before. Withdrawal reflex force increased linearly from 42-52 degrees C. Carrageenan 1% significantly enhanced withdrawal reflexes at 40-46 degrees C, reducing the slope and threshold of the stimulus-response function. This was associated with significant reductions in thermal paw withdrawal latency (Hargreaves test: by 50%), mechanical withdrawal threshold (by 82%) and weight bearing on the injected side (by 81%) measured with independent force plates. Smaller reductions in thermal paw withdrawal latency and mechanical withdrawal threshold, and smaller enhancement of withdrawal reflex force, were observed following 0.1% carrageenan. Intraplantar saline was ineffective. This method allows assessment of hyperalgesic changes in stimulus-response coding over a broad range of noxious stimulus intensities.

Sensitization of the nervous system, induced by two successive hindpaw inflammations, is suppressed by a local anesthetic

Two hindpaw injections of carrageenin were performed 7 days apart and the time-course of the vocalization thresholds to pressure (VTPP) of all the 4 paws of the rat were measured after both injections. The first injection of carrageenin induced an increase in the circumference of the injected paw and a significant reduction of the VTPPs of all the 4 paws. The pain-related behavior of both hindpaws was enhanced, when carrageenin was injected for the second time not only into the previously inflamed, but also into the contralateral hindpaw. However, the abnormal responses of the forepaws were not increased by this second hindpaw inflammation. Both after the first and the second injection of carrageenin, the decrease of the VTPPs of all 4 paws was antagonized by lidocaine with epinephrine (LE) injected into the inflamed paw. This anesthetic effect was of a shorter duration after the second than after the first injection of carrageenin. Presumably, the imprint that is left in the nervous system after the first hindpaw inflammation, takes place at a relatively limited segmental level. In addition, it is possible to influence the established sensitization of the nervous system by treating the peripheral process itself, even when the first injury has primed the nervous system to the second injury. However, there still seems to exist some excitatory influences that cannot be suppressed by the local anesthetic.

The influence of temporal summation on a C-fibre reflex in the rat: effects of lesions in the rostral ventromedial medulla (RVM)

In intact rats, an inhibitory mechanism counteracts the increase in excitability of a flexor reflex seen in spinal animals following high-intensity, repetitive stimulation of C-fibres. We tested the hypothesis that the rostral ventromedial medulla (RVM) is involved in these processes. Electromyographic responses elicited by electrical stimulation of the sural nerve, were recorded from the ipsilateral biceps femoris in halothane-anaesthetised, sham-operated or RVM-lesioned rats. There were no significant differences between the C-fibre reflexes in the two groups in terms of their thresholds, latencies, durations or mean recruitment curves. The excitability of the C-fibre reflex was tested following 20 s of high-intensity homotopic electrical conditioning stimuli at 1 Hz. During the conditioning period, the EMG responses first increased in both groups (the wind-up phenomenon), but then decreased in the sham-operated rats and plateaued in the RVM-lesioned rats. These effects were followed by inhibitions that were very much smaller in the RVM-lesioned rats, both in terms of their magnitudes and their durations. It is concluded that the RVM is involved in inhibitory feedback mechanisms elicited by temporal summation of C-fibre afferents that both counteract the wind-up phenomenon and trigger long periods of inhibition.

Spinal mechanisms underlying persistent pain and referred hyperalgesia in rats with an experimental ureteric stone

Spinal neurons processing information from the ureter have been characterized in rats 1-4 days after the implantation of an experimental ureteric stone and compared with those of normal rats. The effects of a conditioning noxious stimulation of the ureter in the presence of the hyperalgesia evoked by the calculosis also were examined. Extracellular recordings were performed at the T12-L1 segments of the spinal cord. In rats with calculosis, more neurons expressed a ureter input (53 vs. 42% in normal rats); such cells being more likely to show background activity, at a higher rate than normals (6.6 +/- 1.2 vs. 3.2 +/- 0.9 spikes/s; mean +/- SE) and increasing with the continuing presence of the stone. The threshold pressure for a ureteric response was higher than in normal rats (79 +/- 5 vs. 54 +/- 4 mmHg) but the neurons failed to encode increasing intensities of stimulation. Thirty-five percent of the neurons with exclusively innocuous somatic receptive fields had a ureter input in rats with calculosis, whereas none were seen in normal rats. A noxious ureteric distention applied to neurons with ureter input evoked a complex mixture of increases and decreases in somatic receptive field size and/or somatic input properties markedly different from the generalized increases in excitability seen when such a stimulus was applied to normal animals. We conclude that the presence of a ureteric stone evokes excitability changes of spinal neurons (enhanced background activity, greater number of ureter-driven cells, decreased threshold of convergent somatic receptive fields), which likely account for the referred hyperalgesia seen in rats with calculosis. However, further noxious visceral input occurring in the presence of persistent hyperalgesia produces selective changes that cannot be explained by a generalized excitability increase and suggest that the mechanisms underlying maintenance of hyperalgesia include alteration of both central inhibitory and excitatory systems.

Chemical stimulation of the intracranial dura induces enhanced responses to facial stimulation in brain stem trigeminal neurons

Chemical activation and sensitization of trigeminal primary afferent neurons innervating the intracranial meninges have been postulated as possible causes of certain headaches. This sensitization, however, cannot explain the extracranial hypersensitivity that often accompanies headache. The goal of this study was to test the hypothesis that chemical activation and sensitization of meningeal sensory neurons can lead to activation and sensitization of central trigeminal neurons that receive convergent input from the dura and skin. This hypothesis was investigated by recording changes in the responsiveness of 23 [16 wide-dynamic range (WDR), 5 high threshold (HT), and 2 low threshold (LT)] dura-sensitive neurons in nucleus caudalis to mechanical stimulation of their dural receptive fields and to mechanical and thermal stimulation of their cutaneous receptive fields after local application of inflammatory mediators or acidic agents to the dura. Responses to brief chemical stimulation were recorded in 70% of the neurons; most were short, lasting the duration of the stimulus only. Twenty minutes after chemical stimulation of the dura, the following changes occurred: 1) 95% of the neurons showed significant increases in sensitivity to mechanical indentation of the dura: their thresholds to dural indentation changed from 1.57 to 0.49 g (means, P < 0.0001), and the response magnitude to identical stimuli increased by two- to fourfold; 2) 80% of the neurons showed significant increases in cutaneous mechanosensitivity: their responses to brush and pressure increased 2.5- (P < 0.05) and 1. 6-fold (P < 0.05), respectively; 3) 75% of the neurons showed a significant increase in cutaneous thermosensitivity: their thresholds to slow heating of the skin changed from 43.7 +/- 0.7 to 40.3 +/- 0.7 degrees C (P < 0.005) and to slow cooling from 23.7 +/- 3.3 to 29.2 +/- 1.8 degrees C (P < 0.05); 4) dural receptive fields expanded within 30 min and cutaneous receptive fields within 2-4 h; and 5) ongoing activity developed in WDR and HT but not in LT neurons. Application of lidocaine to the dura abolished the response to dural stimulation but had minimal effect on the increased responses to cutaneous stimulation (suggesting involvement of a central mechanism in maintaining the sensitized state). Antidromic activation (current of <30 muA) of dura-sensitive neurons revealed projections to the hypothalamus, thalamus, and midbrain. These findings suggest that chemical activation and sensitization of dura-sensitive peripheral nociceptors could lead to enhanced responses in central neurons and that this central sensitization therefore could result in extracranial tenderness (mechanical and thermal allodynia) in the absence of extracranial pathology. The projection targets of these neurons suggest a possible role in mediating the autonomic, endocrine, and affective symptoms that accompany headaches.

Inflammation-induced Fos protein expression in the rat spinal cord is enhanced following dorsolateral or ventrolateral funiculus lesions

Previous studies have shown an enhanced expression of Fos protein-like immunoreactivity in the lumbar spinal cord of rats with complete spinal transection following persistent hindpaw inflammation. To further locate the spinal pathways responsible for these effects, we compared the inflammation-evoked Fos expression in rats with bilateral lesions of the dorsolateral (DLFX) or ventrolateral (VLFX) funiculus, and with rats with a sham operation. The results indicate that the number of Fos-labeled neurons was significantly increased in all laminae of the dorsal horn ipsilateral to the inflamed hindpaw and in contralateral deep dorsal horn in both DLFX and VLFX rats compared to sham-operated rats. Moreover, when comparing DLFX and VLFX rats, in the ipsilateral spinal cord, DLFX resulted in more Fos expression in the deep dorsal horn; in contrast, a larger number of Fos-labeled cells in superficial laminae was observed in VLFX rats. These results suggest that modulatory systems, which descend in both DLF and VLF pathways, mediate the enhanced net descending nociceptive inhibition after persistent inflammation, although the supraspinal sites of origin of each pathway are likely functionally diverse.

Carrageenan-induced changes in spinal nociception and its modulation by the brain stem

Carrageenan was used to study inflammation-induced changes in spinal nociception and its brain stem modulation in the pentobarbitone-anesthetized rat. Carrageenan was administered intraplantarly into one hindpaw 2 h before the start of electrophysiological single unit recordings of wide-dynamic range (WDR) neurons of the spinal dorsal horn. Carrageenan produced a significant leftward shift in the stimulus-response function for mechanical stimuli, whereas that for noxious heat stimuli was short of statistical significance. Conditioning electrical stimulation in the rostroventromedial medulla (RVM) significantly attenuated noxious heat-evoked, but not mechanically evoked, responses to spinal dorsal horn WDR neurons in the control (contralateral) side. However, in the carrageenan-treated side RVM stimulation had no significant effect on mechanically or noxious heat-evoked responses. Following direct spinal administration of neuropeptide FF (NPFF), noxious heat-evoked responses, but not mechanically evoked responses, were attenuated by RVM-stimulation also in the carrageenan-treated side. This selective NPFF-induced enhancement of brain stem-spinal inhibition was not reversed by naloxone. The results indicate that carrageenan-induced inflammation significantly changes the response properties of spinal nociceptive neurons and their brain stem-spinal modulation. During inflammation, NPFF in the spinal cord produces a submodality-selective potentiation of the antinociceptive effect induced by brain stem-spinal pathways, independent of naloxone-sensitive opioid receptors.

Supraspinal inhibition of nociceptive dorsal horn neurones in the anaesthetized rat: tonic or dynamic?

1. Tonic inhibition of sensory spinal neurones is well known to descend from the rostroventral medulla. It is not clear if this inhibition is dynamically activated by peripheral noxious stimuli. 2. Transection of the ipsilateral dorsolateral funiculus (DLF) removed a descending inhibition of multireceptive spinal neurones and disproportionally prolonged the after-discharge component of their response to a noxious cutaneous stimulus. 3. Microinjection of GABA or tetracaine into the medullary nucleus gigantocellularis pars alpha (GiA) similarly prolonged the after-discharge in response to noxious stimuli. 4. Recordings of GiA cells, initially using minimal surgery, revealed that many had low levels of spontaneous activity and responded vigorously to noxious stimuli applied to any part of the body surface. One hour after the surgery necessary to expose the spinal cord, GiA cells had a high firing rate but responded weakly to noxious stimuli. 5. The response of GiA cells to noxious stimuli was abolished by transection of only the DLF contralateral to the stimulus. 6. It is concluded that the inhibition of multireceptive dorsal horn neurones from GiA is dynamically activated by noxious cutaneous stimuli via a projection in the contralateral DLF. Surgical exposure of the spinal cord tonically activates this inhibition and masks the dynamic component.

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.

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.