Differential Effects of a Distant Noxious Stimulus on Hindlimb Nociceptive Withdrawal Reflexes in the Rat

Recent studies indicate that the nociceptive withdrawal reflexes to individual muscles are evoked by separate reflex pathways. The present study examines whether nociceptive withdrawal reflexes to different muscles are subject to differential supraspinal control in rats. A distant noxious stimulus was used to activate a bulbospinal system which selectively inhibits 'multireceptive' neurons (i.e. neurons receiving excitatory tactile and nociceptive inputs) in the dorsal horn of the spinal cord. Withdrawal reflexes, recorded with electromyographic techniques in single hindlimb muscles, were evoked by standardized noxious pinch. Thirty-seven rats, anaesthetized with halothane and nitrous oxide, were used. Whereas withdrawal reflexes to the extensor digitorum longus and brevis, tibialis anterior and biceps posterior muscles were strongly inhibited, reflexes to interossei muscles were potentiated during noxious pinch of the nose. Reflexes to peronei muscles were not significantly changed. The effects on the reflexes usually had an onset latency of <0.5 s and outlasted the conditioning stimulation by up to 2 s. The monosynaptic la reflex to the deep peroneal nerve, innervating dorsiflexors of the digits and ankle, was not significantly changed during noxious pinch of the nose. Hence, the inhibitory effects on the hindlimb withdrawal reflexes induced by the conditioning stimulation were presumably exerted on reflex interneurons. It is concluded that nociceptive withdrawal reflexes to different hindlimb muscles are differentially controlled by descending pathways activated by a distant noxious stimulus. The results support our previous conclusion that there are separate nociceptive withdrawal reflex pathways to different hindlimb muscles.     

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.     

Spinalization increases the mechanical stimulation-induced withdrawal reflex threshold after a sciatic cut in the rat

The purpose of the present study was to establish whether supraspinal structures modulate mechanical ‘adjacent hyperalgesia'. After a chronic sciatic cut, the paw withdrawal threshold to mechanical stimulation was lower, and the latency of noxious radiant heat-induced withdrawal reflex was shorter at the traumatized side than at the intact side. Then the rats were spinalized, and the withdrawal threshold to mechanical stimulus increased at the injured side, but the withdrawal latency induced by noxious heat decreased at the intact side. No side differences between the injured and the intact side could be detected after spinalization. Thus supraspinal structures may participate in maintenance of mechanically evoked paw withdrawal reflex after a sciatic injury.     

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

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but not when it was 0.5

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. Lidocaine (4%, 0.5

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) 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.     

Intense peripheral electrical stimulation differentially inhibits tail vs. limb withdrawal reflexes in the rat

In an on-going study on mechanisms by which activation of sensory afferents regulates nociception, high-intensity, low-frequency electrical stimulation was applied to previously defined meridian and non-meridian points of the hindlimb or forelimb, and the effects measured on the withdrawal reflex of the tail or limb in the lightly anesthetized rat. Withdrawal was evoked by application of noxious radiant heat to the tip of the tail or to the plantar surface of a hindpaw or forepaw. Parameters of conditioning electrical stimulation were 2 ms pulses at 4 Hz for 20 min at 20 × threshold (20–30 mA) where threshold was the minimum intensity which evoked muscle twitch. In experiments on tail withdrawal, stimulation applied to meridian points fengshi (GB-31), femur-futu (ST-32) and zusanli (ST-36) of the hindlimb or to wai-kuan (TH-5) and hoku (LI-4) of the forelimb increased the latency of the withdrawal reflex to 70–100% of the maximum possible inhibition (MPI) during the stimulation. Inhibition persisted for more than 1 h after the end of stimulation. Bilateral stimulation of hindlimb meridian points evoked a greater inhibition during the stimulation ( > 95% of the MPI); the inhibition persisted for 40 min. Stimulation of non-meridian sites in hindlimb or forelimb inhibited the withdrawal reflexes by 45–50% of the MPI during the stimulation only. Thus, the evoked inhibition has two components, a brief effect elicited by non-meridian point stimulation and a persistent post-stimulation effect produced only upon stimulation of meridian points. Stimulation produced little effect on nociceptive limb withdrawal reflexes. The results suggest that high-intensity, low-frequency electrical stimulation of meridian points produced a long-lasting, extrasegmental inhibition of the tail withdrawal but not of limb withdrawal reflexes. This differential inhibition may be due to differences in neuronal circuitry and CNS modulatory control mechanisms. The persistent inhibition appears to be dependent on the site of stimulation because it is not evoked by stimulation of sites outside of meridian points.     

Suppression of the hindlimb flexor reflex by stimulation of the medial hypothalamus and thalamus in the rat

Pentobarbital-anesthetized rats received electrical hindpaw stimulation every 10 s to elicit a maximal hindlimb withdrawal reflex. The integrated EMG response in the ipsilateral tibialis anterior was sampled by a computer which also controlled the timing of electrical stimuli applied to the brain. A suppression of the evoked flexor activity was obtained with currents below 0.05 mA for stimuli applied in the medial hypothalamic region. A second effective site was located in the paraventricular area of the thalamus. The suppression had an onset latency of 30 ms, increased over a period of 500 ms and was followed by a postinhibitory facilitation (rebound). When the noxious electrical shocks were given over prolonged periods (140 s) the suppression of the flexor reflex was seen to outlast the central stimulation by more than 100 s. Intravenous injection of naloxone or methysergide failed to reverse the effects of the brain stimuli. It is suggested that the hypothalamic induced inhibition of withdrawal reflexes is functionally meaningful in view of the incompatibility between these reflexes and the locomotor behavior which is part of the behavioral responses (i.e. fight or flight) controlled by this area.     

Parametric Studies on Electroacupuncture-Like Stimulation in a Rat Model: Effects of Intensity, Frequency, and Duration of Stimulation on Evoked Antinociception

We have found that electroacupuncture-like stimulation of defined sites in the hindlimb of the rat inhibits a nociceptive withdrawal reflex. The lightly anaesthetized rat was used and tail withdrawal from a noxious radiant heat stimulus was the nociceptive reflex. Standard stimulation of hindlimb meridian points femur-futu (ST-32), fengshi (GB-31), and zusanli (ST-36) consisted of a 2-ms square voltage pulse at 4 Hz for a duration of 20 min, applied at 20 times the threshold to evoke muscle twitch. This produced two types of inhibition of the reflex; one was an increase in the latency of up to 80% during the stimulation, termed the brief antinociception, and the other was a post stimulation increase of up to 60% lasting greater than 1 h, termed the persistent antinociception. When the stimulus intensity was reduced to 10 times threshold, the latency during stimulation increased up to 50%, but the persistent response did not occur. Stimulation at threshold produced neither effect. When the train duration was altered, 10 min of stimulation produced only the brief effect, whereas 40 min of stimulation produced both effects, although the persistent effect lasted only 20 min. Stimulation at 6 Hz produced responses similar to those at 4 Hz, whereas stimulation at 2 Hz produced smaller effects. At 8 Hz, only the brief antinociception was elicited. With a pulse duration of 0.2 ms, the brief response was observed but the persistent response was markedly attenuated, whereas 5 ms produced responses similar to those with 2 ms. These data suggest that high-intensity, low-frequency electrical stimulation of meridian points in the rat hindlimb produces both brief and persistent antinociceptive effects on the tail withdrawal reflex, and both effects are dependent upon the parameters of stimulation. The persistence of the latter effect beyond the period of stimulation suggests events occurring after direct synaptic activity, possibly mediated via plastic changes at spinal and/or supraspinal levels.     

Spinal monoaminergic receptors mediate the antinociception produced by glutamate in the medullary lateral reticular nucleus

Focal electrical stimulation and microinjection of the excitatory amino acid glutamate in the lateral reticular nucleus (LRN) both inhibit the heat-evoked tail flick (TF) reflex in rats. The stimulation-produced inhibition from the LRN has previously been demonstrated to be mediated by spinal monoaminergic receptors. In the present study, inhibition of responses to noxious thermal stimuli by glutamate microinjected into the LRN was examined and characterized; this study is the first to examine the spinal receptors mediating inhibition produced by selective activation of cell bodies in the LRN. Microinjection of glutamate (100 mM) into the LRN in rats lightly anesthetized with pentobarbital produced a transient (less than 5 min) inhibition of the heat-evoked TF reflex, the magnitude of which increased with the volume of glutamate injected (100, 200, or 400 nl). This glutamate-produced inhibition of the TF reflex was antagonized by the intrathecal administration of phentolamine (30 micrograms), yohimbine (15 and 30 micrograms), or methysergide (15 and 30 micrograms) to the level of the lumbar spinal cord, but was not antagonized by prazosin (30 micrograms) or naloxone (20 micrograms). Yohimbine (15 and 30 micrograms) administered to the level of the cervical spinal enlargement did not significantly alter inhibition of the TF reflex produced by glutamate microinjected into the LRN. Microinjection of glutamate (100 mM, 400 nl) into the LRN elevated TF latencies and hindpaw lick latencies in the hot plate test performed on conscious rats. This inhibition of responses to noxious thermal stimuli in conscious rats was short-lasting (less than 5 min), and was also attenuated by intrathecal administration of yohimbine (30 micrograms) or methysergide (30 micrograms), but not by prazosin (30 micrograms) or naloxone (20 micrograms). While it has previously been established that cell bodies in the LRN mediate descending inhibition of spinal nociceptive reflexes, the present results establish that spinal alpha 2-adrenoceptors and serotonin receptors mediate LRN-produced antinociception and extend our understanding of LRN-mediated modulation of nociceptive responses integrated spinally and supraspinally.     

Intense peripheral electrical stimulation evokes brief and persistent inhibition of the nociceptive tail withdrawal reflex in the rat

In a study of modulation of nociception by sensory inputs, electrical stimulation was applied to specific sites in the hindlimb and effects on the nociceptive tail withdrawal reflex were monitored in the lightly anaesthetized rat. Stimulation was applied to previously defined sites in the hindlimb, meridian points femur-futu (ST-32), fengshi (GB-31) and zusanli (ST-36). It consisted of a 4 Hz train of 2 ms square pulses given for 20 min at 20 × the threshold intensity required for muscle twitch. Tail withdrawal was provoked by application of a noxious heat stimulus applied to the tip of the tail. Results were expressed as a percentage of the maximal possible inhibition which is achieved when the post-treatment latency is 2 × the pre-treatment latency otherwise known as the cut off. During stimulation, the latency of the withdrawal increased to ≈ 70% of the maximal possible inhibition. Following stimulation, the inhibition persisted for > 1 h. Stimulation at 2 or 6 Hz elicited similar effects but stimulation at 8 Hz evoked inhibition during the stimulation only. Stimulation applied to sites away from defined meridian points inhibited tail withdrawal during the stimulation; no post-stimulation effect was produced. In acutely transected animals (≤ 48 h), stimulation of meridian points elicited a small, brief increase in latency but during stimulation only. At 7 and 14 days after spinal transection, this response during stimulation was greater in magnitude and a brief post-stimulation increase was also observed. The return of the of this latter effect was coincident with the return of bladder function. These data suggest that high intensity, low frequency electrical stimulation of hindlimb meridian points in the lightly anaesthetized rat produces both brief and persistent inhibitory effects on the nociceptive tail withdrawal reflex. These effects appear to be elicited by different mechanisms. The persistent effect may represent a plastic change in central inhibitory mechanisms. Data from spinal animals indicate a major participation of supraspinal structures but that spinal mechanisms are also capable of sustaining both types of effect.     

A fictive tail flick reflex in the rat

The discharges of motor axons participating in the tail flick reflex were recorded from nerve filaments innervating the medial longissimus muscles of anesthetized rats. The reflex discharges evoked by stimulation of the tail with either noxious radiant heat or pinch were recorded before and after paralysis of the animals. Nociceptive discharges recorded from motor axons in the paralyzed state showed a strong correlation with those observed in the absence of the paralytic agent. For this reason, the electrophysiological response triggered by noxious input was termed a ‘fictive tail flick reflex’. To evaluate the potential applicability of this model in the analysis of pain blocking mechanisms, vaginal stimulation was found to produce a profound reduction of the nociceptive discharges of the fictive tail flick reflex. By eliminating movement artifacts from the experimental paradigm, this model expands our ability to study a basic nociceptive response which was previously limited to behavioral observation.     

Neural organization of the viscero-cardiac reflexes

The reflex responses evoked in the postganglionic nerves to the heart were tested in chloralose-anaesthetized cats. Electrical stimulation of the A delta afferent fibres from the left inferior cardiac nerve evoked spinal and supraspinal reflex responses with the onset latencies of 36 ms and 77 ms respectively. The most effective stimulus was a train of 3-4 electrical pulses with the intratrain frequency of 200-300 Hz. Electrical stimulation of the high threshold afferent fibres (C-fibres) from the left inferior cardiac nerve evoked the reflex response with the onset latency of 200 ms. The C-reflex was present in intact animals and disappeared after spinalization. The most effective stimulus to evoke this reflex was a train of electrical pulses delivered at a frequency of 1-2 Hz with an intratrain frequency of 20-30 Hz. The most prominent property of the C-reflex was its marked increase after prolonged repeated electrical stimulation. We conclude that: (1) viscero-cardiac sympathetic reflexes may be organized at the spinal and supraspinal level; (2) viscero-cardiac sympathetic reflexes evoked by stimulation of the A delta and C afferent fibres from the left inferior cardiac nerve have different central organization.     

Submodality-Selective Hyperalgesia Adjacent to Partially Injured Sciatic Nerve in the Rat is Dependent on Capsaicin-Sensitive Afferent Fibers and Independent of Collateral Sprouting or a Dorsal Root Reflex

We studied submodality dependence of sensory changes produced by unilateral ligation of the sciatic or the saphenous nerve in the rat. We focused especially on sensory changes in the skin area adjacent to the innervation area of the injured nerve. Moreover, we examined the roles of capsaicin-sensitive nociceptive fibers, collateral sprouting and a dorsal root reflex in sensory changes observed behaviorally. Assessment of sensory changes was performed by a pattern of behavioral tests: hot-plate test and hindlimb withdrawal responses induced by radiant heat, hot-water bath, innocuous mechanical stimuli, and noxious mechanical stimuli. In one group, the saphenous nerve ipsilateral to the sciatic ligation was topically treated with capsaicin (1%) at the time of the surgery. A proximal stump of a saphenous nerve strand was orthodromically stimulated to induce a dorsal root reflex (an antidromic volley) in nociceptive fibers of the saphenous nerve trunk. For visualization of plasma extravasation induced by a dorsal root reflex, a dye-labeling (Evans blue) technique was used. A collateral sprouting of nociceptive fibers of the uninjured saphenous nerve was evaluated by determining the plasma extravasation response induced by antidromic stimulation of the saphenous nerve. Three and 10 days following the sciatic constriction injury, the hindlimb withdrawal threshold evoked by noxious mechanical stimulation of the medial side of the paw (the innervation are of the intact saphenous nerve) was significantly decreased. There was no corresponding thermal hyperalgesia adjacent to the injured sciatic nerve. Chronic constriction of the saphenous nerve did not produce any significant hyper- or hypoalgesia to mechanical or thermal stimulation of the uninjured sciatic nerve area. Topical treatment of the ipsilateral (intact) saphenous nerve at the time of the sciatic nerve ligation completely prevented the development of mechanical hyperalgesia in the medial side of the paw (the innervation area of the saphenous nerve). No dorsal root reflex in nociceptive fibers mediating the adjacent hyperalgesia could be evoked. No collateral sprouting of the uninjured nociceptive fibers of the saphenous nerve was observed. The results indicate that the constriction injury of the sciatic nerve produced a selective hyperalgesia to mechanical stimulation in the innervation area of the neighboring saphenous nerve. At the peripheral level, the mechanical hyperalgesia adjacent to the innervation area of the injured nerve was mediated by capsaicin-sensitive nociceptive fibers. Collateral sprouting of nociceptive fibers from the uninjured to the injured innervation area did not contribute to the present sensory findings. The sciatic nerve injury did not induce a dorsal root reflex in nociceptive fibers innervating the hyperalgesic saphenous nerve area.     

Developmental tuning in a spinal nociceptive system: effects of neonatal spinalization.

Recent studies indicate a modular organization of the nociceptive withdrawal reflex system. Each module has a characteristic receptive field, closely matching the withdrawal movement caused by its effector muscle. In the rat, the strength of the sensory input to each module is tuned during the first postnatal weeks, i.e., erroneous spinal connections are depressed, and adequate connections are strengthened. To clarify if this tuning is dependent on supraspinal structures, the effect of a complete neonatal spinal cord transection on the postnatal tuning of withdrawal reflexes was studied. The nociceptive receptive fields of single hindlimb muscles and compound withdrawal reflexes were examined in decerebrate unanesthetized and awake rats, respectively. Noxious thermal CO(2) laser stimulation was used to evoke reflex responses. Neonatal spinal cord transection resulted in a disrupted reflex organization in the adult rat, resembling that previously found in neonatal rats. The receptive fields of single hindlimb muscles exhibited abnormal distribution of sensitivity not matching the withdrawal action of the effector muscles. Likewise, the composite nocifensive movements, as documented in the awake rat, often resulted in erroneous movements toward the stimulus. It is concluded that withdrawal reflexes do not become functionally adapted in rats spinalized at birth. These findings suggest a critical role for supraspinal systems in the postnatal tuning of spinal nociceptive systems.     

Characteristics of sympathetic reflexes evoked by electrical stimulation of phrenic nerve afferents.

In chloralose-anaesthetized cats, sympathetic reflex responses were recorded in left cardiac and renal nerve during stimulation of afferent fibres in the ipsilateral phrenic nerve. In cardiac nerve, a late reflex potential with a mean onset latency of 75.6 +/- 13.8 ms was regularly recorded which, in 20% of the experiments, was preceded by an early, very small reflex component (latency between 35 and 52 ms). In contrast, in renal nerve only a single reflex component after a mean latency of 122.1 +/- 13.1 ms was observed. Bilateral microinjections of the GABA-agonist muscimol into the rostral ventrolateral medulla oblongata resulted in a nearly complete abolition of sympathetic background activity and in an 88% reduction of the late reflex amplitude with only small effects on the latency of the evoked potentials. Under this condition, an early reflex component was never observed to appear. After subsequent high cervical spinalization, the residual small potentials which persisted after bilateral muscimol injections were completely abolished and in cardiac nerve an early reflex potential with a mean latency of 45 +/- 10 ms was observed in all but one experiment. The early reflex was therefore referred to as a spinal reflex component which, however, is suppressed in most animals with an intact neuraxis. In the renal nerve a spinal response was only observed in one experiment after spinalization. The results suggest that sympathetic reflexes evoked by stimulation of phrenic nerve afferent fibres possess similar spinal and supraspinal pathways as previously described for somato-sympathetic and viscero-sympathetic reflexes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Stimulation of the periaqueductal gray matter of the rat produces a preferential ipsilateral antinociception.

The few studies analyzing somatotopic organization of stimulation-produced antinociception (SPA) from the periaqueductal gray matter (PAG) have reported contradictory results. In the present study, the distribution of SPA on the hindquarters was assessed by measuring the threshold for inhibition of withdrawal reflexes to noxious heat applied to the hindpaws and tail in pentobarbital-anesthetized rats. Of the 3 body regions tested, the hindpaw contralateral to the stimulating electrode required the highest level of PAG stimulation to inhibit withdrawal. Reducing the intensity of the heat stimulus applied to the hindpaws caused a concomitant reduction in SPA threshold. As before, a higher stimulation current was needed to inhibit the withdrawal reflex in the contralateral than in the ipsilateral paw. These data indicate the antinociception from PAG stimulation is not equally distributed throughout the body, and that the intensity of the noxious stimulus influences the threshold for SPA.     

Neuropathy reduces viscero-somatic inhibition via segmental mechanisms in rats

The effect of an experimental neuropathy on the viscero-somatic inhibition was studied in lightly anesthetized rats. In controls, colo-rectal distension at noxious intensities produced a multisegmental prolongation of the withdrawal response induced by noxious stimulation of the skin. In rats with a spinal nerve-ligation induced neuropathy this viscero-somatic inhibition was significantly reduced within the neuropathic segment (the hindlimb) but not outside of it (the tail). Naloxone, an opioid antagonist, attenuated this viscero-somatic inhibition in controls and it did not restore the inhibition in neuropathic rats. The results indicate that somatic neuropathy produces a segmental attenuation of viscero-somatic inhibition and this attenuation cannot be explained by a nerve injury-induced release of endogenous opioids. The decreased inhibition of somatic signals may contribute to the hypersensitivity observed in neuropathic conditions.     

Facilitation of the tail-flick reflex by noxious cutaneous stimulation in the rat: antagonism by a substance P analogue

Effects of noxious cutaneous stimulation on the tail flick reflex were examined in the anaesthetized rat. Noxious stimulation was applied by immersing the distal 4 cm of the tail in water at 55 degrees C for 1.5 min. The tail flick reflex was tested at 3 min intervals by applying a noxious radiant heat stimulus to a region of the tail 10 cm proximal to the tip. Tail immersion reduced reaction time to tail flick by 30% and 20% at 0.5 and 3.5 min after immersion, respectively. Reaction time returned to control at 6.5 min and tended to increase above baseline values at 9.5 and 12.5 min. Naloxone (10 mg/kg, i.p.) potentiated the effects of tail immersion on reaction time and prevented the increase above baseline. When the surface temperature of the skin used to evoke the tail flick reflex was raised by 10 degrees C using innocuous radiant heat, reaction time was not significantly different from the control, suggesting that an increase in skin temperature per se is insufficient to account for the response to immersion. Intrathecal administration of a substance P antagonist (1 nmol) attenuated the response to tail immersion. These results indicate that noxious cutaneous stimulation may release an agent in the spinal cord which facilitates the tail flick reflex, and that this agent is antagonized by a substance P antagonist.     

Cutaneous vascular responses evoked by noxious stimulation in rats with the spinal nerve ligation-induced model of neuropathy

Antidromic activation of nociceptive nerve fibres innervating the skin produces an axon reflex that involves extravasation and vasodilation of cutaneous blood vessels. We determined whether the axon reflex of the hindlimb skin is influenced by an experimental model of neuropathy induced by unilateral ligation of spinal nerves L(5) and L(6) in the rat. Ligation of spinal nerves induced symptoms mimicking tactile allodynia, as indicated by a marked decrease of the hindlimb withdrawal threshold to mechanical stimulation. The axon reflex induced by antidromic electrical stimulation of nociceptive fibres innervating the plantar skin ipsilateral to the ligation was attenuated according to determination of extravasation response and blood flow response. Lidocaine block or transection of the sciatic nerve of the neuropathic limb did not induce any change in basal blood flow of the plantar skin. The results indicate that ligation of spinal nerves induces an attenuation of the axon reflex. This attenuation reflects a decrease in the efferent function of primary afferent nociceptors innervating the hypersensitive skin of the hindpaw. The attenuation of antidromically-induced vascular responses was not caused by overriding sympathetic activity, as indicated by lack of blood flow effects by lidocaine blocks or a transection of the sciatic nerve.     

Activity in sympathetic neurons supplying skin and skeletal muscle in spinal cats

In anesthetized cats with intact neuraxis, vasoconstrictor neurons supplying skeletal muscle (MVC) and hairy and hairless skin (CVC), and sudomotor neurons innervating sweat glands (SM), exhibit distinct reflex patterns. MVC and SM are largely under excitatory, CVC under inhibitory control of various afferent input systems from the body surface and from the viscera. In chronic spinal animals all 3 types of sympathetic neurons exhibit some resting activity without cardiac and respiratory modulation. Sixty to 150 days after isolation of the neural circuits within the sympathetic systems within the spinal cord from their descending control systems by spinalization, these reflex patterns are very similar to those in animals with intact neuraxis. Important changes which do occur after spinalization are the following: CVC neurons are excited by stimulation of visceral afferents in spinal animals but inhibited in animals with intact neuraxis; noxious stimulation of skin leads to long-lasting after-effects in CVC and SM neurons in spinal animals. Comparison of reflexes among spinal animals and animals with intact neuraxis indicates that spinal circuits are probably important for the functioning of the sympathetic systems. It is possible that these circuits determine the typical reaction patterns seen in the sympathetic systems by integrating multisensory information from primary afferents and information from spinal descending fiber tracts.     

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 (NGCα), 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 NGCα 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 NGCα 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.