Effects of capsaicin on receptive fields and on inhibitions in rat spinal cord

Interactions between inputs from adjacent digits in somatosensory thalamus and cortex of the raccoon

Interactions between somatosensory afferents arriving from different points in the periphery play an important role in sensory discrimination and also provide the substrate for plasticity following peripheral injury. To examine the extent and time course of such interactions, extracellular recordings were made from neurons in the primary somatosensory cortex and the ventroposterior lateral thalamus of anesthetized raccoons. Interactions between adjacent digits were studied using the conditioning-test paradigm in which a test pulse was delivered to the digit containing the neuron's receptive field (the on-focus digit) at various intervals following conditioning stimulation of an adjacent, off-focus digit. Off-focus stimulation produced predominantly inhibition of the test response with a maximum effect at 20-40 ms in both cortex and thalamus. The mean inhibition was approximately twice as large in the thalamus as in the cortex. Recordings were made in other animals after unmyelinated C fibers had been destroyed in the on-focus digit by subcutaneous injection of capsaicin. This resulted in a doubling of the responses evoked by the test stimulus in both regions, but the spontaneous discharge rate was not changed. The amount of inhibition produced in the cortex was unchanged by capsaicin treatment, but was reduced in the thalamus compared to control animals. This indicates that capsaicin-sensitive peripheral afferents provide a tonic control over interdigit inhibition in the thalamus.

Dorsal Horn Plasticity Following Re-routeing of Peripheral Nerves: Evidence for Tissue-Specific Neurotrophic Influences from the Periphery

Some properties of primary sensory neurons change when they reinnervate new peripheral targets (McMahon et al., Neuroscience, 33, 67 - 75, 1989). We ask here if such influences can extend to the central connectivity of sensory neurons. In adult rats the nerve to the gastrocnemius muscle (GN) and the cutaneous sural nerve (SN) were self- and cross-anastomosed on left- and right-hand sides, respectively, so that they regenerated to either appropriate or inappropriate targets. Ten to 14 weeks later, the distribution and strength of spinal connections of the SN and GN were determined. The unmyelinated afferents in the GN innervating skin increased their connectivity to 286% of that seen for the GN innervating muscle (P < 0.005), and came to resemble normal cutaneous afferents. However, for the SN there was no significant difference between appropriately and inappropriately regenerated nerves by this measure. The ability of myelinated fibres to produce inhibitions and facilitations in dorsal horn cells was also assessed. The intact or self-anastomosed SN produced predominantly inhibitory effects, whilst the GN produced predominantly facilitatory effects. After the SN had regenerated to muscle its central effects became predominantly facilitatory, whilst those of the GN innervating skin became inhibitory. These changes were statistically significant. In conclusion, we have found that major changes in the physiology of central connections in the dorsal horn may occur following peripheral reinnervation of foreign targets. The changes that were seen were appropriate to the new target, and could not easily be explained by non-specific changes due to axotomy, or changes in A-fibre-mediated inhibitions. We suggest that these effects might arise because of trophic influences arising in and specific to different peripheral targets.

Effect of neonatal capsaicin and infraorbital nerve section on whisker-related patterns in the rat trigeminal nucleus

In the present study, we investigated the effect of neonatally administered capsaicin on whisker-related pattern formation in the rat trigeminal complex. Both normal whisker-related patterns of barrelettes and the modified patterns seen after neonatal section of the infraorbital nerve were assessed. Capsaicin caused no change in the pattern or size of cytochrome oxidase (CO) barrelettes in the principal trigeminal nucleus (Vp) or trigeminal nucleus interpolaris (Vi) or caudalis (Vc). Injections of horseradish peroxidase (HRP) or wheatgerm agglutinin conjugated to HRP (WGA-HRP) into the posteroorbital (PO) whisker follicle in vehicle-treated animals showed that WGA labelled a larger number of trigeminal ganglion cells than HRP (203 +/- 23; cf. 158 +/- 19), with an increased labelling of small-diameter neurons (HRP: 25.9 +/- 7.7 microm; WGA: 23.2 +/- 7.2 pm). Capsaicin caused a loss of smaller diameter cells but had no effect on the location, cross-sectional area, or rostrocaudal extent of the transganglionically labelled HRP terminations in Vp, Vi, Vc, and cervical dorsal horn. WGA-HRP labelling revealed similar, but less dense, central terminal areas as HRP and an additional area of superficial terminals in the caudal medulla; these were also unaffected by capsaicin treatment. After infraorbital nerve section, CO patches and transganglionically labelled afferent terminations, corresponding to innervated nonmystacial whiskers, were approximately doubled in size. Capsaicin had no effect on the increased size of these spared whisker patches or their afferent terminal areas. These results suggest that barrelette formation is not dependent on unmyelinated afferents and that the changes in response properties seen after capsaicin, such as increased receptive fields, reflect functional changes rather than anatomical expansion of afferent terminal areas.

Five sources of a dorsal root potential: their interactions and origins in the superficial dorsal horn

The dorsal root potential (DRP) was measured on the lumbar dorsal roots of urethan anesthetized rats and evoked by stimulation of five separate inputs. In some experiments, the dorsal cord potential was recorded simultaneously. Stimulation of the L3 dorsal root produced a DRP on the L2 dorsal root containing the six components observed in the cat including the prolonged negative wave (DRP V of Lloyd 1952). A single shock to the myelinated fibers in the sural nerve produced a DRP on the L6 dorsal root after the arrival in the cord of the afferent volley. The shape of this DRP was similar to that produced by dorsal root stimulation. Repetitive stimulation of the myelinated fibers in the gastrocnemius nerve also produced a prolonged negative DRP on the L6 dorsal root. When a single stimulus (<5 microA; 200 micros) was applied through a microelectrode to the superficial Lissauer Tract (LT) at the border of the L2 and L3 spinal segments, a characteristic prolonged negative DRP (LT-DRP) began on the L2 dorsal root after some 15 ms. Stimulation of the LT evoked DRPs bilaterally. Recordings on nearby dorsal roots showed this DRP to be unaccompanied by stimulation of afferent fibers in those roots. The LT-DRP was unaffected by neonatal capsaicin treatment that destroyed most unmyelinated fibers. Measurements of myelinated fiber terminal excitability to microstimulation showed that the LT-DRP was accompanied by primary afferent depolarization. Repetitive stimulation through a microelectrode in sensorimotor cortex provoked a prolonged and delayed negative DRP (recorded L2-L4). Stimulation in the cortical arm area and recording on cervical dorsal roots showed that the DRP was evoked more from motor areas than sensory areas of cortex. Interactions were observed between the LT-DRP and that evoked from the sural or gastrocnemius nerves or motor cortex. The LT-DRP was inhibited by preceding stimulation of the other three sources but LT stimulation did not inhibit DRPs evoked from sural or gastrocnemius nerves on the L6 dorsal root or from motor cortex on the L3 root. However, LT stimulation did inhibit the DRP evoked by a subsequent Lissaeur tract stimulus. Recordings were made from superficial dorsal horn neurons. Convergence of input from LT sural, and gastrocnemius nerves and cortex was observed. Spike-triggered averaging was used to examine the relationship between the ongoing discharge of superficial dorsal horn neurons and the spontaneous DRP. The discharge of 81% of LT responsive cells was correlated with the DRP.

Neonatal capsaicin treatment alters the metabolic activity of subcortical somatosensory structures of developing rats in response to whisker stimulation

Sprague-Dawley rats neonatally treated with capsaicin exhibited significant changes in 2-deoxy-glucose uptake in the subcortical somatosensory structures following repetitive whisker deflection. There was an increased uptake in the spinal trigeminal nucleus pars caudalis and a decrease in the principal sensory nucleus and the ventral posteromedial thalamic nucleus. The present study suggests that disruption of nociceptive afferents during development causes functional changes to subcortical somatosensory trigeminal nuclei.

The immediate effects of peripheral deafferentation on neurons of the cuneate nucleus in raccoons

Single-unit recordings were obtained from 42 neurons in the cuneate nucleus of 12 anesthetized raccoons. All neurons had receptive fields on the glabrous skin of a forepaw digit. Temporary removal of the dominant excitatory input to a neuron, by injection of lidocaine into the base of the digit, did not result in any expansion of the excitatory receptive field onto adjacent, "off-focus" digits. Similarly, the responses evoked from the off-focus digits by electrical stimulation, which had a longer latency and a higher threshold, were not improved during the lidocaine block. Inhibition was produced in the majority of neurons by high-intensity mechanical stimulation of the off-focus digits, but this was also unchanged when the dominant excitatory input to the neurons was blocked. Since this from of inhibition is not apparent in the somatosensory thalamus before denervation, the spontaneous activity of thalamic neurons must be controlled by inputs other than the cuneate nucleus. These results also indicate that the long-term reorganization seen in the thalamus and cortex is not attributable to a simple unmasking of connections from the adjacent digits within the cuneate nucleus, but may involve strengthening of the connections responsible for longer-latency responses. The only significant change induced in cuneate neurons by temporary denervation was a decrease in the firing rates of 69% of the neurons that had spontaneous activity. Since it is unlikely that any of the large-diameter afferents from touch receptors can account for this finding, mechanically insensitive afferent fibers from the digit may contribute to the spontaneous activity of cuneate neurons, either directly or via a relay in the spinal cord.

Inflammation and hyperalgesia in rats neonatally treated with capsaicin: effects on two classes of nociceptive neurons in the superficial dorsal horn

To address the mechanisms of hyperalgesia and dorsal horn plasticity following peripheral tissue inflammation, the effects of adjuvant-induced inflammation of the rat hindpaw on behavioral nociception and nociceptive neuronal activity in the superficial dorsal horn were examined in neonatally capsaicin-treated rats 6-8 weeks of age. Capsaicin treatment resulted in an 82% loss of unmyelinated fibers in L5 dorsal roots, a dramatic reduction of substance P-like immunoreactivity in the spinal cord, and a significant decrease in the percentage of dorsal horn nociceptive neurons that responded to C-fiber stimulation and noxious heating of the skin. The thermal nociceptive threshold was significantly increased in capsaicin-treated rats, but behavioral hyperalgesia to thermal stimuli still developed in response to inflammation. Following inflammation, there was a significant decrease in mechanical threshold and an increase in response duration to mechanical stimuli in both vehicle- and capsaicin-treated rats, suggesting that a state of mechanical hyperalgesia was also induced. The capsaicin treatment appears to have differential effects on nociceptive specific (NS) and wide-dynamic-range (WDR) neurons in inflamed rats. Expansion of the receptive fields of nociceptive neurons, a measure of the effect of inflammation-induced CNS plasticity, was less extensive for NS than for WDR neurons in capsaicin-treated rats. Compared to vehicle-treated rats, a smaller population of NS neurons, but a similar percentage of WDR neurons, had background activity in inflamed capsaicin-treated rats. C-fiber strength electrical stimulation of the sciatic nerve produced expansion of the receptive fields in a greater portion of NS neurons (53%, P < 0.05) in capsaicin- than in vehicle-treated rats (32%). There was no difference in stimulation-induced expansion of the receptive fields for WDR neurons between vehicle- or capsaicin-treated rats. An N-methyl-D-aspartate receptor antagonist, MK-801, attenuated the behavioral hyperalgesia and reduced the receptive field size of dorsal horn neurons in inflamed capsaicin- and vehicle-treated rats. The data suggest that while capsaicin-sensitive primary afferents may be involved in neuronal plasticity induced by peripheral tissue inflammation, changes in the capsaicin-insensitive WDR and NS populations are sufficient to produce thermal and mechanical hyperalgesia after the loss of capsaicin-sensitive primary afferents.

Effects of neonatal capsaicin treatment on descending modulation of spinal nociception from the rostral, medial medulla in adult rat

Stimulation-produced modulation from the rostral, medial medulla (RMM) on the spinal nociceptive tail-flick (TF) reflex and on lumbar spinal dorsal horn neuron responses to noxious cutaneous stimuli was studied in adult rats treated as neonates with capsaicin or vehicle. In vehicle-treated rats (n = 7), both descending facilitatory and inhibitory influences on the TF reflex were produced from the RMM. At 11/23 sites in the RMM, electrical stimulation produced biphasic modulatory effects. Electrical stimulation facilitated the spinal nociceptive TF reflex at low intensities (5-25 microA) and inhibited the TF reflex at greater intensities (50-200 microA). The mean threshold intensity of stimulation to inhibit the TF reflex (cut-off time = 7.0 s) was 66 microA (n = 11). At 11 of 23 sites, electrical stimulation only inhibited the TF reflex; the mean threshold intensity of stimulation to inhibit the TF reflex was 50 microA (n = 11). At one stimulation site, electrical stimulation only facilitated the TF reflex at the intensities tested (5-100 microA). In capsaicin-treated rats (n = 6), the proportion of sites from which electrical stimulation only inhibited the TF reflex was significantly less (3/27 sites = 11%) than in vehicle-treated rats (11/23 = 48%). The threshold intensity of stimulation to inhibit the TF reflex from these three sites was 50 microA. The number of sites in RMM from which electrical stimulation only facilitated the TF reflex was significantly greater in capsaicin-treated rats (15/27 = 56%) than in vehicle-treated rats (1/23 = 4%). Neither the number of sites in RMM from which electrical stimulation produced biphasic modulatory effects on the TF reflex (48% and 33%, respectively) nor the intensities of stimulation or magnitudes of facilitation or inhibition of the TF reflex significantly differed between vehicle- and capsaicin-treated rats. In electrophysiological experiments, all units studied responded to non-noxious and noxious intensities of mechanical stimulation applied to the glabrous skin of the plantar surface of the ipsilateral hind foot and also to noxious heating of the skin (50 degrees C). The number of sites where electrical stimulation produced only facilitatory effects on responses of spinal dorsal horn neurons to noxious stimulation (thermal or mechanical) of the skin was significantly increased from 13% of the total sites in vehicle-treated rats to 40% in capsaicin-treated rats.(ABSTRACT TRUNCATED AT 400 WORDS)

Activity of deep dorsal horn neurons in the anaesthetized rat during hyperalgesia of the hindpaw induced by ultraviolet irradiation

Thermal hyperalgesia was induced by UV irradiation of the glabrous skin of the hindpaw of adult female Sprague-Dawley rats. We have recorded single cell activity and studied excitability changes in wide dynamic range neurons in the lumbar spinal segments during the early phase (days 1-3) and late phase (days 5-7) of thermal hyperalgesia in animals under urethane anaesthesia. The proportion of spontaneously active wide dynamic range cells was increased following UV irradiation and the degree of spontaneous activity was enhanced during the course of hyperalgesia. In addition there was a significant increase in the total number of spikes evoked by standardized mechanical and noxious heat stimuli when tested at days 1-3 and days 5-7. The duration of the evoked responses was also significantly prolonged in both UV-treated groups. The noxious temperature threshold to radiant heat stimulation was significantly decreased on the UV-treated but not on the contralateral hindpaw. The average size of the receptive fields on the UV-treated paws was expanded in comparison to control. To differentiate between possible central and peripheral components of the hyperactivity of wide dynamic range cells we performed in situ dorsal rhizotomy during the recording. Cutting the dorsal roots (L2-5) evoked a significantly larger and more prolonged discharge in wide dynamic range cells in both UV-treated groups in comparison to control. Spontaneous activity in spinal wide dynamic range neurons was reduced after rhizotomy in each group. However, the decrease was only significant at days 1-3 (P < 0.05) but not at days 5-7.(ABSTRACT TRUNCATED AT 250 WORDS)

Sympathetically-maintained causalgiform disorders in a model for neuropathic pain: a review

Partial nerve injury is the main cause of sympathetically maintained causalgiform pain disorders in humans. We present here an animal model of this condition, produced in rats by a unilateral ligation of about half of the sciatic nerve. Starting hours after the operation and for several months thereafter, the rats developed signs of spontaneous pain, touch-evoked allodynia and hyperesthesia, and mechanical and thermal hyperalgesia in the partially denervated as well as the intact contralateral foot. These disorders were maintained by the sympathetic outflow and disappeared following postoperative sympathectomy. In neonatally capsaicinated rats we found that touch-evoked allodynia and hyperesthesia were mediated by A-fibers whereas thermal hyperalgesia was mediated by C-fibers. These disorders were not due to receptor sensitization of remaining afferent fibers by prostaglandins. We found strain differences and genetic inheritance of these causalgiform disorders which were correlated with the expression of autotomy to hind-paw denervation.

Treatment of reflex sympathetic dystrophy with topical capsaicin. Case report

A 31-year-old woman with intractable reflex sympathetic dystrophy experienced nearly complete, though temporary, resolution of pain following 3 weeks of topical capsaicin. We propose that capsaicin may be a useful treatment for reflex sympathetic dystrophy, either by depleting substance P from primary afferent neurons that mediate allodynia, or by modulating sympathetic efferent activity.

Neonatal capsaicin treatment induces invasion of the substantia gelatinosa by the terminal arborizations of hair follicle afferents in the rat dorsal horn

Capsaicin, administered on the day of birth, was found to alter laminar distribution, but not the receptive field properties or the morphology of the collateral arborizations of hair follicle afferents (HFAs) intra-axonally injected with horseradish peroxidase (HRP). Of the 65 HFA terminal arbors in capsaicin treated rats, 46 (71%) were found to enter the substantia gelatinosa (in control rats, 44/165, 27%). All of the collaterals projected to somatotopically normal areas of cord. Dorsal horn shrinkage (21%), as estimated by planimetric measurements of Nissl and acetylcholinesterase-stained material, was only a partial explanation of this result. This idea was supported by the statistically significant increase (27%, P less than 0.05) in the absolute dorsoventral length of collaterals. The results show that the destruction of unmyelinated fibres during the early postnatal period by capsaicin induces HFA invasion into the area that C fibres normally occupy. This invasion suggests that the laminar termination sites for different primary afferent fibres are not altogether specified and that intact neonatal primary afferents have the capacity to sprout into denervated regions of spinal cord.

Painful dysaesthesias following peripheral nerve injury: a clinical and electrophysiological study

Thirty-three patients with complete median, ulnar or digital nerve transections were studied 4 months to 13 years subsequent to suture or nerve grafting. In all cases, sensory disturbances, in terms of paraesthesia or hypaesthesia, were encountered. Painful or unpleasant symptoms, allodynia or hyperpathia, were observed most frequently in patients with poor recovery. The clinical findings and the patients' subjective complaints were correlated to microneurographic single fibre recordings of regenerated cutaneous mechanoreceptors. In more than 80% of the recordings, discharge properties of regenerated receptors, thresholds and a variety of other electrophysiological data were similar or equal to normal controls. Less than 20% of the receptors exhibited atypical properties suggesting defective steady-state regeneration. The ratio of rapidly adapting (RA-units) to slowly adapting mechanoreceptors (SA-units) was inverse in relation to normals. The density of regenerated RA-receptors was higher in the proximal than in the distal part of the reinnervated area. This paralleled the clinical finding of reduced sensory discrimination in these cases and suggests that SA-units may regenerate preferentially. In painful conditions no single fibres could be recorded, reflecting the relative paucity of fibres and probably the atrophy of the nerve. The results of the microstimulation experiments, although less reliable, revealed some evidence that the central processing of regenerated units is abnormal. Clinical and electrophysiological data supported this concept of central changes underlying some of the phenomena observed during peripheral nerve regeneration.

The neuroanatomy, neurophysiology, and neurochemistry of pain, stress, and analgesia in newborns and children

Beginning with a brief description of mature anatomic pathways and neurotransmitters in the "pain system," this article details their development in the human fetus, neonate, and child. Special emphasis is given to the basic mechanisms and physiologic effects of opioid analgesia. The clinical implications of these data are described, particularly with regard to the maintenance of cardiovascular stability and hormonal-metabolic homeostasis in newborns and children undergoing surgery or other forms of stress.

Lack of sensitization of primary afferent receptors by prostaglandins in a rat model of causalgic chronic pains

Unilateral entrapment of half of the sciatic nerve in a ligature rapidly produces in rats bilateral hyperalgesia: decreased withdrawal thresholds to von-Frey hair touch and to noxious CO2 laser heat pulses and unilateral hyperpathic responses to a supra-maximal noxious heat pulse. These abnormal pain responses last many months and are very similar to those seen in humans with causalgia. In the present study we determined whether the underlying mechanism involves bilateral sensitization of primary afferent receptors. Since the responses of rats with partial sciatic injury to stimulation of rostral areas (forepaws, muzzle and auricles) were normal, we presume that the hyperalgesia at the contralateral hindpaw could not be due to receptor sensitization, but to rapid central plasticity. Moreover, since indomethacin did not prevent the bilateral hyperalgesia, we conclude that the causalgiform pain disorders seen in the ipsilateral hindpaw did not derive from receptors sensitized by prostaglandins.

Diminished dorsal root GABA sensitivity following chronic peripheral nerve injury

The depolarizing effect of gamma-aminobutyric acid (GABA) on rat lumbar dorsal roots was studied in a sucrose gap chamber following axotomy or crush injury of the sciatic nerve or dorsal root. The mean depolarization elicited by GABA on normal dorsal roots (3.96 +/- 0.71 mV, N = 14) was significantly reduced following chronic sciatic axotomy (2.02 +/- 0.99 mV, N = 15). Chronic sciatic crush injury had no significant effect on dorsal root GABA sensitivity. The amplitudes of the dorsal root compound action potentials were the same from rats with normal and injured sciatic nerves, indicating that axons proximal to the sciatic nerve lesion did not undergo appreciable degeneration. A marked loss of dorsal root GABA sensitivity was also seen following dorsal root axotomy or crush injury (1.02 +/- 0.98 mV (N = 10) and 0.69 +/- 0.70 mV (N = 9), respectively). These results indicate that GABA sensitivity of dorsal roots is attenuated following peripheral nerve lesions in which regeneration and functional reconnection with peripheral targets are prevented. Previous work indicates that the primary afferent depolarization is reduced under similar conditions. The reduction in GABA sensitivity of dorsal root fibers described here may have a contributory role in the reduced primary afferent depolarization that follows peripheral nerve transection, which has pathophysiologic implications in chronic pain syndromes.

Cutaneous hyperalgesia: contributions of the peripheral and central nervous systems to the increase in pain sensitivity after injury

This study assesses the contributions of the peripheral and central nervous systems in the development of hyperalgesia (increased pain sensitivity) after an injury. Experiments were carried out to examine the role of C-fiber afferents, the spinal cord and sympathetic efferents on inflammation, primary hyperalgesia and referred hyperalgesia produced in rats by a heat injury. A peripheral mechanism was indicated since both primary hyperalgesia and inflammation after a heat injury were significantly attenuated by blocking C-fiber afferents with local capsaicin. In addition, a central mechanism was indicated since the spread of hyperalgesia to the paw contralateral to a heat injury was prevented by either spinal anesthesia or the blocking of sympathetic efferents by guanethidine. A further role for central mechanisms was indicated since referred hyperalgesia--the enhancement of self-mutilation (autotomy) of a denervated limb which had previously sustained a heat injury--was reduced by spinal anesthesia or a combined blocking of C-fiber afferents and sympathetic efferents with intrathecal capsaicin + guanethidine. The results strongly suggest that referred hyperalgesia after a heat injury is dependent on increased spinal cord activity. However, autotomy in rats that did not undergo a previous injury was unaffected by either spinal anesthesia or intrathecal capsaicin. This suggests that spinal cord hyperactivity, although it plays a role in hyperalgesia following a heat injury, is not a crucial factor in producing pain and hyperalgesia after a nerve injury.

Deafferentation and chronic pain in animals: an evaluation of evidence suggesting autotomy is related to pain

This paper examines evidence which suggests that the self-mutilation of deafferented limbs exhibited by laboratory animals is a response to pain or dysesthesia and is therefore an adequate model of chronic pain. Evidence from studies using physiological, pharmacological and behavioral methods provides strong support that autotomy reflects chronic pain. New evidence presented in this paper demonstrates that specific treatments can be used to manipulate the extent of autotomy, causing increases or decreases, as well as restricting it to specific parts of a denervated foot. This evidence argues that autotomy scores are an appropriate measure of the degree of pain or dysesthesia which results from the deafferentation of a limb.

Autotomy following sciatic and saphenous nerve sections: sparing of the medial toes after treatment of the sciatic nerve with capsaicin

Autotomy, or self-mutilation of the foot following sciatic and saphenous nerve lesions, was examined in rats after pretreatment of the sciatic nerve with capsaicin. This pretreatment produced an alteration in autotomy behavior which resulted in the sparing of the medial side of the foot. The effect occurred following a long (12-week) pretreatment-test interval, but not after shorter (1- and 4-week) intervals. The effect also depended on the successive transecting of the saphenous and sciatic nerves. Sparing of the medial side of the foot occurred only when the saphenous nerve was transected at the time of the sciatic nerve treatment with capsaicin. Because the side of the foot innervated by the saphenous nerve was spared by treating the sciatic nerve with capsaicin, we suggest that capsaicin alters the course of autotomy by preventing collateral innervation of the saphenous region by the intact sciatic nerve during the pretreatment-test interval. The fact that this occurs only after a 12-week interval suggests that capsaicin's effect on collateral innervation is a gradual process that requires a long time to develop.

Electrophysiological studies on the effects of intrathecal morphine on nociceptive neurones in the rat dorsal horn

We have studied the effects of intrathecal morphine on the responses of 38 dorsal horn neurones in the intact rat under halothane anaesthesia to A and C fibre electrical stimulation and to natural stimuli applied to their receptive fields. Morphine selectively reduced the C fibre and pinch evoked activity in a dose-dependent naloxone-reversible manner with an ED50 of 7 nmoles. The 'wind-up' of neurones to repetitive stimulation was little altered except with the highest doses (50-150 nmoles) tested. By contrast, the A fibre evoked responses of the neurones were only slightly reduced by morphine and both the tactile responses and receptive field size to innocuous stimuli enhanced for certain cells. The results are discussed in relation to the spinal actions of opiates and their clinical applications.

Loss of spinothalamic tract neurons following neonatal treatment of rats with the neurotoxin capsaicin

The purpose of the present experiments was to determine whether the organization of spinothalamic tract (STT) cells of adult rats was altered following the loss of most of their small-diameter peripheral afferent fibers, resulting from the neonatal administration of capsaicin. Rat pups were randomly assigned to serve as normal controls, to serve as vehicle controls, or to receive subcutaneous injections of capsaicin (50 mg/kg) on postnatal day (PND) 1, 2, 7, or 15; or an injection on PND 1, 3, and 5. When 60 days old, they were anesthetized and received 0.1-microliter thalamic injections of wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP) in the area of the central lateral nucleus (CL), the posterior group (PO), and the ventrobasal complex (VB), or the area of CL or VB. Following a survival time of 48 hr, the animals were perfused, and neuronal HRP reaction product was visualized with tetramethylbenzidine. The number and distribution of WGA:HRP-labeled STT neurons varied in treated animals with the time of capsaicin injection. Rats injected with capsaicin on or before PND 7 demonstrated a significant reduction of labeled STT neurons from the superficial laminae of the spinal cord. Additionally, lamina I neurons were unlabeled in animals treated before PND 7 even with large thalamic injections. Differences in the distribution of labeled STT neurons could not be demonstrated for animals injected with capsaicin on PND 7 or PND 15, though there was a decrement in the number of labeled neurons in PND 7 animals. In order to make certain that absence of labeled STT neurons was not due to some technical error or to insufficient spread of WGA:HRP at the site of injection, six injections of WGA:HRP were placed in the thalamus of PND 1 and normal adult animals. Where the dense core of reaction product did not extend caudal to the posterior commissure, WGA:HRP-positive neurons were located and distributed similarly to those cases described for large thalamic injections. Neurons in superficial laminae of the nucleus proprius and lamina I of the contralateral spinal cord were labeled where the dense core of the thalamic injection extended into the mesencephalon of PND 1 animals. These studies indicate that the number and distribution of the cells of origin of the STT are altered in adult rats following their neonatal treatment with the neurotoxin capsaicin, and that this effect is limited to a critical postnatal period.

Expansion of receptive fields in the mouse cortical barrelfield after administration of capsaicin to neonates or local application on the infraorbital nerve in adults

A possible role fore peripheral unmyelinated fibres on the establishment and maintenance of stable receptive fields mediated by myelinated afferents has been studied in the mouse cortical barrelfield. The barrels are cytoarchitectonic units consituting a visible and particularly precise somatotopic map of the facial vibrissae. Barrelfields were mapped electrophysiologically in adults using two experimental setups: (i) after destruction of unmyelinated peripheral fibres by systemic administration of capsaicin neonatally; and (ii) after inactivation of these fibres by local application of capsaicin to the nerve subserving the vibrissae in the adult. The latter procedure is known not to destroy fibres. Control animals received the vehicle used to dissolve capsaicin. The effectiveness of capsaicin on the peripheral fibres was assessed histochemically on their terminals in the substantia gelatinosa. Both methods of application of capsaicin resulted in a large expansion of cortical receptive fields: units within a particular barrel were driven by a statistically significant, larger number of vibrissae than in controls. No morphological alterations of the barrels were found in cortical tangential sections. We concluded that capsaicin treatment 'defocusses' the normal physiological precision without destroying the general plan of representation and without alteration of the barrel pattern.

Injury-induced plasticity of the flexor reflex in chronic decerebrate rats

The hindlimb-flexor-withdrawal reflex elicited by stimulation of the skin of the hindpaw has been examined in chronic decerebrate rats. This flexor reflex manifests as a typical phasic avoidance response when measured either behaviourally in the decerebrate rat or electrophysiologically in the decerebrate-spinal preparation. Once the threshold of the cutaneous flexor-reflex afferents in the skin have been exceeded a brief burst of activity with only a short afterdischarge occurs in the flexor motoneurones. The response to sustained stimuli adapts rapidly. In the absence of any treatment to the hindlimb the threshold, duration and responsiveness of the reflex remains stable when tested repeatedly. Thermal or chemical stimuli of sufficient intensity to produce tissue injury and prolonged local inflammation in a hindpaw of the chronic decerebrate rat result in marked and long-lasting (several weeks) alterations in the ipsilateral withdrawal reflex. The mechanical threshold necessary to elicit the reflex by stimulation of the hindpaw falls so that light touch or brush can now elicit a response instead of the firm pressure or pinch required pre-injury. Suprathreshold stimuli to the inflamed skin generate a sustained oscillating pattern of flexion in contrast to the brief flicking movement found in control animals. Electromyographic recordings from the hamstring flexor muscles ipsilateral to the inflamed hindpaw show decreased mechanothresholds, increased spontaneous activity, prolonged afterdischarges to brief stimuli and a slowly adapting tonic response to sustained stimulation. Populations of single cutaneous mechanoreceptive C-primary afferents recorded both from untreated decerebrate rats and from rats with an inflamed hindpaw are indistinguishable in terms of their response properties. There is no difference in threshold, spontaneous activity or afterdischarge between the two populations. The possible mechanisms responsible for the conversion of the high threshold phasic flexor reflex into a low threshold tonic reflex are discussed as are the possible implications for sensory disorders that accompany chronic injury in man.

Hypothalamic thermosensitivity in capsaicin-desensitized rats

In rats, we tested the hypothesis that capsaicin desensitization reduces hypothalamic warm thermosensitivity. We locally heated and cooled the hypothalamus using water-perfused thermodes while observing thermoregulatory variables. In untreated rats, a small dose of capsaicin had profound effects on thermoregulation. However, desensitizing rats to capsaicin had no effect on hypothalamic thermosensitivity for metabolic rate or changes in body temperature due to displacements of hypothalamic temperature. Contrary to current opinion, we conclude that capsaicin desensitization does not alter hypothalamic thermosensitivity to warm or cold.

Nerve growth factor counteracts the neurophysiological and neurochemical effects of chronic sciatic nerve section

The sciatic nerve was sectioned unilaterally in rats and nerve growth factor (NGF) applied locally to the nerve stump for the following 10-14 days using an indwelling osmotic pump. The aim of the experiment was to test whether NGF had any effect on the previously reported neurophysiological and neurochemical events that occur central to a peripheral nerve lesion. The method of application allowed the sciatic nerve on the other side to be used as a control. Primary afferent depolarization fell, as expected, to 13% of its control value after chronic nerve section but if NGF was administered it fell to only 43.5% of control. Chronic nerve section is also known to result in expansion of the receptive fields of deafferented dorsal horn cells. NGF treatment reduced the number of such large receptive fields by 50%. The normal depletion of fluoride resistant acid phosphatase from the cut nerve terminals in the dorsal horn did not occur following NGF treatment. Radioimmunoassay of substance P revealed that the 30% reduction in dorsal horn levels that follows chronic sciatic nerve section did not occur when NGF was applied and that the accompanying 60% decrease in dorsal root ganglion levels was changed to a 64% increase by NGF. The results show that chronic NGF treatment of a cut sciatic nerve does partially reverse the central changes that normally follow deafferentation.

Effect of capsaicin upon fluoride sensitive acid phosphatases in selected ganglia and spinal cord and upon neuronal size and number in dorsal root ganglion

A quantitative method for the analytical separation of the fluoride sensitive acid phosphatases of rat spinal cord and peripheral nervous tissues into tartrate-sensitive and tartrate-resistant forms (TSAP and TRAP, respectively) is described. Evidence supporting the use of L-(+)-tartrate rather than fluoride as an inhibitor is presented. The method is used for the quantitative description of the consequences of neonatal capsaicin treatment, and the results appear to justify the attribution of the TRAP activity to the capsaicin sensitive neurons. In the first experiment, rats were killed at weekly intervals after neonatal capsaicin treatment. In controls, both TRAP and TSAP activities in dorsal root ganglia (DRG) increased during the second postnatal week and remained constant thereafter. At all ages (1-4 weeks) TRAP activity was reduced 50-60% in capsaicin-treated DRG. Reduction of TSAP activity was much less. In a second experiment, rats were treated neonatally with capsaicin or vehicle (control) and allowed to grow to adulthood (4 months). TRAP activity was found to be decreased 38% in the dorsal half of the lumbar spinal cord (L3-L5), decreased 33% in nodose ganglion, and unchanged in superior cervical ganglion of the capsaicin treated animals. TSAP activity was unchanged in dorsal spinal cord and superior cervical ganglion and decreased 33% in nodose ganglion. The number of neurons in C8 DRG was found to be reduced 28% in capsaicin treated animals. The loss appeared to be among the small neurons. The number of large neurons was actually increased in ganglia from capsaicin-treated animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of ES52, an enkephalinase inhibitor, on responses of ventrobasal thalamic neurons in rat

The effects of ES52, a highly potent derivative of Thiorphan, an inhibitor of enkephalinase, at doses of 5 and 10 mg/kg IV were studied on the responses to cutaneous stimuli of 18 "nociceptive" (N), 10 "convergent" (NNn) and 4 "non-nociceptive" (Nn) neurons recorded in the ventrobasal (VB) complex of the rat. The responses of neurons exclusively driven by noxious mechanical and thermal stimuli (N neurons) were depressed by 56% by ES52 15 min after the injection of 5 or 10 mg/kg IV. This depressive effect was reversed by naloxone for half the neurons. For the ten neurons driven by both noxious and non-noxious stimuli (convergent NNn neurons), the responses to noxious heat were decreased by 42% at 15 min. By contrast, there was a marked enlargement of their receptive fields to light tactile stimuli, which was not naloxone-reversible. The receptive fields of neurons exclusively driven by non-noxious stimuli (Nn neurons) were also greatly expanded by ES52. These results show that ES52 can depress the responses of VB thalamic neurons to noxious stimuli; the effects on receptive field size underlines the complexity of the endogenous opiate systems.

The effects of capsaicin applied to peripheral nerves on responses of a group of lamina I cells in adult rats

In adult rats, the sciatic and saphenous nerves on one side were treated topically with capsaicin. The capsaicin treatment had the effect of increasing the latency for withdrawal of the foot from hot water; 11-22 days later, the animals were decerebrated, and cells in the superficial dorsal horn of the lumbar cord with axons projecting in the contralateral dorsolateral funiculus (DLF) were examined electrophysiologically on the treated and untreated sides of the cord. HRP was applied to cut axons of the DLF at C4, in other rats, and retrograde labelling of cells in the lumbar cord indicated that most or all of the recordings in the capsaicin-treated animals were likely to originate from lamina 1. The dorsal horn cells, with receptive fields on the foot, showed decreased responses to electrically evoked afferent impulses in C fibres and grossly altered receptive fields. After capsaicin treatment, the proportion of cells responding to C afferents fell from 83% to 14%. The proportion responding only to C afferents and not to A afferents fell from 9% to 0%. The receptive fields (RFs) of these cells showed two gross abnormalities; 32% of the cells on the treated side had no apparent RF or an ill-defined, intermittent RF, whereas such cells were rare on the untreated side or in intact animals. By contrast 49% of the cells had grossly expanded RFs with an average area of 430 mm2 against the normal average size of 130 mm2.(ABSTRACT TRUNCATED AT 250 WORDS)

Deafferentation in animals as a model for the study of pain: an alternative hypothesis

The notion that post-deafferentation autonomy is a pain response is unsupported by the results of studies with neurotoxins. The selective massive destruction of a fiber system considered essential to normal nociception--unmyelinated primary afferent axons--prior to deafferenting nerve lesions did not stop or even significantly impede post-denervation DI despite massive evidence from humans and animals that pain following nerve lesions originates in the periphery and is generated by abnormal discharges in the injured nerve. In addition, when a reduction in abnormal impulse discharges of both large and small injured sensory axons could be inferred following neonatal sympathectomy, DI was not reduced in incidence or severity. This latter observation (1) provides further support for a dissociation between DI and pain, since any contribution of myelinated primary afferent axons to painful pathology probably was substantially reduced by sympathectomy and (2) suggests that DI also may be unrelated to non-painful sensory pathology attributable to abnormal activity in the thick-diameter fiber population. These findings and an evaluation of other relevant observations suggest that DI may not be a manifestation of deafferentation pain and perhaps this animal model for the experimental study of pain should be discarded. An alternative view of DI, reconcilable with known properties of this behavior, is that it reflects a proclivity in some species and circumstances to shed a functionally-impaired insensate appendage.

Cutaneous inputs to dorsal horn neurones in adult rats treated at birth with capsaicin

Single unit electrical activity has been recorded from dorsal horn neurones in the lumbar spinal cord of adult rats which had been treated at birth with either capsaicin (50 mg kg-1) or with the solvent-vehicle only. The responses of these neurones to electrical stimulation of A- and C-fibres in the sural nerve and to natural stimulation of their cutaneous receptive fields have been studied. In vehicle-injected rats, 54% of the units driven by electrical stimulation of the A-fibres in the sural nerve could also be driven by stimulation of the C-fibres in this nerve. In capsaicin-treated animals, only 30% of such units had a C-fibre input from the sural nerve. In vehicle-injected rats, 51.5% of the neurones with a C-fibre input showed a 'wind-up' effect on repetitive C-fibre stimulation of the sural nerve at 1 Hz. A similar proportion of neurones (55%) displayed this effect in capsaicin-treated rats. There were fewer neurones with very intense 'wind-up' in capsaicin-treated compared to vehicle-treated rats. In capsaicin-treated animals, greater proportions of neurones with 'wind-up' were superficially located in the dorsal horn, had small receptive fields and were driven only by cutaneous nociceptors. The proportions of neurones driven by innocuous mechanical stimulation of the skin, by noxious mechanical stimulation or by both forms of stimulation were similar in vehicle-injected and capsaicin-treated animals. In capsaicin-treated rats, more neurones had 'medium-sized' receptive fields than in vehicle-injected rats. In capsaicin-treated rats, more neurones had receptive fields in the foot and ankle than in vehicle-injected animals, where receptive fields in the toes were predominant. Some neurones showed expanded receptive fields after repetitive electrical stimulation of C-fibres at 1 Hz. This expansion occurred more often in neurones recorded from capsaicin-treated animals than in those of vehicle-injected rats. These results are discussed in relation to the role of afferent C-fibres in sensory mechanisms.