Janus molecule I: dichotomous effects of COMT in neuropathic vs nociceptive pain modalities

The enzyme catechol-O-methyltransferase (COMT) has been shown to play a critical role in pain perception by regulating levels of epinephrine (Epi) and norepinephrine (NE). Although the key contribution of catecholamines to the perception of pain has been recognized for a long time, there is a clear dichotomy of observations. More than a century of research has demonstrated that increasing adrenergic transmission in the spinal cord decreases pain sensitivity in animals. Equally abundant evidence demonstrates the opposite effect of adrenergic signaling in the peripheral nervous system, where adrenergic signaling increases pain sensitivity. Viewing pain processing within spinal and peripheral compartments and determining the directionality of adrenergic signaling helps clarify the seemingly contradictory findings of the pain modulatory properties of adrenergic receptor agonists and antagonists presented in other reviews. Available evidence suggests that adrenergic signaling contributes to pain phenotypes through α(1/2) and β(2/3) receptors. While stimulation of α(2) adrenergic receptors seems to uniformly produce analgesia, stimulation of α(1) or β receptors produces either analgesic or hyperalgesic effects. Establishing the directionality of adrenergic receptor modulation of pain processing, and related COMT activity in different pain models are needed to bring meaning to recent human molecular genetic findings. This will enable the translation of current findings into meaningful clinical applications such as diagnostic markers and novel therapeutic targets for complex human pain conditions.

Consumption of soy diet before nerve injury preempts the development of neuropathic pain in rats

BACKGROUND

A previous report using a partial sciatic nerve ligation (PSL) model for neuropathic pain in rats demonstrated that consumption of soy-containing diets preoperatively and postoperatively suppressed development of mechanical and heat allodynia, as well as hyperalgesia. The current study examined whether dietary soy suppresses these neuropathic sensory disorders when consumed either before or after PSL injury.

METHODS

Male Wistar rats were grouped into seven different feeding regimens. These rats were fed SOY (RMH-1000, PMI Feeds, St. Louis, MO), a diet containing 85% soy protein since weaning, and were then switched to noSOY (Bio-Serv Co., Frenchtown, NJ), a diet devoid of soy at certain time points before PSL injury (14, 7, 1 days, or 15 and 0 h). Postoperatively, these rats were fed SOY or noSOY diets. Levels of mechanical and heat allodynia and hyperalgesia were determined preoperatively and 3, 8, and 14 days after PSL injury.

RESULTS

Compared with groups fed preoperative noSOY, consumption of SOY before PSL injury significantly blunted postoperative levels of allodynia and hyperalgesia. Administering the SOY diet both before and after PSL injury provided no additional suppression of neuropathic pain. No pain suppression was noted in rats fed a noSOY diet preoperatively and SOY diet after PSL injury. Switching from SOY to noSOY feeding within 15 h of PSL injury was sufficient to allow for the full development of allodynia and hyperalgesia.

CONCLUSIONS

Consumption of a soy-containing diet suppressed the development of neuropathic pain after PSL injury. The pain-suppressing properties of dietary soy were the result of a preemptive effect (i.e., when consumed preoperatively), but not a palliative effect (i.e., when consumed postoperatively). This effect of soy-containing diets appears to be short-lived, since switching to a noSOY diet 15 h before ligation abrogated the suppressive effect of soy.

Soy-containing diet suppresses chronic neuropathic sensory disorders in rats

Partial sciatic nerve ligation (PSL) in rodents produces chronic neuropathic sensory disorders resembling neuropathic pain in humans. We previously reported that levels of allodynia and hyperalgesia after PSL injury were markedly attenuated by consumption of soy-containing diets. Here we aimed to show that dietary effect on pain behavior is not specific to a certain laboratory. For this purpose, experiments were conducted in a different laboratory (Baltimore rather than Jerusalem) and a different rat strain (Wistar rather than Sabra), with additional and different testing methods (radiant heat from a lamp rather than a CO(2) laser). Rats were fed two soy-free diets and a soy-containing one for 28 days. The sensitivity of rats to nonnoxious and noxious stimuli was determined before PSL injury, and levels of neuropathic sensory disorders were determined after it. We found that consuming the soy-containing diet prevented development of tactile and heat allodynia, but not mechanical hyperalgesia. This dietary effect was not correlated with calorie intake and weight gain or dietary concentration of fat and carbohydrates. We conclude that, regardless of experimental site, diet markedly affects chronic neuropathic sensory disorders in rats and should be standardized in animal models of pain.

IMPLICATIONS

Levels of chronic sensory disorders in a rat model of allodynia and hyperalgesia after partial sciatic nerve ligation depend on the consumption of a soy-containing diet. Further studies are needed to determine the role of diet in humans with chronic pain.

Heat hyperalgesia following partial sciatic ligation in rats: interacting nature and nurture

As in humans, levels of neuropathic pain produced by nerve injury are highly variable among animals. This variability was attributed to genetic and environmental factors. For example, we reported that chronic neuropathic sensory disorders developing following total (autotomy) or partial nerve injury (allodynia and hyperalgesia) depended on the diet rats consumed. Here we investigated the interaction between genetic and dietary factors in the development of heat hyperalgesia in rats following partial sciatic ligation (the PSL model). We show that heat sensitivity of intact rats and levels of heat hyperalgesia of PSL-injured rats were highly variable across eight different rat strains and seven different diets. Thus, genetic and environmental variables interact in determination of levels of chronic neuropathic sensory disorders in rats.

Correlation of intact sensibility and neuropathic pain-related behaviors in eight inbred and outbred rat strains and selection lines

In some rat strains, total hindpaw denervation triggers autotomy, a behavior of self mutilation presumably related to neuropathic pain. Partial sciatic ligation (PSL) in rats produces tactile allodynia and heat hyperalgesia but not autotomy. Our aims in this study were to examine: (1) whether sensibility of intact rats to noxious and non-noxious stimuli is strain-dependent; (2) whether sensibility of intact rats could predict levels of autotomy, or of allodynia and hyperalgesia in the PSL model; and (3) whether autotomy levels are correlated with levels of allodynia or hyperalgesia. Here we report that in two inbred rat strains (Lewis and Fisher 344), two outbred rat strains (Sabra and Sprague-Dawley) and four selection lines of rats (Genetically Epilepsy-Prone Rats, High Autotomy, Low Autotomy and Flinders Sensitive Line), tactile sensitivity and response duration to noxious heat of intact animals were strain-dependent. Levels of autotomy following hindpaw denervation and of allodynia and hyperalgesia in the PSL model were also strain-dependent. Thus, these traits are determined in part by genetic factors. Sensory sensibility of intact rats was not correlated with levels of autotomy following total denervation, or allodynia and hyperalgesia following partial denervation. We suggest that preoperative sensibility of intact rats is not a predictor of levels of neuropathic disorders following nerve injury. Likewise, no correlation was found between autotomy, allodynia and hyperalgesia, suggesting that neuropathic pain behaviors triggered by nerve injury of different etiologies are mediated by differing mechanisms.

Preoperative open field behavior predicts levels of neuropathic pain-related behavior in mice

Exploratory open field (OF) activity was assessed in seven different mouse strains and selection lines. We counted the number of beam interruptions made by three cagemate mice at a time. This assay tests reactivity to aversive stimuli, anxiety and emotionality. One hindlimb was then totally denervated by transecting the sciatic and saphenous nerves on one side, and autotomy, a behavior thought to be related to neuropathic pain, was quantified over 35 days. We report that OF activity and autotomy are highly variable across different strains/lines. These results reaffirm the genetic control of these behaviors. We also found that these behaviors are inversely and significantly correlated. We suggest that common genetically-determined neural mechanisms may underlie anxiety, emotionality and neuropathic pain in mice.

A novel computerized system for analyzing motor and social behavior in groups of animals

We present here the VMB Tracking System, a novel method for tracking locomotor activity, posture, thigmotactic scanning behavior and social interactions of up to eight animals at a time, at a high resolution (up to+/-0.1 mm). We used a commercially available computerized system that is considerably cheaper than other available methods. This system utilizes a basic personal computer controlling three transponders ('towers') fixed in space above the tested area, where animals as small as rats stroll freely in their normal habitat or in an experimental arena. Each tower emits infra-red (IR) pulses to a transponder ('button') adhered to a plastic mount glued to a shaved area of skin on the animal's back. When the button detects the IR pulses it responds with a button-specific ultrasonic signal that is fed back to the towers. The 3D location of the buttons is calculated by triangulation. Movement parameters of each button, such as displacement trajectory, time, speed and acceleration, can be displayed on-line and stored for off-line analysis. This system can be used to track animals in any lighting conditions, and to assess drug effects on the CNS, neuromuscular junction or muscle. As an example we demonstrate the ataxic effects of pentobarbital in rats.

Dietary supplementation with the inhibitory amino acid taurine suppresses autotomy in HA rats

Taurine is an inhibitory amino acid in the CNS. When supplied to rats it produces analgesia in some acute pain tests. Here we examined the effect of taurine supplementation on sensitivity to pain in intact rats, and whether perioperative dietary supplementation with taurine in rats would suppress autotomy, a behavior produced by peripheral neurectomy and related to neuropathic pain. Thermal pain sensitivity of intact rats consuming 1% taurine in the drinking solution for 2 weeks was not significantly different from that of control rats. Autotomy levels, determined in rats consuming taurine pre-, post- or perioperatively were significantly lower than in matching control groups. We conclude that taurine plays an important role in the autotomy model, presumably by protecting inhibitory neurons in the CNS against an excitotoxic damage triggered by injury discharge and ectopic input from the severed nerves.

Brief electrical stimulation of c-fibers in rats produces thermal hyperalgesia lasting weeks

Nerve injury produces neuropathic pain in some humans, but the nature of the signal triggering the pain is still unknown. When injured, many afferent fibers emit a prolonged discharge of action potentials. This input triggers pain disorders in rats following some, but not other types of total and partial nerve injury. In the present study we report that similar sensory disorders develop in rats by electrically stimulating an intact sciatic nerve. Robust thermal hyperalgesia and weak mechanical allodynia developed rapidly, lasting 3-5 weeks thereafter, but only when activating C-fibers. Thus, a discharge mimicking injury discharge in C-fibers, in the absence of any intended damage to the stimulated nerve, is sufficient to alter chronically central processing of sensory input.

Differential sensory-motor effects of pentobarbital in intact rats genetically selected for high vs. low neuropathic pain-related behaviour

Denervation of the hindpaw in rodents triggers autotomy, a behaviour of licking, scratching and self-mutilation of the denervated paw. This behaviour has been used as a model of paraesthesia, dysaesthesia and neuropathic pain. HA and LA rats are lines that have been genetically selected for high or low levels of autotomy, respectively. Compared to intact LA rats, HA rats are more sensitive to convulsions induced by pentylenetetrazol (PTZ), a blocker of the chloride channel associated with the GABA(A) receptor. Here we tested whether an acute administration of a sedative but not anaesthetic dose of pentobarbital (PB) would differentiate between these rat lines, in a number of sensory and motor tests performed in intact rats. This drug was tested since in contrast to PTZ, PB enhances central nervous system (CNS) inhibition by increasing chloride flux through the same channel. We found that PB was significantly more ataxic, antinociceptive, and reduced touch sensitivity in LA rats, compared to HA rats. These results suggest that HA and LA rats genetically differ in the levels of central inhibitions mediated by the GABA system presumably at the chloride channel. This difference may be associated with the dichotomous expression of neuropathic pain in these rat lines.

Neuropathic pain following partial nerve injury in rats is suppressed by dietary soy

Some humans with partial nerve injury present a syndrome of neuropathic sensory disorders which depend on the sympathetic activity (sympathetically-maintained pain, SMP). Several years ago we introduced a rat model for SMP, produced by tightly ligating 1/3-1/2 of the sciatic nerve, leading to a partial denervation of the hindpaw (Partial Sciatic Ligation, PSL model) [Seltzer, Z., Dubner, R. and Shir, Y., Pain, 43 (1990) 245-250]. After working with this model for several years we encountered difficulties in replicating it although rat strain, vendor, gender, age and weight, surgical approach and sensory testing procedures were not changed. We report here that this variability can be attributed, at least in part, to the diet the animals consumed. Rats fed perioperatively with soy-containing diets expressed significantly weaker neuropathic sensory disorders compared to rats fed on soy-free diets. We conclude that diet may greatly affect experimental outcome in the PSL model.

Diet can modify autotomy behavior in rats following peripheral neurectomy

Partial sciatic nerve ligation in rats (PSL) produces neuropathic pain disorders [Seltzer, Z., Dubner, R. and Shir, Y., Pain, 43 ( 1990) 205-18 (corrected) ]. Recently we reported that diet markedly affected the levels of these disorders. Here we questioned whether diet also affects neuropathic pain-related behavior in another model, produced by total denervation of the hindpaw following peripheral sciatic and saphenous neurectomy. Sabra rats and HA line rats were fed for 2-3 weeks preoperatively and up to 58 days postoperatively (PO) with one of five different diet formulas. We found that the autotomy behavior differed significantly between the diet groups. Surprisingly, in some diets the effects on autotomy and PSL models were different and even contrasting. Modulation of diet in humans may emerge as a novel therapy of neuropathic pain.

The effect of neonatal capsaicin treatment on gustatory behavior in the albino rat

Small-diameter fibers present in gustatory peripheral nerves have historically been suspected of relaying information about the bitter quality of a taste stimulus. Neonatally injected capsaicin irreversibly destroys a proportion of unmyelinated C- and some A delta-fibers. Consummatory responses to increasing concentrations of quinine and other chemical solutions following neonatal capsaicin injection were compared to those of untreated and vehicle-injected control Sabra albino rats. Capsaicin-treated rats significantly increased their withdrawal thresholds to noxious, CO2 laser-generated heat pulses verifying treatment effectiveness. Furthermore, neonatal capsaicin treatment diminished sensitivity to pungent capsaicin solutions in mature rats. However, there were no group differences in quinine intake, suggesting that the full array of unmyelinated fibers associated with taste buds is not essential for the transmission of bitter taste. Capsaicin-treated animals showed a significant reduction in intake of normally highly preferred sodium chloride and sucrose concentrations. These results were probably not due to loss of peripheral unmyelinated afferent fibers per se, but rather to secondary central changes.

Inherited propensity for neuropathic pain is mediated by sensitivity to injury discharge

We reported previously that injury discharge (ID), a burst of impulses fired following nerve injury, plays a role in triggering autotomy, a neuropathic pain-related behavior in rats. Here we affirm this link using two lines of rats, derived by selective breeding from the Sabra strain to express high (HA) or low (LA) levels of autotomy following hindpaw denervation. Blocking ID in HA rats before injury suppressed autotomy. Correspondingly, artificial prolongation of ID in LA rats just prior to neurectomy, increased autotomy. The autotomy in these HA and LA rats was like that of their Sabra ancestors. This suggests that the underlying basis for selection of these lines was differential sensitivity of the CNS to the effects of ID.

Suppression of neuropathic pain behavior in rats by a non-psychotropic synthetic cannabinoid with NMDA receptor-blocking properties

HU211 is a novel synthetic derivative of tetrahydro-cannabinol (THC), the active marijuana ingredient. The stereochemistry of HU211 is enantiomeric to that of THC. In contrast to THC, HU211 is not psychotropic. This agent exhibits other types of biological activities; it is a non-competitive NMDA receptor blocker and has antinociceptive activity when injected with cupric chloride. This study examined its effects in autotomy, a behavioral model of neuropathic pain. Autotomy, a behavior of self-mutilation of denervated areas, was induced in Sabra rats by cutting the sciatic and saphenous nerves. We found that injections of HU211 (2.5 mg/kg) with cupric chloride (0.8 mg/kg) every 2nd day markedly suppressed autotomy during the injection period by delaying its average onset day and reducing the incidence of severe autotomy. Moreover, suppression of autotomy was retained in the postinjection period (for at least 30 days) but only when the drug was injected intraperitoneally. Lesser effects were achieved by subcutaneous injections. Cupric chloride or HU211 alone were ineffective. The general behavior and open field motor activity indicated that the effects of HU211 with Cu++ on autotomy were not due to sedation or ataxia but presumably due to antinociception mediated by NMDA receptor blockade.

Neuropathic pain behavior in rats depends on the afferent input from nerve-end neuroma including histamine-sensitive C-fibers

Sciatic and saphenous neurectomy in rats produces nerve-end neuromas, known to be a source of afferent input. Concurrently rats self-injure the denervated hindpaw ('autotomy'), a behavior related to neuropathic pain in humans. Here we show that surgical resection of the neuromas in various groups of rats, each at a different postoperative time (days 22, 33, 48) suppress autotomy. This recalls the pain relief in humans following resection of painful neuromas. We also show that daily injections of astemizole, a peripheral anti-histamine which blocks histamine H1-receptors, suppress autotomy. Since mostly C-fibers in rat neuroma are sensitive to histamine, these results corroborate the suggestion that autotomy is driven by afferent neuroma input, mainly in histamine-sensitive C-fibers.

Mechano- and thermo-sensitivity in rats genetically prone to developing neuropathic pain

By selective breeding, lines of rats were derived which consistently expressed high (HA) or low (LA) levels of autotomy following sciatic nerve injury, autotomy being a behavior pattern presumed to reflect the presence of neuropathic paraesthesias and pain. We report here that intact (unoperated) HA and LA rats differ in their responsiveness to cutaneous mechanical and thermal stimuli. Thus, the autotomy trait, which was identified by its expression under conditions of nerve injury, shares determinants with sensory processing channels in the intact animal.

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.

A-fibers mediate mechanical hyperesthesia and allodynia and C-fibers mediate thermal hyperalgesia in a new model of causalgiform pain disorders in rats

Unilateral tight ligation of about half of the sciatic nerve in rats rapidly produces sympathetically dependent neuropathic pain which lasts many months and resembles causalgia in humans. The sensory abnormalities detected at the plantar side of the hindpaws include: (1) nocifensive responses to repetitive light touch (allodynia); (2) bilateral reduction in withdrawal thresholds to repetitive von-Frey hair stimulation (mechanical hyperesthesia); (3) bilateral reduction in withdrawal thresholds to CO2 laser heat pulses; and (4) unilateral increase in response duration to an intense laser heat pulse (thermal hyperalgesia). Using neonatal capsaicin treatment, we determined the type of afferent fiber remaining in the partially injured nerve, which mediates these disorders. Capsaicin, which destroys most C- and some A delta-fibers in peripheral nerves, had no effect on the touch-evoked allodynia and mechanical hyperesthesia that are produced by partial sciatic nerve injury. These disorders were, therefore, mediated by myelinated fibers. In contrast, thermal hyperalgesia failed to develop in capsaicin-treated rats following partial nerve injury. Thus, thermal hyperalgesia produced by partial nerve injury appears to be mediated by heat-nociceptive C-fibers.

Subarachnoid spinal cord transplantation of adrenal medulla suppresses chronic neuropathic pain behavior in rats

Several weeks after transection of the sciatic and saphenous nerves, rats respond by self-injury of the denervated limb ('autotomy'). This behavior serves as a model of neuropathic pain. In this study we allografted fragments of rat adrenal medulla into the subarachnoid space of other rats, at lumbar spinal cord level, in an attempt to suppress autotomy behavior. The results show that autotomy was reduced by an average of 63% throughout the 8 week observation period. Catecholamine (CA) histofluorescent staining performed up to 120 days postoperatively (P0) revealed viable transplants in 75% of the rats. Transplant viability correlated with suppression of autotomy. This suggests that medullary chromaffin cells function as a local, long-lasting source of anti-nociceptive agents at the spinal segments which process input from the injured nerves.

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.

Effect of hyperglycemia on pain perception and on efficacy of morphine analgesia in rats

The effect exerted by different hyperglycemic states on the pain threshold and on the analgesic potential of morphine was studied in male Sabra rats with the hot plate device. Hyperglycemia induced by an intraperitoneal injection of 0.014 mol/kg glucose or an acute or chronic diabetic state induced by streptozocin injection did not significantly alter the pain threshold. However, states of acute and chronic diabetes markedly blunted the analgesic effect of morphine (5 mg/kg). Sabra rats maintained on a cocktail of glucose-saccharin, thought to activate the release of endogenous opioids, demonstrated an increased pain threshold and rapidly developed resistance to the analgesic effect of morphine. Previous studies have shown that glucose in high concentration may interfere with the interaction of morphine on the opiate receptor. The influence of the diabetic state on beta-endorphin synthesis and concentration in the central nervous system is another factor that might change pain perception in diabetes. We propose that in diabetes, generally, the pain threshold is adequately maintained, despite the antagonistic effect of glucose, partly due to a compensatory increased secretion of endogenous opioid peptides. We hypothesize that in patients with chronic painful diabetic neuropathy, these normal analgesic response mechanisms may be overwhelmed either by an excess of nociceptive impulses from diseased peripheral nerves or conceivably by a failure of endogenous opioid secretory response to the hyperglycemia.

A chronically implanted delivery system of drugs to a nerve-end neuroma: effects on a behavioural chronic pain model

Autotomy has been suggested as an animal model of chronic pain. It starts about a week or two postoperatively and develops until 10 weeks after nerve section. This behaviour is thought to be triggered by activity of sensory fibres ending in a neuroma. Here we suggest to utilize it in combination with a novel drug delivery system which enables a direct and exclusive access of the drug to the neuroma. Alteration in the autotomy behaviour can then be related to the exclusive topical action on the sensory fibres within the neuroma. The sciatic nerve is transsected and its proximal end inserted into a PE tube sealed distally. A second, smaller tube originates in a wound exit in the back of the animal and subcutaneously leads into the large tube, where it is fixed by glue to the inner wall. Thus, the end of the smaller tube is juxtaposed to the nerve end. During the following weeks a neuroma develops within the tube. The resulting autotomy scores are then examined weekly. At various times after the operation, under light anaesthesia, drugs can be injected into the tube and the effect on the autotomy behaviour is monitored. An example is given, describing the autotomy suppressive effects of glycerol and alcohol, injected to different groups of rats immediately after the operation and compared to an injection 14 days postoperatively. This method is suggested as a pharmaco-behavioural assay for the assessment of the analgetic efficacy of drugs for chronic pain.

Effect of nerve section on the spinal distribution of neighboring nerves

The spinal cord distribution of axonal terminals of peripheral nerves that innervate the skin of the upper medial thigh was examined in rats using transganglionic transport of horseradish peroxidase (HRP) and wheat-germ agglutinin-conjugated HRP (WGA-HRP). Chronic transection of the sciatic nerve or both the sciatic and saphenous nerves did not alter this distribution. Therefore, long-distance sprouting of intact 'thigh nerve' afferents in the dorsal horn is apparently not the mechanism whereby spinal dorsal horn neurons deafferented by sciatic and saphenous neurectomy, gain novel receptive fields in the cutaneous distribution of neighbouring intact nerves of the thigh.

The cells of origin of the dorsal column postsynaptic projection in the lumbosacral enlargements of cats and monkeys

Dorsal column postsynaptic (DCPS) neurons in the lumbosacral enlargements of cats and macaque monkeys were retrogradely labeled by placing HRP on their severed axons within the dorsal columns. The enlargements of both species contained 800-1,100 labeled DCPS neurons. The DCPS projection is thus as large as the feline spinocervical tract. It is very probable that most of these neurons project to the dorsal column nuclei and thereby constitute one of the major sources of somatosensory input to the brain. In the cat, DCPS neurons were concentrated in a band centered in lamina IV that swept down through laminae V-VI along the medial border of the dorsal horn. A second concentration of labeled cells was found in dorsomedial lamina VII. In the monkey, DCPS neurons were concentrated in a relatively broader band in laminae III-IV, and scattered cells were consistently observed throughout laminae V-VII and X and in the dorsolateral white matter. The prominence of the monkey's DCPS projection suggests that humans also have such a projection.

Autotomy following peripheral nerve lesions: experimental anaesthesia dolorosa

(1) When hindlimb peripheral nerves are cut across in rats and mice, there is a tendency for the animal to attack the anaesthetic limb. We have called this attack "autotomy". In this paper we describe the time course and degree of autotomy following various types of nerve injury. (2) Four different types of lesion were applied to the sciatic nerve of rats. The most serious autotomy was produced by section of the nerve and encapsulation of its cut end in a polythene tube. Section followed by immediate resuturing also produced serious autotomy. Simple ligation of the nerve end was followed by less autotomy than encapsulation or cut and resuture. A crush lesion caused only minimal attack. (3) Section of the saphenous branch of the femoral nerve produced no autotomy. However, if the saphenous and sciatic nerves were ligated at the same time so that the entire foot became anaesthetic there was a great increase of autotomy over that seen when the sciatic nerve alone was ligated. This increase with the double lesion occurred even if the saphenous nerve was ligated more than 100 days after the sciatic nerve had been cut. (4) Mice showed autotomy very similar to that seen in rats but the onset was somewhat faster. (5) Reasons are given to propose that autotomy is triggered by an abnormal afferent barrage generated in the cut end of the nerve. Autotomy from peripheral nerve lesions is a different phenomenon from that seen after dorsal root section. Autotomy occurs under conditions which produce anaesthesia dolorosa in man. This simple model may be suitable for studies of the prevention of irritations originating from chronic lesions of peripheral nerves.

Ephaptic transmission in chronically damaged peripheral nerves

Several weeks after damage of the sciatic nerve in adult rats, a stable electrical (ephaptic) interaction forms between pairs of injured sensory and motor axons. Fiber-fiber interaction occurs when the nerve ends in a neuroma, after end-to-end nerve suture and after nerve crush injury. Unlike the transient "artificial synapse" created acutely on section of a nerve, this form of crosstalk is long-lasting. Its existence lends support to the hypothesis that ephaptic interaction is an important factor in neurologic pathophysiology.

Two modes of cutaneous reinnervation following peripheral nerve injury

The return of sensation to the foot following sciatic nerve crush injury was analyzed behaviorally and electrophysiologically in the rat. Functional recovery begins within four days. Its early phase is accounted for by expansion of the functional distribution of intact neighboring fibers of the saphenous n. It occurs even if the sciatic n. is ligated, and it disappears with section of the saphenous n. Accompanying this functional expansion we began to encounter in electrophysiological recordings from the saphenous n., fibers with unusually large receptive fields (RF's) extending onto the plantar surface of the foot, well beyond their limits in intact rats. All of the expanded RF's were high threshold mechanoreceptors. On about the twentieth day after crushing, the regenerating sciatic n. began to make a functional contribution. This was seen by return of sensation to zones not invaded by the saphenous n. and by the onset of sensation in rats in which the saphenous n. had previously been ligated. With return of the sciatic n. the expanded distribution of the saphenous n. went back to its original boundaries. Correspondingly, we could no longer find expanded saphenous n. RF's. We conclude that cutaneous reinnervation begins with the collateral expansion of high threshold afferents from intact neighboring nerves. This alien innervation is later replaced upon regeneration of the original nerve.