Induction of the gene encoding pro-dynorphin by experimentally induced arthritis enhances staining for dynorphin in the spinal cord of rats

Neurophysiological evidence for increased kappa opioidergic control of spinal cord neurons in rats with unilateral inflammation at the ankle

The role of the endogenous kappa opioid system in the control of neuronal activity has been studied in the spinal cord of normal rats and in rats with Freund's adjuvant induced unilateral inflammation of the ankle under barbiturate anaesthesia. During recordings from neurons with ankle input the kappa receptor agonist U50,488H and/or the kappa antagonist nor-binaltorphamine were administered ionophoretically using multibarrel electrodes. In most neurons tested U50,488H reduced the responses evoked by pressure applied across the ankle whereas smaller proportions of neurons showed increased activity or were not affected. The kappa opioid antagonist nor-binaltorphamine affected more neurons in rats with inflammation than in control rats. Ongoing activity was increased in 7 of 19 (37%) neurons in control rats, in 16 of 24 (67%) neurons in the acute phase of inflammation (2 days post inoculation) and in 15 of 23 (65%) neurons in the chronic phase of inflammation (16-20 days post inoculation). During application of nor-binaltorphamine in control rats, the responses to pressure were increased in 9 cells (36%), reduced in 7 cells (28%) and unaffected in 9 cells (36%). In the acute phase of inflammation significantly more neurons (11 of 15, 73%) showed enhanced responses to pressure during ionophoresis of nor-binaltorphamine but not in the chronic phase. These results show that spinal cord neurons with ankle input are influenced by the endogenous kappa opioid system particularly under inflammatory conditions. The upregulation of this system under inflammatory conditions may serve to counteract inflammation-induced hyperexcitability.

Spinal cord dynorphin: positive region-specific modulation during pregnancy and parturition

In laboratory animals and humans, pregnancy is associated with opioid-mediated elevations in the threshold for responsiveness to aversive stimuli. Previous pharmacological analysis has demonstrated that this analgesia results, at least in part, from the activation of spinal cord kappa opioid receptors utilizing dynorphin as the major opioid substrate. The present report demonstrates that during late pregnancy, the content of spinal dynorphin A(1-17 and 1-8) is altered in a region-specific fashion. As a result, levels of dynorphin peptides are elevated, but only in the lumbar spinal region. In parturient animals, lumbar levels of dynorphin A(1-8) remained elevated but there was an additional increment in the content of dynorphin A(1-17). During late gestation, spinal content of Met-enkephalin and its precursor are also elevated, but, in contrast to dynorphin peptides, there is no interaction between condition and spinal level. Possible analgesic synergy between mu-delta and kappa opioid receptor systems is discussed. It is concluded that some parameter(s) of the pregnant condition triggers the activation of a spinal cord dynorphin system that attenuates the pain associated with late pregnancy and labor.

Regional specificities in the distribution, chemical phenotypes, and coexistence patterns of neuropeptide containing nerve fibres in the human anal canal

Despite the pivotal clinical significance of the human anal canal, little is known about its total and specific innervation. This study assessed the comparative distribution and histotopology of nerve fibres immunoreactive for neural markers and a variety of regulatory active neuropeptides in the human anal canal by light microscopic immunohistochemistry. Depending on the epithelial zone and region of the anal canal, the neural elements were differentially immunoreactive for the pan-neural marker protein gene product 9.5, the catecholamine marker tyrosine hydroxylase, the neuroendocrine marker chromogranin A, and various neuropeptides. Protein gene product 9.5-immunoreactive nerve fibres were ubiquitously abundant in the anal canal. In the anal transitional zone, ectopic epithelial types were supplied by the same pattern of peptidergic nerves as the respective type of epithelium in normotopic location. In the dermis of the squamous zone and in the perianal epidermis, unusual distribution patterns of nerve fibres, referred to as areas of high nerve fibre density, were encountered. Double immunohistochemistry revealed region-specific coexistence patterns of neuropeptidergic nerve fibres, and novel peptide coexistence patterns were detected in anal nerve fibres. Subsets of nerve fibres formed close spatial relationships with chromogranin A-positive neuroendocrine cells, most frequently in the anal transitional zone. Chromogranin-A positive cells were shown to be present in the epithelium of perianal eccrine sweat glands. The differential distribution, peptide phenotypes and coexistence patterns of different nerve fibre populations in the human anal canal may reflect topospecific regulatory functions of neurally released neuropeptides in health and disease.

CSF neuropeptides in cancer pain: effects of spinal opioid therapy

The cerebrospinal fluid (CSF) levels of the opioid peptides met-enkephalin (ME), beta-endorphin (BE) and dynorphin (DYN) as well as the putative sensory neuropeptides substance P (SP), somatostatin (SOM), calcitonin gene related peptide (CGRP) and vasoactive intestinal polypeptide (VIP) were determined in 10 patients with severe nociceptive pain due to malignancy, before and after initiation of spinal opioid therapy, and in 10 control patients. Pain intensity, evaluated by means of a 100-mm visual analog scale (VAS), was reduced from 39 +/- 9 to 18 +/- 10 for continuous pain and from 70 +/- 10 to 10 +/- 8 for intermittent pain (means +/- s.e.mean). Lumbar CSF immunoreactive ME and DYN concentrations were significantly increased (P = 0.05) and BE and VIP were significantly decreased (P < or = 0.05) in the pain patients. A slight, but non-significant (P = 0.06) decrease in SP-like immunoreactivity was found after initiation of spinal opioid therapy. Visceral pain seemed to be associated with low immunoreactive SP and ME concentrations compared to somatic pain. A highly significant correlation was found between SP and ME (P < 0.001) and to a lesser extent also between other peptides. We conclude that the concentrations of the endogenous opioids were more affected by nociceptive pain states than the non-opioid peptides. The origin of pain may also influence the results. The postulated inhibition of peptide release by spinal opioid application seemed to be present for SP, but could otherwise not be confirmed.

C-fos expression in rat lumbar spinal cord following peripheral stimulation in adjuvant-induced arthritic and normal rats

Our previous data reported a maximal expression of the c-fos immediate-early gene in the lumbar spinal cord of the non-stimulated polyarthritic rat neurons, three weeks after Freund's adjuvant injection. The present study utilises c-fos expression to judge the reactivity of spinal neurons to calibrated mechanical pressure applied to the ankle joint, in both normal and arthritic rats under ketamine anesthesia. The results indicate that the number of Fos-like immunoreactive neurons (1) is slightly decreased in ketamine-anesthetized non-stimulated arthritic rats as compared to the non-anesthetized non-stimulated ones, (2) is significantly higher in both stimulated normal and arthritic animals as compared to non-stimulated animals, particularly in laminae I, II, V and VI of L3 and L4, and (3) is significantly increased in stimulated arthritic as compared to stimulated normal rats, in all laminae of lumbar spinal segments. The appearance of 'basal' Fos labeling during the adjuvant-induced arthritic disease and the increased number of Fos-like immunoreactive neurons in stimulated arthritic rats compared to stimulated normal animals indirectly suggests that these neurons are abnormally active and thus involved in the hyperalgesia of arthritic disease. Therefore the use of Fos-like immunoreactivity in the chronic pain model seems to be an appropriate tool to study possible effects of various pharmacological compounds, such as analgesics and anti-inflammatory drugs.

Contribution of central neuroplasticity to pathological pain: review of clinical and experimental evidence

Peripheral tissue damage or nerve injury often leads to pathological pain processes, such as spontaneous pain, hyperalgesia and allodynia, that persist for years or decades after all possible tissue healing has occurred. Although peripheral neural mechanisms, such as nociceptor sensitization and neuroma formation, contribute to these pathological pain processes, recent evidence indicates that changes in central neural function may also play a significant role. In this review, we examine the clinical and experimental evidence which points to a contribution of central neural plasticity to the development of pathological pain. We also assess the physiological, biochemical, cellular and molecular mechanisms that underlie plasticity induced in the central nervous system (CNS) in response to noxious peripheral stimulation. Finally, we examine theories which have been proposed to explain how injury or noxious stimulation lead to alterations in CNS function which influence subsequent pain experience.

Dynamic changes in the receptive field properties of spinal cord neurons with ankle input in rats with chronic unilateral inflammation in the ankle region

The aim of this study was to determine the discharge and receptive field properties of spinal cord neurons with ankle input in spinal segments L4-6 in the rat, both under control conditions and during the course of an adjuvant-induced unilateral inflammation in the ankle. The extent of receptive fields in the skin and deep tissue was assessed using brush, pinch and compression stimuli. Neurons were categorized as nociceptive-specific or wide-dynamic-range neurons on the basis of their response thresholds and responses to suprathreshold stimuli. At all stages of inflammation (2, 6, 13 and 20 days post inoculation) the population of neurons with ankle input showed differences from the population of neurons with ankle input in control rats. There was a reduction in the number of neurons that appeared as nociceptive specific and a concomitant increase in the number of neurons showing a wide-dynamic-range response profile. The receptive fields of the neurons with ankle input were markedly larger in rats with inflammation in the ankle region and mainly spread proximally on the ipsilateral hindlimb and also to the abdomen and tail in some cases. There was also an increase in the number of neurons with contralateral excitatory inputs. The mechanical thresholds at the ankle joint and proximal parts of the ipsilateral hindlimb were less in arthritic rats than in controls. The proportion of spontaneously active neurons was also increased in rats during the initial and later stages of inflammation, although there was no significant increase in the mean spontaneous discharge frequency. These data show that there are long-term changes in the receptive field and response properties of neurons in intact rats with chronic unilateral adjuvant-induced inflammation similar to those described previously in spinal cats with acute inflammation (Neugebauer and Schaible 1990). It is presumed that similar afferent and spinal mechanisms are at work under acute and chronic inflammatory conditions which produce hyperexcitability in spinal neurons with joint input.

Monoarthritis induces complex changes in mu-, delta- and kappa-opioid binding sites in the superficial dorsal horn of the rat spinal cord

Recently, an experimental model of monoarthritis was described in the rat induced by injection with Freund's adjuvant of the tibio-tarsal joint of one hindlimb. After injection, the clinical and behavioural signs of arthritis are stable from weeks 2 to 6 post-injection. Our purpose was to study the regulation of mu-, delta- and kappa-opioid binding sites in the superficial layers (laminae I-II) of the lumbar and cervical enlargements of the spinal cord 2, 4 and 6 weeks post-injection. Using quantitative receptor autoradiography and highly selective opioid ligands, we found complex changes consisting of a bilateral increase in specific [3H]DAMGO (Tyr*-D-Ala-Gly-NMe-Phe-Gly-ol) and [3H]pCl-DPDPE (Tyr*-D-Pen-Gly-Cl-Phe-D-Pen) binding at 2 weeks post-injection and a bilateral decrease in [3H]U-69593 ((5 alpha,7 alpha,8 beta)-(-)-N-methyl-N-[7-(1-pyrrolidinyl)-1- oxaspiro(4,5)dec-8-yl]) specific binding at 4 weeks post-injection. These changes were restricted to the lumbar level. At 6 weeks post-injection, there was a bilateral increase in [3H]pCl-DPDPE specific binding at both lumbar and cervical levels. Altogether, these results suggest that, after probable local changes in endogenous opioid peptides, the three types of opioid binding sites are differentially involved in the development of the pathological process. These results contrast with the lack of significant modification in mu-, delta- and kappa-opioid binding classically reported at various levels of the spinal cord in polyarthritic rats at 3 weeks post-injection and verified for 2, 4 and 6 weeks post-injection in the present study.

Orofacial pain increases mRNA level for galanin in the trigeminal nucleus caudalis of the rat

The changes of preprogalanin mRNA levels in the superficial dorsal horn neurons (laminae I and II) of the trigeminal nucleus caudalis in response to orofacial pain induced by the injection of 5% formalin into the lips of rats was investigated and compared to those of preproenkephalin A mRNA and preprodynorphin mRNA in the same region by means of in situ hybridization histochemistry. Rapid and marked increases of preprogalanin and preprodynorphin mRNA were observed on the side of the injection, but the increase of preproenkephalin A mRNA level was less pronounced than that of the other two mRNAs, indicating that these peptides have different roles in the dorsal horn analgesic mechanism and that galanin, in addition to opioid peptides, may have a highly specific role in this mechanism.

Differential influence of naloxone on the responses of nociceptive neurons in the superficial versus the deeper dorsal horn of the medulla in the rat

Naloxone (200 micrograms/kg, i.v.) reduced the noxious thermal stimuli-evoked responses of 16/25 nociceptive neurons in the superficial laminae whereas it enhanced the responses of 6/10 nociceptive neurons in the deeper dorsal horn. However, a different picture emerged when selectivity of neuronal responsivity (nocireceptive or multireceptive) was considered. In the superficial dorsal horn, naloxone reduced the responses of the majority of (15/18) selectively nocireceptive neurons. The reduction in responses became apparent within 60 sec following naloxone administration and returned to control level within 48 min. In contrast, the responses of the majority of multireceptive neurons in the superficial (6/7), or the deeper (6/10) dorsal horn, were enhanced. The excitatory action in the superficial dorsal horn persisted for only 6-15 min, whereas it persisted for 40-70 min in the deeper dorsal horn. The firing of the majority of cold-receptive neurons (6/8) in the superficial dorsal horn was not altered. These effects were stereoselective since (+)-naloxone, the inactive isomer of naloxone, did not affect the responses of 14/16 nociceptive neurons. It is concluded that naloxone differentially, and selectively, affects the firing of nociceptive neurons in the superficial versus the deeper dorsal horn, and the firing of selectively nocireceptive versus multireceptive neurons. The relevance of these findings to the behavioral effects of naloxone, hyperalgesia and analgesia, is discussed.

Activity-dependent neuronal plasticity following tissue injury and inflammation

Increases in neuronal activity in response to tissue injury lead to changes in gene expression and prolonged changes in the nervous system. These functional changes appear to contribute to the hyperalgesia and spontaneous pain associated with tissue injury. This activity-dependent plasticity involves neuropeptides, such as dynorphin, substance P and calcitonin gene-related peptide, and excitatory amino acids, such as NMDA, which are chemical mediators involved in nociceptive processing. Unilateral inflammation in the hindpaw of the rat results in an increase in the expression of preprodynorphin and preproenkephalin mRNA in the spinal cord, which parallels the behavioral hyperalgesia associated with the inflammation. Cellular intermediate-early genes, such as c-fos, are also expressed in spinal cord neurons following inflammation and activation of nociceptors. Peripheral inflammation results in an enlargement of the receptive fields of many of these neurons. Dynorphin applied to the spinal cord also induces an enlargement of receptive fields. NMDA antagonists block the hyperexcitability produced by inflammation. A model has been proposed in which dynorphin, substance P and calcitonin gene-related peptide enhance excitability at NMDA receptor sites, leading first to dorsal horn hyperexcitability and then to excessive depolarization and excitotoxicity.

Unmyelinated primary afferent fiber stimulation depletes dynorphin A (1-8) immunoreactivity in rat ventral horn

The present study demonstrates many dynorphin (DYN)-immunoreactive fibers and presumed presynaptic terminals in rat lumbar ventral horn. The fibers and terminals seem to arise largely from DYN-containing intrinsic neurons in the dorsal horn. The majority of the presumed terminals closely surround a subpopulation of motoneurons that tend to be located in flexor motoneuron columns. Acute C fiber, but not A fiber, primary afferent stimulation depletes the ventral horn DYN immunostaining. We interpret these findings to indicate that the spinal DYN neurons are well positioned to serve both as modulators of nociceptive input and as interneurons in motor reflexes. We further hypothesize that the depletion of DYN-immunoreactivity that follows either acute C fiber stimulation or intense nociceptive stimuli may be the trigger for the upregulation in spinal cord DYN that occurs in models of chronic pain states.

Widespread release of peptides in the central nervous system: quantitation of tannic acid-captured exocytoses

Tannic acid methods have been applied to capture the exocytosis of peptide-containing granules from peptidergic neurons. The captured exocytoses have been quantitated to assess the proportion and amount of peptide released at different parts of the neuronal membrane. Examination of hypothalamic synaptic boutons shows that only about one-half of the peptidergic vesicles is exocytosed into the synaptic cleft and also that exocytosis also occurs from undilated peptidergic axons. Study of the magnocellular neurosecretory system reveals that all parts of their extensive terminal arborization appear to be equally capable to exocytose peptide. Only about one-half of their peptide is released from their nerve endings, which about the capillaries. The remainder is released much deeper in the lobules of secretory tissue where its principal target(s) could be the pituicytes and/or neurosecretory axons. Dendrites of magnocellular neurons are also capable of releasing peptide by exocytosis and dendrites could release sufficient oxytocin and vasopressin to account for the peptide known to be released into the hypothalamus. We conclude that peptidergic neurons release substantial amounts of peptides from all of their processes and that this must be taken into account when considering what functions those peptides might serve.

Up-regulation of opioid gene expression in spinal cord evoked by experimental nerve injuries and inflammation

Opioid systems modulate nociceptive input at several levels of the CNS. At the spinal cord level neurons are present that express the genes coding for the precursors of the dynorphin and enkephalin opioid peptide families. We found that two conditions in rats, a chronic constriction injury to the sciatic nerve and peripheral inflammation, have a common consequence centrally: they evoke a large, rapid and sustained up-regulation of preprodynorphin mRNA. Both are also characterized by signs of hyperalgesia and increased primary afferent input. In contrast, there is little or no up-regulation of preprodynorphin mRNA following complete transection of the sciatic nerve or sciatic nerve crush. Furthermore, only minor alterations in the levels of preproenkephalin mRNA occur in any of the conditions, except for inflammation where the elevation is relatively small compared to that of preprodynorphin mRNA. These data imply that specific regulatory processes that include stimulation of opioid gene expression are strongly engaged in the spinal cord in certain types of peripheral nerve injuries and inflammation, but not in others. Marked and sustained up-regulation of the spinal cord dynorphin system distinguishes the chronic constriction injury model from other nerve injury models of pain.

No evidence for endorphin deficiency in fibromyalgia following investigation of cerebrospinal fluid (CSF) dynorphin A and Met-enkephalin-Arg6-Phe7

The CSF levels of Met-enkephalin-Arg6-Phe7 and dynorphin A were measured in patients with fibromyalgia. The mean CSF Met-enkephalin-Arg6-Phe7 level was 35.1 +/- 2.4 fmol/ml (mean +/- S.E.M.). The mean CSF level of dynorphin A was 14.3 +/- 0.9 fmol/ml. Regression analysis showed a statistically significant correlation between Met-enkephalin-Arg6-Phe7 and dynorphin A (r = 0.5369, P = 0.001). When correlated to the previously measured CSF levels of beta-endorphin, a statistically significant correlation was found with Met-enkephalin-Arg6-Phe7 (r = 0.5055, P = 0.03) but not with dynorphin A (P greater than 0.05). The Met-enkephalin-Arg6-Phe7 and dynorphin A levels are elevated compared to the levels available for comparison groups. Therefore, a lack of endorphin secretion does not seem to be the basis for the hyperalgesia observed in these patients.

Dynorphin expression and Fos-like immunoreactivity following inflammation induced hyperalgesia are colocalized in spinal cord neurons

Fos and Fos-related proteins are increased in spinal dorsal horn neurons following noxious stimulation. The laminar location of neurons that exhibit this increase is coincident with those that exhibit an increase in dynorphin in a rat model of peripheral inflammation and hyperalgesia. In order to determine whether the increase in Fos or related proteins and dynorphin occurs in the same dorsal horn neurons, two kinds of double-labeling methods were used: in situ hybridization histochemistry to label dynorphin mRNA autoradiographically, and immunocytochemistry to label Fos and Fos-related proteins, or a double immunocytochemical method that labeled Fos and Fos-related proteins and dynorphin peptide with distinct chromagens. With both methods more than 80% of the neurons in laminae I, II, V and VI exhibiting an increase in either dynorphin mRNA or peptide following peripheral inflammation also colocalized increased nuclear Fos-like immunoreactivity. However, the number of neurons displaying increased Fos-like immunoreactivity was substantially greater than the number of neurons colocalizing increased dynorphin. These data suggest that the activation of nuclear Fos and Fos-related proteins may be related to the induction of dynorphin gene expression in a subpopulation of spinal cord neurons following peripheral inflammation and hyperalgesia.

Periaqueductal gray stimulation produces a spinally mediated, opioid antinociception for the inflamed hindpaw of the rat

The objective of the present study was to characterize stimulation-produced antinociception from the periaqueductal gray matter (PAG) in rats with unilateral hindlimb inflammation induced by an intraplantar injection of Freund's complete adjuvant. Rats were chronically implanted with a bipolar stimulating electrode in the PAG. Nociception was assessed using a paw pressure test. Prior to inflammation, PAG stimulation significantly increased paw pressure threshold in both paws compared to non-stimulated controls. Following inflammation, PAG stimulation inhibited nociception in the inflamed, but not the non-inflamed paw. Systemic administration of naloxone blocked antinociception from ventral, but not dorsal PAG stimulation sites. Intrathecal, but not subcutaneous, administration of quaternary naltrexone completely blocked stimulation-produced antinociception from the PAG. The known increased levels of endogenous opioids occurring in the spinal cord ipsilateral to the site of inflammation suggest a mechanism for the selective antinociceptive effect of ventral PAG stimulation seen for the inflamed paw.

Opioid systems in the response to inflammatory pain: sustained blockade suggests role of kappa- but not mu-opioid receptors in the modulation of nociception, behaviour and pathology

One day after intraplantar inoculation of Mycobacterium butyricum into the right hind-paw, unilaterally inflamed and control rats were implanted subcutaneously with osmotic mini-pumps delivering naloxone at 0.16 or 3.0 mg/kg/h or vehicle. As determined three days after implantation, 0.16 mg/kg/h of naloxone completely antagonized the antinociceptive action of the mu-agonist, morphine, but did not affect antinociception evoked by the kappa-agonist, U69,593. In contrast, at 3.0 mg/kg/h, naloxone blocked both morphine- and U69,593-induced antinociception. Thus, 0.16 mg/kg ("low dose") and 3.0 mg/kg ("high dose") of naloxone block mu, or mu- plus kappa-opioid receptors, respectively. Pumps were removed one week following their implantation. Inoculation was associated with a sustained hyperalgesia of the inflamed paw to noxious pressure, and elevation in resting core temperature, a loss of body weight, hypophagia, hypodipsia and a reduction in mobility. These parameters were differentially modified by the high as compared to the low dose of naloxone. Two days following implantation of pumps delivering the high dose of naloxone, the hyperalgesia of the inflamed paw was potentiated: by six days, this effect was lost. Further, one day after removal of pumps yielding the high dose, the inflamed paw showed a normalization of thresholds, that is a "rebound antinociception". One day later, this effect had subsided. In distinction, at no time did the low dose of naloxone modify nociceptive thresholds. The high dose of naloxone enhanced the reduction in body weight and food intake shown by unilaterally inflamed rats whereas the low dose was ineffective. Neither dose affected the reduction in water intake or hypothermia of unilaterally inflamed animals. The high dose of naloxone reduced the mobility of unilaterally inflamed rats whereas the low dose was ineffective. Finally, by 10 days following pump removal, pathology had transferred to the contralateral paw. In rats which had received the high but not the low dose, this transfer was blocked. It is concluded that blockade of kappa-opioid receptors with a high dose of naloxone experts pronounced functional effects in unilaterally inflamed rats. In distinction, selective blockade of mu-receptors with a low dose is ineffective. The changes seen include not only an enhancement of the hyperalgesia of the inflamed tissue, but also an exacerbation of variables (body weight, food intake and motility) which reflect pain states.(ABSTRACT TRUNCATED AT 400 WORDS)

Opioids from immunocytes interact with receptors on sensory nerves to inhibit nociception in inflammation

Exogenous opioids can produce localized opioid receptor-mediated antinociception in peripheral inflamed tissue. Previous studies show that activation of endogenous opioids by a cold water swim in rats with hind paw inflammation results in a similar local antinociceptive effect but suggest that pituitary-adrenal opioid pools are not directly involved in producing this effect. Here we show increased amounts of opioid peptides in immune cells infiltrating the inflamed tissue. Furthermore, we demonstrate immunoreactive opioid receptors on peripheral terminals of sensory neurons. The local administration of antibodies against opioid peptides or receptors or systemic pretreatment with the immunosuppressant cyclosporine blocks cold water swim-induced antinociception. These findings suggest that antinociception in inflammation can be brought about by endogenous opioids from immune cells interacting with opioid receptors on peripheral sensory nerves.

Dynorphin increases in the dorsal spinal cord in rats with a painful peripheral neuropathy

It is known that painful tissue injury evokes an increase in dynorphin in spinal neurons. It is not known, however, whether dynorphinergic systems respond similarly to the pain that accompanies peripheral neuropathy. Radioimmunoassays and immunocytochemistry were used to evaluate changes in dynorphin A(1-8) in the spinal cord of rats with a painful peripheral neuropathy. The neuropathy is the result of a constriction injury that is created by tying loose ligatures around the common sciatic nerve. Signs of abnormal pain sensations, hyperalgesia, allodynia (pain after normally innocuous stimuli), and spontaneous pain (or dysesthesia), are first detected 2-5 days after injury, reach peak severity in about 10 days, and persist for 2-3 months (Bennett, G. J.; Xie, Y.-K. Pain 33:87-107; 1988). Dynorphin increased by 5 days in cells in laminae I-II and V-VII in the lumbar spinal cord ipsilateral to the injury. This increase, maximal at 10 days (262%), was still present 20 days after the injury but was now seen only in neurons in the deep laminae (V-VII). Thus, the spinal dynorphinergic system appears to respond to neuropathic pain. Furthermore, our results suggest that dynorphinergic cells in the superficial and deep laminae may have different roles in nociception.

Peripheral opioid receptors mediating antinociception in inflammation. Activation by endogenous opioids and role of the pituitary-adrenal axis

This study investigated the involvement of endogenous opioid peptides in mediating cold water swim (CWS) stress-induced antinociception (SIA) in rats with unilateral hind paw inflammation induced by Freund's complete adjuvant (FCA). Following 0.5, 1 and 2 min of CWS, there was a duration-dependent elevation of paw pressure threshold (PPT) in both inflamed and non-inflamed paws which was maximal immediately after CWS and returned to control values within 15 min. The antinociception elicited in the inflamed paw was significantly greater than that elicited in the non-inflamed paw. The antinociception induced by a 1 min CWS was dose dependently antagonized by tertiary naloxone (0.125-1 mg/kg s.c.) and completely reversed by tertiary naltrexone (0.5 mg/kg). Quaternary naltrexone (5-40 mg/kg s.c.) was similarly effective in reversing the elevation of inflamed PPT induced by a 1 min CWS stress. In contrast, similar doses of quaternary naltrexone had no effect against centrally mediated morphine antinociception in non-inoculated rats. Adrenalectomy was without effect on the pattern of SIA seen in FCA-treated rats. Surgical hypophysectomy completely abolished the differential antinociception induced by 0.5 and 1 min durations of CWS but had little effect on that following 2 min of CWS stress. Inhibition of hypophysial corticotrophic cell secretion with dexamethasone (300 micrograms/kg) injected s.c. 120 min prior to CWS completely abolished the differential SIA at all durations of CWS tested. beta-Endorphin 12.5 micrograms/kg administered i.v. in non-stressed rats also caused a greater elevation of PPT in inflamed than in non-inflamed paws. This effect was not reversed by concomitant i.v. administration of (-) tertiary naloxone 5 mg/kg or quaternary naltrexone 20 mg/kg.

Spinal c-fos induction by sensory stimulation in neonatal rats

C-fos immunocytochemistry was used to investigate the functional connectivity between primary afferent nociceptive fibres and second-order neurons in the spinal cord of the anaesthetised neonatal rat. Subcutaneous injection of 50 mg/kg capsaicin potently induced c-fos in the spinal cord on the first postnatal day (P1), whilst plantar injection of dilute formalin was much less effective until P3. In contrast with the adult, there was no c-fos response of the neonatal rat spinal cord to cutaneous application of the C-fibre irritant, mustard oil. It is concluded that spinal c-fos expression induced by noxious sensory stimulation can occur before the development of mature functional synaptic contact with first-order neurones.