Nitric oxide in the stress axis

Calcitonin gene-related peptide in sensory primary afferent neurons has an excitatory effect on postsynaptic neurons and potentiates the effect of substance P in the rat spinal dorsal horn. It has been established that calcitonin gene-related peptide expression in dorsal root ganglion neurons is depressed, and the effect of calcitonin gene-related peptide on dorsal horn neurons is attenuated, following peripheral nerve injury. We report here that a subpopulation of injured dorsal root ganglion neurons show increased expression of calcitonin gene-related peptide. Using in situ hybridization and the retrograde tracer, FluoroGold, we detected an increased number of medium- to large-sized rat dorsal root ganglion neurons projecting to the gracile nucleus that expressed alpha-calcitonin gene-related peptide messenger RNA following spinal nerve transection. Immunohistochemistry revealed a significant increase in calcitonin gene-related peptide immunoreactivity in the gracile nucleus and in laminae III-IV of the spinal dorsal horn. These results indicate that a subpopulation of dorsal root ganglion neurons express alpha-calcitonin gene-related peptide messenger RNA in response to peripheral nerve injury, and transport this peptide to the gracile nucleus and to laminae III-IV of the spinal dorsal horn. The increase of the excitatory neuropeptide, calcitonin gene-related peptide, in sites of primary afferent termination may affect the excitability of postsynaptic neurons, and have a role in neuronal plasticity following peripheral nerve injury.     

Glial cell responses, complement, and clusterin in the central nervous system following dorsal root transection

We have examined the glial cell response, the possible expression of compounds associated with the complement cascade, including the putative complement inhibitor clusterin, and their cellular association during Wallerian degeneration in the central nervous system. Examination of the proliferation pattern revealed an overall greater mitotic activity after rhizotomy, an exclusive involvement of microglia in this proliferation after peripheral nerve injury, but, in addition, a small fraction of proliferating astrocytes after rhizotomy. Immunostaining with the phagocytic cell marker ED1 gradually became very prominent after rhizotomy, possibly reflecting a response to the extensive nerve fiber disintegration. Lumbar dorsal rhizotomy did not induce endogenous immunoglobulin G (IgG) deposition or complement expression in the spinal cord dorsal horn, dorsal funiculus, or gracile nucleus. This is in marked contrast to the situation after peripheral nerve injury, which appears to activate the entire complement cascade in the vicinity of the central sensory processes. Clusterin, a multifunctional protein with complement inhibitory effects, was markedly upregulated in the dorsal funiculus in astrocytes. In addition, there was an intense induction of clusterin expression in the degenerating white matter in oligodendrocytes, possibly reflecting a degeneration process in these cells. The findings suggest that 1) complement expression by microglial cells is intimately associated with IgG deposition; 2) axotomized neuronal perikarya, but not degenerating central fibers, undergo changes which induce such deposition; and 3) clusterin is not related to complement expression following neuronal injury but participates in regulating the state of oligodendrocytes during Wallerian degeneration.     

Partial and complete sciatic nerve injuries induce similar increases of neuropeptide Y and vasoactive intestinal peptide immunoreactivities in primary sensory neurons and their central projections

Partial nerve injury is more likely to cause neuropathic pain than complete nerve injury. We have compared the changes in neuropeptide expression in primary sensory neurons which follow complete and partial injuries to determine if these might be involved. Since more neurons are damaged by complete injury, we expected that complete sciatic nerve injury would simply cause greater increases in neuropeptide Y and vasoactive intestinal peptide than partial injury. We examined neuropeptide Y and vasoactive intestinal peptide immunoreactivities in L4 and L5 dorsal root ganglia, the dorsal horn of L4-L5 spinal cord, and the gracile nuclei of rats killed 14 days after unilateral complete sciatic nerve transection, partial sciatic nerve transection and chronic constriction injury of the sciatic nerves. In all three groups of rats, neuropeptide Y- and vasoactive intestinal peptide-immunoreactive neurons were increased in the ipsilateral L4 and L5 dorsal root ganglion when compared with the contralateral side. Most neuropeptide Y-immunoreactive neurons were of medium and large size, but a few were small. Neuropeptide Y-immunoreactive axonal fibers were increased from laminae I to IV, and vasoactive intestinal peptide-immunoreactive axonal fibers were increased in laminae I and II, of the ipsilateral dorsal horn of L4-L5 spinal cord. The increases of neuropeptide Y and vasoactive intestinal peptide immunoreactivities in the dorsal horn were similar among the three groups. However, only after constriction injury were some vasoactive intestinal peptide-immunoreactive neurons seen in the deeper laminae of the ipsilateral dorsal horn. Robust neuropeptide Y-immunoreactive axonal fibers and some neuropeptide Y-immunoreactive cells were seen in the ipsilateral gracile nuclei of all three groups of animals, but neuropeptide Y-immunoreactive cells were more prominent after constriction injury. Contrary to our expectations, partial and complete sciatic nerve injuries induced similar increases in neuropeptide Y and vasoactive intestinal peptide in lumbar dorsal root ganglion neurons and their central projections in the dorsal horn and the gracile nuclei two weeks after injury. Some neurons whose axons were spared by partial injury may also increase neuropeptide Y or vasoactive intestinal peptide expression. Altered neuropeptide release from these functional sensory neurons may play a role in neuropathic pain.     

Changes in brain-derived neurotrophic factor immunoreactivity in rat dorsal root ganglia, spinal cord, and gracile nuclei following cut or crush injuries

In the present study, we evaluated changes in brain-derived neurotrophic factor (BDNF) immunoreactivity in the rat lumbar (L) 5 dorsal root ganglion (DRG) and areas where afferents from the DRG terminate, the L5 spinal cord and gracile nuclei, following unilateral sciatic nerve transection or crush. From 3 days to 4 weeks following cut or crush injury, the percentage of medium and large BDNF-immunoreactive neurons in the ipsilateral DRG increased significantly compared with those on the contralateral side. Following cut injury, there was no significant change in the percentage of small BDNF-immunoreactive neurons in the ipsilateral DRG; however, the intensity of immunoreactivity of these cells decreased. Following crush injury, however, both the percentage and intensity of small BDNF-immunoreactive neurons in the ipsilateral DRG significantly increased. Following cut injury, the expression of BDNF-immunoreactive axonal fibers decreased markedly in the ipsilateral superficial laminae of the L5 spinal cord and increased significantly in the ipsilateral deeper laminae of the spinal cord and gracile nuclei. Crush injury induced a marked increase in the expression of BDNF-immunoreactive axonal fibers in the superficial laminae of the spinal cord and gracile nuclei. These differences in BDNF response in the DRG and spinal cord after cut or crush injuries may reflect differences in trophic support to the injured DRG neurons and altered neuronal activity in the spinal cord and gracile nuclei following different types of peripheral nerve injury.     

Differential expression of galanin immunoreactivities in the primary sensory neurons following partial and complete sciatic nerve injuries

Neuropeptide expression in primary sensory neurons is highly plastic in response to peripheral nerve axotomy. While neuropeptide changes following complete sciatic nerve injury have been extensively studied, much less is known about the effects of partial sciatic nerve injuries on neuropeptide plasticity. Galanin. a possible endogenous analgesic peptide, was up-regulated in primary sensory neurons following complete sciatic nerve injury. We investigated the effects of partial sciatic nerve injuries on galanin expression in primary sensory neurons, and compared this effect with that after complete sciatic nerve injury. Complete transection, partial transection and chronic constriction injury were made, respectively, on the sciatic nerves of three groups of rats at high thigh level. Animals were allowed to survive for four and 14 days before being killed. L4 and L5 dorsal root ganglia, L4 5 spinal cord and lower brainstem were processed for galanin immunocytochemical staining. After all three types of sciatic nerve injuries, galanin-immunoreactive neurons were significantly increased in the ipsilateral dorsal root ganglia, and galanin-immunoreactive axonal fibres were dramatically increased in the superficial laminae of the dorsal horn and the gracile nuclei, compared to the contralateral side. However, in partial injury models, the percentages of galanin-immunoreactive dorsal root ganglion neurons were significantly higher than in complete nerve transection. Size frequency distribution analysis detected that more medium- and large-size galanin-immunoreactive dorsal root ganglion neurons were present after partial nerve transection and constriction injury than after complete nerve transection. Using a combined approach of retrograde tracing of flurorescent dyes and galanin immunostaining, we found that a partial transection increased the proportions of galanin-immunoreactive neurons among both axotomized and non-axotomized neurons. Galanin-immunoreactive axonal fibres were not only detected in the superficial laminae, but also in the deeper laminae of the dorsal horn of partial injury animals. Furthermore, more galanin-immunoreactive axonal fibres were observed in the ipsilateral gracile nuclei of partially injured rats than in completely injured rats. We conclude that partial sciatic nerve injuries induced greater galanin up-regulation in medium- and large-size dorsal root ganglion neurons than complete sciatic nerve injury. Galanin expression in primary sensory neurons seems to be differentially regulated following partial and complete sciatic nerve injuries.     

Somatotopic redistribution of c-fos expressing neurons in the superficial dorsal horn after peripheral nerve injury

The functional somatotopic reorganization of the lumbar spinal cord dorsal horn after nerve injury was studied in the rat by mapping the stimulus-evoked distribution of neurons expressing proto-oncogene c-fos. In three different nerve injury paradigms, the saphenous nerve was electrically stimulated at C-fibre strength at survival times ranging from 40 h to more than six months: 1) Saphenous nerve stimulation from three weeks onwards after ipsilateral sciatic nerve transection resulted in an increase in the number of Fos-immunoreactive neurons within the dorsal horn saphenous territory in laminae I-II, and an expansion of the saphenous territory into the denervated sciatic territory until 14 weeks postinjury. 2) Saphenous nerve stimulation from five days onwards after ipsilateral sciatic nerve section combined with saphenous nerve crush resulted in an increase in the number of Fos-immunoreactive neurons within the dorsal horn saphenous nerve territory, and an expansion of the saphenous nerve territory into the denervated sciatic nerve territory. 3) Stimulation of the crushed nerve (without previous adjacent nerve section) at five days, but not at eight months resulted in a temporary increase in the number of Fos-immunoreactive neurons within the territory of the injured nerve, and no change in area at either survival time. The results indicate that nerve injury results in an increased capacity of afferents in an adjacent uninjured, or regenerating nerve, to excite neurons both in its own and in the territory of the permanently injured nerve in the dorsal horn. The onset and duration of the increased postsynaptic excitability and expansion depends on the types of nerve injuries involved. These findings indicate the complexity of the central changes that follows in nerve injuries that contain a mixture of uninjured, regenerating and permanently destroyed afferents.     

Increased uptake and transport of cholera toxin B-subunit in dorsal root ganglion neurons after peripheral axotomy: possible implications for sensory sprouting

In the present study we show that, in contrast to the rat, injection of cholera toxin B-subunit (CTB) into the intact sciatic nerve of Macaca mulatta monkey gives rise to labelling of a sparse network of fibers in laminae I-II of spinal cord and of some mainly small dorsal root ganglion (DRG) neurons. Twenty days after sciatic nerve cut, the percentage of CTB-positive lumbar 5 (L5) DRG neuron profiles increased from 11% to 73% of all profiles. In the spinal cord, a marked increase in CTB labelling was seen in laminae I, II, and the dorsal part of lamina III. In the rat L5 DRGs, 18 days after sciatic nerve cut, the percentage of CTB- and CTB conjugated to horseradish peroxidase (HRP)-labelled neuron profiles increased from 45% to 81%, and from 54% to 87% of all neuron profiles, respectively. Cell size measurements in the rat showed that most of the CTB-positive neuron profiles were small in size after axotomy, whereas most were large in intact DRGs. In the rat spinal dorsal horn, a dense network of CTB-positive fibers covered the whole dorsal horn on the axotomized side, whereas CTB-labelled fibers were mainly seen in laminae III and deeper laminae on the contralateral side. A marked increase in CTB-positive fibers was also seen in the gracile nucleus. The present study shows that in both monkey and rat DRGs, a subpopulation of mainly small neurons acquires the capacity to take up CTB/CTB-HRP after axotomy, a capacity normally not associated with these DRG neurons. These neurons may transganglionically transport CTB and CTB-HRP. Thus, after peripheral axotomy, CTB and CTB-HRP are markers not only for large but also for small DRG neurons and, thus, possibly also for both myelinated and unmyelinated primary afferents in the spinal dorsal horn. These findings may lead to a reevaluation of the concept of sprouting, considered to take place in the dorsal horn after peripheral nerve injury.     

NK1, NMDA, 5HT1a, and 5HT2 receptor binding sites in the rat lumbar spinal cord: modulation following sciatic nerve crush

Quantitative receptor binding autoradiography was used to study the NK1, NMDA, 5HT1a, and 5HT2 receptor binding densities in the adult rat lumbar spinal cord from 3 days to 20 weeks following a unilateral crush lesion of the sciatic nerve. NK1 binding density increased unilaterally in the superficial dorsal horn on the side of the sciatic crush to reach levels 60% above controls by 4 weeks following the lesion and returned to control values by 12 weeks. NMDA binding density increased bilaterally and equally in both the dorsal and ventral horns to reach 300% of control values at 2 weeks following the crush and returned to near control values by 20 weeks following the lesion. Serotonergic receptor binding did not change. The changes in NK1 receptor binding density on postsynaptic dorsal horn cells are consistent with a response to the decrease and recovery in the synthesis and transport of tachykinins by the dorsal root ganglion cells following peripheral nerve injury. the bilateral changes in NMDA receptor binding are more likely mediated by polysynaptic pathways in the spinal cord that respond to the changes in metabolic events of the dorsal root ganglion cells evoked by axotomy and regeneration.     

Treatments of leprosy at the Ryukyu University Hospital

The competence of neurons to regenerate depends on their ability to initiate a program of gene expression supporting growth and on the growth-permissive properties of glial cells in the distal stump of the injured nerve. Most studies on intrinsic molecular mechanisms governing peripheral nerve regeneration have focussed on the lesion-induced expression of proteins promoting growth cone motility, neurite extension, and adhesion. However, little is known about the expression of intrinsic chemorepulsive proteins and their receptors, after peripheral nerve injury and during nerve regeneration. Here we report the effect of peripheral nerve injury on the expression of the genes encoding sema III/coll-1 and its receptor neuropilin-1, which are known to be expressed in adult sensory and/or motor neurons. We have shown that peripheral nerve crush or transection results in a decline in sema III/coll-1 mRNA expression in injured spinal and facial motor neurons. This decline was paralleled by an induction in the expression of the growth-associated protein B-50/GAP-43. As sema III/coll-1 returned to normal levels following nerve crush, B-50/GAP-43 returned to precrush levels. Thus, the decline in sema III/coll-1 mRNA coincided with sensory and motor neuron regeneration. A sustained decline in sema III/coll-1 mRNA expression was found when regeneration was blocked by nerve transection and ligation. No changes were observed in neuropilin-1 mRNA levels after injury to sensory and motor neurons, suggesting that regenerating peripheral neurons continue to be sensitive to sema III/coll-1. Therefore we propose that a decreased expression of sema III/coll-1, one of the major ligands for neuropilin-1, during peripheral nerve regeneration is an important molecular event that is part of the adaptive response related to the success of regenerative neurite outgrowth occurring following peripheral nerve injury.     

Synthesis and expression of a DNA encoding the Fv domain of an anti-lysozyme monoclonal antibody, HyHEL10, in Streptomyces lividans

The effect of peripheral axotomy on the expression of the class III beta-tubulin gene in adult dorsal root ganglion (DRG) neurons was examined. Of the 5 isotypic classes of beta-tubulin expressed in the mammalian nervous system, only the class III beta-tubulin is neuron specific. While information about the expression of several of the tubulin genes during neuronal development and regeneration has become available recently, very little is known about the expression of beta III-tubulin during axonal regeneration. To explore this issue, we examined axotomy-induced changes in beta III-tubulin mRNA levels in adult rat lumbar dorsal root ganglion (DRG) neurons at different times (1-28 days) after unilateral sciatic nerve crush using northern blotting of total RNA and quantitative in situ hybridization. These studies showed an initial decrease in beta III-tubulin mRNA levels in axotomized DRG neurons as compared to contralateral controls at 1 day after injury followed by robust increases in beta III-tubulin mRNA levels relative to contralateral controls from 1 to 4 weeks after injury. We postulate that beta III-tubulin may play an essential role in axonal growth because of its unique neuron-specific pattern of expression and its substantial increase in neurons that have been stimulated to regrow their axons.     

Cardiac output and blood pressure during active and passive standing

The present study was conducted to demonstrate immunohistochemically, the sites of c-fos protein expression in the brains of mice subjected to acute and chronic social defeat stress. To induce social stress, mice were placed in situations of species-specific intermale aggression either only once or five times at 24 h intervals. Two hours after the single or fifth defeat stress, many c-fos immunoreactive neurons were observed in a variety of brain regions including the limbic system and sensory relay nuclei. The c-fos immunoreactive neurons in the brains of acute defeat mice decreased in number with time and the c-fos staining pattern of acute defeat mice became indistinguishable from that of normal control mice by 24 h after the single defeat stress. In contrast, chronic defeat stress induced persistent c-fos expression in the forebrain and brainstem even 24 h after the fifth defeat stress. In the forebrain of chronic defeat mice, the olfactory bulb, cingulate cortex, hippocampus, entire hypothalamus, septal nuclei and the amygdaloid complex, except for the central nucleus, were labeled intensely with c-fos antiserum. In the lower brainstem, nerve cells with c-fos immunoreactivity were seen mainly in ascending and descending sensory relay nuclei relevant to auditory and vestibular transmission, epicritic sensation (gracile and external cuneate nuclei), pain inhibition (central gray and raphe nuclei), and viscerosensory transmission (solitary tract nucleus). The differences in c-fos expression among the normal control, acute and chronic defeat mice were evaluated by an enumeration of the immunopositive neurons within each brain nucleus examined, and they were confirmed subsequently by statistical analysis. There was little c-fos expression in the nucleus putamen, lateral, ventral and posterior thalamic nuclei, pretectal nuclei, medial geniculate nucleus, red nucleus, substantia nigra, cerebellum, spinal cord, or cranial nerve nuclei. These findings suggest that chronic but not acute defeat stress causes persistent c-fos expression in more widespread brain regions than do any other stresses so far investigated. The present study may shed light on the central mechanisms by which behavioral abnormalities and/or chronic sociopsychological stress leads to the occurrence of abnormal behavior and/or psychosomatic disorders in experimental animals and humans.     

Peripheral nerve stimulation increases Fos immunoreactivity without affecting type II Ca2+/calmodulin-dependent protein kinase, glutamic acid decarboxylase, or GABAA receptor gene expression in cat spinal cord

Expression patterns of the immediate early gene c-fos and of other genes including those for the alpha-subunit of type II Ca2+/calmodulin-dependent protein kinase (CaMKII alpha), 67-kDa glutamic acid decarboxylase (GAD), and the alpha 1-, beta 2-, and gamma 2-subunits of the GABAA receptor were described in the spinal cord of normal cats and following peripheral nerve stimulation. As revealed by in situ hybridization histochemistry, CaMKII alpha messenger RNA (mRNA) is normally distributed only in cells of Rexed's laminae I-IV, whereas GAD mRNA is expressed by subpopulations of cells in all laminae, with the heaviest hybridization signal found in laminae I-III and medial parts of laminae V and VI. The three GABAA receptor subunits have varying expression patterns in the laminae. All of them are expressed by many cells located in the base of the dorsal horn and the intermediate zone, but only the gamma 2-subunit is intensely expressed by motoneurons. Single-pulse, electrical stimulation of the sciatic or median and ulnar nerve of anesthetized cats at a pulse rate of 1/s for 6-8 h failed to induce observable changes in gene expression for CaMKII alpha, GAD, or for the three subunits of the GABAA receptor; although immunoreactivity for the protein products of c-fos (or c-fos-related genes) was markedly upregulated in some neurons of the dorsal horn and the intermediate zone. Therefore, under the present experimental conditions, upregulation of the immediate early gene c-fos (or c-fos-related genes) is not associated with changes in expression of late-effector genes potentially involved in central nervous system plasticity.     

Neuronal injury increases retrograde axonal transport of the neurotrophins to spinal sensory neurons and motor neurons via multiple receptor mechanisms

We investigated the retrograde axonal transport of 125I-labeled neurotrophins (NGF, BDNF, NT-3, and NT-4) from the sciatic nerve to dorsal root ganglion (DRG) sensory neurons and spinal motor neurons in normal rats or after neuronal injury. DRG neurons showed increased transport of all neurotrophins following crush injury to the sciatic nerve. This was maximal 1 day after sciatic nerve crush and returned to control levels after 7 days. 125I-BDNF transport from sciatic nerve was elevated with injection either proximal to the lesion or directly into the crush site and after transection of the dorsal roots. All neurotrophin transport was receptor-mediated and consistent with neurotrophin binding to the low-affinity neurotrophin receptor (LNR) or Trk receptors. However, transport of 125I-labeled wheat germ agglutinin also increased 1 day after sciatic nerve crush, showing that increased uptake and transport is a generalized response to injury in DRG sensory neurons. Spinal cord motor neurons also showed increased neurotrophin transport following sciatic nerve injury, although this was maximal after 3 days. The transport of 125I-NGF depended on the expression of LNR by injured motor neurons, as demonstrated by competition experiments with unlabeled neurotrophins. The absence of TrkA in normal motor neurons or after axotomy was confirmed by immunostaining and in situ hybridization. Thus, increased transport of neurotrophic factors after neuronal injury is due to multiple receptor-mediated mechanisms including general increases in axonal transport capacity.     

Primary afferent-evoked release of immunoreactive galanin in the spinal cord of the neuropathic rat

We have determined if peripheral nerve stimulation altered the increased spontaneous release of immunoreactive (ir)-galanin that is found in the superficial dorsal horn of the spinal cord of neuropathic rats. Using the antibody microprobe technique to study the localized sites of ir-galanin release in vivo, we found that high intensity electrical stimulation of the injured nerve resulted in a further increase in ir-galanin release in the superficial dorsal horn, with no significant persistence of ir-galanin after release. Release of ir-galanin at stimulus strengths sufficient to activate C fibres, in an area of the spinal cord thought to be concerned with nociceptive transmission, indicates a possible role for this peptide in the spinal modulation of pain after peripheral nerve injury.     

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

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

Down-regulation of mu-opioid receptors in rat and monkey dorsal root ganglion neurons and spinal cord after peripheral axotomy

To understand the role of opioids and their receptors in chronic pain following peripheral nerve injury, we have studied the mu-opioid receptor in rat and monkey lumbar 4 and 5 dorsal root ganglion neurons and the superficial dorsal horn of the spinal cord under normal circumstances and after peripheral axotomy. Our results show that many small neurons in rat and monkey dorsal root ganglia, and some medium-sized and large neurons in rat dorsal root ganglia, express mu-opioid receptor-like immunoreactivity. Most of these neurons contain calcitonin gene-related peptide. The mu-opioid receptor was closely associated with the somatic plasmalemma of the dorsal root ganglion neurons. Both mu-opioid receptor-immunoreactive nerve fibers and cell bodies were observed in lamina II of the dorsal horn. The highest intensity of mu-opioid receptor-like immunoreactivity was observed in the deep part of lamina II. Most mu-opioid receptor-like immunoreactivity in the dorsal horn originated from spinal neurons. A few mu-opioid receptor-positive peripheral afferent terminals in the rat and monkey dorsal horn were calcitonin gene-related peptide-immunoreactive. In addition to pre- and post-junctional receptors in rat and monkey dorsal horn neurons, mu-opioid receptors were localized on the presynaptic membrane of some synapses of primary afferent terminals in the monkey dorsal horn. Peripheral axotomy caused a reduction in the number and intensity of mu-opioid receptor-positive neurons in the rat and monkey dorsal root ganglia, and of mu-opioid receptor-like immunoreactivity in the dorsal horn of the spinal cord. The decrease in mu-opioid receptor-like immunoreactivity was more pronounced in the monkey than in the rat dorsal root ganglia and spinal cord. It is probable that there was a parallel trans-synaptic down-regulation of mu-opioid-like immunoreactivity in local dorsal horn neurons of the monkey. These data suggest that one factor underlying the well known insensitivity of neuropathic pain to opioid analgesics could be due to a marked reduction in the number of mu-opioid receptors in the axotomized sensory neurons and in interneurons in the dorsal horn of the spinal cord.