Do neuropeptides in the dorsal horn change if the dorsal root ganglion cell death that normally accompanies peripheral nerve transection is prevented?

Constitutive expression of the low-affinity neurotrophin receptor and changes during axotomy-induced death of sensory neurones in the neonatal rat dorsal root ganglion

Sensory neurones in the dorsal root ganglion (DRG) of the neonatal rat express the 75-kDa low-affinity neurotrophin receptor (p75NTR) and these neurones degenerate rapidly after axotomy. p75NTR belongs to the tumour necrosis factor superfamily, several members of which have a role in cell death and it is constitutively expressed within a subpopulation of DRG neurones. p75NTR has been implicated in mediating the degeneration of these neurones after axotomy. In this study, we characterize the expression of p75NTR in sensory neurones of the newborn rat DRG using immunohistochemistry. Furthermore, we investigate the change in constitutive expression pattern of p75NTR in these neurones following axotomy. In the C7 and C8 DRG of the newborn rat, p75NTR is expressed in approximately 70% of DRG neurones. Those expressing p75NTR can be classified into subpopulations with moderate or intense p75NTR expression, each present in approximately equal proportions. Whilst p75NTR expression is observed in neurones throughout the entire neuronal diameter range, a correlation exists between neuronal diameter and p75NTR expression intensity. We also found that the most vulnerable population following axotomy were those sensory neurones which constitutively express the highest levels of p75NTR, i.e. the large-diameter neurones.

Calcitonin gene-related peptide increase in the rat spinal dorsal horn and dorsal column nucleus following peripheral nerve injury: up-regulation in a subpopulation of primary afferent sensory neurons

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.

Islet amyloid polypeptide and calcitonin gene-related peptide expression are down-regulated in dorsal root ganglia upon sciatic nerve transection

Islet amyloid polypeptide (IAPP) is structurally related to calcitonin gene-related peptide (CGRP) and has been implicated in glucose homeostasis and diabetes pathogenesis because it is expressed in insulin cells and forms amyloid in pancreatic islets from type II diabetic patients. IAPP is also constitutively co-expressed with CGRP in rat sensory neurons. Whether expression of IAPP is altered by nerve injury with or without regeneration was investigated in adult rats subjected to unilateral sciatic axotomy; IAPP and CGRP expression were determined by quantitative in situ hybridization and immunocytochemistry at days 3, 10 and 30 after axotomy. In ipsilateral L4-L5 dorsal root ganglia (DRG), the percentages of nerve cell profiles labelled for IAPP and CGRP mRNA were reduced at all time points studied. IAPP and CGRP mRNA expression were lower in nerve cell profiles in ipsilateral DRGs compared to the contralateral side after axotomy alone whereas epineurial nerve suture maintained or restored IAPP and CGRP expression. The numbers of IAPP- and CGRP-immunoreactive DRG nerve cell profiles and dorsal horn fibers were reduced on the ipsilateral side at all time points. Thus, IAPP and CGRP expression are down-regulated upon axotomy. Nerve repair maintains or restores IAPP and CGRP expression in individual neurons but does not prevent the loss of CGRP/IAPP phenotype of some of these neurons in response to axotomy.

Sensory neurons regulate immunoglobulin secretion of spleen cells: cellular analysis of bidirectional communications between neurons and immune cells

The effects of primary sensory neurons in the dorsal root ganglia (DRG) on immunoglobulin (Ig)-secreting activity of spleen cells were investigated in culture. The conditioned medium (CM) of normal spleen cells stimulated DRG neurons to release neurokinin A which increased the number of Ig-secreting spleen cells. In contrast, the CM of concanavalin A-induced suppressor spleen cells induced the release of vasoactive intestinal peptide, an inhibitor of Ig secretion of spleen, from DRG neurons. These findings indicate that sensory neurons can help and suppress Ig secretion, and these bidirectional activities are controlled by the factors released from immune cells.

Evolution of morphological and histochemical changes in the adult cat cuneate nucleus following forelimb denervation

Morphological and histochemical changes were studied in the ipsilateral cuneate nucleus between one and 52 weeks after forelimb denervation in adult cats. The deafferented nucleus and neighboring fasciculus were noticeably reduced in size within four weeks and decreased further by 13 weeks. The intensity of acetylcholinesterase staining decreased within one week and was further reduced one month after nerve transections. This reduction in acetylcholinesterase staining was transient, approaching control levels within one year. Parvalbumin immunostaining was also altered by the nerve transections; on the deafferented side, the neuropil staining in the cuneate nucleus and fasciculus decreased, but the number of parvalbumin-positive cells was consistently greater than in the contralateral side. These cell counts returned to normal levels within one year. One month after the injury, cytochrome oxidase activity was reduced. This reduction persisted and was even more apparent after one year. In parallel, the cell clusters of the nucleus became progressively less distinct. These observations in an adult mammal indicate that peripheral nerve injury imposes molecular and morphological changes on second-order sensory neurons which evolve differentially with time. Although some changes developed rapidly after deafferentation, the onset of others was slower; and whereas some seemed irreversible, others eventually regressed. Taken together with the functional studies of others, these findings suggest that early molecular changes observed in cuneate neurons reflect adaptive reactions to lesion-induced alterations in afferent activity. Permanent deprivation of the normal input, however, would eventually lead to chronic, and perhaps irreversible, degenerative changes.

Blockade of capsaicin-induced reduction of GABA-immunoreactivity by spantide in cat spinal superficial dorsal horn

In our previous study, perineural application of capsaicin not only produced release and depletion of substance P from primary nociceptive afferent terminals, but also reduced GABA immunoreactivity in the superficial dorsal horn. The aim of the present study was to determine whether the release of GABA is triggered by substance P released from primary nociceptive afferent terminals by capsaican. GABA and substance P immunoreactivity in the lumbar dorsal horn was examined in two groups: in the first group the tibial nerve was treated with 3% capsaicin, and in the second group the dorsal surface of the lumbar cord was infused with spantide (50 nM), a substance P receptor antagonist, before application of capsaicin to the tibial nerve. Following perineural treatment of capsaicin for 30 min, both the GABA-immunostaining density and the number of GABA immunoreactive neurons were reduced significantly in the ipsilateral laminae I-II at L5 through L7. GABA immunoreactivity was reduced by 54.12%, 44.46% and 31.0% in the medial, central and lateral parts of the ipsilateral laminae I-II at L7, respectively. With pre-application of spantide to the spinal cord, GABA immunoreactivity was reduced only to 14.4%, 16.4% and 10.16%, respectively, in the medial, central and lateral parts of laminae I-II at L7 and no reduction of GABA immunoreactive neurons was observed. Additionally, capsaicin-induced reduction of substance P immunoreactivity was partially blocked by spantide. These results suggest that capsaicin produces substance P release from primary nociceptive afferent terminals, and that substance P, in turn, activates the second-order GABAergic interneurons in the dorsal horn. The functional significance of capsaicin-induced activation of GABAergic neurons in modulation of spinal nociception is discussed.

Co-expression of nociceptor properties in dorsal root ganglion neurons from the adult rat in vitro

The cell body of sensory neurons in vitro has been used as a model to study the electrophysiological properties of afferent terminals. A limitation of this approach has been the ability to identify the function of the neuron studied. In the present study, we have tested the hypothesis that a putative nociceptor can be identified in vitro based on the expression of properties associated with nociceptors in vivo. A combination of patch-clamp electrophysiological and immunohistochemical techniques were used to describe the expression of nociceptor properties in acutely cultured dorsal root ganglion neurons from the adult rat. These properties include: a small cell body diameter; the presence of the neuropeptides substance P and calcitonin-gene related peptide; a shoulder (inflection) on the falling phase of the somal action potential, a response to the algogenic agent capsaicin, and sensitization in response to prostaglandin E2. Our results indicate that the frequency of expression of each of these properties varies in a manner consistent with that predicted from observations made in vivo, and that when one property is present in any given neuron, the other properties are also likely to be present. These data support the suggestion that the cell body of adult rat dorsal root ganglion neurons in vitro can be used to study the electrophysiological properties of nociceptors.

Neurotransmitters in the spinal cord dorsal horn in a model of painful neuropathy and in nerve crush

We tested the hypothesis that neurochemical changes in the spinal cord dorsal horn associated with neuropathic pain states differ from those seen in association with non-painful neuropathies. Immunohistochemistry was performed on spinal cord sections from rats with a chronic constriction injury (CCI), which develop hyperalgesia, and from animals with a nerve crush injury, which do not develop hyperalgesia or other signs of a painful syndrome. Immunohistochemistry was quantified by computer-assisted densitometry. Calcitonin gene-related peptide (CGRP) immunoreactivity and substance P (SP) immunoreactivity were decreased from 1 to 4 weeks after injury in CCI and from 2 to 6 weeks in crush. Gamma-aminobutyric acid immunoreactivity was unchanged in both conditions at all time points. Met-enkephalin (Met-enk) immunoreactivity was increased in CCI and unchanged in crush. Although SP and CGRP are involved in pain transmission, we conclude that their decrease in immunoreactivity is not specific for the CCI model, but rather a more general event in nerve de- and regeneration. The increase in immunoreactivity for the opioid peptide Met-ink, however, was only seen in the late phase of CCI, and may be specific for conditions associated with neuropathic pain and its resolution.

Galanin expression in carotid body afferent neurons

The present study examined expression and plasticity of the neuropeptide, galanin, in carotid body afferent neurons in the petrosal ganglion of the adult rat. The pattern of galanin expression was compared with that of tyrosine hydroxylase, a selective marker of dopaminergic carotid body afferents in the petrosal ganglion. In normal animals, only 3% of tyrosine hydroxylase-containing petrosal ganglion neurons co-expressed galanin. Retrograde labeling studies, in which FluoroGold was injected into the vascularly isolated carotid body, demonstrated that all tyrosine hydroxylase-positive-galanin-positive cells in the petrosal ganglion project to this target. In addition, however, we unexpectedly found that galanin expression was markedly increased in the petrosal ganglion following FluoroGold injection into the carotid body. On the other hand, tyrosine hydroxylase expression was unchanged, indicating that monoaminergic and peptidergic traits can be differentially regulated in these cells. In summary, these data demonstrate that monoaminergic chemoafferent neurons can co-express a peptidergic trait, similar to catecholaminergic neurons within the central and autonomic nervous systems, and that these cells retain the potential for phenotypic plasticity in adulthood.

Quantitative evaluation of calcitonin gene-related peptide and substance P levels in rat spinal cord following peripheral nerve injury

Levels of calcitonin gene-related peptide immunoreactivity (CGRP-ir) and substance P immunoreactivity (SP-ir) in the lumbar dorsal spinal cord of rats with either sciatic nerve transection or chronic constriction injury (CCI) were measured using radioimmunoassay. Significant decreases in CGRP-ir and SP-ir occurred in the ipsilateral spinal cord at 10 and 31 days after nerve transection. An ipsilateral decrease in SP-ir occurred 60 days after CCI. In addition, contralateral decreases in CGRP-ir and SP-ir occurred 31 days after transection and 60 days after CCI. Transection of the sciatic nerve produced greater decreases in peptide levels than did the CCI. Changes in spinal levels of these peptides may be involved in the appearance of neuropathic signs associated with nerve injury.

Long-term age-related consequences of forelimb damage upon expression of primary afferent phenotypes in the cervical dorsal horn

Rats that sustained forelimb removal on either embryonic day 16 (E-16) or the day of birth (P-0), or transection of the brachial plexus in adulthood, had sections through the cervical dorsal horn stained for galanin, calcitonin gene-related peptide (CGRP), or the plant lectin Bandieria simplicifolia-I (BS-I) 35-50 days after these lesions. The results of these experiments demonstrated age-related differences in the effects of peripheral nerve damage upon the distributions of each of these three primary afferent markers in the dorsal horn. Damage to the brachial plexus in adulthood caused a significant increase in the density of galanin immunoreactivity in the medial portion of layers I and II and the appearance of galanin immunoreactivity in layers III and IV of the cervical dorsal horn. Such lesions resulted in significant reductions in the density of CGRP immunoreactivity in layers I and II and of BS-I binding in lamina II. Forelimb removal on the day of birth resulted in no significant change in the density of galanin immunoreactivity in layers I and II, but in the appearance of galanin-immunoreactive fibers in layers III-V. Neonatal forelimb removal resulted in no significant change in the density of CGRP immunoreactivity in layers I and II, but in a significant reduction in the density of BS-I binding in the medial portion of lamina II. Removal of the forelimb on E-16 caused a significant increase in the density of galanin immunoreactivity in layers III-V, but had no significant effect on the density or distribution of either CGRP immunoreactivity or BS-I binding in the cervical dorsal horn. These results suggest that peripheral nerve damage at all ages may cause an up-regulation of galanin in a wider distribution of ganglion cell types than was previously thought to be the case, and that there are different sensitive periods for lesion-induced, long-term changes in the innervation of the dorsal horn by CGRP- and BS-I-positive primary afferent axons.

Effects of dorsal rhizotomy on depressor response to spinal cord stimulation mediated by endogenous calcitonin gene-related peptide in the pithed rat

The effects of acute and chronic dorsal rhizotomy on vasodilation induced by spinal cord stimulation were investigated in the pithed rat in vivo. Pithed rats were treated intravenously with hexamethonium (2 mg/kg/min) to block autonomic outflow, and mean arterial blood pressure was maintained at approximately 100 mm Hg with methoxamine (10 to 15 micrograms/kg/min). Electrical stimulation (2 or 4 Hz, 10 V, 1 msec) of the lower thoracic spinal cord (T9-12) via the pithing rod caused a frequency-dependent depressor response without a change in heart rate. The depressor response to spinal cord stimulation was inhibited by the intravenous administration of human calcitonin gene-related peptide (CGRP) [8-37] (60 nmol/kg/min) or tetrodotoxin (100 micrograms/kg). In the pithed rat with acute or chronic bilateral dorsal root rhizotomy at lower thoracic levels (T8-12), spinal cord stimulation at 2 and 4 Hz caused no depressor response. These results suggest that the depressor response to spinal cord stimulation is mediated by endogenous CGRP, which is released from CGRP-containing nerves. The present results also suggest an outflow of CGRP-containing nerves from the spinal cord via the dorsal roots.

Autoradiographic localization of [125I-Tyr8]-bradykinin receptor binding sites in the guinea pig spinal cord

The present study aimed to localize and characterize [125I-Tyr8]-BK binding sites in all major segments of the guinea pig spinal cord using in vitro quantitative receptor autoradiography. [125I-Tyr8]-BK specific binding sites were localized predominantly in superficial layers of the dorsal horn, with lamina II depicting the highest labelling. The density of specific binding in laminae I and III was moderate, whereas in other areas, i.e., laminae IV-X, lower amounts of labelling were noticed. The B2 receptor antagonists D-Arg[Hyp3,Thi5,D-Tic7,Oic8]-BK (Hoe 140), D-Arg[Hyp3,D-Phe7,Leu8]-BK, Tyr0,D-Arg[Hyp3,D-Phe7,Leu8]-BK, D-Arg[Tyr3,D-Phe7,Leu8]-BK, D-Arg[Hyp2,Thi5,8,D-Phe7]-BK, D-Arg[Hyp3,Leu8]-BK and D-Arg[Hyp3,Gly6,Leu8]-BK as well as unlabelled [Tyr8]-BK inhibited [125I-Tyr8]-BK binding with respective Ki values of 0.04, 12.4, 23.4, 34.5, 43.5, 33.5, 23.0, and 0.6 nM while B1 related molecules (Tyr0,des-Arg10-kallidin and [Leu8]-des-Arg9-BK) did not significantly inhibit [125I-Tyr8]-BK binding up to micromolar concentrations. These results indicate that the specific [125I-Tyr8]-BK binding sites present in the guinea pig spinal cord belong to the B2 receptor subtype. The high density of B2 binding sites in the substantia gelatinosa provides an anatomical evidence in favour of a role for BK as a modulator of nociceptive information.

Restoration of substance P and calcitonin gene-related peptide in dorsal root ganglia and dorsal horn after neonatal sciatic nerve lesion

Dorsal root ganglion (DRG) neurons decrease their substance P (SP) synthesis after peripheral nerve lesions. Levels in the dorsal horn also decline but return to normal if regeneration is successful. In adults, when regeneration is prevented, recovery of SP in the dorsal horn is slow and incomplete, whereas in newborns, recovery is rapid and complete even though retrograde cell death of DRG neurons is greater than in adults. We have examined the mechanisms that might account for the rapid and complete recovery of SP and calcitonin-gene related peptide (CGRP) in the dorsal horn after peripheral nerve injury in newborns. Peptides were compared in the L4 and L5 DRG and spinal cord segments of normal rats and in rats surviving 6 days to 4 months after sciatic nerve section/ligation within 24 hours of birth. Sciatic nerve section/ligation produced 50% neuron death in L4 and L5 DRGs, but immunocytochemical methods showed that both SP-immunoreactivity (-IR) and CGRP-IR recovered completely in dorsal horn. Radioimmunoassay confirmed that recovery of SP was not an artefact due to shrinkage. beta-Preprotachykinin (PPT)-mRNA hybridization and SP-IR were observed mostly in small neurons; alpha-CGRP-mRNA-hybridized and CGRP-IR neurons were more heterogeneous. The percentage of DRG neurons that contained SP (approximately 25%) or CGRP (approximately 50%) was the same in normal newborn and adult rats. Neither selective cell survival nor change in neuron phenotype was likely to contribute to the recovery seen in the dorsal horn, and DRG neurons ipsilateral to the lesion exhibited the same level of hybridized beta-PPT-mRNA and alpha-CGRP-mRNA as intact DRG neurons. Because neither the constitutive level of expression of the genes nor peptide levels increased above those observed in intact DRG neurons, these mechanisms were also not responsible. Axotomized DRG neurons, however, contributed to recovery. Recovery was also due to sprouting by neurons in intact DRGs rostral and caudal to L4 and L5.

Quantitative analysis of substance P and calcitonin gene-related peptide immunohistochemical staining in the dorsal horn of neuropathic MK-801-treated rats

An animal model of peripheral neuropathy resulting in a unilateral hyperalgesia has recently been developed. The N-methyl-D-aspartate (NMDA) antagonist MK-801 reduces the thermal hyperalgesia observed in this model. The goal of the present study was to determine whether the immunohistochemical changes in dorsal horn peptides shown by neuropathic animals could also be modified by MK-801. Changes in immunostaining densities of substance P (SP) and calcitonin gene-related peptide (CGRP) within the spinal cord of untreated (reference population) neuropathic rats and that of neuropathic rats treated for 7 days with MK-801 were quantified and compared. The reference neuropathic animals demonstrated thermal hyperalgesia and an ipsilateral decrease in SP staining density without an accompanying change in CGRP staining density. MK-801-treated animals showed a dose-dependent attenuation of the thermal hyperalgesia. The expected ipsilateral decrease in SP was prevented in neuropathic animals treated with a low dose (0.5 mg/kg) of MK-801, while a higher dose of MK-801 (1 mg/kg) resulted in an increase in SP staining ipsilateral to the injury. MK-801 treatment in naive rats caused a global increase in both SP and CGRP staining in the dorsal horn. However, this global increase failed to mask the changes in staining density in neuropathic animals following MK-801 treatment. The results suggest a functional interaction between excitatory amino acids (EAAs) and SP, with activation of NMDA receptors mediating depletion of SP in neuropathic animals. It is suggested that SP-containing interneurons are a target of the EAAs in the dorsal horn.

Increased number of dorsal root ganglion neurons in vitamin-E-deficient rats

Quantitative and morphometric observations were carried out on neurons of L3-L6 dorsal root ganglia (DRGs) in control and vitamin-E-deficient rats at different ages. Controls were fed a standard diet and sacrificed at 1 or at 5 months of age; deficient rats were fed a diet without vitamin E from 1 to 5 months of age and then sacrificed. No significant difference in total number of neurons was found, but an increase in neuron sizes, a decrease in nucleus-cytoplasm ratio, and a more circular neuron shape were found in controls with increasing age (from 1 to 5 months). In L3-L6 DRGs of vitamin-E-deficient rats (5 months of age), a higher number of neurons was found than in those of either young or adult controls. Moreover, some morphometric characteristics of neurons in the deficient rats were similar to those of neurons in 1-month-old controls. The findings suggest that vitamin E deficiency can trigger events resulting in appearance of new neurons, possibly anticipating phenomena that normally occur in aging.

Changing pattern of c-FOS expression in spinal cord neurons after electrical stimulation of the chronically injured sciatic nerve in the rat

Immunocytochemical technique was used to study the distribution of c-FOS protein immunoreactive cells in the spinal cord and gracile nuclei 2 h after electrical stimulation of the sciatic nerve in ketamine/xylazine/acepromazine-anesthetized adult rats. Quantitative examination of the c-fos-labeled cells in the spinal cord laminae was made in unoperated and sham operated controls, after sciatic nerve transection without electrical stimulation, and after electrical stimulation at C-fiber or A alpha/beta-fiber intensity, both in normal animals and at various survival times after chronic sciatic nerve injury (transection and ligation) or crush. Unoperated animals showed very few c-fos-labeled cells, and sham operated controls showed labeled cells located mainly outside the sciatic nerve projection territory. A small increase in number of c-fos protein positive cells was seen after sciatic nerve transection without electrical stimulation. Stimulation of the normal sciatic nerve at C-fiber intensity resulted in c-fos protein-positive cells within the sciatic projection territory in the ipsilateral dorsal horn. Labeled cells were seen in all spinal cord laminae except lamina IX, with the vast majority in lamina I and outer lamina II. No labeled cells were seen in the gracile nucleus. Stimulation at A alpha/beta fiber intensity resulted in no or only a very small number of c-fos-positive neurons. Electrical stimulation of the injured sciatic nerve at C-fiber intensity, using the uninjured contralateral side as control, resulted in significant decreases in c-fos-immunoreactive cells in lamina I plus the outer portion of lamina II at 12 and 39 days survival after injury. A non-significant decrease was seen in these laminae also after 21 days. Significant increases were seen in laminae III and IV at 21 days. Decreases in laminae V, VI and more ventral laminae were significant at 21 and 39 days after injury. At longer survival times, the difference between the normal and injured side seen weeks after injury tended to disappear. Stimulation at A alpha/beta fiber intensity 21 days after injury resulted in increases in the numbers of labeled cells in ipsilateral laminae II, III and IV and in the gracile nucleus. Sciatic nerve stimulation after crush injury resulted in more variable side differences, with tendencies for the same alterations as those noted after chronic transection-ligation.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of A- and C-fiber stimulation on patterns of neuropeptide immunostaining in the rat superficial dorsal horn

The present study determines the effects of sciatic nerve stimulation at intensities that activate A-fibers alone or both A- and C-fibers on immunostaining for substance P (SP), cholecystokinin-octapeptide (CCK-8), galanin (GAL), dynorphin (DYN) and vasoactive intestinal polypeptide (VIP) in the superficial dorsal horn of the rat spinal cord. The goal of this study is to provide a more precise spatial localization of the sites of release or accumulation of these compounds in relation to specific types of stimuli. Following A-fiber stimulation, there was no significant change in immunostaining for any of these compounds. However, A- and C-fiber stimulation resulted in major changes. For SP, CCK-8, GAL and DYN there was a large and significant loss of immunostaining in medial regions of the dorsal horn. This is the area where sciatic nerve primary afferent fibers terminate and the depletion is probably correlated with activity in these fibers. By contrast, VIP immunostaining is increased in the lateral part of the superficial cord, which is outside of the central sciatic afferent fiber terminations. This indicates that the increase is not in the fine sciatic sensory axons that are directly stimulated. As a final point, the fact that C-fiber but not A-fiber stimulation causes marked changes in the immunocytochemical distribution of all these compounds is further evidence, albeit indirect, that they are involved in nociceptive information processing.

Evidence that fine primary afferent axons innervate a wider territory in the superficial dorsal horn following peripheral axotomy

Peripheral axotomy initiates changes in central primary afferent receiving areas of the dorsal horn of the spinal cord. Most of the presently known changes are degenerative in nature and consist of such things as cell and axon death or declines in peptides or enzymes. Other changes are regenerative in nature and because most of these occur in the superficial dorsal horn, which is where fine primary afferents end, we wished to ask whether peripheral axotomy results in a change in the distribution in these fine afferents. Using recently available markers for fine primary afferent axons and small dorsal root ganglion cells, we demonstrate that peripheral axotomy results in a considerable increase in the immunolabeled area for these compounds. Our interpretation is that there may be an extension of fine primary afferent fibers into lamina III and possibly lamina IV following peripheral axotomy. If further work bears out this conclusion, this would provide a possible explanation for the chronic pain states that sometimes follow peripheral nerve damage.