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Peripheral nerve injury is associated with spinal microgliosis which plays a pivotal role in the development of neuropathic pain behavior. Several agents of primary afferent origin causing the microglial reaction have been identified, but the type(s) of primary afferents that release these mediators are still unclear. In this study, specific labeling of C-fiber spinal afferents by lectin histochemistry and selective chemodenervation by capsaicin were applied to identify the type(s) of primary afferents involved in the microglial response. Comparative quantitative morphometric evaluation of the microglial reaction in central projection territories of intact and injured peripheral nerves in the superficial (laminae I and II) and deep (laminae III and IV) spinal dorsal horn revealed a significant, about three-fold increase in microglial density after transection of the sciatic or the saphenous nerve. Prior perineural treatment of these nerves with capsaicin, resulting in a selective defunctionalization of C-fiber afferent fibers failed to affect spinal microgliosis. Similarly, peripheral nerve injury-induced increase in microglial density was unaffected in rats treated neonatally with capsaicin known to result in a near-total loss of C-fiber dorsal root fibers. Perineural treatment with capsaicin per se did not evoke a significant increase in microglial density. These observations indicate that injury-induced spinal microgliosis may be attributed to phenotypic changes in injured myelinated primary afferent neurons, whereas the contribution of C-fiber primary sensory neurons to this neuroimmune response is negligible. Spinal myelinated primary afferents may play a hitherto unrecognized role in regulation of neuroimmune and perisynaptic microenvironments of the spinal dorsal horn.

The long-term effect of perineural capsaicin treatment on the distribution of substance P (SP) binding sites was studied in the rat spinal dorsal horn using 125I-labelled Bolton-Hunter-SP. Three months after local application of capsaicin onto the sciatic nerve quantitative evaluation of the autoradiograms revealed a significant decrease in the density of SP binding sites of up to 48% in regions of laminae I and II of the spinal dorsal horn somatotopically related to the capsaicin treated sciatic nerve. It is suggested that reduction in SP binding may result from transganglionic and transsynaptic degenerative changes affecting postsynaptic structures. Changes in the distribution of SP binding sites may significantly contribute to functional alterations observed after perineural treatment with capsaicin.

Local treatment of rat peripheral nerves with capsaicin induces permanent impairment of afferent C-fiber functions and changes in the response properties of spinal dorsal horn neurons. In this study a new experimental approach, the "capsaicin gap" technique, has been utilized in an attempt to unravel pathomorphological alterations which commence in the domain of primary sensory neurons as a consequence of perineural treatment with capsaicin. The technique relies on the facts that peripheral nerves in the spinal dorsal horn are represented in a strict somatotopic manner, and on the observation that in the adult rat systemic injection of appropriate doses of capsaicin results in a selective degeneration of primary afferent fibers within Rexed's laminae I and II of the spinal cord. We have assumed that, if treatment of a peripheral nerve with capsaicin results in an irreversible destruction of primary afferent terminals related to that nerve, then the corresponding spinal projection areas will be free of degeneration following a subsequent systemic injection of capsaicin. The topographical distribution of degenerating axon terminals, induced by a systemic injection of capsaicin, was studied within the lumbar spinal cord of rats whose sciatic nerves had been treated with capsaicin or its vehicle one to three months previously. In the dorsal horn ipsilateral to the vehicle-treated nerve, axon terminal degeneration was found in the entire mediolateral extent of Rexed's laminae I and II. In contrast, in the dorsal horn ipsilateral to the capsaicin-treated nerve, virtually no degeneration was observed in areas which correspond to the projection territories of sciatic afferents. It is concluded that the central terminals of capsaicin-sensitive sciatic afferents underwent transganglionic degeneration as a result of prior perineural treatment with capsaicin, and a subsequent systemic injection of this neurotoxin therefore failed to cause axon terminal degeneration in somatotopic areas served by the treated nerve. Comparative quantitative morphometric analysis of cell populations of dorsal root ganglia related to capsaicin- or vehicle-treated nerves disclosed (1) a marked reduction in the proportion of small-sized neurons, (2) a fall of about 80% in the percentage of neurons which undergo degeneration after the systemic injection of capsaicin, and (3) a marked decrease in the total number of neurons in ganglia ipsilateral to the capsaicin-treated nerves. Quantitative electron microscopic studies on saphenous nerves treated perineurally with capsaicin revealed a 32% reduction in the number of unmyelinated axons as compared with the controls, whereas the number of myelinated fibers was unchanged.(ABSTRACT TRUNCATED AT 400 WORDS)

The capsaicin-sensitive afferent innervation of the urinary bladder and the central nervous system distribution of urinary bladder afferents have been studied in the rat. Capsaicin-sensitive primary sensory neurones supplying the urinary bladder have been found in two groups of spinal ganglia located in the Th13-L2 and L6-S1 segments. Capsaicin-sensitive primary sensory afferents from the bladder terminate within Rexed's laminae I, V and X, and in the dorsal gray commissure of the lumbosacral spinal cord. In addition, the results point to a possible vagal sensory innervation of the urinary bladder.

After neonatal treatment of rats with capsaicin, the spinal cord, the spinal trigeminal nucleus and spinal and trigeminal ganglia were analysed with immunohistochemistry using antisera to several peptides and 5-hydroxytryptamine. A marked decrease was observed in substance P-, cholecystokinin-, somatostatin- and VIP-like immunoreactivity present in the central branches of primary sensory neurons in the spinal cord and in substance P- and somatostatin-like immunoreactivity in sensory ganglion cells. No definite depleting effect of capsaicin could be established on 5-hydroxytryptamine and peptides, such as enkephalin and neurotensin, present in centrally originating fibres in the dorsal horn of the spinal cord. The results demonstrate that the effects of capsaicin are not confined to substance P immunoreactive primary sensory neurons. The possibility is discussed that capsaicin effects specifically functioning rather than chemically specific primary sensory neurons.

Using indirect immunofluorescence technique, avian pancreatic polypeptide (APP) immunoreactive cell bodies and fibres have been observed in the superficial laminae of the dorsal horn of the spinal cord and of the spinal trigeminal nucleus. Fibres were also seen in the ventral horns, in low numbers at the cervical and thoracic levels and in high numbers at the lower lumbar and upper sacral levels. Neither total cord transection, nor dorsal rhizotomy, nor capsaicin treatment seemed to affect the APP systems described above. The present findings suggest that an APP-like peptide may be involved in processing of sensory information at the level of the first relay station.

The axon terminals of some capsaicin-sensitive sensory neurons in the spinal cord of the rat contain high amounts of acid phosphatase (EC 3.1.3.1) activity. We quantitated this activity in control and capsaicin-treated rats and mice in a biochemical assay using beta-glycerophosphate (beta-GP) and thiamine monophosphate (TMP), which have both been used in previous histological investigations, as substrates and measured the amount of phosphate liberated from particular fractions. The ventral spinal cord of rats yielded 209 +/- 9 (mean +/- S.E.M.) nmol phosphate/mg protein/h from beta-GP and 18 +/- 5 nmol from TMP; the values for the upper dorsal horn are 544 +/- 42 and 198 +/- 12 for beta-GP and TMP respectively. Values for mouse spinal cord tissue are quite similar; the spinal cord of guinea pigs contains lower amounts of beta-GPase and very little TMPase activity per mg protein. There was a fairly broad pH optimum between 5.4 and 6.3. After capsaicin (50 mg/kg s.c.) pretreatment, beta-GPase activity in the upper dorsal horn was decreased by 29% in rats and by 17% in mice; TMPase activity was reduced by 48% and 37% respectively. Values in the ventral spinal cord were unchanged. It is proposed that biochemical measurement of TMPase activity might be useful in quantitative investigations of acid phosphatase activity (e.g. "FRAP") in capsaicin-sensitive sensory neurons.

The distribution of spinal primary afferent terminals labeled transganglionically with the choleratoxin B subunit (CTB) or its conjugates changes profoundly after perineural treatment with capsaicin. Injection of CTB conjugated with horseradish peroxidase (HRP) into an intact nerve labels somatotopically related areas in the ipsilateral dorsal horn with the exceptions of the marginal zone and the substantia gelatinosa, whereas injection of this tracer into a capsaicin-pretreated nerve also results in massive labeling of these most superficial layers of the dorsal horn. The present study was initiated to clarify the role of C-fiber primary afferent neurons in this phenomenon. In L5 dorsal root ganglia, analysis of the size frequency distribution of neurons labeled after injection of CTB-HRP into the ipsilateral sciatic nerve treated previously with capsaicin or resiniferatoxin revealed a significant increase in the proportion of small neurons. In the spinal dorsal horn, capsaicin or resiniferatoxin pretreatment resulted in intense CTB-HRP labeling of the marginal zone and the substantia gelatinosa. Electron microscopic histochemistry disclosed a dramatic, ∼10-fold increase in the proportion of CTB-HRP-labeled unmyelinated dorsal root axons following perineural capsaicin or resiniferatoxin. The present results indicate that CTB-HRP labeling of C-fiber dorsal root ganglion neurons and their central terminals after perineural treatment with vanilloid compounds may be explained by their phenotypic switch rather than a sprouting response of thick myelinated spinal afferents which, in an intact nerve, can be labeled selectively with CTB-HRP. The findings also suggest a role for GM1 ganglioside in the modulation of nociceptor function and pain. [ABSTRACT FROM AUTHOR]