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Polyethylene cuffs of varying inner diameters were applied to the rat sciatic or sural nerve with the aim of inducing a standardized nerve injury, as assessed by morphometric analyses of fiber-size spectrum alterations, associated with behavioral manifestations of neuropathic pain. The temporal sequence of axonal degeneration and regeneration was examined in parallel with behavioral analyses of pain initiation and recovery over a 6-week postoperative (PO) period. Cuffs of 0.028-0.030" inner diameter loosely enclosed sciatic nerves of young rats and elicited relatively uniform axonal degeneration and 'pain'. Large myelinated axons underwent an early and sustained numerical depletion. Both the thinly myelinated and unmyelinated axon populations were initially diminished, but later rose to levels significantly greater than control values, likely the result of: (1) demyelination, (2) early stages of remyelination, (3) regenerative sprouting, and/or (4) collateral sprouting of undamaged unmyelinated axons. Pathological alterations of the injured nerve included edematous swelling, hypertrophy of the perineurial sheath, infiltration of fibroblasts and collagen into the intraneurial compartment, increasing interaxonal space and decreasing order and density of axonal packing. Animals displayed maximal pain-related behaviors, including gait and postural asymmetries and hypersensitivity to mechanical compression and cold, during the 2nd week PO and had largely recovered by approximately 4 weeks PO. Consistent behavioral manifestations of pain were achieved over a wide range of fiber spectrum alteration; however, with the largest cuffs or 'bracelets' used in this study, a substantial axonal fiber spectrum change was produced without inducing pain-related behavior, suggesting that decrement in the number of myelinated axons was not always sufficient to elicit pain. Similar morphometric and pathological results were achieved with sural neuropathy after 0.010" ID cuffs and 14 days PO survival. Considering the lack of correlation between axonal alterations and pain, modification in the local intraneurial microenvironment at the site of injury may be a key component of peripheral pain mechanisms; these include changes in the biochemical milieu, increased intraneurial pressure, and altered nociceptor sensitivity or impulse propagation in the relatively intact unmyelinated axon population.

A whole-mount preparation is described for visualizing unmyelinated peptide immunoreactive axons in two specialized tissues, thin enough for tracing axons to their terminal knobs. These tissues, the cornea and the mucosal surface of the tympanic membrane, are known to possess specialized nociceptor innervation and can be used for selecting sensory endings labeled by peroxidase reaction product and embedded in plastic for more detailed analysis. Rat calcitonin gene-related peptide antibody proved a particularly robust primary antibody found in a substantial proportion of sensory unmyelinated axons in a variety of fixation and preparative conditions.

The sparse distribution of thin, principally unmyelinated sensory axons confined largely to the planar tunica vasculosa of the testis provides a suitable model for examining the fine structure of electrophysiologically characterized nerve fiber terminals. The marked sites of polymodal receptors of canine testis using the in vitro preparation devised by Kumazawa et al. (1987) were examined in serial sections traced to the terminal with the electron microscope, revealing the first micrographs of a characterized polymodal receptor ending. The inferred role of these terminals in nociception, their organelle content, and the problems encountered in interpreting our initial findings are considered in the context of the variety of morphological patterns and functional roles of thin sensory axons.

Primary afferent sprouting in the spinal cord was evaluated by comparing the central projection of horseradish peroxidase (HRP)-labeled sciatic nerve afferent axons in nonlesioned control rats, and in rats subjected to acute or chronic partial spinal hemisections as adults. The lesions were performed at various levels from T10 to L3, and removed supraspinal and varying amounts of descending propriospinal afferents to lumbar segments receiving the maximal sciatic projection. The hemisections typically involved all but the dorsal column, although in some cases a portion of the dorsal column, including the corticospinal tract, was also transected. The distribution pattern and density of spinal HRP reaction product was not significantly different in experimental and control preparations in any segment below the lesion, regardless of the quantity of denervation, or the density of the normal sciatic projection in a given terminal region. These results, together with our previous finding concerning an absence of primary afferent sprouting following long-term dorsal root ganglionectomies, suggest that current concepts concerning collateral sprouting as a factor in functional plasticity in the mature mammalian spinal cord warrant re-evaluation.

Calcitonin gene-related peptide (CGRP) is a recently characterized neuroactive substance that is expressed in a large proportion of small- to medium-diameter sensory ganglion neurons whose central terminals lie in the superficial spinal and medullary dorsal horn. This restricted distribution within the peripheral nervous system suggests a prominent role for the peptide in nociceptive processing. The mammalian tooth pulp, which receives a relatively homogeneous afferent input from thin (putative nociceptive) fibers originating from this subpopulation of trigeminal ganglion cells, thus affords an ideal target zone in which to examine peripheral nociceptive mechanisms. The large percentage of these neurons displaying CGRP-like immunoreactivity (CGRP-LI) furthermore provides a valuable tool to study its thin-fiber afferent innervation. CGRP-LI has been localized within intact, decalcified specimens of rat, cat, monkey, and human teeth and associated dental structures. A remarkably robust CGRP-LI innervation of molar pulp and dentin was revealed in all species, with fibers coursing both in fascicles and individually, in variable relation to blood vessels and pulpal stroma. Our methods enabled tracing of a large number of axons through Raschkow's plexus and odontoblast layer into dentinal tubules. Paralleling anterograde axonal transport studies, a greater share of fibers was found in coronal vis-à-vis radicular dentin. In the rat, this fiber pattern stood in contrast both to incisor dentin, which appeared devoid of CGRP-LI, and to the abundant labeled axons in gingiva and periodontal tissues. Surgical deafferentation of rat mandible resulted in widespread depletion of CGRP-LI, while superior cervical ganglionectomy was without effect, confirming the sensory nature of the CGRP-LI fibers. Neonatal capsaicin treatment greatly attenuated the immunostaining, providing evidence for CGRP-LI localization in chemosensitive unmyelinated afferents. The great density of CGRP-LI axons demonstrated is considered in contrast to the restricted range and extent of sensory stimuli to which teeth are presumably subjected, and in relation to the diverse ongoing trophic, regulatory, and reparative processes in tooth structures. It is therefore suggested that these fibers may be subserving prominent efferent roles in dental pulp not directly related to nociception.