SUBSTANCE P-, CALCITONIN-GENE-RELATED PEPTIDE, GROWTH-ASSOCIATED PROTEIN-43, AND NEUROTROPHIN RECEPTOR-LIKE IMMUNOREACTIVITY ASSOCIATED WITH UNMYELINATED AXONS IN FELINE VENTRAL ROOTS AND PIA MATER

The spinal pia mater receives a rich innervation of small sensory axon s via the ventral roots. Other sensory axons enter the ventral roots b ut end blindly or turn abruptly in hairpin loop-like formations and co ntinue in a distal direction. In the present study, the content of sub stance P (SP)-, calcitonin gene-related peptide (CGRP)-, growth-associ ated protein (GAP-43)-, and low-affinity neurotrophin receptor protein (p75NGFr)-like immunoreactivity (-LI) associated with these different types of sensory axons was assessed with light and electron microscop ic immunohistochemical techniques. In addition, the binding of antibod ies against synthetic peptides representing unique sequences of residu es in the products of the trh and trkB protooncogenes was analyzed. Th ese genes encode membrane spanning proteins, which have been shown to constitute specific high affinity binding sites for several members of the nerve growth factor family of neurotrophic factors. The results o f the present study imply that the ventral root afferents comprise sev eral different types of sensory axons, which all contain SP-, CGRP-, G AP-43-, and p75NGFr-like immunoreactivities. In addition, at least som e of the presumed sensory fiber bundles in ventral roots and the pia m ater were immunoreactive for the trkB gene product. Moreover, leptomen ingeal cells and nonneuronal cells of the ventral roots were shown to bind antibodies to both the trh and trkB gene products. The ventral ro ot afferents seem to share their immunohistochemical pattern with pain -transducing axons at some other locations, such as the tooth pulp. Th e contents of SP- and CGRP-LI in sensory axons that reach the central nervous system (CNS) through the ventral root indicate that ventral ro ot afferents may be involved in sensory mechanisms, such as the ventra l root pain reaction, as well as in the control of the pial blood vess els. The demonstration of GAP-43 and neurotrophin receptor-immunoreact ivities associated with unmyelinated fibers in ventral roots and the p ia mater is discussed in relation to previous reports on postnatal pla sticity in these axonal populations.