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

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Abstract
The spinal pia mater receives a rich innervation of small sensory axons via the ventral roots. Other sensory axons enter the ventral roots but end blindly or turn abruptly in hairpin loop‐like formations and continue in a distal direction. In the present study, the content of substance P (SP)‐, calcitonin gene‐related peptide (CGRP)‐, growth‐associated 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 microscopic immunohistochemical techniques. In addition, the binding of antibodies against synthetic peptides representing unique sequences of residues in the products of the trk and trkB protooncogenes was analyzed. These 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 of the present study imply that the ventral root afferents comprise several different types of sensory axons, which all contain SP‐, CGRP‐, GAP‐43‐, and p75NGFr‐like immunoreactivities. In addition, at least some of the presumed sensory fiber bundles in ventral roots and the pia mater were immunoreactive for the trkB gene product. Moreover, leptomenin‐geal cells and nonneuronal cells of the ventral roots were shown to bind antibodies to both the trk and trkB gene products. The ventral root afferents seem to share their immunohistochemical pattern with pain‐transducing axons at some other locations, such as the tooth pulp. The contents of SP‐ and CGRP‐LI in sensory axons that reach the central nervous system (CNS) through the ventral root indicate that ventral root afferents may be involved in sensory mechanisms, such as the ventral root pain reaction, as well as in the control of the pial blood vessels. The demonstration of GAP‐43 and neurotrophin receptor‐immunoreactivities associated with unmyelinated fibers in ventral roots and the pia mater is discussed in relation to previous reports on postnatal plasticity in these axonal populations.

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