Analysis of calcitonin gene‐related peptide‐like immunoreactivity in the cat dorsal spinal cord and dorsal root ganglia provide evidence for a multisegmental projection of nociceptive C‐fiber primary afferents
Authors: Traub Richard J. , Allen Barbara , Humphrey Emma , Ruda M. A. 
Published: January 2005

Recent studies have suggested that calcitonin gene‐related peptide (CGRP) can be used as a marker for a subpopulation of nociceptive primary afferents. Consequently, CGRP‐immunoreactive (CGRP‐IR) primary afferents have been reported to project many segments rostral to their segment of entry and to send collaterals into the superficial and deep laminae of the dorsal horn. This study reports that some CGRP‐IR primary afferents of sacral origin project rostral through the ipsilateral lumbar enlargement in the cat. The ultrastructure of these multisegmentally projecting primary afferent axons and terminals identified in a partially denervated cat was examined and compared to the ultrastructure of CGRP‐IR afferents from an intact cat.

Retrograde transport of wheatgerm agglutinin‐colloidal gold injected into the cat L4 spinal cord resulted in labeling of primary afferent cell bodies in the ipsilateral L4‐S1 dorsal root ganglia (DRG). Analysis of every fourth section through the ipsilateral S1 DRG revealed as many as 1,000 retrogradely labeled neuronal cell bodies. One third of these cell bodies were double labeled for CGRP‐like immunoreactivity. The number of single‐ and double‐labeled cells increased in ganglia closer to the injection site (L4–L7).

At the ultrastructural level, in the lumbosacral dorsal spinal cord of a normal cat, most CGRP‐IR axons were unmyelinated, while the rest were small myelinated axons. In both the superficial dorsal horn and lamina V, CGRP‐IR varicosities were dome shaped, scallop shaped, or elongated. The CGRP‐IR varicosities contained small agranular vesicles and frequently a few dense core vesicles. These labeled varicosities formed asymmetric synapses on unlabeled dendritic spines, shafts, or neuronal somata.

One cat received multiple unilateral dorsal rhizotomies (S1–L4) and an ipsilateral hemisection (mid L4). CGRP‐IR axons and terminals were found within each of the rhizotomized segments, although their density was greatly reduced compared to that in the intact animals. In Lissauer's tract the majority (>90%) of CGRP‐IR fibers were unmyelinated. In laminae I and V, the remaining CGRP‐IR varicosities were mainly the dome‐shaped varicosities morphologically similar to those observed in the normal spinal cords. They contained small agranular vesicles and a few dense core vesicles and formed asymmetric synapses on unlabeled dendritic shafts and spines.

These data demonstrate that unmyelinated, presumably C‐fiber nociceptive primary afferents and some small myelinated A‐delta nociceptive primary afferents of sacral origin project rostral through the cat lumbar enlargement and make synaptic connections in both the superficial and deep laminae of the cat dorsal spinal cord. Furthermore, the varicosities that arise from these multisegmentally projecting nociceptive afferents seem to represent a morphologically distinct, subpopulation of CGRP‐IR varicosities normally present in laminae I and V of the cat dorsal horn. On the basis of these data it is hypothesized that the multisegmentally projecting CGRP‐IR afferents innervate the pelvic viscera.