Immunohistochemical studies on the distribution and origin of candidate peptidergic primary afferent neurotransmitters in the spinal cord of an elasmobranch fish, the atlantic stingray (Dasyatis sabina)

Abstract

The distribution and origin of four peptide neurotransmitter candidates of primary afferents (substance P, SP; somatostatin, SS; cholecystokinin, CCK; and vasoactive intestinal polypeptide, VIP) were studied in the stingray with peroxidase‐antiperoxidase (PAP) immunohistochemistry. This elasmobranch has virtually no unmyelinated primary afferents, having instead only large and small myelinated afferents. SP‐like immunoreactivity was distributed densely in the superficial aspect of the substantia gelatinosa (SG), particularly laterally, and was scattered in the nucleus proprius, the intermediate zone, and the ventral horn. The distributions of SS‐, CCK‐, and VIP‐like immunoreactivities were similar to each other, but different from that of SP. Stained fibers appeared to issue from a prominent tract in the dorsolateral funiculus to form a plexus at the lateral margin of the nucleus proprius. The fibers spread dorsally and medially through the SG to terminate in a thin band at the superifical margin of the SG. Both SS and CCK were more dense in the lateral third of the SG, while VIP was more diffusely distributed within this structure. The remaining regions of the spinal gray matter contained immunoreactive fibers and terminals in variable densities. Many SS‐positive cell bodies were observed in the ventral horn, in the deep dorsal horn, and in the ependymal layer. CCK‐positive cells were observed in the medial ventral horn, and VIP‐positive cells were observed subjacent to the SG and within the dorsolateral funiculus. After unilateral dorsal rhizotomies, SP‐like immunoreactivity in the SG was depleted, while SP staining elsewhere and all SS, CCK, and VIP staining was indistinguishable from control. Thus all four peptides are present in the stingray spinal cord, although only SP appears to be a candidate primary afferent transmitter.