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).
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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 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 superficial margin of the SG. Both SS and CCK were more dense 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 latyer. 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 SP appears to be a candidate primary afferent transmitter.