Organization of the spinal cord in four species of elasmobranch fish: Cytoarchitecture and distribution of serotonin and selected neuropeptides

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Abstract
An analysis of Nissl stained sections of the spinal cord taken from four species of elasmobranch showed that seven distinct cytoarchitectonic laminae are present. These laminae are compared with laminae described previously in the spinal cord of other vertebrates.The distribution of immunoreactivity to serotonin, substance P, somatostatin, calcitonin gene‐related peptide, neuropeptide Y, and bombesin was determined in the brown stingray (Dasyatis fluviorum), the eagle ray (Aetobatis narinari), the shovelnose ray (Rhinobatis battilum), and the black‐tip shark (Carcharhinus melanopterus). In all species, dense immunoreactivity to most substances tested was found in the outer part of the substantia gelatinosa. Many fibres and varicosities immunoreactive to substance P, calcitonin gene‐related peptide, and bombesin were found in this region and smaller numbers of fibres were found in the nucleus proprius. Immunoreactivity to somatostatin consisted of coarse fibre bundles that entered the dorsal horn at the nucleus proprius and radiated dorsally to the substantia gelatinosa. Axons and varicosities immunoreactive to serotonin and neuropeptide Y were found in all regions of the dorsal horn but were concentrated in the outer part of the substantia gelatinosa. The distribution of immunoreactivity to met‐enkephalin in the shovelnose ray was concentrated in the lateral third of the substantia gelatinosa and to a lesser extent in the nucleus proprius.The distribution of these substances is compared with that described in other vertebrates. Although the sensory information reaching the elasmobranch spinal cord is limited, compared with that of mammalian species, the distribution of these neuroactive factors in the dorsal horn of the two groups is strikingly similar.