Immunohistochemical localization of serotonin, leu-enkephalin, tyrosine hydroxylase, and substance P within the visceral sensory area of cartilaginous fish

We examined the distribution of immunoreactivity to serotonin (5-HT), leu-enkephalin (LENK), tyrosine-hydroxylase (TH), and substance P (SP) within the primary visceral sensory region of cartilaginous fish. Two genera of sharks, Squalus and Heterodontus, a skate, Raja, a ray, Myliobatis, and a holocephalian, Hydrolagus, were used. Cranial nerves, VII, IX, and X enter the visceral sensory complex from the lateral aspect and divide it into lobes. Based on sagittally cut sections, there are four lobes in Hydrolagus and five in Squalus, corresponding to the number of gill arches. The neurochemicals are differentially distributed within each lobe. LENK+ and 5-HT+ fibers are located in all regions within the visceral sensory complex. SP+ fibers are extremely dense in a dorsolateral subdivision and do not extend as far ventrally as 5-HT+ and LENK+ fibers. The lobes lack 5-HT+ cells, but contain a few LENK+ and SP+ cells. Many TH+ cells are distributed in dorsomedial portions of the complex, but there are few TH+ fibers. Thus, the visceral sensory area of cartilaginous fish contains several divisions that can be distinguished by their neurochemical content.