Immunohistochemical demonstration of serotonin neuron system in the central nervous system of the Japanese dogfish, Scyliorhinus torazame (Chondrichthyes).

The distribution of serotonin immunoreactivity in the brain of the Japanese dogfish (Scyliorhinus torazame) was studied with the peroxidase-antiperoxidase immunohistochemical method with serotonin antiserum. The somata of the serotonin neurons were located in the raphe regions of the brain stem and the spinal cord. A small number of serotonergic neurons were also distributed as cerebrospinal-fluid contacting neurons in the organum vasculosum hypothalami (OVH) and the paraventricular organ (PVO). These neurons were classified into nine groups (S1-S9): spinal cord (S1), rhombencephalon (S2-S6), mesencephalon (S7), and prosencephalon (OVH, S8 and PVO, S9). Processes of the serotonin neurons were widely distributed in the central nervous system, forming dense networks in various regions. The highest concentrations of these fibers were in the dorsal area of the vagal lobe, the nucleus isthmi, the tuberculum posterium, the distal part of the PVO, and the dorsal regions of the olfactory tubercle.