Immunohistochemical demonstration of the serotonin neuron system in the central nervous system of the bullfrog, Rana catesbeiana.

The distribution of serotonin immunoreactivity in the brain of the bullfrog (Rana catesbeiana) was studied, using the peroxidase-antiperoxidase (PAP) immunohistochemical method with serotonin antiserum. The somata of the serotonin neurons were mainly located in the raphe regions of the brain stem from the level of the caudal mesencephalon to that of the spinomedullary junction. A small number of serotonin neurons were also distributed as cerebrospinal-fluid contacting neurons in the preoptic recess organ (PRO), the paraventricular organ (PVO), and the nucleus infundibularis dorsalis (Nid). In the raphe region, these serotonin neurons formed nearly-continuous bilaterally-symmetrical cell columns along the midline of the brain stem, divided into lateral and medial groups. The medial group was further subdivided into rostral and caudal parts. Processes of the serotonin neurons were widely distributed in the central nervous system, forming dense networks in various regions. The greatest concentrations of these fibers were in the nucleus medialis septi, lateral portion of striatum, nucleus corporis geniculi, nucleus entopeduncularis, periventricular gray of ventral hypothalamus, optic tectum, nucleus isthmi, nucleus interpeduncularis, dorsal edge of medulla oblongata, and fasciculus solitarius.