The location of the somata, course of the main tracts, and fiber distribution of the serotonin neurons in the turtle brain were studied using the peroxidase-antiperoxidase (PAP) immunohistochemical method with antibodies against serotonin (5-hydroxytryptamine). The somata of the serotonin neurons were distributed in the reticular formation of the brainstem from the mesencephalon to the lower medulla level and in a resticted region of the hypothalamus, viz. the paraventricular organ (PVO). In the PVO the serotonin neurons were seen to have the appearance of cerebrospinal fluid-contacting neurons. Analysis of serial sections cut in the frontal and sagittal planes revealed a widespread distribution of the serotonin immunoreactive fibers in the turtle brain. Prominent concentrations of the serotonin immunoreactive fibers were found in the lateral portion of the striatum, the ventral portion of the septum, the nucleus corporis geniculati lateralis, the nucleus pretectalis, the nucleus isthmi parvocellularis, the optic tectum, and the lateral edge of the reticular formation of the brainstem. Ascending and descending serotonin pathways could be defined: the ascending pathway originated mainly from the nucleus profundus mesencephali caudalis, nucleus lemnisci lateralis, nucleus reticularis isthmi and, less prominently, from the nucleus raphe superior pars lateralis, and the descending pathway arose predominantly from the nucleus raphe inferior. The fibers of the ascending pathway projected widely in the prosencephalon and mesencephalon, via the medial forebrain bundle. The descending pathway ran through the ventral and lateral portion of the medulla oblongata and spinal cord.
