Brainstem location of serotonin neurons projecting to the caudal neurosecretory complex

Serotonergic fibers in the caudal neurosecretory complex (CNc) of poeciliids originate from neurons within, and extrinsic to this spinal cord nucleus. In the present study, retrograde tracing and immunofluorescence techniques were combined to localize extrinsic serotonergic projection neurons. The entire spinal cord and brain were sectioned after Fast Blue (FB) or horseradish peroxidase (HRP) was implanted in the CNc. No HRP or FB filled neurons were found in the spinal cord. Retrogradely filled neurons were found bilaterally in dorsolateral and ventromedial reticular nuclei, and the dorsal midbrain tegmentum. Fusiform cells in the medullary fasciculus longitudinalis medialis filled with FB but not HRP. Serotonin immunopositive neurons were found surrounding the third ventricle, in the raphe and in medullary reticular nuclei. Double labelled neurons in the medial reticular nucleus were determined to be the source of serotonergic projections to the CNc. Reticular projection nuclei are strategically situated to receive visceral sensory input from rhombencephalic cranial nerves. These putative pathways may provide an anatomical substrate by which visceral sensory information is transmitted to the CNc.