Distribution of the pro-opiomelanocortin-immunoreactive axons in relation to the serotoninergic neurons in the dorsal raphe nucleus of the rat.

The anatomical relationships between pro-opiomelanocortin-containing axons and serotonin neurons in the nucleus raphe dorsalis (NRD) of the rat were examined at the light microscope level with antibodies against CLIP (corticotropin-like intermediate lobe peptide), alpha-MSH (alpha-melanocyte-stimulating hormone) and serotonin. Sequential double labeling was performed with either immunofluorescence or peroxidase-antiperoxidase techniques. It was observed that the network of POMC-immunoreactive axons displayed a gradient of decreasing density from rostral to caudal levels and from dorsal to ventral parts or the NRD. The examples of close proximity between immunoreactive axons and serotonin cell bodies or dendrites were rather scarce. On the whole, the immunoreactive fibers seemed to run quasi-independently of the serotonin neurons.