The organization of indoleamine neurons in the brain of the rhesus monkey (Macaca mulatta).

The organization of indoleamine-containing neurons throughout the CNS of Macaca mulatta was examined with Falck-Hillarp histofluorescence and radioenzymatic biochemical techniques. Indoleamine-containing cell bodies, corresponding to the serotonin-containing groups B1 to B9, were observed within the brain stem. The pontine cell populations, however, were far more numerous than in the rat and represented caudal subpopulations of cells within the nucleus raphe dorsalis (B7) and nucleus centralis superior (B8). Additionally, the pontine indoleamine-containing cells in M. mulatta extended laterally through the tegmentum such that they were often adjacent to catecholamine-containing neurons of the locus coeruleus complex. Ascending indoleamine-containing fiber bundles, similar to those defined in nonprimate mammals, were also observed within the pons and mesencephalon. Regional differences in the distribution of serotonin within hypothalamic nuclei and other forebrain regions of M. mulatta were revealed through a highly sensitive radioenzymatic assay in conjunction with microdissection. Significant concentrations of serotonin were evident in most areas, although the levels of this neurotransmitter were much lower in the median eminence region and nucleus suprachiasmaticus than in the rat. These differences may reflect variations in the control neuroendocrine events in the two species.