Distribution of gonadal steroid receptor-containing neurons in the preoptic-periaqueductal gray-brainstem pathway: A potential circuit for the initiation of male sexual behavior

The present study used anterograde and retrograde tract tracing techniques to examine the organization of the medial preoptic-periaqueductal gray-nucl eus paragigantocellularis pathway in the male rat. The location of neurons containing estrogen (alpha subtype; ER alpha) and androgen receptors (AR) w ere also examined. We report here that injection of the anterograde tracer biotinylated dextran amine (BDA) into the medial preoptic (MPO) produced de nse labeling within the periaqueductal gray (PAG); anterogradely labeled fi bers terminated in close juxtaposition to neurons retrogradely labeled from the nucleus paragigantocellularis (nPGi). Dual immunostaining for Fluoro-G old (FG) and ER alpha or FG and AR showed that over one-third of MPO effere nts to the PAG contain receptors for either estrogen or androgen. In additi on, approximately 50% of PAG neurons retrogradely labeled from the nPGi wer e immunoreactive for either ER alpha or AR. These results are the first to establish an MPO-->PAG-->nPGi circuit and further indicate that gonadal ste roids can influence neuronal synaptic activity within these sites. We repor ted previously that nPGi reticulospinal neurons terminate preferentially wi thin the motoneuronal pools of the lumbosacral spinal cord that innervate t he pelvic viscera. Together, we propose that the MPO-->PAG-->nPGi circuit f orms the final common pathway whereby MPO neural output results in the init iation and maintenance of male copulatory reflexes. J. Comp. Neurol. 438:19 1-212, 2001. (C) 2001 Wiley-Liss, Inc.