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We have shown recently that the medial preoptic area (MPO) robustly innervates discrete columns along the rostrocaudal axis of the midbrain periaqueductal gray (PAG). However, the location of PAG neurons responsive to MPO activation is not known. Anterograde tract tracing was used in combination with Fos immunohistochemistry to characterize the MPO --> PAG pathway anatomically and functionally within the same animal. Focal electrical or chemical stimulation of MPO in anesthetized rats induced extensive Fos expression within the PAG compared with sham controls. Fos-positive neurons were organized as 2–3 longitudinal columns. The organization and location of these columns overlapped remarkably well with the distribution of fibers and terminals in PAG labeled by Phaseolus vulgaris leucoagglutinin (PHA-L) injected into the same MPO stimulation site. This indicates that MPO inputs may terminate on the soma or proximal dendrites of neurons exhibiting elevated Fos. A second series of experiments investigated whether MPO stimulation excited PAG neurons with descending projections to the medulla. Retrograde labelling of PAG neurons projecting to the medial and lateral regions of the rostroventral medulla (RVM) was combined with MPO-induced Fos expression. The results showed that a substantial population (37–53%) of Fos-positive PAG neurons projected to the ventral medulla. This indicates that MPO stimulation engages PAG- medullary output neurons. Taken together, these results suggest that the MPO --> Pag --> RVM projection constitutes a functional pathway. This circuit may coordinately regulate neuroendocrine, motor, and autonomic adjustments necessary for the elaboration of sexual behaviors.

Medial preoptic area afferents to periaqueductal gray medullo-output neurons: a combined Fos and tract tracing study