MEDIAL PREOPTIC AREA AFFERENTS TO PERIAQUEDUCTAL GRAY MEDULLO-OUTPUT NEURONS - A COMBINED FOS AND TRACT TRACING STUDY

We have shown recently that the medial preoptic area (MPG) robustly in nervates discrete columns along the rostrocaudal axis of the midbrain periaqueductal gray (FAG). However, the location of FAG neurons respon sive to MPO activation is not known. Anterograde tract tracing was use d in combination with Fos immunohistochemistry to characterize the MPG --> FAG pathway anatomically and functionally within the same animal. Focal electrical or chemical stimulation of MPO in anesthetized rats induced extensive Fos expression within the FAG compared with sham con trols. 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 FAG labeled by Phaseolus vulgaris leucoagglutinin (PHA-L) injected into the same MPG stimulation site. This indicates that MPO inputs may terminate on the soma or proximal dendrites of neurons exhibiting elevated Fos. A secon d series of experiments investigated whether MFO stimulation excited F AG neurons with descending projections to the medulla. Retrograde labe ling of FAG neurons projecting to the medial and lateral regions of th e rostroventral medulla (RVM) was combined with MPG-induced Fos expres sion. The results showed that a substantial population (37-53%) of Fos -positive FAG neurons projected to the ventral medulla. This indicates that MFO stimulation engages FAG-medullary output neurons. Taken toge ther, these results suggest that the MFO --> PAG --> RVM projection co nstitutes a functional pathway. This circuit may coordinately regulate neuroendocrine, motor, and autonomic adjustments necessary for the el aboration of sexual behaviors.