Journal of Neuroscience, Vol 16, 333-344, Copyright © 1996 by Society for Neuroscience
Medial preoptic area afferents to periaqueductal gray medullo-output neurons: a combined Fos and tract tracing study
TA Rizvi, AZ Murphy, M Ennis, MM Behbehani and MT Shipley
Department of Anatomy, University of Maryland School of Medicine, Baltimore 21201, USA.
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.
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