THE HYPOTHALAMIC VENTROMEDIAL NUCLEUS SENDS A MET-ENKEPHALIN PROJECTION TO THE PREOPTIC AREAS PERIVENTRICULAR ZONE IN THE FEMALE RAT

The female but not the male rat possesses a dense network of methionin e enkephalin (m-Enk) fibers in the periventricular zone of the preopti c area (pePOA). The potential source of these fibers was determined by injection of the tracer fluorogold, FAu, into the preoptic area of ad ult female rats. Twenty-four hours before they were killed, the rats w ere administered colchicine (intraventricularly) to enable immunocytoc hemical visualization of m-Enk cells. Upon examination of the brains w ith fluorescence microscopy, double-labeled cells showing fluorogold a nd immunofluorescence for m-Enk were consistently observed in the preo ptic area, the ventrolateral division of the ventromedial nucleus of t he hypothalamus (VMHv1) and nearby medial tuberal area (MTA), the arcu ate nucleus, periventricular area of the hypothalamus, perifornical ar ea, and dorsomedial nucleus of the hypothalamus. A series of lesion an d knife cut experiments using glass, Halasz, and wire knives determine d that the pePOA m-Enk fibers arose from the hypothalamus, near or wit hin the VMH. Ibotenic acid lesions further determined that the source of the m-Enk projection was the VMHv1 with a possible additional contr ibution from the MTA.