TESTOSTERONE PROPIONATE ADMINISTRATION PREVENTS THE LOSS OF NEURONS WITHIN THE CENTRAL PART OF THE MEDIAL PREOPTIC NUCLEUS

In the rat, the central part of the medial preoptic nucleus (MPNc) of the male is larger in volume and has a greater number of neurons than that of the female. The nucleus of the female, however, can be ''sex reversed'' by exposing the rat to gonadal steroids perinatally. The purpose of the present study was to examine the development of the MPNc to determine when the sex difference first appears and whether this difference occurs due to the relative accumulation of neurons into the compact part of the MPNc of the male and sex-reversed female rat or to the loss of MPNc neurons in the control female. Pregnant, female Sprague-Dawley rats were given an injection of [H-3]methyl thymidine on embryonic day 18 (E18). Rats were exposed to testosterone propionate (TP) or vehicle from E20 to postnatal day 10 (PN10) or until the time of sacrifice. Pups from three groups [males (oil), females (oil), and sex-reversed females (TP)] were sacrificed on PN2, PN4, PN7, PN10, or PN30. The volume of the compact part of the MPNc increased in males and sex-reversed females after PN4 but the volume in the nucleus of females remained relatively constant. The number of neurons and [H-3] thymidine-labeled cells remained elevated from PN2-PN30 in males or sex-reversed females but decreased dramatically in oil-treated females between PN4 and PN7, reaching a minimal number by PN10. Cell cross-sectional area increased with age while cell density decreased. These observations are consistent with the hypothesis that the lack of growth of the compact part of the MPNc of the female is due to a loss of neurons while the increase in volume of the male's nucleus is due at least in part to growth of its constituent neurons.