Influence of sex and estrus cycle on the sexual dimorphisms of the hypothalamic ventromedial nucleus: Stereological evaluation and Golgi study

Neurons in the ventromedial nucleus of the hypothalamus (VMN) display struc tural and biochemical sex differences in response to estrogen. Despite this fact, reports on sex differences in the morphology of the VMN are restrict ed to its volume and synaptic patterning. The aim of this study was to char acterize the neuroanatomical sexual dimorphisms in the VMN and to investiga te whether endogenous changes in ovarian steroid secretion influence such d imorphisms. The VMN of adult male rats and intact, aged-matched female rats killed on proestrus and diestrus day 1 was examined by using stereological methods applied to conventionally stained sections and Golgi-impregnated m aterial. The VMN contained 55,000 neurons in rats of both sexes, but its vo lume was, on average, 1.25 times larger in males than in females. The volum e was greater in proestrus than in diestrus rats due to parallel changes in the neuronal somatic size. Unlike the dorsomedial division, neurons in the ventrolateral division had longer dendritic trees in proestrus than in die strus females and males. The spine density was consistently higher in femal es than in males in both VMN divisions. In addition, in the ventrolateral p art the magnitude of the sex differences varied across the estrus cycle, an d reached the greatest value when females were in proestrus. The volume of the neuropil was significantly larger in males than in females, and was not affected by the estrus phase. Our results reveal that the magnitude of the neuroanatomical sex differences in the VMN vary across the estrus cycle du e to the trophic influence of estrogen upon its neurons. They also show tha t the fundamental sex difference in the structure of the VMN is accounted f or by the neuropil components. (C) 2001 Wiley-Liss, Inc.