The medial preoptic area/ anterior hypothalamus (MPOA/AH), a region known to be
critical for the expression of masculine sexual behavior in most mammalian species (7), is
clearly one region to consider. The MPOA/AH comprises a steroid sensitive brain region that
contains high concentrations of androgen and estrogen receptors (8). Conversion of testosterone
to estradiol by cytochrome P450 aromatase within the MPOA/AH is an important part of the
mechanism by which androgens facilitate male sexual behaviors (9). Higher concentrations of
aromatase have been measured in the MPOA/AH of female-oriented rams than of male-oriented
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rams (10). Within the MPOA/AH of several species, including humans, sexually dimorphic cell
groups have been identified that are significantly larger in males than in females (11). In rats,
Gorski et al. (12;13) identified a sexually dimorphic nucleus in the preoptic area (SDN-POA).
The SDN-POA has been associated with two distinct aspects of male sexual behavior, male
copulatory motor patterns (7) and sexual partner preference (14;15). In male rats treated
perinatally with the aromatase inhibitor ATD (1,4,6-androstatriene-3, 17-dione), SDN volume
correlates positively with male-typical sexual behavior and female-directed partner preference
(16). The third interstitial nucleus of the anterior hypothalamus (INAH3) in humans, which
exhibits positional and cytoarchitectonic similarities to the SDN-POA of the rat (17), has been
found to be significantly larger in heterosexual men than in homosexual men and women (18)