Neurosteroids: regulatory mechanisms in male rat brain during heterosexual exposure.

Pregnenolone (delta 5-P) and dehydroepiandrosterone (DHEA) were measured in the limbic system of young adult male rats (M) exposed for several days to the scent of cycling females but in absence of sexual contacts (M/F), and compared to levels obtained in males similarly exposed to other males (M/M). delta 5-P was highest in the olfactory bulbs of M/M, as compared to other regions of the limbic system. It decreased greater than 50% in M/F olfactory bulbs, but was identical in the olfactory tubercles, the amygdalas and the hypothalamus of M/M and M/F, as well as in the plasma, the adrenals and the spleen (taken as a representative non-endocrine organ). In comparison with M/M levels, DHEA selectively increased in the hypothalamus of M/F. These results demonstrate very different steroid concentrations in different regions of the brain, and they reveal their selective and eventually opposite changes upon heterosexual exposure. Therefore, they suggest regulatory mechanisms specific to various parts of the brain which are not directly related to the hormonal levels in the blood, and which could be part of the response to still undefined signals emitted by animals of the other sex.