Synergic effects of estradiol and progesterone on regulation of the hypothalamic neuronal nitric oxide synthase expression in ovariectomized mice.

Nitric oxide (NO) is a gaseous neurotransmitter that plays an important role in the regulation of sexual behavior in rodents. NO is produced, within the central nervous system, by the enzyme neural NO synthase (nNOS) whose expression is influenced by gonadal hormones. In previous studies, we demonstrated that part of the nitrergic hypothalamic and limbic system is influenced, in physiological conditions, by the hormonal fluctuations during the estrous cycle, but we were unable to distinguish among the role played by progesterone (P) or estradiol (E(2)) in inducing these changes. In the present study, we investigated the effects of E(2) and P (alone or together) on the nitrergic system of gonadectomized female mice, following a timing of administration that emulates the different phases of estrous cycle. In parallel, we tested the influence of the two hormones on sexual behavior, confirming that P works in synergistic fashion with E(2) to facilitate female receptivity. The quantitative analysis of nNOS-ir system demonstrated a statistically significant variation in the number of positive cells only in those part of the limbic-hypothalamic nitrergic system that are affected in cycling females, i.e. the bed nucleus of the stria terminalis, the arcuate nucleus and the medial preoptic area, with the highest number of positive neurons observed in E(2)+P group. The variable effects of E(2) and P may depend on the different distribution of their receptors within the analyzed nuclei, but the relationships among variations of estrogen and progesterone levels and in vivo modulation of nNOS expression remain unknown and needed further investigations.