Rapid changes in preoptic estradiol concentration during male sexual behavior

Estrogens such as estradiol (E2) exert pleiotropic effects on physiological and behavioral responses such as neuroprotection, aggression or reproduction. Estrogens derived from local brain synthesis (neuroestrogens) are critical for the regulation of different functions including the control of male sexual behavior. Classically, E2 acts through effects initiated in the nucleus to regulate male sexual function. Along with these long-term effects, E2 also acts rapidly (within minutes) via membrane-initiated events. These effects are thought to depend on short-term variations in the local production of estrogens, through rapid fluctuations of the enzymatic activity of brain aromatase. In Japanese quail, rapid modulations of brain aromatase activity (AA) have been reported after sexual interactions or exposure to an acute stress. These changes take place mainly in the medial preoptic nucleus (POM), a sexually differentiated structure that plays a key role in the control of male sexual behavior and where aromatase is densely expressed. Yet, it has recently been shown that, in the short term, AA does not always reflect local E2 concentration. This study was designed to determine by in vivo microdialysis whether local E2 concentrations fluctuate during sexual interactions and test whether these changes parallel the decrease in AA observed ex vivo after copulation. We first conducted a series of experiments to validate the microdialysis and E2 assay. When dialysis probes were placed in successive baths containing known increasing amounts of E2, proportional changes in E2 concentration were measured in the dialysate. Moreover, a rise in E2 concentration was detected after in vivo retrodialysis of testosterone only if the probe was located within the POM and, after a peripheral injection of E2, a sharp rise of E2 was detected regardless of the probe location. Together these results show that in vivo microdialysis is a valid method to assess endogenous fluctuations of brain E2 concentrations in behaving animals. Two independent experiments then identified a rise in E2 concentrations in POM during sexual interactions. This increase occurred within 10 min after the initiation of the sexual interaction and was specific to the POM as there was no increase in E2 concentrations in males that had their cannula outside of this area. Together these data confirm that rapid changes in AA measured ex vivo cannot be considered as a reliable proxy for E2 concentrations. The discrepancies could originate either from the different time resolution related to the two techniques or from differences in the microenvironment in which aromatase functions in vivo and during ex vivo assays.