Interactions of hormones, aging and sexual experience on masculine sexual behavior and hormone receptor expression in the hypothalamus
Age-related declines of androgens and libido in males have been observed for decades. This dissertation sought to elucidate the mechanisms by which hormones may act differentially upon their receptors in the hypothalamus of aging compared to young males. I also examined how sexual experience modulates the ability of hormones to facilitate sexual behavior with aging. Experiment one measured androgen receptors (AR) and estrogen receptor α (ERα) cells in male rats at young, middle-aged and old age. I found that AR cell numbers in hypothalamic regions studied underwent significant age-related increases. Numbers of heavily ERα labeled cells, but not total ERα cells, increased with age. This study demonstrates that the aging brain has the capacity to synthesize hormone receptors which is increased possibly due to decreased testosterone concentrations. Experiment two examined the effect of sexual experience on serum hormones and cells of AR and ERα in hypothalamic regions in young and middle-aged males. The results showed that AR cell numbers increased with aging but did not change with experience. No age- or experience-related alteration in ERα expression occurred. However, serum testosterone increased and estradiol decreased with age. Experience increased total and free testosterone. Interactions of age and experience on total testosterone, estradiol, and luteinizing hormone were found. These results show long-lasting effects of sexual experience on hormones, but not on their receptors in the hypothalamus. Experiment three investigated effects of exogenous testosterone on sexual behavior in young and middle-aged males. The results showed a decline in sexual behavior parameters with age. After castration with testosterone treatment, there were few differences in sexual behavior measures between young and middle-aged males. AR cell numbers were higher and ERα cell numbers lower in testosterone compared to vehicle-treated males of both ages, and few effects of age occurred. These findings indicate that testosterone and aging interact in a complex manner to control numbers of cells expressing hormone receptors in the brain and on the subsequent control of sexual behavior. This insight provides a better understanding of the relationship between molecular changes in the brain and behavior, and suggests new therapeutic targets to human testosterone treatment.