Photoperiod and steroid-dependent adjustments in hypothalamic gonadotropic hormone-releasing hormone, dopamine, and norepinephrine content in male deer mice

Abstract

The purpose of this study was twofold: 1) to examine photoperiod- dependent and steroid-dependent adjustments in hypothalamic GnRH and catecholamine content in male deer mice, Peromyscus maniculatus, and 2) to evaluate whether the commonly used measure of GnRH accumulation serves as an accurate indicator of altered reproductive function in short-day-housed rodents. Deer mice provide a unique animal model in which to evaluate the effect of short day length on the neural substrates directing reproductive function, since individual deer mice respond to short photoperiod with a range of neuroendocrine, endocrine, metabolic, and thermoregulatory adjustments. These adjustments, in part, result in two distinct reproductive phenotypes; reproductively competent 'nonresponsive' mice, and gonad- regressed 'responsive' mice. Reproductively mature males were maintained on either long (16L:8D) or short (8L:16D) photoperiod for 8 wk. Thereafter, mice were phenotyped and either remained intact, castrated, or castrated and provided with an s.c. testosterone-filled silicone elastomer capsule releasing long-day or twice long-day levels of testosterone; mice were then housed for an additional 4 wk. At autopsy, plasma levels of LH and testosterone, hypothalamic GnRH content, and hypothalamic dopamine and norepinephrine concentrations were determined. Short photoperiod caused a similar increase in GnRH content in both short-day phenotypes, and this increase was primarily localized to the medial basal hypothalamus (MBH). Castration decreased GnRH content within the MBH of mice housed on both a long and short photoperiod. However, castration also reduced GnRH content within the medial preoptic area (MPOA) only among the short-day phenotypes. Steroid replacement at levels similar to those of intact long-day males restored GnRH content in all brain regions to levels of intact mice. On the other hand, steroid replacement at twice long-day levels restored GnRH content among long-day mice but reduced GnRH content among both short-day reproductive response groups, specifically within the MBH and MPOA. There were no dramatic effects of photoperiod or steroid treatment on catecholamine concentrations. But our results do suggest that the lateral preoptic area (LPOA) and MBH may be important sites of steroid feedback among mice exposed to short photoperiods and that the LPOA and diagonal band of Broca-septal regions may specifically be associated with catecholamine-directed photoperiod-induced adjustments of reproductive responsive mice. Taken together, these data show that GnRH accumulation is not the proximate signal directing seasonal changes in pituitary-gonadal function and cast doubt on the use of this factor as an accurate indicator of altered reproductive function.