Regulation of norepinephrine in the medial preoptic area of Siberian hamsters by gonadal steroids.

Photoperiod has profound effects upon the neuroendocrine axis underlying reproductive physiology in seasonally breeding mammals. For long-day (LD) breeders, such as the Siberian hamster, exposure to a short-day (SD) photoperiod results in declines in circulating levels of gonadal steroids, luteinizing hormone (LH), and prolactin (PRL). The current study sought to investigate the effects of photoperiod and steroid levels on norepinephrine (NE), one of the major neurochemical regulators of gonadotropin-releasing hormone (GnRH) function. Since NE release within the medial preoptic area (mPOA) has been shown to stimulate the activity of GnRH cells, it was hypothesized that exposure to a short photoperiod would decrease NE levels. Furthermore, since gonadal steroids show negative feedback on GnRH function, it was hypothesized that gonadectomy would result in increased levels of NE. Adult male and female Siberian hamsters were gonadectomized and implanted with silastic capsules containing either cholesterol (C) or a mixture of estradiol (E) or testosterone (T). Microdialysis sampling within the mPOA was conducted after 8 weeks of exposure to either an LD or an SD photoperiod. Blood samples were analyzed for LH and PRL, while dialysis samples were analyzed for NE and its major metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG). The results revealed significant suppression of LH and PRL by exposure to the SD photoperiod in both males and females. For LH, the steroid implants suppressed circulating hormone levels under both photoperiods, whereas for PRL, steroid treatment facilitated circulating levels. In contrast, there were no significant effects of photoperiod on NE or MHPG release for either males or females, but there was a significant decrease in extracellular levels of these neurochemicals in steroid-treated animals. These data suggest that photoperiodic modulation of GnRH neuronal function by NE is achieved largely through the indirect effects of photoperiod on circulating gonadal steroids.