| Summary: | Animals exhibit physiological and behavioral changes in anticipation of and response to changing environmental conditions. One example of this is the influence of photoperiod on the reproductive cycle of temperate zone seasonal breeders, such as Syrian hamsters. In addition to physiological changes which prevent ovulation and reduce gonadal steroid secretion, photoperiod can influence behavioral responses to gonadal steroid replacement, suggesting that photoperiods which inhibit reproduction alter neural responsiveness to estradiol and progesterone. The experiments described here were designed to address possible mechanisms by which photoperiod might produce these changes. In Experiment 1, immunocytochemistry was used as a semi-quantitative and anatomically specific technique to explore the hypothesis that reduced neural responsiveness to gonadal steroids was due to changes in the number of estrogen and progestin receptors in nuclei mediating hormonal effects on reproductive behavior and physiology. Exposure to a short photoperiod was associated with decreased progestin receptor immunoreactivity (Experiment 1B) but not with changes in estrogen receptor immunoreactivity (Experiments 1A, 1D), suggesting that neural responsiveness to estradiol is reduced in short photoperiods but that this effect is not due to changes in estrogen receptor number. Changes in responsiveness to estradiol are not dependent on peripheral effects of photoperiod on estradiol metabolism, as progestin receptor-immunoreactivity was reduced in short days even when estradiol was implanted directly into the mediobasal hypothalamus (Experiment 1C). Experiment 2 utilized Fos as a marker of neuronal activity to identify specific neural sites where photoperiod might act to influence neural responsiveness to steroid hormones and sociosexual cues responsible for induction of lordosis. Effects of photoperiod on lordosis and Fos were highly variable among experiments, but exposure to a short photoperiod was associated in three of the four experiments (Experiment 2A, 2B, and 2D) with increased Fos immunoreactivity in the ventromedial hypothalamus. Exposure to a short photoperiod may increase the responsiveness of the ventromedial hypothalamus to one or more hormonal or sociosexual cues to which hamsters were exposed.
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