Studies are designed to clarify the neuroendocine mechanisms underlying the regulation of reproduction by daylength in a seasonal breeder, the Syrian golden hamster. Exposure to photoperiods of less than 12.5h light/day causes gonadal regression by a mechanism which depends on the pineal gland. The mediation of photoperiodic influences on reproduction by the paraventricular nucleus (PVN) will be examined in light of its anatomical connections and recent evidence that its destruction blocks the regulation of reproduction and pineal melatonin content by daylength. The collapse of gonadal function in short days is correlated with an increase in testosterone's (T) ability to suppress LH release. Two experiments will determine whether these changes arise from photoperiodic influences on gonadal steroid processing. Exchange assays will be used to quantify T+receptor levels in cell nuclei of neuroendocrine tissues of hamsters maintained in short or long days. Nuclear 3H-steroid levels will also be determined in brain and hypophysial tissues after the administration of 3H-1Alpha,2AlphaT to castrates maintained in stimulatory or inhibitory daylengths. If photoperiod influences steroid processing in these tissues, further studies will examine daylength's effects on T receptors and metabolism. If daylength's effects on T's neuroendocrine actions are not accompanied by detectable changes in its processing, steroid-independent influences of photoperiod on LH secretion will be examined. Specifically, daylength effects on the frequency and amplitude of pulsatile LH secretion will be studied in castrated hamsters. If photoperiod is found to modulate episodic LH release, the influences of steroids and ascending monoaminergic projections to the hypothalamus on the neural LH-pulse generator will be explored under long-and shortday conditions. These studies will provide a better understanding of environmental influences on brain-pituitary-gonadal interrelationships. They exploit the advantages of seasonal breeding as a model of reversible fertility and may lead to improvements in food production and human contraception. Investigation of the PVN's role in the neurocircuitry governing pineal function may explain aspects of rhythmicity of sympathetic function of which the photoperiodic control of reproduction is only a part. Finally, study of the neuroendocrine mechanisms by which daylength drives annual breeding cycles may contribute to our understanding of brain plasticity.