Regulation of seasonal neuroendocrine events is likely accomplished by the daily assessment of photoperiod and the consequent adjustment of various periodic functions wihin the central nervous system. Current evidence strongly implicates the pineal gland as part of the mechanism for monitoring daily changes in photoperiod, and also implicates the brain opioids (methionine enkephalin, met-enk) as potential mediators of pineal function. Thus, the underlying concept of this proposal is that circadian secretion of pineal factors may influence the periodic fluctuations of other components within the hypothalamico-pituitary axis. The ultimate effect of these minor daily changes is alteredgonadotropin secretion leading to reproductive collapse or recrudescence. Therefore, the experimental approach of this proposal will be to examine changes in key factors within the pineal-hypothalamico-pituitary axis at different times of the day and different seasons or artificial photoperiods. This will be done by measuring hypothalamic met-enk and GnRH (via push-pull perfusion) as well as circulating LH, FSH and melatonin at certain times of the day, throughout different seasons or artificial photoperiods in intact and pinealectomized animals. Additionally, attempts will be made to alter the chain of events initiated by the pineal (through pinealectomy, melatonin treatment, or by manipulation of the endogenous opioids) and the resultant changes in gonadotropin secretion monitored. Since it is a thesis of this proposal that temporal interactions between pineal factors and the hypothalamico-pituitary axis are critical, it is important to address the timing of these interactions and attempt to change the period, but not to abolish the rhythm. The test animal of this proposal, the pony mare, is a highly seasonal animal which displays an ovary independent seasonal pattern of gonadotropin secretion, permitting concentration of effort within the pineal-hypothalamico-pituitary axis. The long term objective of this proposal is to obtain a better understanding of the relative importance of opioid-GnRH interactions and pineal-opioid interactions in response to seasonal photoperiodic changes. The biological significance of these interactions in man and food-producing animals remains to be determined, but is doubtless a significant health factor.