EFFECTS OF PHOTOPERIOD ON ESTROGEN-RECEPTOR, TYROSINE-HYDROXYLASE, NEUROPEPTIDE-Y, AND BETA-ENDORPHIN IMMUNOREACTIVITY IN THE EWE HYPOTHALAMUS

The neural components underlying the influence of photoperiod upon rep roductive functioning are poorly understood. In this study, we have us ed immunocytochemistry to examine whether changes in photoperiod may i nfluence specific neuronal cell populations implicated in mediating go nadal steroid feedback actions on GnRH neurons. Short day (SD) exposed exposed in the midluteal stage of the estrous cycle and long day (LD) anestrous ewes were perfused in pairs and hypothalamic brain sections immunostained for tyrosine hydroxylase (TH), neuropeptide Y (NPY), be ta-endorphin (beta E), and the estrogen receptor (ER). The number of E R-immunoreactive cells detected within the preoptic area, but not the hypothalamus, was approximately 20% higher (P < 0.05) in LD ewes compa red with SD animals. The numbers of TH-immunoreactive neurons comprisi ng the A12, A14, and A15 cell groups were not different between LD and SD ewes, and the percentage of A12 (similar to 15%) and A14 (similar to 25%) neurons expressing ERs was similarly unaffected by photoperiod . The number of beta E neurons detected in the arcuate nucleus was 50% lower (P < 0.05) in SD vs. LD ewes, whereas NPY-immunoreactive cell n umbers in the median eminence were 300% higher (P < 0.05). Approximate ly 3% of NPY neurons in the median eminence, and 10% in the arcuate nu cleus, expressed ER immunoreactivity in a photoperiod-independent mann er. These studies indicate that changes in photoperiod may regulate ER expression within the preoptic area and suggest that hypothalamic NPY and beta E neurons are involved in the seasonal regulation of reprodu ctive activity in the ewe.