EVIDENCE FOR SEASONAL PLASTICITY IN THE GONADOTROPIN-RELEASING-HORMONE (GNRH) SYSTEM OF THE EWE - CHANGES IN SYNAPTIC INPUTS ONTO GNRH NEURONS

In the Suffolk ewe, seasonal reproductive transitions are due primaril y to changes in the responsiveness of the GnRH neurosecretory system t o the negative feedback influence of estradiol. As GnRH neurons in the sheep, like those in other mammals, lack estrogen receptors, the infl uence of estradiol on GnRH neurosecretory activity is probably conveye d via afferents. As a possible structural basis for seasonality, we ex amined the ultrastructure and synaptic inputs of GnRH neurons in the p reoptic area of ewes during the breeding season and seasonal anestrus. GnRH neurons were examined in both ovary-intact ewes and. ovariectomi zed ewes bearing implants that produced constant levels of estradiol t o eliminate a changing hormonal milieu as a factor in any seasonal var iations. We found that preoptic GnRH neurons in breeding season ewes r eceived more than twice the mean number of synaptic inputs per unit of plasma membrane as GnRH neurons in anestrous animals. Although GnRH d endrites received more synaptic input than GnRH somas, significant sea sonal differences were seen in both axodendritic and axosomatic inputs . In contrast, unidentified neurons in the preoptic area showed no sig nificant seasonal changes in their synaptic inputs. Seasonal changes i n synaptic inputs onto GnRH neurons were seen in both intact animals a nd ovariectomized ewes bearing estradiol implants. Consequently, these seasonal alterations are unlikely to be due to changing levels of end ogenous sex steroids, but may instead reflect changes in the environme ntal photoperiod and/or the expression of an endogenous circannual rhy thm.