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 primarily to changes in the responsiveness of the GnRH neurosecretory system to the negative feedback influence of estradiol. As GnRH neurons in the sheep, like those in other mammals, lack estrogen receptors, the influence of estradiol on GnRH neurosecretory activity is probably conveyed via afferents. As a possible structural basis for seasonality, we examined the ultrastructure and synaptic inputs of GnRH neurons in the preoptic area of ewes during the breeding season and seasonal anestrus. GnRH neurons were examined in both ovary-intact ewes and ovariectomized ewes bearing implants that produced constant levels of estradiol to eliminate a changing hormonal milieu as a factor in any seasonal variations. We found that preoptic GnRH neurons in breeding season ewes received more than twice the mean number of synaptic inputs per unit of plasma membrane as GnRH neurons in anestrous animals. Although GnRH dendrites received more synaptic input than GnRH somas, significant seasonal differences were seen in both axodendritic and axosomatic inputs. In contrast, unidentified neurons in the preoptic area showed no significant seasonal changes in their synaptic inputs. Seasonal changes in synaptic inputs onto GnRH neurons were seen in both intact animals and ovariectomized ewes bearing estradiol implants. Consequently, these seasonal alterations are unlikely to be due to changing levels of endogenous sex steroids, but may instead reflect changes in the environmental photoperiod and/or the expression of an endogenous circannual rhythm.