Oestrogen-induced changes in the synaptology of the monkey (Cercopithecus aethiops) arcuate nucleus during gonadotropin feedback.

To assess their role in the regulation of gonadotropin secretion in primates, we determined the number of synaptic connections on gondotropin releasing hormone (GnRH)- and non-GnRH neurones of the arcuate nucleus of ovariectomized (OVX) and OVX plus oestradiol benzoate-treated African green monkeys. After 24 h (day 1), 48 h (day 2) and 8 days (day 8), we performed immunostaining for GnRH. Using electron microscopy, synapses on GnRH- and randomly selected non-GnRH neurones were counted and characterized according to the classification of Gray (symmetric/inhibitory or asymmetric/excitatory). Serum concentrations of oestradiol (OVX) needed to 232 pg/ml on day 1, 63 pg/ml on day 2 and 45 pg/ml on day 8. Concentrations of luteinizing hormone (LH) fell after ovariectomy to 9 microg/ml on day 1, surged to 93 microg/ml on day 2 and declined again by day 8. (a) Ten days after ovariectomy, there were no synapses on GnRH neurones, whereas non-GnRH cells received substantial inhibitory innervation and moderate excitatory input. (b) On day 1, GnRH neurones had highest numbers of inhibitory synapses, while inhibitory synapses on non-GnRH neurones decreased, whereas numbers of excitatory synapses remained relatively unchanged compared to OVX monkeys. (c) By day 2, synapses on GnRH neurones decreased, while synapses increased on non-GnRH cells compared to day 1. (d) On day 8, the most pronounced alteration on GnRH cells was an elevated inhibitory input while non-GnRH neurones received the fewest synapses compared to day 2. We conclude that during an oestrogen-induced LH surge, synapses on GnRH- and mixed non-GnRH neurones are differentially regulated. These findings suggest that oestrogen modulation of arcuate nucleus synapses may be important in the regulation of gonadotropin secretion in monkeys.