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 primate s, we determined the number of synaptic connections on gondotropin releasin g hormone (GnRH)- and non-GnRH neurones of the arcuate nucleus of ovariecto mized (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 immunost aining 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). Ser um concentrations of oestradiol (OVX) needed to 232 pg/ml on day 1, 63 pg/m l on day 2 and 45 pg/ml on day 8. Concentrations of luteinizing hormone (LH ) fell after ovariectomy to 9 mug/ml on day 1, surged to 93 mug/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 inh ibitory innervation and moderate excitatory input. (b) On day 1, GnRH neuro nes had highest numbers of inhibitory synapses, while inhibitory synapses o n non-GnRH neurones decreased, whereas numbers of excitatory synapses remai ned relatively unchanged compared to OVX monkeys. (c) By day 2, synapses on GnRH neurones decreased, while synapses increased on non-GnRH cells compar ed 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 s ynapses compared to day 2. We conclude that during an oestrogen-induced LH surge, synapses on GnRH- and mixed non-GnRH neurones are differentially reg ulated. These findings suggest that oestrogen modulation of arcuate nucleus synapses may be important in the regulation of gonadotropin secretion in m onkeys.