Effect of androgen on sexual differentiation of synaptic organization in the hypothalamic arcuate nucleus: an ontogenetic study.

Wistar female rats were treated with 1.25 mg testosterone propionate (TP) on day 5 (day of birth = day 1). The hypothalamic arcuate nucleus (ARCN) in the brains of normal and neonatally TP-treated (androgenized) females was examined ultrastructurally at 6, 10, 20, 45 and 100 days of age. Axodendritic shaft and spine synapses were counted in a field of 10,000 micrometer2 of the neuropil in the middle part of the ARCN in each brain. The mean numbers of shaft and spine synapses both in normal and androgenized females increased from days 6 to 45, and reached a plateau by day 45. The mean number of shaft synapses in androgenized females was not significantly different from that in normal controls at each age. No significant difference in the number of spine synapses between normal and androgenized rats was seen 1 day after TP administration (sacrificed at 6 days of age). However, the incidence of spine synapses in androgenized females 5 days after TP injection (sacrificed at 10 days of age) was significantly smaller than that in the controls. This tendency became much clearer during the course of postnatal development. Since the synaptic organization of the ARCN in normal adult males is quite homologous to that observed in the androgenized adult females in the present study, it is suggested that androgen given neonatally is responsible for sexual differentiation of the synaptic pattern in the ARCN and that this synaptic difference develops from the early postnatal period.