Effect of estrogen of early postnatal development of synaptic formation in the hypothalamic arcuate nucleus of female rats

Female rats were given injections of estradiol benzoate for the first 30 days of their postnatal life; i.e., 2 μg for the first 10 days and 4 μg and 8 μg for the middle and last 10 days, respectively. On the day following the last injection (31 days of age), the hypothalamic arcuate nucleus (ARCN) was examined ultrastructurally and the axodendritic and axosomatic synapses were counted in a field of 10,117.5 μm² in the middle part of the ARCN. The mean number of axodendritic synapses in the ARCN of the neonatally estrogenized females was about two-fold greater than that of oil-treated controls. As for the number of axosomatic synapses, of which incidence was low, there was no significant difference between the estrogenized and control animals. From these results it is suggested that an increase in the number of axodendritic synapses in the ARCN is caused by neonatal treatment with estrogen.