Synapse formation in response to estrogen in the medial amygdala developing in the eye

Medial amygdaloid tissue, taken from female rats immediately after birth, was transplanted into the anterior chamber of the eye in adult ovariectomized host rats in order to elucidate the influence of estrogen on synapse formation without contribution of neural afferents. After injections of estradiol benzoate or oil vehicle to the hosts for 20 successive days, the grafts were processed for semiquantitative electron microscopic study to examine synaptic density in the neuropil. The number of synapses on dendritic shafts vs. dendritic spines was not significantly different in the control group. In contrast, in the grafts exposed to estrogen, shaft synapses occurred more frequently than spine synapses. Synaptic density on shafts was significantly greater in these grafts than that in the controls, although the density on spines did not differ between the two groups. These data show that estrogen affects the medial amygdaloid neurons themselves and specifically facilitates the formation of dendritic shaft synapses in oculo. Our previous report raises the possibility that the specific increase of shaft synapses induced by sex steroids is involved in the process of sexual differentiation of neuronal networks from the inherently feminine pattern to the masculine pattern in the medial amygdala. Therefore, the present findings may provide evidence that sexual differentiation triggered by sex steroids is accomplished by intrinsic factors in the neurons of the medial amygdala.