Synaptogenic effect of estrogen on the hypothalamic arcuate nucleus of the adult female rat.

In order to examine the effect of estrogen on the synaptic structures in the hypothalamic arcuate nucleus (ARCN), semi-quantitative studies were performed by counting synapses in an 18,000 micron 2 area in the middle part of the ARCN in each brain. In ovariectomized female rats injected with 2 micrograms of estradiol benzoate (EB) for three weeks, the mean numbers of axodendritic and axosomatic synapses were not significantly different from those in the intact and ovariectomized controls. When the medial basal hypothalamus (MBH) including the ARCN was isolated by use of a Halász knife (MBH island), the mean number of axodendritic synapses was decreased to about half of the controls. However, EB treatment for three weeks from the day of surgery effectively restored the axodendritic synaptic population of the deafferented ARCN. This may suggest that estrogen has a facilitatory effect on axodendritic synapse formation in the deafferented ARCN, presumably by stimulating axonal sprouting and synaptic regeneration of intact axons in the MBH island.