Role of neural afferents as mediators of estrogen effects on the hypothalamic ventromedial nucleus

The effects of estrogens on the ventrolateral division of the hypothalamic ventromedial nucleus (VMNvl) are essential for its role in the regulation of female sexual behavior. Enhanced synaptogenesis and induction of progesterone receptors (PRs) are hallmarks of the actions of estrogens on the VMNvl. To investigate the influence of neural afferents in mediating these effects, we estimated the number of spine and dendritic synapses per neuron and the total number of PR-immunoreactive neurons in ovariectomized rats treated with either estradiol benzoate or vehicle, after unilateral VMN deafferentation. The estimates were performed independently in the VMNvl of the deafferented and contralateral sides, and in the VMNvl of unoperated rats (controls). The administration of estradiol benzoate did not induce any increase in the number of synapses of the deafferented VMNvl. In the contralateral VMNvl, the synaptogenic effects of estrogen were apparent, but still reduced relative to the control VMNvl, where a 25% increase in the total number of synapses was observed after estrogenic stimulation. In the absence of estrogenic stimulation, i.e., in basal conditions, deafferentation reduced the number of dendritic and spine synapses, but particularly the latter. The reduction was also visible, but less marked, in the contralateral VMNvl. Contrary to synapses, the estrogen induction of PRs was unaffected by deafferentation, and the total number of PR-immunoreactive neurons was similar in the control, deafferented and contralateral VMNvl. The results show that estrogens enhance synaptogenesis in the VMNvl by acting through neural afferents and induce PR expression by acting directly upon VMN neurons.