Neuronal plasticity in the deafferented hypothalamic arcuate nucleus of adult female rats and its enhancement by treatment with estrogen.

The hypothalamic arcuate nucleus (ARCN) was examined ultrastructurally 3 or 21 days after complete deafferentation of the medial basal hypothalamus (MBH) in ovariectomized adult female rats. Axodendritic shaft (SHS) and spine synapses (SPS) were counted in a field of 18,000 mu m2 in the middle part of the ARCN in each brain. The mean numbers of SHS and SPS at 3 or 21 days after deafferentation were reduced to about half of those in intact control animals. When the ARCNs of the ovariectomized MBH-island rats were examined 3 days after treatment with estradiol benzoate (EB, 2 mu g/day), the numbers of SHS and SPS did not differ from those in ovariectomized MBH-island rats without EB treatment. However, EB treatment for 21 days produced a marked increase in the number of both SHS and SPS in the ovariectomized MBH-island females, with the number of SHS in these females being restored to almost 75% of the intact level; the incidence of SPS was also significantly greater than that in the intact control animals. In these EB-treated, ovariectomized MBH-island rats, double synapses (spine-spine and spine-shaft double synapses) were frequently observed. In ovariectomized females without MBH deafferentation, however, estrogen failed to increase the numbers of SHS, SPS, and double synapses, which were almost comparable to those in intact and ovariectomized controls. These results suggest that estrogen has a facilitatory effect on SHS and SPS formation in the deafferented ARCN, presumably stimulating not only axonal sprouting but also dendritic spine formation by intact arcuate neurons in the MBH island.