Ultrastructural and morphometric analysis of neurons in the arcuate nucleus of the female rat hypothalamus following estradiol.

Recent evidence suggests that E2 may act in the ventromedial nucleus of the hypothalamus (VMN) and arcuate nucleus (ARC) through mechanisms partly unique to each area. To examine this hypothesis, we investigated early and discontinuous effects of E2 action on neurons within the ARC from an ultrastructural and morphometric perspective. In contrast to our findings of dramatic, rapid effects of E2 on the structure of VMN neurons, no changes in any of the ultrastructural parameters examined, including the nucleolus, nucleus, nucleoplasm, and the nuclear envelope-RER system, were observed in ARC neurons after either 2 hours of E2 or a discontinuous schedule of 2 h on/7 h off/2 h on of E2. No changes were noted in any of the quantitative parameters examined, including nucleolar, nuclear, and somal area, nuclear shape, and nuclear envelope length after either 2 hours of E2 or a discontinuous schedule of 2 h on/7 h off/2 h on of E2. These morphological results support the hypothesis of differential actions of E2 on various E2-concentrating nuclear groups in the brain.