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¹ Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya 464-8601, Japan
² Department of Neurosurgery, Farber Institute for Neurosciences, Thomas Jefferson University, 900 Walnut Street, Suite 400, Philadelphia, Pennsylvania 19107, USA
³ Department of Biology, Washington and Lee University, Lexington, Virginia 24450-03, USA

(Requests for offprints should be addressed to B A S Reyes; Email: bsr103{at}jefferson.edu)

Fasting-induced LH suppression is augmented by estrogen in female rats. We investigated the temporal changes in the number of estrogen receptor (ER)-immunoreactive (ir) cells in various brain regions in ovariectomized rats fasted for 6, 24, 30, and 48 h, commencing at 1300 h. We also determined the anatomical relationship of ER immunoreactivity and dopamine-ß-hydroxylase (DBH) neurons in the A2 region of the nucleus of the solitary tract (NTS) and the paraventricular nucleus (PVN). The number of ER-ir cells significantly increased after 30 h from the onset of fasting in the PVN and NTS compared with the unfasted controls and was sustained until 48 h. In the A2 region of 48-h fasted rats, 46.75% DBH-ir cells expressed ER, and this was significantly higher than in unfasted controls (8.16% DBH-ir cells expressed ER). In the PVN, most ER-ir neurons were juxtaposed with DBH-ir varicosities. These results suggest that ER is expressed in specific brain regions at a defined time from the onset of fasting. In addition, the anatomical relationship of noradrenergic and ER-ir neurons in the A2 region and PVN may suggest a role for estrogen in increasing the activity of noradrenergic neurons in the A2 region and enhancing sensitivity of the PVN to noradrenergic input arising from the lower brainstem and thereby augmenting the suppression of LH secretion during fasting.