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The Journal of Neuroscience, May 15, 2003, 23(10):4261-4269
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Osmotic Regulation of Estrogen Receptor- in Rat Vasopressin and Oxytocin Neurons
Suwit J. Somponpun and
Celia D. Sladek
Department of Physiology and Biophysics, University of Colorado Health Sciences Center, Denver, Colorado 80262
The vasopressin (VP) magnocellular neurosecretory cells (MNCs) in the supraoptic and paraventricular (PVN) nuclei are regulated by estrogen and exhibit robust expression of estrogen receptor (ER)- . In contrast, only  7.5% of oxytocin (OT) MNCs express ER-. We examined the osmotic regulation of ER- mRNA expression in MNCs using quantitative in situ hybridization histochemistry. Hyper-osmolality induced via 2% hypertonic saline ingestion significantly decreased, whereas sustained hypo-osmolality induced via d-D-arginine VP and liquid diet increased ER- mRNA expression in MNCs (p < 0.05). Thus, the expression of ER- mRNA correlated inversely with changes in plasma osmolality. Because hyper-osmolality is a potent stimulus for VP and OT release, this suggests an inhibitory role for ER- in MNCs. Immunocytochemistry demonstrated that the decrease in ER- mRNA was translated into depletion of receptor protein content in hyper-osmotic animals. Numerous MNCs were positive for ER- in control animals, but they were virtually devoid of ER--immunoreactivity (IR) in hyper-osmotic animals. The osmotically induced decrease in ER- expression was selective for MNCs because ER--IR remained unaltered in PVN parvocellular neurons. Plasma estradiol and testosterone were not correlated with ER- mRNA expression after osmotic manipulation, suggesting that ER- expression was not driven by ligand availability. Expression of FOS-IR in MNCs with attenuated ER--IR, and the absence of FOS-IR in parvocellular neurons that retain ER--IR suggest a role for neuronal activation in the regulation of ER- expression in MNCs. Thus, osmotic modulation of ER- expression in MNCs may augment or attenuate an inhibitory effect of gonadal steroids on VP release.
Key words: in situ hybridization; oxytocin; vasopressin; salt loading; hyponatremia; FOS; supraoptic nucleus; paraventricular nucleus; parvocellular neurons; magnocellular neurosecretory neurons
Received Dec. 26, 2002;
revised Feb. 20, 2003;
accepted Feb. 26, 2003.
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