Anorectic effects of estrogen may be mediated by decreased neuropeptide-Y release in the hypothalamic paraventricular nucleus.

There is a considerable body of evidence to suggest that estrogen suppresses food intake and body weight gain by an action in the hypothalamus. However, the neurotransmitter/neuromodulator mediating the anorectic effects of estrogen are unknown. Neuropeptide-Y (NPY) is the most potent orexigenic signal known, and NPY-producing neurons in the hypothalamus concentrate 17 beta-estradiol (E2). In these studies we tested the hypothesis that estrogen-induced anorectic effects may be due to decreased NPY levels and release in hypothalamic sites previously implicated in the control of food intake. The results show that uninterrupted physiological levels of E2 in ovariectomized rats suppressed daily food intake and body weight gain. Evaluation of NPY concentrations in five hypothalamic sites showed that NPY levels were decreased selectively in the paraventricular nucleus (PVN) and neighboring perifornical nucleus of E2-treated rats. In contrast, concentrations of beta-endorphin, another less potent orexigenic peptide, were not changed by E2 in any hypothalamic site. In the next experiment, the effects of similar E2 treatment on NPY release in vitro from the PVN and ventromedial nucleus were studied in rats killed at the onset of the dark phase when food intake increases in conjunction with increased PVN NPY secretion. The results show that basal and KCl-induced NPY release were significantly decreased from the PVN of E2-treated compared to those in control rats. In contrast, both basal and KCl-induced NPY release from the ventromedial nucleus of E2-treated rats were similar to those in control rats. Collectively, these results show that estrogen suppresses NPY levels and release selectively from the PVN. As NPY levels and release in the PVN have been shown to be highly correlated with appetite status, and the PVN is one of the important sites of NPY action, these findings imply that the anorectic effects of estrogen may be mediated by decreased NPY release from the PVN NPY innervations.