The central effect of estrogen on the regulation of food intake and osmoregulation.

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1.Effect of estrogen replacement on food intake behavior in ovariectomized rats.Seven-week-old rats were ovariectomized and were assigned into estradiol- (E2-T) and placebo-treated (Pla-T) groups. The daily food intake and increase in body weight in the E2-T group were significantly lower than the Pla-T group. Food intake during both light and dark phases in the Pla-T group was larger than the E2-T group, and the difference between the two groups was more prominent during light phase. The number of c-Fos ir cells in suprachiasmatic nucleus (SCN) in the Pla-T group was not different from the E2-T group at the beginning of dark phase, but was lower than the E2-T group at the beginning of light phase. The c-Fos expressing cell number at the arcuate nucleus (ARC) in E2-T rats was lower than Pla-T rats in both light and dark phases. The responses of feeding and c-Fos expression at ARC were not different between the two groups. These results suggest that 1)estrogen tonically inhibits ARC neu … More ronal activity and feeding, 2)estrogen might play a role in the production of circadian SCN neuronal activity rhythm and attenuation of feeding during light phase, and 3)estrogen modulates spontaneous feeding but does not influence fasting-induced feeding response.2.Effect of estrogen replacement on osmoregulation in ovariectomized rats.To clarify the effect of estrogen on osmotically-induced water intake and its central mechanism, we examined the responses of water intake and hypothalamic neuronal activation (c-Fos expression) to plasma hyperosmolality and central angiotensin II (ANGII) administration. Ovariectomized rats were assigned into E2-T and Pla-T groups. Two weeks after the ovariectomy, rats were infused with hypertonic saline (1.5 M;0.33 ml/100g/BW ; i.v.). Access to water was provided 30-min after the infusion, and water intake was measured for 30 min. A few days later, we also examined c-Fos expression immunohistochemically. The same protocols were employed in other rats to examine the responses to central ANGII injection (5ng ; i.c.v.). E2-T rats drank significantly less amount of water than Pla-T rats in response to both hyperosmotic challenge and to central ANGII administration. The number of c-Fos ir cells in the OVLT in response to either osmotic or ANGII stimulations was not different between the E2- and Pla-T groups. The E2-T group showed lower number of c-Fos ir cells in the lateral part of the PVN after osmotic challenge compared to the Pla-T group. E2 treatment decreased AngII-induced c-Fos expression in the SFO and the SON as compared to Pla treatment. Thus, estrogen attenuates osmoregulatory water intake, and the site of this inhibitory action might be located on the pathway between the osmoreceptors and hypothalamic nuclei related to body fluid regulation. Central ANGII might be involved in the estrogen effect on osmoregulatory drinking. Less