Estradiol selectively reduces central neural activation induced by hypertonic NaCl infusion in ovariectomized rats

Abstract

We recently reported that the latency to begin drinking water during slow, intravenous infusion of a concentrated NaCl solution was shorter in estradiol-treated ovariectomized rats compared to oil vehicle-treated rats, despite comparably elevated plasma osmolality. To test the hypothesis that the decreased latency to begin drinking is attributable to enhanced detection of increased plasma osmolality by osmoreceptors located in the CNS, the present study used immunocytochemical methods to label fos, a marker of neural activation. Increased plasma osmolality did not activate the subfornical organ (SFO), organum vasculosum of the lamina terminalis (OVLT), or the nucleus of the solitary tract (NTS) in either oil vehicle-treated rats or estradiol-treated rats. In contrast, hyperosmolality increased fos labeling in the area postrema (AP), the paraventricular nucleus of the hypothalamus (PVN) and the rostral ventrolateral medulla (RVLM) in both groups; however, the increase was blunted in estradiol-treated rats. These results suggest that estradiol has selective effects on the sensitivity of a population of osmo-/Na(+)-receptors located in the AP, which, in turn, alters activity in other central areas associated with responses to increased osmolality. In conjunction with previous reports that hyperosmolality increases blood pressure and that elevated blood pressure inhibits drinking, the current findings of reduced activation in AP, PVN, and RVLM-areas involved in sympathetic nerve activity-raise the possibility that estradiol blunts HS-induced blood pressure changes. Thus, estradiol may eliminate or reduce the initial inhibition of water intake that occurs during increased osmolality, and facilitate a more rapid behavioral response, as we observed in our recent study.

Fig. 1

Body weight (g) of OIL- (gray circles) and EB- (black circles) treated rats during the 4-day protocol. Weight depended upon the interaction between hormone and time (F_(2,44)=18.63, p<0.001), with weight of OIL-treated rats increasing during the protocol, while weight of EB-treated rats decreased. 1=significantly greater than OIL day 1 (p<0.001); 2=significantly greater than OIL day 2 (p<0.001); 4*=significantly greater than EB day 4 (p<0.001); 1*=significantly less than EB day 1 (p<0.01).

Fig. 2

Number of neurons labeled for fos (fos⁺ neurons) in the organum vasculosum of the lamina terminalis (OVLT; top) and the subfornical organ (SFO; bottom) of OIL-(gray bars) and EB- (black bars) treated rats after iv infusion of 0.15 M NaCl (ISO; left bars) or 2 M NaCl (HS; right bars). The number of fos⁺ neurons was not affected by hormone condition or infusion in either area.

Fig. 3

Number of fos⁺ neurons in the area postrema (AP; top) and the nucleus of the solitary tract (NTS; bottom) of OIL- (gray bars) and EB- (black bars) treated rats after iv infusion of ISO (left bars) or HS (right bars). The number of fos⁺ neurons in the NTS was not affected by hormone condition or infusion. In contrast, the number of fos⁺ neurons in the AP of OIL-treated rats after HS infusion was significantly greater than that in the AP of EB-treated rats (2*=significantly greater than EB-HS; p<0.05; planned comparison with Bonferroni correction).

Fig. 4

Line drawing (top; adapted from [40]) illustrating the location of the area postrema (AP; red highlighting) and the nucleus of the solitary tract (NTS; green highlighting). Digital photomicrographs from OIL- (middle) and EB- (bottom) treated rats after iv HS infusion showing fos⁺ neurons in the AP and NTS. Scale bars indicate 100 μm. Photomicrographs were adjusted for brightness and contrast.

Fig. 5

Number of fos⁺ neurons in the hypothalamic supraoptic nucleus (SON; top) and paraventricular nucleus (PVN; bottom) of OIL- (gray bars) and EB- (black bars) treated rats after iv infusion of ISO (left bars) or HS (right bars). The number of fos⁺ neurons in the SON depended upon infusion (F_(1,19)=114.15, p<0.001), but was not affected by hormone condition. In contrast, the number of fos⁺ neurons in the PVN depended upon the interaction between hormone condition and infusion (F_(1,19)=5.41, p<0.05), with the HS-induced increase blunted in EB-treated rats. 1=significantly greater than OIL-ISO (OIL-HS p<0.001; EB-HS p<0.05); 1*=significantly greater than EB-ISO (OIL-HS p<0.001; EB-HS p<0.05); 2*=significantly greater than EB-HS (p<0.01).

Fig. 6

Line drawing (right; adapted from [40]) illustrating the location of the paraventricular nucleus (PVN; red highlighting). Digital photomicrographs from OIL- (middle) and EB-(right) treated rats after iv HS infusion showing fos⁺ neurons in the PVN. Scale bars indicate 100 μm. Photomicrographs were adjusted for brightness and contrast.

Fig. 7

Number of fos⁺ neurons in the caudal ventrolateral medulla (CVLM; top) and rostral ventrolateral medulla (RVLM; bottom) of OIL- (gray bars) and EB- (black bars) treated rats after iv infusion of ISO (left bars) or HS (right bars). The number of fos⁺ neurons in the CVLM was not affected by hormone condition or infusion. In contrast, the number of fos⁺ neuronsin the RVLM depended upon the interaction between hormone condition and infusion (F_(1,19)=8.94, p<0.01), with an HS-induced increase in OIL-treated rats, but not in EB-treated rats. 1=significantly greater than OIL-ISO (p<0.01); 2*=significantly greater than EB-HS (p<0.01).

Fig. 8

Line drawing (right; adapted from [40]) illustrating the location of the rostral ventrolateral medulla (RVLM; red highlighting). Digital photomicrographs of the RVLM from OIL- (middle) and EB- (right) treated rats after HS infusion showing neurons labeled for fos (dark brown-black nuclear accumulation) and dopamine-β-hydroxylase (DBH; bright green cytoplasmic accumulation), and neurons double-labeled for fos and DBH. These images were obtained by overlaying brightfield and darkfield images. Scale bars indicate 100 μm. Photomicrographs were adjusted for brightness and contrast.