Bidirectional effects of estradiol on the control of water intake in female rats

Abstract

The inhibitory effect of estradiol (E2) on water intake has been recognized for 50 years. Despite a rich literature describing this phenomenon, we report here a previously unidentified dipsogenic effect of E2 during states of low fluid intake. Our initial goal was to test the hypothesis that the anti-dipsogenic effect of E2 on unstimulated water intake is independent of its anorexigenic effect in female rats. In support of this hypothesis, water intake was reduced during estrus, compared to diestrus, when food was present or absent. Water intake was reduced by E2 in ovariectomized rats when food was available, demonstrating a causative role of E2. Surprisingly, however, when food was removed, resulting in a significant reduction in baseline water intake, E2 enhanced drinking. Accordingly, we next tested the effect of E2 on water intake after an acute suppression of intake induced by exendin-4. The initial rebound drinking was greater in E2-treated, compared to Oil-treated, rats. Finally, to reconcile conflicting reports regarding the effect of ovariectomy on water intake, we measured daily water and food intake, and body weight in ovariectomized and sham-operated rats. Predictably, ovariectomy significantly increased food intake and body weight, but only transiently increased water intake. Together these results provide further support for independent effects of E2 on the controls of water and food intake. More importantly, this report of bidirectional effects of E2 on water intake may lead to a paradigm shift, as it challenges the prevailing view that E2 effects on fluid intake are exclusively inhibitory.

Keywords: Drinking microstructure; Estrogens; Food intake; Ovariectomy.

Figure 1.. Water and food intake during E was reduced compared to intake during D2.

(A) Water intake was lower during E, compared to D2, regardless of food access. Regardless of cycle stage, water intake was greater when food was available. (B) The magnitude of the reduction in water intake (wi) between D2 and E was not affected by food availability. (C) Food intake was lower during E, compared to D2. *Less than D2, p < 0.05. +Less than when food was available, p < 0.05.

Figure 2.. The direction of the fluid intake effect of E2 was influenced by food access in OVX rats.

(A) When food was available, EB treatment reduced water intake. When food was not available, EB treatment increased water intake. Regardless of hormone treatment, water intake was greater when food was available. (B) When food was available, EB-treatment reduced licks for water. When food was not available, EB-treatment increased licks for water. Regardless of hormone treatment, licks for water were greater when food was available. (C) EB-treatment reduced food intake. (D) When food was not available, both EB and EB+P treatment increased water intake. (E) When water was not available, EB treatment reduced food intake. *Less than Oil, p < 0.05. #Greater than Oil, p < 0.05. +Less than when food was available, p < 0.05.

Figure 3.. The anti-dipsogenic effect of E2 decreased water intake by reducing burst number.

Lick analysis was conducted in Oil- and EB-treated rats in the presence of food. (A/B) There was no effect of EB-treatment on burst number or burst size across the entire test period. (C) EB-treatment did not reduce licks for water in any specific 6 h bin. The reduction in licks was due to a main effect of EB-treatment. Licks for water during the dark phase (grey bar) were significantly greater than licks for water during the light phase. (D/E) During the last 6 h bin (bin 4), EB treatment reduced burst number, with no effect on burst size. +Greater than light phase, p < 0.05. *Less than Oil, p < 0.05.

Figure 4.. The dipsogenic effect of E2 increased water intake by increasing burst number.

Lick analysis was conducted in Oil- and EB-treated rats in the absence of food. (A/B) EB-treatment increased burst number, but had no effect on burst size, across the entire test period. (C) EB-treatment increased licks for water during the first two 6h bins. Licks for water during the dark phase (grey bar) were significantly greater than licks for water during the light phase. (D/E) During the second 6 h bin (bin 2), EB treatment increased burst number, with no effect on burst size. *Greater than Oil, p < 0.05. +Greater than light phase, p < 0.05.

Figure 5.. E2 increased water intake after an acute suppression by Exendin-4.

(A) Ex-4 temporarily suppressed water intake. Drinking resumed during the 5^th h of the dark phase. During this time, intake in EB-treated rats was greater than intake in Oil-treated rats. Intake during the 7^th h of the dark phase was also greater in EB-treated rats. Intake during the 6^th h of the dark phase was greater in Oil-treated, compared to EB-treated, rats. (B) The latency to consumed 1 ml of water was significantly shorter in EB-treated rats. *Greater than Oil, p < 0.05. #Greater than EB, p < 0.05. +Greater than bins 1–4, p < 0.05.

Figure 6.. Ovariectomy increased body weight and food intake.

(A) At 22 days post-surgery, OVX rats weighed significantly more than SHAM rats, a different that remained for the duration on the study. (B/C) Daily food intake was significantly greater in OVX compared to SHAM rats. (D/E) Daily water intake was not significantly different between OVX and SHAM rats throughout the entire experiment. During the 12-day period post-surgery, water intake in OVX rats was greater than in SHAM rats. The vertical line in each graph denotes the surgery. *Greater than SHAM, p < 0.05.