The anti-dipsogenic and anti-natriorexigenic effects of estradiol, but not the anti-pressor effect, are lost in aged female rats

Abstract

Estradiol (E2) inhibits fluid intake in several species, which may help to defend fluid homeostasis by preventing excessive extracellular fluid volume. Although this phenomenon is well established using the rat model, it has only been studied directly in young adults. Because aging influences the neuronal sensitivity to E2 and the fluid intake effects of E2 are mediated in the brain, we tested the hypothesis that aging influences the fluid intake effects of E2 in female rats. To do so, we examined water and NaCl intake in addition to the pressor effect after central angiotensin II treatment in young (3-4 months), middle-aged (10-12 months), and old (16-18 months) ovariectomized rats treated with estradiol benzoate (EB). As expected, EB treatment reduced water and NaCl intake in young rats. EB treatment, however, did not reduce water intake in old rats, nor did it reduce NaCl intake in middle-aged or old rats. The ability of EB to reduce blood pressure was, in contrast, observed in all three age groups. Next, we also measured the gene expression of estrogen receptors (ERs) and the angiotensin type 1 receptor (AT1R) in the areas of the brain that control fluid balance. ERβ, G protein estrogen receptor (GPER), and AT1R were reduced in the paraventricular nucleus of the hypothalamus in middle-aged and old rats, compared to young rats. These results suggest the estrogenic control of fluid intake is modified by age. Older animals lost the fluid intake effects of E2, which correlated with decreased ER and AT1R expression in the hypothalamus.

Keywords: NaCl intake; drinking; estrogen receptor; water intake.

FIGURE 1

Experimental Timeline. OVX, telemetry implantation, and ICV surgeries occurred during Weeks 1–3. Testing occurred during Weeks 4–5 and brains were collected during Week 6

FIGURE 2

Age influenced the anti‐dipsogenic and anti‐natriorexigenic effects of E2. (a) In young rats, estradiol benzoate (EB) treatment reduced the total number of licks during the first and second 15 min of testing. (b/c) This change was driven by an EB‐mediated reduction in 0.26 M NaCl licks during the first 15 min of testing and a reduction in water licks during the second 15 min of testing. (d) In middle‐aged rats, EB treatment had no effect on the total number of licks, (e) but did reduce licks for water during the second 15‐min of testing. (f) There was no effect of EB treatment on 0.26 M NaCl licks in middle‐aged rats. (g/h) In old rats, EB treatment had no effect on the total number of licks or licks for water. (i) EB treatment enhanced licks for 0.26 M NaCl during the second 15 min of testing. Sample size = 11–12/group. Three‐factor ANOVA: *Less than oil treatment, p < 0.05. +Greater than oil treatment, p < 0.05

FIGURE 3

The inhibitory effect of estradiol benzoate (EB) on drinking microstructure was influenced by age. (a/b) EB reduced licks for water in young rats during the second 15 min of testing, by reducing the burst number, but not the burst size. However, EB reduced licks for water in middle‐aged rats by reducing burst size, not burst number. (c/d) In young rats, EB reduced licks for 0.26 M NaCl during the first 15 min of testing; however, there was no significant reduction in either burst size or burst number. In old rats, EB increased licks for 0.26 M NaCl during the second 15 min of testing, which was mediated by an increase in burst number, with no change in burst size. Sample size = 11–12/group. t‐test: *Less than oil treatment, p < 0.05. ⁺Greater than oil treatment, p < 0.05

FIGURE 4

Estradiol benzoate (EB) treatment reduced blood pressure. In (a/b) young, (c/d) middle‐aged, and (e/f) old rats, AngII increased mean arterial pressure (MAP) during the first 32 min of the test period (Bins 1 and 2), compared to BL. MAP was reduced, in EB‐ compared to oil‐treated rats, during Bins 1 and 2 in young rats, Bin 1 in middle‐aged rats, and Bins 1–3 in old rats. In old rats, EB treatment also reduced BL MAP. Sample size = 6–8/group. Three‐factor ANOVA: *Less than oil‐treatment, p < 0.05. ^#Greater than BL, p < 0.05

FIGURE 5

Age influenced estrogen receptor (ER) and angiotensin type 1 receptor (AT1R) expression in discrete brain regions. (a) In the subfornical organ (SFO), there was no influence of age on ERα, GPER, or AT1R expression. (b) In the AV3V, there was no influence of age on ERα, ERβ, or AT1R expression. (c). In the paraventricular nucleus of the hypothalamus (PVN), both middle‐aged and old rats had significantly less ERβ and AT1R expression, compared to young rats. Old rats also had significantly less GPER expression than young rats. Sample size = 6–9/group. Kruskal–Wallis Test (SFO: ERα and GPER)/one‐way ANOVA: *Less than young, p < 0.05