Differential effects of estradiol on drinking by ovariectomized rats in response to hypertonic NaCl or isoproterenol: Implications for hyper- vs. hypo-osmotic stimuli for water intake

Abstract

We examined the effects of estradiol on behavioral responses to osmotic challenges in ovariectomized (OVX) rats to test the hypothesis that estradiol enhances sensitivity to gradual changes in plasma osmolality (pOsm) in stimulating water intake. Despite comparably elevated pOsm after a slow infusion of 2 M NaCl, the latency to begin water intake was significantly less in estradiol-treated OVX rats compared to that in oil vehicle-treated rats. Other groups of OVX rats were injected with isoproterenol, which increases circulating angiotensin II. These rats then were given 0.15 M NaCl to drink instead of water, to prevent decreased pOsm associated with water ingestion. Isoproterenol stimulated 0.15 M NaCl intake by both groups; however, estradiol-treated rats consumed less 0.15 M NaCl than did oil-treated rats, findings that are similar to those reported when estradiol-treated rats consumed water. The estradiol enhancement of sensitivity to increased, but not to decreased, pOsm suggests that estradiol has directionally-specific effects on osmoregulatory drinking. Moreover, the estradiol attenuation of 0.15 M NaCl intake after isoproterenol suggests that estradiol effects on osmoregulatory drinking are independent of those on volume regulatory drinking.

Figure 1

Latency to begin water intake (min; A), duration of first water intake bout (min; B) and water consumed in the first bout (ml; C) during iv infusion of 2 M NaCl (35 μl/min). In OVX rats, EB treatment (black bars) significantly decreased the latency to begin drinking (p < 0.05). Individual rats are indicated by filled triangles (OIL-treated) or open diamonds (EB-treated). The error bars are obscured by the individual points for the EB-treated rats, which are tightly clustered around ∼15 min, and partially obscured by the individual points for the OIL-treated rats, which span a wider range. EB treatment did not affect the duration of the first bout or the volume of water consumed in the first bout.

Figure 2

Total water intake (ml) by OVX rats given OIL (white bars) or EB (black bars) treatment during 60-min adaptation or 60-min iv infusion of 2 M NaCl (HS) at 35 μl/min. Water intake was significantly increased during HS infusion (p < 0.001), but there were no differences between the hormone conditions.

Figure 3

Intake of 0.15 M NaCl (ml) by OVX rats given OIL (squares) or EB (circles) treatment in 60-min tests after administration of Isoproterenol (ISOP; filled symbols, solid lines) or the 0.15 M NaCl vehicle (SAL; open symbols, dotted lines). Intake of 0.15 M NaCl was significantly increased by ISOP (p < 0.001) regardless of hormone condition. However, in the last 30 min of the test, EB-treated rats consumed less 0.15 M NaCl after ISOP than did OIL-treated rats. d = p < 0.001 vs. corresponding time after ISOP within hormone condition; t = p < 0.01 vs. previous time after OIL-ISOP; t* = p < 0.001 vs. previous time after OIL-ISOP; h = p < 0.01 vs. EB-ISOP at corresponding time; h* = p < 0.001 vs. EB-ISOP at corresponding time.