Roles of the subfornical organ and area postrema in arterial pressure increases induced by 48-h water deprivation in normal rats

Abstract

In rats, water deprivation (WD) increases arterial blood pressure (BP) in part due to actions of elevated osmolality in the brain to increase vasopressin levels and sympathetic activity. However, the osmoreceptors that mediate this response have not been identified. To test the hypothesis that osmoregulatory circumventricular organs are involved, BP and heart rate (HR) were continuously recorded telemetrically during 48 h of WD in normal rats with lesions (x) or sham lesions (sham) of the subfornical organ (SFO) or area postrema (AP). Although WD increased BP in SFOx and SFOsham rats, no significant difference in the hypertensive response was observed between groups. HR decreased transiently but similarly in SFOx and SFOsham rats during the first 24 h of WD. When water was reintroduced, BP and HR decreased rapidly and similarly in both groups. BP (during lights off) and HR were both lower in APx rats before WD compared to APsham. WD increased BP less in APx rats, and the transient bradycardia was eliminated. Upon reintroduction of drinking water, smaller falls in both BP and HR were observed in APx rats compared to APsham rats. WD increased plasma osmolality and vasopressin levels similarly in APx and APsham rats, and acute blockade of systemic V1 vasopressin receptors elicited similar depressor responses, suggesting that the attenuated BP response is not due to smaller increases in vasopressin or osmolality. In conclusion, the AP, but not the SFO, is required for the maximal hypertensive effect induced by WD in rats.

Keywords: Area postrema; blood pressure; heart rate; subfornical organ; water deprivation.

Figure 1.

Photomicrographs of 40‐μm coronal sections of the rat brain with an intact area postrema (A) and after lesion of the area postrema (B) (left panels), and midsagittal sections of the rat brain with an intact subfornical organ (C) and after lesion of the subfornical organ (D) (right panels). Scale bar = 500 μm.

Figure 2.

Representative tracings from an APx rat (top panel), a SFOx rat (middle panel), and a sham‐operated rat (bottom panel) showing mean arterial pressure (MAP) and heart rate (HR) before, during, and after a period of 48‐h water deprivation. Gray stripes indicate the lights off segment of the day–night cycle.

Figure 3.

Mean arterial pressure (MAP) and heart rate responses to 48‐h water deprivation in rats with an intact subfornical organ (SFOsham, filled squares) and with lesion of the SFO (SFOx, open squares) during the lights on (left) and lights off (right) segments of the day–night cycle. (*P < 0.05 compared to baseline).

Figure 4.

Mean arterial pressure (MAP) and heart rate (HR) responses to 48‐h water deprivation in rats with an intact area postrema (APsham, closed circles) and with lesion of the AP (APx, open circles) during the lights on (left) and lights off (right) segments of the day–night cycle. While WD did not decrease HR compared to baseline, HR was suppressed (P < 0.05) in the APsham rats, but not APx rats, after 24 h of WD (lights on and lights off) compared to the recovery period. (*P < 0.05 compared to baseline; ^†P < 0.05 between groups).