Functional separation of brain AT1A and AT1B receptors in AT1A knockout mice
Angiotensin II (AII) type 1 receptors (AT1) are located in brain areas known to be involved in central AII-mediated cardiovascular and fluid balance regulation. Two genetically distinct subtypes of the AT1 receptor, AT1A and AT1B, have been identified and localized to different regions of the rodent brain, but it is not known if the differential distribution of the subtypes corresponds to divergent functional roles. The aim of this study was to examine the relative roles of AT1A and AT1B in central AII-induced blood pressure (BP) and drinking responses (DR). Since there are no pharmacological tools to functionally separate these receptors, we performed these experiments in mice with a homozygous gene-targeted deletion of the AT1A receptor (AT1A-/-). Mice were instrumented with carotid arterial catheters and intracerebroventricular (ICV) cannulae, allowed 4d recovery, and BP and DR were measured in the conscious unrestrained state. Due to the small quantity of water intake by mice, DR was quantified by the number of episodes of drinking that occurred by 20min post-AII injection. ICV injection of vehicle (0.5μl artificial cerebrospinal fluid) did not elicit BP or DR in any of the mice. ICV administration of AII (50, 100, 200ng, 0.5μl each) caused dose-dependent pressor responses in wildtype controls (8±1, 19±3, 32±7, ΔBP, mmHg, n=4, p<.01 all doses) but had no significant effect on BP at any of these doses in AT1A-/- mice (1±1, 4±1, 5±2, ΔBP mmHg, n=8, p>.05 all doses). In contrast, DR elicited by the same ICV doses of AII were not abolished in AT1A-/-mice (3±1, 3±2, 4±2, n=4) compared to wildtype controls (2±1, 5±2, 8±2, n=3, p<05 AT1A-/- vs. control, 200ng dose). However, following ICV pre-treatment with the AT-1 receptor antagonist losartan (10μg, 0.5μl), ICV AII (200 ng)-induced drinking was abolished in both groups. These results suggest that whereas central AII-mediated BP effects can be ascribed selectively to AT1A receptors, both receptor subtypes may play a role in central AII-elicited drinking behavior.
Davisson, RL; Beltz, TG; Oliverio, MI; Johnson, AK; Coffman, TM; Sigmund, CD
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