Diverse Roles of Angiotensin Receptor Intracellular Signaling Pathways in the Control of Water and Salt Intake

Angiotensin II (AngII) is a key component in the maintenance of body fluid homeostasis. It has potent dipsetic and natriorexigenic effects when injected into the brain of rats. This ingestive response to AngII is so reliable that AngII is frequently used to verify proper placement of cannulae in forebrain ventricles, even by laboratories with little interest in fluid intake. The actions of AngII have been an important focus in the study of fluid intake, but have also been particularly informative in other fields of study. Much of our knowledge about the means by which peripherally derived peptides act in the brain, for example, has been greatly influenced by earlier studies that identified central targets of AngII.

The central targets of AngII are numerous. Perhaps the most interesting among them are the forebrain circumventricular organs (CVOs), which reside on the anterior wall of the third ventricle. The subfornical organ (SFO) and the organum vasculosum of the lamina terminalis (OVLT) (for review, see Daniels and Fluharty 2004; Phillips 1987) are critical for many central actions of AngII. The hypothesis that AngII acts directly on CVOs is supported by a variety of experimental approaches. Lesion techniques have shown that ablation of the SFO or the periventricular area containing the OVLT dramatically reduces the water or NaCl intake stimulated by AngII (Buggy and Johnson 1978; Morris et al. 2002; Simpson et al. 1978). Small amounts of AngII injected directly into the SFO or OVLT, on the other hand, increase water and NaCl intake (Mangiapane and Simpson 1979, 1980). Experiments using receptor autoradiography demonstrated high levels of AngII receptor expression in CVO structures (for review, see Allen et al. 2000) and studies of brain activation using deoxyglucose (Kadekaro et al. 1989) or Fos immunohistochemistry (McKinley et al. 1995; Rowland et al. 1994a,b, 1995) highlight these areas as central AngII targets. Electrophysiological recordings have also been useful in the study of central responses to AngII. Early whole-cell patch clamp recordings from SFO neurons demonstrated AngII-induced excitation (Li and Ferguson 1993a,b) and suggested that the excitation occurred by inhibition of transient outward currents (Ferguson and Li 1996). More recent reports have supported these earlier findings (e.g., Ono et al. 2005), firmly establishing the SFO as a brain area that is sensitive to AngII.

Although many studies have confirmed the importance of the forebrain CVOs in the actions of AngII, these areas initially were an attractive focus primarily because they lack a blood–brain barrier and AngII does not cross the blood–brain barrier (Harding et al. 1988). AngII receptors have been found in numerous brain areas and, other than the CVOs, these brain areas do not appear accessible to peripherally derived AngII. The apparent paradox of AngII responsive brain areas without an endogenous source of AngII has been solved by the more recent discovery that the brain contains components of the renin–angiotensin system (Sakai and Sigmund 2005). In what appears to be a fascinating coincidence, cells in the same structure that responds to AngII made in the periphery, the SFO, use AngII as a peptide transmitter (Li and Ferguson 1993a,b).

The receptors for AngII fall into two main subtypes, type 1 (AT₁) and type 2 (AT₂) receptors. The intake effects of AngII appear to be exclusively driven by action at the AT₁ receptor. AT₁ receptors are expressed at relatively high levels in the CVOs and other CNS structures (Bunnemann et al. 1992; Rowe et al. 1992; Song et al. 1992), and experimental manipulation of these receptors has notable effects on fluid intake. Mice with genetic disruption of the gene for the AT₁ receptor have a severely impaired water intake response to AngII injection (Li et al. 2003), and AT₁-specific antagonists or antisense oligonucleotides attenuate the intake responses to AngII (Beresford and Fitzsimons 1992; Sakai et al. 1994, 1995; Weisinger et al. 1997). Although most commonly referred to as the AT₁ receptor, this nomenclature actually comprises two receptor isoforms, the AT_1a and AT_1b receptors. These isoforms are identical in the number of amino acids (359 in both the rat and mouse), but have 17 residues that differ in the rat and 22 residues that differ in the mouse. Although there are several reports of anatomical localization of the two AT₁ receptor subtypes, inconsistencies in the reports and the potential for species differences remain to be resolved. For example, an early study in rats used polymerase chain reaction to demonstrate expression of AT_1a and AT_1b receptors in CVO structures and hypothalamus (Kakar et al. 1992), but subsequent in situ hybridization studies indicate that the AT_1a receptor is the predominant or the only receptor type in rat CVO structures and in the hypothalamus, whereas the AT_1b receptor is expressed in the cerebral cortex and the hippocampus (Johren et al. 1995). Studies conducted predominantly in mice, however, have revealed interesting differences in the role of these AT₁ receptor isoforms and their regulation by perturbations in fluid balance. Specifically, selective gene targeting found that the AT_1b receptor is critical for the drinking response to central injections of AngII in mouse (Davisson et al. 2000), and other studies found that dehydration increased AT_1a receptor expression in forebrain CVOs, but AT_1b receptor expression was unaffected (Chen and Morris 2001). The latter of these studies, however, reported expression of both AT_1a and AT_1b receptors in CVO structures, suggesting that the distribution in the mouse and the rat may be different. A different anatomical distribution and different roles of the receptor isoforms may explain the differences between mice and rats in their responsiveness to AngII. Specifically, lateral ventricle injections of ~5 ng of AngII are sufficient to stimulate water intake in rats, but far greater amounts (~100–200 ng) are needed to stimulate water intake in mice and AngII is surprisingly not dipsetic when injected peripherally in mice (Rowland et al. 2003). Accordingly, it is appropriate to be cautious when attempting to apply findings generated in one species to another species.