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1. Angiotensin (Ang)II is involved in responses to hypovolaemia, such as sodium appetite and increase in blood pressure. Target areas subserving these responses for AngII include the cardiovascular system in the periphery and the circumventricular organs in the brain. 2. Conflicting data have been reported for the role of systemic versus brain AngII in the mediation of sodium appetite. 3. The role for systemic AngII and systemic AngII receptors in the control of blood pressure in hypovolaemia is well established. In contrast with systemic injections, i.c.v injections of AngII non-peptide AT1 and AT2 receptor antagonists, such as losartan and PD123319, do not reduce arterial pressure in sodium-depleted (furosemide injection plus removal of ambient sodium for 24 h) rats. Thus, brain AngII receptors are likely not important for cardiovascular responses to hypovolaemia induced by sodium depletion. 4. Intracerebroventricular injections of losartan or PD123319 increase arterial pressure when injected at relatively high doses. This hypertensive effect is unlikely to be an agonist effect on brain AngII receptors. Increases in arterial pressure produced by i.c.v. losartan are attenuated by lesions of the tissue surrounding the anterior third ventricle (AV3V). The hypertensive effect of i.c.v. AngII is abolished by lesions of the AV3V. 5. Hypertension induced by AngII receptor antagonists is consistent with hypotension induced by AngII acting in the brain. However, the full physiological significance of this hypotensive effect mediated by brain AngII receptors remains to be determined.

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Non-adrenergic ligands that bind to imidazoline receptors (I-R), a selective ligand that binds to = 2 -adrenoceptors (= 2 -AR) and mixed ligands that bind to both receptors were tested for their action on water intake behavior of 24-h water-deprived rats. All drugs were injected into the third cerebral ventricle. Except for agmatine (80 nmol), mixed ligands binding to I-R/= 2 -AR such as guanabenz (40 nmol) and UK 14304 (20 nmol) inhibited water intake by 65% and up to 95%, respectively. The selective non-imidazoline = 2 -AR agonist, = -methylnoradrenaline, produced inhibition of water intake similar to that obtained with guanabenz, but at higher doses (80 nmol). The nonadrenergic I-R ligands histamine (160 nmol, mixed histaminergic and imidazoline ligand) and imidazole-4-acetic acid (80 nmol, imidazoline ligand) did not alter water intake. The results show that selective, nonimidazoline = 2 -AR activation suppresses water intake, and suggest that the action on imidazoline sites by non-adrenergic ligands is not sufficient to inhibit water intake.