Dehydration and fluid balance: central effects of angiotensin

Central effects of dehydration are stimulated by osmotic stimuli, the reduced input of volume receptors, and angiotensin II. The subfornical organ (SFO) and organum vasculosum laminae terminalis (OVLT) have become accepted as putative receptor sites for angiotensin II in the brain. The exact quantitative relationship between the hours of water deprivation and the amount of angiotensin generated peripherally and whether that amount is sufficient to induce thirst centrally have not been established, but there is no question that when animals are dehydrated their angiotensin levels rise and the animals are thirsty. Attempts to block centrally the contribution of angiotensin II to thirst have been variable and cholinergic inputs have to be blocked at the same time. Various stimuli for thirst interact in a parallel fashion, and when one stimulus is blocked the other stimuli are still effective. Plasma angiotensin II may induce natural thirst, but how it enters the brain still remains to be explained. Although the SFO and OVLT have no blood-brain barrier, the blood supply to these organs acts as a limited perfusion system whereby blood-borne proteins cannot diffuse far from the capillary bed. A second set of receptors is found on the ventricular surface of the OVLT, as shown by fluorescence labeled angiotensin II. The connection between the SFO and OVLT was cut by discrete knife cuts. Drinking to angiotensin II intraventricularly was not significantly altered but the pressor response was reduced by 50%. These results can be explained by a circuit for drinking passing down below the level of the knife cut and a separate pressor pathway passing dorsally through the area that was cut by the knife. Thirst and pressor neural circuits beginning with angiotensin receptors could explain some of the data accumulated with the AV3V syndrome that occurs when the OVLT and nucleus medianas are destroyed.