The role of mesencephalic structures in thirst induced by centrally administered angiotensin II.

(1) In 27 animals microinjection of 25--100 ng of angiotensin II through chronic cannulae implanted in the preoptic region initiated drinking and in subsequent acute experiments influenced the spontaneous discharge rate of single neurons in the ipsi-lateral mesencephalon. Of 148 neurons for which recordings were made, 52 (35%) increased their frequency of spike potentials following administration of angiotensin II, 2 (1%) showed inhibition and 94 (64%) showed no change in firing rate. (2) In another series of 44 animals, unilateral or bilateral lesions of the midbrain ventral tegmentum or reticular formation were found to have little or no effect on water intake elicited by the microinjection of angiotensin II into the preoptic region. (3) In contrast to the effects of tegmental and reticular lesions, unilateral lesions located dorsally and laterally to the mammillary peduncle, in the area of passage of the medial forebrain bundle, significantly attenuated the dipsogenic response to either contralateral or ipsilateral injections of angiotensin II into the preoptic region. With bilateral lesions this effect was permanent. (4) Since the more caudal lesions were relatively ineffective in disrupting the elicited drinking, it is suggested that signals from angiotensin II receptors in the preoptic region are transmitted along pathways which diverge in the midbrain. (5) The possibility of a forebrain-hypothalamus-midbrain circuit mediating thirst initiated by activation of the renin-angiotensin system is discussed.