1. Intracerebroventricularly (i.c.v.) administered vasopressin (0.001-1.0 u) in dogs anaesthetized with chloralose produced adose-dependent increase in urine flow with a concomitant decrease in the levels of antidiuretic hormone (ADH) in jugular vein blood. 2. Higher doses of vasopressin (1.5-2.0 u, i.c.v) on the other hand had an antidiuretic effect and produced an increase in blood ADH level. 3. Pretreatment (i.c.v.) with a beta-adrenoceptor antagonist completely blocked the diuretic response of low doses of vasopressin (i.c.v.) but did not influence the antidiuretic response obtained with high doses. 4. Repeated administration of vasopressin (1.0 u, i.c.v.) induced tachyphylaxis; central catecholamine depletion with tetrabenazine significantly inhibited the vasopressin-induced diuretic response. 5. It is concluded that intracerebroventricular vasopressin-induced changes in ADH secretion are mediated through the release of catecholamines in the central nervous system
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