The posterior hypothalamus of anaesthetized cats were superfused with CSF through a push-pull cannula and electrically stimulated with its tip. Hypothalamic stimulation elicited a pressor response. Superfusion of the hypothalamus with histamine-containing CSF elicited a concentration-dependent enhancement of the pressor response. The enhancing effect of histamine was abolished, when the hypothalamus was pre-superfused with metiamide, whereas pre-superfusion with mepyramine only slightly inhibited the effect of histamine. Hypothalamic superfusion with dimaprit caused a concentration-dependent increase in the pressor response. The effect of dimaprit was abolished by pre-superfusion of the hypothalamus with metiamide, but it was not affected by pre-superfusion with mepyramine. Superfusion of the hypothalamus with 2-methylhistamine did not affect the pressor response. It is concluded that histamine increases the pressor response to hypothalamic stimulation. The enhancing effect of histamine is mainly due to its action on H2-receptors of the posterior hypothalamus.
