1. Cats were anaesthetized with pentobarbitone sodium. The hypothalamus was superfused with drugs through a push-pull cannula and electrically stimulated with the non-insulated tip of the cannula. In another series of experiments, the hypothalamus was stimulated with a monopolar electrode and the nucleus of the solitary tract superfused with drugs through a push-pull cannula.
2. Superfusion of the hypothalamus with alpha-adrenoreceptor blocking agents (piperoxan or tolazoline) decreased the pressor responses to hypothalamic stimulation. Superfusion with low concentrations of clonidine increased, whereas superfusion with higher concentrations of this drug diminished, the rise of blood pressure during hypothalamic stimulation. Muscarinic agents (oxotremorine, muscarine or AHR 602) decreased the pressor responses to hypothalamic stimulation. The inhibitory effects of the muscarinic compounds were abolished by superfusing the hypothalamus with methylatropine. Superfusion with nicotinic agents (1,1-dimethyl-4-phenylpiperazinium, or carbachol) enhanced the pressor responses to hypothalamic stimulation. The enhancement of the pressor responses by these drugs was completely inhibited by superfusion of the hypothalamus with hexamethonium.
3. Superfusion of the nucleus of the solitary tract with tolazoline enhanced the pressor responses to hypothalamic stimulation. The pressor responses were reduced during superfusion of the nucleus of the solitary tract with clonidine.
