Effects on blood pressure of noradrenaline and isoprenaline administered into the third ventricle of the brain of anaesthetized and conscious cats.

1 Noradrenaline administered into the third ventricle of the brain (IIIv) of both conscious and anaesthetized cats induced increases in blood pressure accompanied by small and variable heart rate effects. The pressor responses were reduced after autonomic ganglion blockade indicating their likely central origin. 2 Noradrenaline when administered either into a lateral cerebral ventricle (i.c.v.) in conscious and anaesthetized cats or into the cisterna magna (i.c.) in anaesthetized cats induced falls in blood pressure accompanied by bradycardia. 3 Pressor responses to IIIv noradrenaline in conscious cats were partly blocked by either propranolol or thymoxamine indicating a possible involvement in the responses of excitatory adrenoreceptors of both alpha and beta types. 4 Pressor responses induced by IIIv isoprenaline in conscious cats were blocked by propranolol but not by thymoxamine suggesting the effect is mediated solely via excitatory beta-adrenoreceptors. 5 In anaesthetized cats prior i.c. administration of noradrenaline reduced the pressor responses induced by IIIv noradrenaline. 6 In conscious cats i.v. clonidine reduced pressor responses to IIIv noradrenaline without depressing peripheral vascular noradrenaline sensitivity. 7 The results suggest the involvement of excitatory and inhibitory alpha-adrenoreceptors and of excitatory beta-adrenoreceptors in central blood pressure control. It is also concluded that activation of inhibitory alpha-adrenoreceptors in the hind brain region can suppress the cardiovascular effects of stimulating excitatory alpha- and beta-adrenoreceptors located in the region of the third ventricle.