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.

Drug administration into the third ventricle of the brain of anesthetized and conscious cats.

A method is described for the administration of drugs into the third ventricle of the brain of conscious and anesthetized cats. Noradrenaline administered by this route produced cardiovascular stimulant effects as against depressor effects when administered into a lateral cerebral ventricle. The pressor responses evoked by third ventricular administration of noradrenaline were antagonized by spinal section in anesthetized cats and by hexamethonium in conscious animals. Conscious cats were more sensitive to the pressor effects of third ventricular noradrenaline than chloralose-anesthetized animals.

Recording of splanchnic (sympathetic) nerve activity in conscious and anaesthetised cats.

The method described allows of continuous recording of electrical activity from the splanchnic nerves of conscious unrestrained cats. The activity recorded was phasic and corresponded with respiration as reported by previous workers. Splanchnic nerve activity varied with the active state of the animal and, in general, closely paralleled changes in arterial blood pressure and heart rate. Noradrenaline administered intravenously raised arterial blood pressure but splanchnic nerve activity was depressed while acetylcholine produced the opposite effects.