Action and specificity of ventral medullary vasopressor neurones in the cat

An investigation has been made into the mode and specificity of action of ventral medullary pressor neurones. These were activated by microinjections of excitant amino acid into the ventral brain surface of chloralose-anaesthetized, artificially ventilated cats, and a number of autonomic responses were measured. Indirect assessment of cardiac output (by CO2 delivery to the lungs) suggested that it was either unchanged or fell during pressor responses. The inference that activating the pressor neurones caused vasoconstriction was confirmed directly for hindlimb and mesenteric vascular beds, by a rise in inflow pressure when they were perfused at constant flow. Sympathetic activity also increased in cervical, splanchnic and inferior cardiac nerves. Bradycardia often (but not always) accompanied pressor responses, but this was abolished by vagotomy, although not by cutting the sinus and aortic nerves. In vagotomized cats, tachycardia could be produced during pressor responses even after either bilateral adrenalectomy or removal of the stellate ganglia, indicating both direct sympathetic drive to the heart and release of adrenal catecholamines. Plasma adrenaline levels were measured and found to increase by up to 20.2 times control values, plasma noradrenaline up to 12.6 times, and dopamine by a smaller amount. Activating ventral medullary pressor neurones appeared to have no significant action on pupils, nictitating membranes or piloerection. In three adrenalectomized, vagotomized cats, only small, inconsistent effects were measured on intestinal motility following pressor neurone excitation. However, large electrodermal responses could be evoked from the ventral medulla, but from a distinct area medial to the pressor neurones.(ABSTRACT TRUNCATED AT 250 WORDS)