The role of the central nervous system in the cardiovascular responses to yohimbine.

Yohimbine injected intravenously or intracerebroventricularly in conscious dogs produced behavioural excitation accompanied by a rise in blood pressure and heart rate. The cardiovascular effects were reduced or abolished by hexamethonium, phenoxybenzamine and reserpine. In conscious cats intravenously administered yohimbine was depressor but intracerebroventricular administration in these animals caused a rise in blood pressure accompanied by behavioural depression. In anaesthetized or decerebrate cats yohimbine was always depressor. Yohimbine injected intracerebroventricularly produced a rise in blood pressure and heart rate in both conscious and anaesthetized rats. When administered intravenously to these animals there was a fall in blood pressure. It was concluded that the pressor action of yohimbine in conscious dogs was central in origin via the sympathetic nervous system. The different pattern of cardiovascular responses in the dog, cat and rat may be related to differences in the balance between medullary effects and effects on higher brain centres of the three species.