[Morphine antagonizes the central hypotensive effect of clonidine in the anesthetized cat]

A dose of 1 microgram/kg of clonidine injected intracerebroventricularly (I.C.V.) in the normotensive anaesthetized Cat elicits a significant and long lasting drop in the mean arterial blood pressure (23 +/- 5 mmHg) and the heart rate (34 +/- 5 beats/min.). A pretreatment with morphine (100 micrograms/kg, I.C.V.) completely prevents the central hypotensive effect of clonidine, whereas the bradycardia is only weakly influenced. Administered after clonidine, morphine reverses the hypotensive effect of clonidine whereas morphine does not affect its bradycardic action. These data demonstrate that morphine antagonizes the central hypotensive effect of clonidine.