Depression of reflex vagal bradycardia by a central action of phentolamine in the spinal cat.

Cats were anaesthetized with a mixture of alpha-chloralose and urethane and artificially ventilated. The spinal cord was transected at the C-1 level and the fourth cerebral ventricle cannulated. Phentolamine (500 microgram) administered into the fourth cerebral ventricle depressed the reflex vagal bradycardic responses elicited by intravenous pressor doses of noradrenaline. Enhancement of reflex bradycardia occurred following intracerebroventricular administration of L-DOPA (3.0 mg) which was reversed by subsequent administration of phentolamine into the fourth cerebral ventricle of spinal cats. These results suggest that central noradrenergic stimulation enhances and noradrenergic blockade suppresses the reflex vagal activation. In midcollicular decerebrate cats, intracerebroventricular administration of phentolamine reduced the reflex bradycardic responses elicited by intravenous noradrenaline. It is suggested that the action of phentolamine to depress baroreceptor mediated reflex vagal activation is on sites within the pontomedullary areas of the cat.