Effect of intraventricular phentolamine on hyper- and hypotensive vasomotor reflexes

Injection of small amounts of phentolamine into the lateral ventricle of the cat abolishes the blood pressure fall, evoked by vagal stimulation, and finally converts it into a pressure rise; all other vasomotor responses tested remain unchanged. In contrast, intraventricular pentobarbitone blocks both the pressor and depressor reactions following sciatic stimulation, although the vagal hypotension is again affected first. It appears probable that the depressor neurons, reached by afferent vagus fibers, are situated at the surface of the floor of the fourth ventricle and are thus accessible to intraventricular phentolamine. These neurons are activated by a -adrenergic transmission.