Cardiovascular reactivity in adult rats pretreated at a suckling age with intracerebroventricular 6-hydroxydopamine.

To investigate functional differences in noradrenergic mechanisms of cardiovascular regulation between the hypothalamus and the brainstem, cardiovascular responses to various stimuli were examined in mature rats pretreated with 6-hydroxydopamine (6-OHDA) given i.c.v. at a suckling age which is known to provoke low norepinephrine concentrations in the hypothalamus but not in the brainstem or in peripheral organs. Under alpha-chloralose anesthesia the resting mean arterial pressure (MAP) was significantly lower in the 6-OHDA treated group than in the control group. Pressor response and associated reflexbradycardia produced by i.v. injection of phenylephrine in the two groups were similar. Likewise, carotid occlusion response and clonidine-induced hypotension and bradycardia did not differ between the two groups. Electrical stimulation of the sciatic nerve elicited increases in MAP and heart rate in both groups. However, pressor responses in the treated rats were significantly less than in the controls. Increases in heart rate during stimulation in the treated group were larger than in the controls. These results indicate that a long-lasting depletion of norepinephrine in the rat hypothalamus leads to a deterioration of basal level of blood pressure and somatosympathetic reflexes, yet baroreflexes remain intact.