Involvement of presynaptic dopamine receptors in the antihypertensive response to 2-NN-dimetnylamino-5,6-dihydroxy-1,2,3,4-tetrahydronaphthalene(M-7)

M-7, 1 and 3 mg kg^?1 s.c., elicits an antihypertensive response and bradycardia in conscious spontaneously hypertensive rats (SHR) and causes inhibition of stimulation-evoked pressor responses and tachycardia in pithed SHR. Metoclopramide (30 mg kg^?1 i.p.), but not piperoxan (5 mg kg^?1 i.p.), abolishes the antihypertensive effect and inhibition of stimulation-evoked pressor responses produced by M-7 (1 mg kg^?1 s.c.) in SHR. Conversely, piperoxan, but not metoclopramide, reduces the bradycardia and inhibition of stimulation-evoked tachycardia produced by M-7. Metoclopramide (30 mg kg^?1 i.p.) did not affect the cardiovascular responses elicited by intracerebroventricular administration of either clonidine (1 μg) or M-7 (3 μg). These results suggest that the antihypertensive effect of M-7 may be mediated by stimulation of presynaptic dopamine receptors on sympathetic nerves to the vasculature and is independent of the bradycardia, which is probably due to stimulation of presynaptic α₂-adrenoceptors on cardiac sympathetic nerve endings.