Adrenoceptors in intracerebral resistance vessels

1. The effects of tyramine and isoprenaline on hypothalamic blood flow (HBF) were measured in conscious rabbits. 2. Injections of small doses of tyramine caused an increase in HBF while larger doses caused a decrease in HBF. 3. Isoprenaline injections also produced an increase in HBF. 4. The vasodilatation induced by isoprenaline and the small dose of tyramine was blocked by propranolol. 5. The vasoconstriction induced by the larger doses of tyramine was abolished by phenoxybenzamine. 6. Chemical sympathectomy of the hypothalamus with 6-hydroxydopamine and depletion of biogenic amines by reserpine also abolished tyramine-induced vasocoonstriction. 7. These results suggest the presence of alpha- and beta-adrenoceptors in cerebral resistance vessels, and that these receptors may be activated by released (endogenous) noradrenaline.