Effects of noradrenaline injected into the brainstem on reflex vasodilatation in the pinna of rabbits.
Reflex vasodilation in one ear of a rabbit following radiant heating of the opposite ear was observed by measuring the heat loss by a change in the heat flux when either noradrenaline or porcaine hydrochloride was administered locally into various diencephalic or mesencephalic loci in the brain. Hypothalamic cooling resulted in less reflex vasodilatation, while no vasodilatatory effect was observed immediately after an intravenous administration of atropin sulphate (0.5 mg). Five mug of noradrenaline injected intrahypothalamically either eliminated or reduced the development of reflex vasodilatation to the same extent as did an injection of procaine hydrochloride (0.5 mg). On the other hand, the magnitude of the reflex vasodilatation increased markedly with an injection of either procaine hydrochloride (1.5 mg) or noradrenaline (10 mug) into the reticular formation of the midbrain. Increased reflex vasodilatation was also obtained with a localized administration of procaine hydrochloride into the medial forebrain bundle of the hypothalamus. The results suggest that the noradrenaline in the preoptic/anterior hypothalamic (PO/AH) area and the midbrain reticular formation exerted an inhibitory effect on the action of the neural structure in connection with the heat dissipation system.