Sympathetic Neural Influence on Norepinephrine Vasoconstriction in Brain Vessels

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Abstract
The mass effect of norepinephrine on the brain circulation was studied using changes in cerebral blood volume (CBV) of mice as the experimental model. Tyramine (which releases norepinephrine from the rich plexus of perivascular adrenergic nerves) significantly reduced CBV in a dose-dependent manner. This vasoconstriction was abolished by previous cranial sympathectomy. Administration of levarterenol (into the tail vein) was ineffective unless the animal was subjected to cranial sympathectomy at least 24 hours before the injection, ie, when a sufficient degree of denervation supersensitivity ensued. The results imply that a norepinephrineinduced vasoconstriction in the circulation of the brain depends on the quantitative access of the amine to the adrenergic receptor area. The vasoconstrictor response may be influenced by such features as the amount of adrenergic innervation, the types of adrenergic receptors present, and the properties of the barrier.