Autonomic Receptors in Human Brain Arteries

Pial and brain parenchymal arteries down to the arteriolar level are richly innervated by sympathetic nerves. The main transmitter in these nerves appears to be norepinephrine (NE). The smooth muscle membranes in pial and parenchymal vessels are equipped with a-and 13-adrenergic receptors. The effect on cerebrovascular tone of NE depends on which segment along the vascular tree is studied, and whether the monoamine reaches the vessel wall from the adventitial side (owing to release from the sympathetic nerves and, during pathological conditions, possibly also from NE neurons in the brain parenchyma) or from the intimai side (i.e., via the blood stream). During physiological conditions, the morphological and enzymatic blood—brain barrier (BBB) effectively prevents circulating catecholamines from reaching beyond the endothelial lining and thus to the outer layer of the smooth musculature, where the majority of receptors for catecholamines are present.