Innervation of the cerebral vasculature

With the development of specific antibodies to vasoactive peptides and application of immunohistochemistry and radioimmunoassay methods, knowledge of vascular innervation has grown rapidly. In the cerebral circulation, four possible neurotransmitters are present: norepinephrine, acetylcholine, vasoactive intestinal peptide (VIP), and substance P. There is a dense adrenergic innervation of cerebral arteries, but contractile responses to nerve stimulation or circulating catecholamines are relatively small both in vitro and in vivo. Recent studies using radioligand binding techniques indicate a lack of specific 3H-prazosin binding in cerebral arteries, in contrast to other vascular beds. Thus a lack of alpha1-adrenergic receptors in cerebral arteries may account for weak responsiveness to sympathetic stimulation. Both VIP and acetylcholine may be vasodilator neurotransmitters, but blockade of cholinergic responses does not alter neurogenic vasodilation. The lack of specific VIP antagonists hampers efforts to explore this system more fully. Substance P-containing nerves are affected by capsaicin, supporting the hypothesis that these are primary sensory afferents, perhaps mediating pain. Future work in this area may focus on defining the pathways of these nerves and exploring the role of co-transmitters and possible interactions between nerves. With this basic information, experiments can be designed to elucidate more clearly the functional roles these nerves play.