Effect of acetylcholine and vasoactive intestinal peptide on cerebral blood flow

These experiments were performed to determine whether the neurotransmitters acetylcholine (ACh) and vasoactive intestinal peptide (VIP) increase cerebral blood flow (CBF). In anesthetized dogs, infusion of very large doses of ACh (500 microgram/min) into the carotid artery increased CBF (measured with microspheres) 50%. Anatomical limitations prevented adequate testing of cerebrovascular responses in dogs. In anesthetized rabbits, infusion of ACh (10 microgram/min) into the carotid artery produced a twofold increase in CBF. Vessels in gray matter were more responsive than vessels in white matter. ACh did not alter cerebral oxygen consumption, which indicates that the dilator effect of the drug is not an indirect effect mediated through an increase in metabolism. The vasodilator response to ACh was attenuated by atropine (1 mg iv). Intracarotid infusion of VIP in rabbits increased blood flow to cerebral gray matter: infusion of VIP (10 microgram/min) incresed flow 33%. We conclude that 1) cerebral vasodilator responses to ACh and VIP are larger than previously recognized and 2) responses to these drugs occur primarily in cerebral gray matter. Because the neurotransmitters increase CBF significantly, these studies should encourage further studies concerning effects of cholinergic and peptidergic nerves on cerebral vessels.