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Glutamate and its synthetic analogues N-methyl-d-aspartate (NMDA), kainate, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) are potent dilator agents in the cerebral circulation. The close linkage between neural activity-based release and actions of glutamate on neurons and the related decrease in cerebral vascular resistance is a classic example in support of the concept of tight coupling between increased neural activity and cerebral blood flow. However, mechanisms involved in promoting cerebral vasodilator responses to glutamatergic agents are controversial. Here we review the development and current status of this important field of research especially in respect to cerebrovascular responses to NMDA receptor activation.

Objective Mitochondrial depolarization following ATP-sensitive potassium (mitoKATP) channel activation has been shown to induce cerebral vasodilation by the generation of ‘calcium sparks’ in smooth muscle. It is unclear, however, if mitochondrial depolarization in endothelial cells is capable of promoting vasodilation by releasing vasoactive factors. Therefore, we studied the effect of endothelial mitochondrial depolarization by mitoKATP channel activators, BMS-191095 (BMS) and diazoxide, on endothelium-dependent vasodilation. Methods and Results Diameter studies in isolated rat cerebral arteries showed BMS and diazoxide induced vasodilations that were diminished by endothelial denudation. Mitochondrial depolarization-induced vasodilation was reduced by inhibition of mitoKATP channels, phosphoinositide-3 kinase (PI3K) or nitric oxide synthase (NOS). Scavenging of reactive oxygen species (ROS), however, diminished vasodilation induced by diazoxide but not by BMS. Fluorescence studies in cultured rat brain microvascular endothelial cells (CMVECs) showed that BMS elicited mitochondrial depolarization, and enhanced nitric oxide (NO) production; diazoxide exhibited largely similar effects, but unlike BMS, increased mitochondrial ROS production. Measurements of intracellular calcium ([Ca2+]i) in CMVECs and arteries showed that both diazoxide and BMS increased endothelial [Ca2+]i. Western blot analyses revealed increased phosphorylation of Akt and endothelial NOS (eNOS) by BMS and diazoxide. Increased phosphorylation of eNOS by diazoxide was abolished by PI3K inhibition. Electron spin resonance spectroscopy confirmed vascular NO generation in response to diazoxide and BMS. Conclusions Pharmacological depolarization of endothelial mitochondria promotes activation of eNOS by dual pathways involving increased [Ca2+]i as well as by PI3K-Akt-induced eNOS phosphorylation. Both mitochondrial ROS-dependent and –independent mechanisms mediate activation of eNOS by endothelial mitochondrial depolarization.

Cortical spreading depression (CSD) leads to dramatic changes in cerebral hemodynamics. However, mechanisms involved in promoting and counteracting cerebral vasodilator responses are unclear.Here we review the development and current status of this important field of research especially with respect to the role of perivascular nerves and nitric oxide (NO). It appears that neurotransmitters released from the sensory and the parasympathetic nerves associated with cerebral arteries, and NO released from perivascular nerves and/or parenchyma, promote cerebral hyperemia during CSD. However, the relative contributions of each of these factors vary according to species studied. Related to CSD, axonal and reflex responses involving trigeminal afferents on the pial surface lead to increased blood flow and inflammation of the overlying dura mater. Counteracting the cerebral vascular dilation is the production and release of constrictor prostaglandins, at least in some species, and other possibly yet unknown agents from the vascular wall. The cerebral blood flow response in healthy human cortex has not been determined, and thus it is unclear whether the cerebral oligemia associated with migraines represents the normal physiological response to a CSD-like event or represents a pathological response. In addition to promoting cerebral hyperemia, NO produced during CSD appears to initiate signaling events which lead to protection of the brain against subsequent ischemic insults. In summary, the cerebrovascular response to CSD involves multiple dilator and constrictor factors produced and released by diverse cells within the neurovascular unit, with the contribution of each of these factors varying according to the species examined.