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Nitric oxide (NO) is a highly versatile gasotransmitter that has first been shown to regulate cardiovascular function and then to exert tight control over a much broader range of processes, including neurotransmitter release, neuronal excitability, and synaptic plasticity. Endothelial NO synthase (eNOS) is usually far from the mind of synaptic neurophysiologists, who have focused most of their attention on neuronal NO synthase (nNOS) as the primary source of NO at the neurovascular unit (NVU). Nevertheless, the available evidence suggests that eNOS could also contribute to generating the burst of NO that, serving as volume intercellular messenger, is produced in response to neuronal activity in the brain parenchyma. Herein, we review the role of eNOS in both the regulation of cerebral blood flow and of synaptic plasticity and discuss the mechanisms by which cerebrovascular endothelial cells may transduce synaptic inputs into a NO signal. We further suggest that eNOS could play a critical role in vascular-to-neuronal communication by integrating signals converging onto cerebrovascular endothelial cells from both the streaming blood and active neurons. [ABSTRACT FROM AUTHOR]

Basal forebrain neurons control cerebral blood flow (CBF) by releasing acetylcholine (Ach), which binds to endothelial muscarinic receptors to induce nitric (NO) release and vasodilation in intraparenchymal arterioles. Nevertheless, the mechanism whereby Ach stimulates human brain microvascular endothelial cells to produce NO is still unknown. Herein, we sought to assess whether Ach stimulates NO production in a Ca2+‐dependent manner in hCMEC/D3 cells, a widespread model of human brain microvascular endothelial cells. Ach induced a dose‐dependent increase in intracellular Ca2+ concentration ([Ca2+]i) that was prevented by the genetic blockade of M5 muscarinic receptors (M5‐mAchRs), which was the only mAchR isoform coupled to phospholipase Cβ (PLCβ) present in hCMEC/D3 cells. A comprehensive real‐time polymerase chain reaction analysis revealed the expression of the transcripts encoding for type 3 inositol‐1,4,5‐trisphosphate receptors (InsP3R3), two‐pore channels 1 and 2 (TPC1–2), Stim2, Orai1–3. Pharmacological manipulation showed that the Ca2+ response to Ach was mediated by InsP3R3, TPC1–2, and store‐operated Ca2+ entry (SOCE). Ach‐induced NO release, in turn, was inhibited in cells deficient of M5‐mAchRs. Likewise, Ach failed to increase NO levels in the presence of l‐NAME, a selective NOS inhibitor, or BAPTA, a membrane‐permeant intracellular Ca2+ buffer. Moreover, the pharmacological blockade of the Ca2+ response to Ach also inhibited the accompanying NO production. These data demonstrate for the first time that synaptically released Ach may trigger NO release in human brain microvascular endothelial cells by stimulating a Ca2+ signal via M5‐mAchRs. Acetylcholine induces Ca2+‐dependent nitric oxide release in human brain microvascular endothelial cells by binding to M5 muscarinic receptors. This finding sheds novel light on the mechanism by which acetylcholine triggers neurovascular coupling in the brain. [ABSTRACT FROM AUTHOR]

The neurotransmitter glutamate increases cerebral blood flow by activating postsynaptic neurons and presynaptic glial cells within the neurovascular unit. Glutamate does so by causing an increase in intracellular Ca2+ concentration ([Ca2+]i) in the target cells, which activates the Ca2+/Calmodulin‐dependent nitric oxide (NO) synthase to release NO. It is unclear whether brain endothelial cells also sense glutamate through an elevation in [Ca2+]i and NO production. The current study assessed whether and how glutamate drives Ca2+‐dependent NO release in bEND5 cells, an established model of brain endothelial cells. We found that glutamate induced a dose‐dependent oscillatory increase in [Ca2+]i, which was maximally activated at 200 μM and inhibited by α‐methyl‐4‐carboxyphenylglycine, a selective blocker of Group 1 metabotropic glutamate receptors. Glutamate‐induced intracellular Ca2+ oscillations were triggered by rhythmic endogenous Ca2+ mobilization and maintained over time by extracellular Ca2+ entry. Pharmacological manipulation revealed that glutamate‐induced endogenous Ca2+ release was mediated by InsP3‐sensitive receptors and nicotinic acid adenine dinucleotide phosphate (NAADP) gated two‐pore channel 1. Constitutive store‐operated Ca2+ entry mediated Ca2+ entry during ongoing Ca2+ oscillations. Finally, glutamate evoked a robust, although delayed increase in NO levels, which was blocked by pharmacologically inhibition of the accompanying intracellular Ca2+ signals. Of note, glutamate induced Ca2+‐dependent NO release also in hCMEC/D3 cells, an established model of human brain microvascular endothelial cells. This investigation demonstrates for the first time that metabotropic glutamate‐induced intracellular Ca2+ oscillations and NO release have the potential to impact on neurovascular coupling in the brain. Glutamate induces intracellular Ca2+ oscillations in mouse brain microvascular endothelial cells. Glutamate‐induced intracellular Ca2+ oscillations lead to robust, although delayed, nitric oxide release. Brain microvascular endothelial cells could play a crucial role in neurovascular coupling during synaptic activity. [ABSTRACT FROM AUTHOR]