Blood flow into the brain is dynamically regulated to satisfy the changing metabolic requirements of neurons, but how this is accomplished has remained unclear. Here, we demonstrate a central role for capillary endothelial cells in sensing neural activity and communicating it to upstream arterioles in the form of an electrical vasodilatory signal. We further demonstrate that this signal is initiated by extracellular potassium (K+)—a byproduct of neural activity—which activates capillary endothelial cell inward-rectifier K+ (KIR2.1) channels to produce a rapidly propagating retrograde hyperpolarization that causes upstream arteriolar dilation, increasing blood flow into the capillary bed. Our results establish brain capillaries as an active sensory web that converts changes in external K+ into rapid, ‘inside-out’ electrical signaling to direct blood flow to active brain regions.

For decades it has been known that external potassium (K+) ions are rapid and potent vasodilators that increase cerebral blood flow (CBF). Recent studies have implicated the local release of K+ from astrocytic endfeet—which encase the entirety of the parenchymal vasculature—in the dynamic regulation of local CBF during neurovascular coupling (NVC). It has been proposed that the activation of strong inward rectifier K+ (KIR) channels in the vascular wall by external K+ is a central component of these hyperemic responses; however, a number of significant gaps in our knowledge remain. Here, we explore the concept that vascular KIR channels are the major extracellular K+ sensors in the control of CBF. We propose that K+ is an ideal mediator of NVC, and discuss KIR channels as effectors that produce rapid hyperpolarization and robust vasodilation of cerebral arterioles. We provide evidence that KIR channels, of the KIR2 subtype in particular, are present in both the endothelial and smooth muscle cells of parenchymal arterioles and propose that this dual positioning of KIR2 channels increases the robustness of the vasodilation to external K+, enables the endothelium to be actively engaged in neurovascular coupling, and permits electrical signaling through the endothelial syncytium to promote upstream vasodilation to modulate CBF.

We recently reported that the inward-rectifier Kir2.1 channel in brain capillary endothelial cells (cECs) plays a major role in neurovascular coupling (NVC) by mediating a neuronal activity-dependent, propagating vasodilatory (hyperpolarizing) signal. We further demonstrated that Kir2.1 activity is suppressed by depletion of plasma membrane phosphatidylinositol 4,5-bisphosphate (PIP2). Whether cECs express depolarizing channels that intersect with Kir2.1-mediated signaling remains unknown. Here, we report that Ca2+/Na+-permeable TRPV4 (transient receptor potential vanilloid 4) channels are expressed in cECs and are tonically inhibited by PIP2. We further demonstrate that depletion of PIP2 by agonists, including putative NVC mediators, that promote PIP2 hydrolysis by signaling through Gq-protein-coupled receptors (GqPCRs) caused simultaneous disinhibition of TRPV4 channels and suppression of Kir2.1 channels. These findings collectively support the concept that GqPCR activation functions as a molecular switch to favor capillary TRPV4 activity over Kir2.1 signaling, an observation with potentially profound significance for the control of cerebral blood flow.

One hundred and twenty five years ago, Roy and Sherrington made the seminal observation that neuronal stimulation evokes an increase in cerebral blood flow.1 Since this discovery, researchers have attempted to uncover how the cells of the neurovascular unit-neurons, astrocytes, vascular smooth muscle cells, vascular endothelial cells and pericytes-coordinate their activity to control this phenomenon. Recent work has revealed that ionic fluxes through a diverse array of ion channel species allow the cells of the neurovascular unit to engage in multicellular signaling processes that dictate local hemodynamics. In this review we center our discussion on two major themes: (1) the roles of ion channels in the dynamic modulation of parenchymal arteriole smooth muscle membrane potential, which is central to the control of arteriolar diameter and therefore must be harnessed to permit changes in downstream cerebral blood flow, and (2) the striking similarities in the ion channel complements employed in astrocytic endfeet and endothelial cells, enabling dual control of smooth muscle from either side of the blood-brain barrier. We conclude with a discussion of the emerging roles of pericyte and capillary endothelial cell ion channels in neurovascular coupling, which will provide fertile ground for future breakthroughs in the field.

Brain capillaries play a critical role in sensing neural activity and translating it into dynamic changes in cerebral blood flow to serve the metabolic needs of the brain. The molecular cornerstone of this mechanism is the capillary endothelial cell inward rectifier K (Kir2.1) channel, which is activated by neuronal activity-dependent increases in external K concentration, producing a propagating hyperpolarizing electrical signal that dilates upstream arterioles. Here, we identify a key regulator of this process, demonstrating that phosphatidylinositol 4,5-bisphosphate (PIP) is an intrinsic modulator of capillary Kir2.1-mediated signaling. We further show that PIP depletion through activation of G protein-coupled receptors (GPCRs) cripples capillary-to-arteriole signal transduction in vitro and in vivo, highlighting the potential regulatory linkage between GPCR-dependent and electrical neurovascular-coupling mechanisms. These results collectively show that PIP sets the gain of capillary-initiated electrical signaling by modulating Kir2.1 channels. Endothelial PIP levels would therefore shape the extent of retrograde signaling and modulate cerebral blood flow.

Significance When neurons become active, they signal to local arterioles via intermediate glial cells, called astrocytes, to evoke dilation. This increases local blood flow and provides the oxygen and glucose necessary to support ongoing neuronal function. This process is termed neurovascular coupling. We demonstrate that chronic stress—which is a contributing factor for many diseases—impairs neurovascular coupling in the amygdala, a region involved in stressor processing. Our results further indicate that this dysfunction is due to the loss of arteriolar inwardly rectifying potassium (K + ) channel function, which makes vessels less able to respond to vasodilatory K + ions released by astrocytes during periods of increased neuronal activity. This neurovascular coupling impairment may contribute to the pathology of a range of brain disorders.

Healthy brain function depends on the finely tuned spatial and temporal delivery of blood-borne nutrients to active neurons via the vast, dense capillary network. Here, using in vivo imaging in anesthetized mice, we reveal that brain capillary endothelial cells control blood flow through a hierarchy of IP3 receptor–mediated Ca2+ events, ranging from small, subsecond protoevents, reflecting Ca2+ release through a small number of channels, to high-amplitude, sustained (up to ~1 min) compound events mediated by large clusters of channels. These frequent (~5000 events/s per microliter of cortex) Ca2+ signals are driven by neuronal activity, which engages Gq protein–coupled receptor signaling, and are enhanced by Ca2+ entry through TRPV4 channels. The resulting Ca2+-dependent synthesis of nitric oxide increases local blood flow selectively through affected capillary branches, providing a mechanism for high-resolution control of blood flow to small clusters of neurons.

Cerebral small vessel diseases (SVDs) are a central link between stroke and dementia—two comorbidities without specific treatments. Despite the emerging consensus that SVDs are initiated in the endothelium, the early mechanisms remain largely unknown. Deficits in on-demand delivery of blood to active brain regions (functional hyperemia) are early manifestations of the underlying pathogenesis. The capillary endothelial cell strong inward-rectifier K+ channel Kir2.1, which senses neuronal activity and initiates a propagating electrical signal that dilates upstream arterioles, is a cornerstone of functional hyperemia. Here, using a genetic SVD mouse model, we show that impaired functional hyperemia is caused by diminished Kir2.1 channel activity. We link Kir2.1 deactivation to depletion of phosphatidylinositol 4,5-bisphosphate (PIP2), a membrane phospholipid essential for Kir2.1 activity. Systemic injection of soluble PIP2 rapidly restored functional hyperemia in SVD mice, suggesting a possible strategy for rescuing functional hyperemia in brain disorders in which blood flow is disturbed.