Acid-Sensing Ion Channels: Novel Mediators of Cerebral Vascular Responses.

Abstract

Precise regulation of cerebral blood flow is critical for normal brain function. Insufficient cerebral blood flow contributes to brain dysfunction and neurodegeneration. Carbon dioxide (CO₂), via effects on local acidosis, is one of the most potent regulators of cerebral blood flow. Although a role for nitric oxide in intermediate signaling has been implicated, mechanisms that initiate CO₂-induced vasodilation remain unclear. Acid-sensing ion channel-1A (ASIC1A) is a proton-gated cation channel that is activated by extracellular acidosis. Based on work that implicated ASIC1A in the amygdala and bed nucleus of the stria terminalis in CO₂-evoked and acid-evoked behaviors, we hypothesized that ASIC1A might also mediate microvascular responses to CO₂. To test this hypothesis, we genetically and pharmacologically manipulated ASIC1A and assessed effects on CO₂-induced dilation of cerebral arterioles in vivo. Effects of inhalation of 5% or 10% CO₂ on arteriolar diameter were greatly attenuated in mice with global deficiency in ASIC1A (<i>Asic1a</i>^-/-) or by local treatment with the ASIC inhibitor, psalmotoxin. Vasodilator effects of acetylcholine, which acts via endothelial nitric oxide synthase were unaffected, suggesting a nonvascular source of nitric oxide may be key for CO₂ responses. Thus, we tested whether neurons may be the cell type through which ASIC1A influences microvessels. Using mice in which <i>Asic1a</i> was specifically disrupted in neurons, we found effects of CO₂ on arteriolar diameter were also attenuated. Together, these data are consistent with a model wherein activation of ASIC1A, particularly in neurons, is critical for CO₂-induced nitric oxide production and vasodilation. With these findings, ASIC1A emerges as major regulator of microvascular tone.