Acid-sensing ion channels as sensors of brain metabolic state

Acid-sensing ion channels (ASICs) are widely recognized as proton (H⁺)-gated cation channels that respond to extracellular acidification associated with pathological states such as ischemia, inflammation, and epilepsy. However, an emerging body of evidence compels a broader conceptual reframing: ASICs function as dynamic sensors of metabolic state, integrating real-time signals of neural activity, including CO₂-derived H⁺, lactate, and nano-domain pH transients, to regulate neuronal excitability and intercellular communication. Here, we review the mechanisms by which activity-dependent pH shifts arise in the brain, how lactate potentiates ASIC gating through divalent-cation chelation and direct channel modulation, and how the tripartite neuron-astrocyte-vascular unit functions as a spatially organized pH-sensing system. We discuss how astrocytes simultaneously buffer extracellular pH and express ASICs, placing them at the center of a bidirectional metabolic feedback loop, and how ASICs in cerebrovascular cells may link neural metabolic load to vascular tone. Finally, we identify key open questions, including the spatial scale of physiologically relevant pH nano-domains, the role of ASIC tachyphylaxis as a "use-history" sensor, and whether ASIC-dependent metabolic signaling shapes circuit-level dynamics during sleep-wake transitions and high-demand cognitive states. This perspective reframes ASICs not merely as damage sensors but as constitutive physiological transducers of brain metabolic activity.