σ-1 Receptor modulation of acid-sensing ion channel a (ASIC1a) and ASIC1a-induced Ca²⁺ influx in rat cortical neurons

Acid-sensing ion channels (ASICs) are proton-gated cation channels found in peripheral and central nervous system neurons. The ASIC1a subtype, which has high Ca²⁺ permeability, is activated by ischemia-induced acidosis and contributes to the neuronal loss that accompanies ischemic stroke. Our laboratory has shown that activation of σ receptors depresses ion channel activity and [Ca²⁺]_i dysregulation during ischemia, which enhances neuronal survival. Whole-cell patch-clamp electrophysiology and fluorometric Ca²⁺ imaging were used to determine whether σ receptors regulate the function of ASIC in cultured rat cortical neurons. Bath application of the selective ASIC1a blocker, psalmotoxin1, decreased proton-evoked [Ca²⁺]_i transients and peak membrane currents, suggesting the presence of homomeric ASIC1a channels. The pan-selective σ-1/σ-2 receptor agonists, 1,3-di-o-tolyl-guanidine (100 μM) and opipramol (10 μM), reversibly decreased acid-induced elevations in [Ca²⁺]_i and membrane currents. Pharmacological experiments using σ receptor-subtype-specific agonists demonstrated that σ-1, but not σ-2, receptors inhibit ASIC1a-induced Ca²⁺ elevations. These results were confirmed using the irreversible σ receptor antagonist metaphit (50 μM) and the selective σ-1 antagonist BD1063 (10 nM), which obtunded the inhibitory effects of the σ-1 agonist, carbetapentane. Activation of ASIC1a was shown to stimulate downstream Ca²⁺ influx pathways, specifically N-methyl-D-aspartate and (±)-α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid/kainate receptors and voltage-gated Ca²⁺ channels. These subsequent Ca²⁺ influxes were also inhibited upon activation of σ-1 receptors. These findings demonstrate that σ-1 receptor stimulation inhibits ASIC1a-mediated membrane currents and consequent intracellular Ca²⁺ accumulation. The ability to control ionic imbalances and Ca²⁺ dysregulation evoked by ASIC1a activation makes σ receptors an attractive target for ischemic stroke therapy.