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 sigma 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 sigma 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 sigma-1/sigma-2 receptor agonists, 1,3-di-o-tolyl-guanidine (100 mu M) and opipramol (10 mu M), reversibly decreased acid- induced elevations in [Ca2+](i) and membrane currents. Pharmacological experiments using sigma receptor-subtype-specific agonists demonstrated that sigma-1, but not sigma-2, receptors inhibit ASIC1a-induced Ca2+ elevations. These results were confirmed using the irreversible sigma receptor antagonist metaphit (50 mu M) and the selective sigma-1 antagonist BD1063 (10 nM), which obtunded the inhibitory effects of the sigma-1 agonist, carbetapentane. Activation of ASIC1a was shown to stimulate downstream Ca2+ influx pathways, specifically N-methyl-D-aspartate and (+/-)-alpha-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 sigma-1 receptors. These findings demonstrate that sigma-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 sigma receptors an attractive target for ischemic stroke therapy.