Functional Characterization Of Acid-sensing Ion Channels In Neurons Of Rat Inferior Colliculus

Acid-sensing ion channels (ASICs), which are widely distributed in the mammalian brain, the spinal cord and the peripheral sensory organs, are ligand-gated cation channels activated by extracellular protons. Abundant experimental evidence shows that ASICs play important roles in physiological/pathological conditions, such as sensory transduction, learning/memory, retinal function, seizure and ischemia. In the auditory system, however, there are only a few studies available describing ASICs in hair cells, the spiral ganglion and the vestibular ganglion. In particular, functional ASICs have not been assessed in the central auditory region, although there is evidence to show their transcription in the inferior colliculus (IC). In the present study, we have studied the electrophysiological and pharmacological properties of acid-induced current in cultured IC neurons, identified the possible ASIC subunits mediating the current.1. Reverse transcriptase polymerase chain reaction (RT-PCR) experiment demonstrates that four ASIC isoforms, ASIC1a, ASIC1b, ASIC2a and ASIC2b are present in the rat IC, while ASIC3 is absent.2. Our electrophysiological data showed that a rapidly decaying inward current was induced by exogenous application of acidic solution in cultured IC neurons with a response threshold around pH 6.9 and a half activation pH value at 5.92. The current was sensitive to amiloride (IC₅₀ = 20.4±0.4μM), an ASIC blocker, and its reversal potential was close to the theoretical Na⁺ equilibrium potential, indicating that the recorded current was mediated by ASICs.3. Further experiments revealed the presence of homomeric ASIC1a channels in IC neurons: (1) the ASIC-like current was partially carried by Ca²⁺ as demonstrated with an ion-substitution protocol and Ca²⁺ imaging; (2) the current was inhibited by the tarantula venom PcTX1, a specific blocker for homomeric ASIC1a channels; (3) the current could be inhibited by extracellular Ca²⁺ (IC₅₀= 2.31 mM) and Pb²⁺ (10μM), confirming the presence of ASIC1a subunit. The presence of functional ASIC2a containing channels was revealed by the Zn²⁺(300μM)-induced enhancement of ASIC-like currents and the absence of functional AS1C3 channels was indicated by the insensitivity of ASIC-like currents to salicylate (1mM), an ASIC3 subunit blocker.4. Finally, we show that activation of ASICs by a pH drop could induce membrane depolarization and evoke neuronal firing in IC neurons. Our study clearly demonstrates that functional homomeric ASIC1a channels and ASIC2a-containing channels, but not ASIC3 channels, are present in the IC. We suggest that ASICs should be taken into consideration for their possible functional roles in information processing and pathological processes in the central auditory system.