Impact of Recovery from Desensitization on Acid-sensing Ion Channel-1a (ASIC1a) Current and Response to High Frequency Stimulation

ASIC1a is a neuronal sodium channel activated by external H+ ions. To date, all the characterization of ASIC1a has been conducted applying long H+ stimuli lasting several seconds. Such experimental protocols weaken and even silence ASIC1a currents to repetitive stimulation. In this work, we examined ASIC1a currents by methods that use rapid application and removal of H+. We found that brief H+ stimuli, < 100 ms, even if applied at high frequency, prevent desensitization thereby generate full and steady peak currents of human ASIC1a. Kinetic analysis of recovery from desensitization of hASIC1a revealed two desensitized states: short- and long-lasting with time constants of tau(Ds) <= 0.5 and tau(Dl) = 229 s, while in chicken ASIC1a the two desensitized states have similar values tau(D) 4.5 s. It is the large difference in stability of the two desensitized states that makes hASIC1a desensitization more pronounced and complex than in cASIC1a. Furthermore, recovery from desensitization was unrelated to cytosolic variations in pH, ATP, PIP2, or redox state but was dependent on the hydrophobicity of key residues in the first transmembrane segment (TM1). In conclusion, brief H+-stimuli maintain steady the magnitude of peak currents thereby the ASIC1a channel is well poised to partake in high frequency signals in the brain.