ASIC-like, proton-activated currents in rat hippocampal neurons

The expression of mRNA for acid sensing ion channels (ASIC) subunits ASIC1a, ASIC2a and ASIC2b has been reported in hippocampal neurons, but the presence of functional hippocampal ASIC channels was never assessed. We report here the first characterization of ASIC-like currents in rat hippocampal neurons in primary culture. An extracellular pH drop induces a transient Na+ current followed by a sustained non-selective cation current. This current is highly sensitive to pH with an activation threshold around PH 6.9 and a pH(0.5) of 6.2. About half of the total peak current is inhibited by the spider toxin PcTX1, which is specific for homomeric ASIC1a channels. The remaining PcTX1-resistant ASIC-like current is increased by 300 muM Zn2+ and, whereas not fully activated at PH 5, it shows a pH(0.5) of 6.0 between pH 7.4 and 5. We have previously shown that Zn2+ is a co-activator of ASIC2a-containing channels. Thus, the hippocampal transient ASIC-like current appears to be generated by a mixture of homomeric ASIC1a channels and ASIC2a-containing channels, probably heteromeric ASICIa+2a channels. The sustained non-selective current suggests the involvement of ASIC2b-containing heteromeric channels. Activation of the hippocampal ASIC-like current by a PH drop to 6.9 or 6.6 induces a transient depolarization which itself triggers an initial action potential (AP) followed by a sustained depolarization and trains of APs. Zn2+ increases the acid sensitivity of ASIC channels, and consequently neuronal excitability. It is probably an important co-activator of ASIC channels in the central nervous system.