Investigation of acid sensing ion channels (ASIC) of stellate and bushy neurons in the cochlear nucleus

Acid sensing ion channels (ASICs) are voltage–independent and proton–gated channels commonly found in the both central and peripheral nervous systems. In this study, it was aimed to test the hypothesis that whether ASICs could have important roles in processing of auditory signals by affecting excitability of cells in the cochlear nucleus. In this study, ASIC gene expressions in the cochlear nuclei of albino BALB/mice were determined in terms of mRNA levels by using real time polymerase chain reaction and ASIC currents in stellate and bushy cells were characterized by using path clamp technique. ASIC1, ASIC2 and ASIC3 gene expressions were detected in 16 day-old, 75 day-old and 2 year-old mice. In the voltage-clamp experiments, inward currents were recorded upon application of acidic solution to the cochlear nucleus cells, whose half–maximum activation pH values (pH50) were 5,84 and 5,81 for stellate and bushy cells respectively. Amiloride, which is ASICs blockers, inhibited the acid induced-currents in a dose–dependent manner. Half–maximal inhibition concentration (IC50) of amiloride was 32,5 ± 0,4 µM (n=30) for stellates and 32,1 ± 0,2 µM (n=12) for bushy cells. ASIC current was significantly depressed (%87,5, p<0.05) (n=13) in sodium free solution, which indicates that the ASIC current is primarily carried by Na+. Inhibition of the ASIC currents by extracellular Ca2+ and Pb2+ (10 μM) was a significant evidence for the existence of homomeric ASIC1a subunits. ASIC currents were increased by 20 % upon extracellular application of Zn2+ (300 μM) (p<0.05, n=13), which indicates the presence of functional homomeric ASIC2a subunits. Sodium salicylate (1 mM) did not significantly affect the amplitudes of ASIC currents (p>0.05 n= 13), which indicates that there are no ASIC3 channels. In current–clamp experiments, application of acidic extracellular solution resulted in depolarization of stellate and bushy cells of the cochlear nucleus. The results shows that the ASIC currents in the cochlear nucleus cells is carried by the ASIC1a and ASIC2a channels. ASIC channels affects excitability of the stellate and bushy cells and therefore they likely to have a role in processing of auditory information and / or pathological processes in the central auditory system.