Endogenous Acidification of the Inhibitory Synapse: Proton Amplification of Gabaa-mediated Neurotransmission

Maintenance of external pH is critical to ensuring proper CNS function. Recent work in excitatory transmission suggests that in vivo synaptic proton buffering is not sufficient to rigidly maintain an extracellular pH of 7.4. To extend this work to inhibitory synapses, I recorded miniature GABA inhibitory post-synaptic currents (mIPSCs) from cultured rat cerebellar granule cells (CGCs) under varying pH and proton buffering conditions. Consistent with previous findings in rat hippocampal neurons, mIPSCs recorded from CGCs under rigid extracellular pH control (achieved by using 24 mM HEPES) exhibited an inverse relationship between extracellular pH and event size, such that raising pH from 6.8 to 8.0 resulted in a nearly 50% decrease in charge transfer. Kinetically, alkalinization sped both the mIPSC rise-time and fast component of decay while slowing the slow component. More intriguingly, I found that while maintaining external pH at 7.4, increasing the proton buffering capacity of the extracellular solution from 3 mM to 24 mM HEPES mimicked changes in amplitude and kinetics consistent with alkalinization. Supplementing physiological buffer concentrations (24 mM HCO3/5% CO2) with 10 mM HEPES also reduced mIPSCs in a manner consistent with alkalinization, providing