Endogenous Acidification of Central Inhibitory Synapses

Abstract

In the brain, extracellular pH is rigidly maintained to ensure proper CNS function. To assess pH fluctuation at central synapses, we recorded miniature GABA inhibitory post-synaptic currents (mIPSCs) from cultured cerebellar granule cells under varying pH and proton buffering conditions. We found an inverse relationship between extracellular pH and mIPSC amplitude and charge transfer, resulting in over a 100% increase in size of events recorded at pH6.8 vs. pH8.0. Acidification also slowed the kinetics of rise time and fast component of decay, while speeding the slow decay component. We find that lowering the pH buffering capacity of the extracellular solution from 24 to 3mM HEPES at pH7.4, results in a similar enhancement of mIPSC size, mimicking changes in kinetics induced by acidification. The effects of diminished buffering capacity on mIPSC were negated by lowering extracellular pH to 6.8. To probe these effects with physiological buffers, we measured mIPSCs using 24mM of bicarbonate and compared them with those recorded in 24mM bicarbonate supplemented with 10mM HEPES . We found that physiological concentrations of bicarbonate produced mIPSCs that were similar in size and kinetics to those found with 3mM HEPES and were similarly altered with addition of HEPES, confirming the physiological relevance of our findings. To determine the possible contribution of Na+/H+ exchanger to synaptic acidification we inhibited the exchanger with amiloride (20μM), and in a parallel set of experiments replaced extracellular sodium with lithium. Both of these treatments caused changes in mIPSCs that mirrored increased buffering capacity, and the effects were negated by acidification to pH6.8 or by increasing HEPES buffering capacity to 24mM. We conclude that GABAergic synaptic pH in vivo may be quite labile and subject to rapid and pronounced acidification from the Na+/H+ exchanger with the net effect of enhancing synaptic transmission.