Mitochondrial Ca(2+) buffering regulates synaptic transmission between retinal amacrine cells

The diverse functions of retinal amacrine cells are reliant on the physiological properties of their synapses. Here we examine the role of mitochondria as Ca(2+) buffering organelles in synaptic transmission between GABAergic amacrine cells. We used the protonophore p-trifluoromethoxy-phenylhydrazone (FCCP) to dissipate the membrane potential across the inner mitochondrial membrane that normally sustains the activity of the mitochondrial Ca(2+) uniporter. Measurements of cytosolic Ca(2+) levels reveal that prolonged depolarization-induced Ca(2+) elevations measured at the cell body are altered by inhibition of mitochondrial Ca(2+) uptake. Furthermore, an analysis of the ratio of Ca(2+) efflux on the plasma membrane Na-Ca exchanger to influx through Ca(2+) channels during voltage steps indicates that mitochondria can also buffer Ca(2+) loads induced by relatively brief stimuli. Importantly, we also demonstrate that mitochondrial Ca(2+) uptake operates at rest to help maintain low cytosolic Ca(2+) levels. This aspect of mitochondrial Ca(2+) buffering suggests that in amacrine cells, the normal function of Ca(2+)-dependent mechanisms would be contingent upon ongoing mitochondrial Ca(2+) uptake. To test the role of mitochondrial Ca(2+) buffering at amacrine cell synapses, we record from amacrine cells receiving GABAergic synaptic input. The Ca(2+) elevations produced by inhibition of mitochondrial Ca(2+) uptake are localized and sufficient in magnitude to stimulate exocytosis, indicating that mitochondria help to maintain low levels of exocytosis at rest. However, we found that inhibition of mitochondrial Ca(2+) uptake during evoked synaptic transmission results in a reduction in the charge transferred at the synapse. Recordings from isolated amacrine cells reveal that this is most likely due to the increase in the inactivation of presynaptic Ca(2+) channels observed in the absence of mitochondrial Ca(2+) buffering. These results demonstrate that mitochondrial Ca(2+) buffering plays a critical role in the function of amacrine cell synapses.