The Journal of Neurophysiology Vol. 87 No. 3 March 2002, pp. 1426-1439
Copyright ©2002 by the American Physiological Society

Mitochondrial Ca²⁺ Buffering Regulates Synaptic Transmission Between Retinal Amacrine Cells

Department of Biological Sciences, Louisiana State University, Baton Rouge, Louisiana 70803

Medler, Kathryn and Evanna L. Gleason. Mitochondrial Ca²⁺ Buffering Regulates Synaptic Transmission Between Retinal Amacrine Cells. J. Neurophysiol. 87: 1426-1439, 2002. The diverse functions of retinal amacrine cells are reliant on the physiological properties of their synapses. Here we examine the role of mitochondria as Ca²⁺ 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 Ca²⁺ uniporter. Measurements of cytosolic Ca²⁺ levels reveal that prolonged depolarization-induced Ca²⁺ elevations measured at the cell body are altered by inhibition of mitochondrial Ca²⁺ uptake. Furthermore, an analysis of the ratio of Ca²⁺ efflux on the plasma membrane Na-Ca exchanger to influx through Ca²⁺ channels during voltage steps indicates that mitochondria can also buffer Ca²⁺ loads induced by relatively brief stimuli. Importantly, we also demonstrate that mitochondrial Ca²⁺ uptake operates at rest to help maintain low cytosolic Ca²⁺ levels. This aspect of mitochondrial Ca²⁺ buffering suggests that in amacrine cells, the normal function of Ca²⁺-dependent mechanisms would be contingent upon ongoing mitochondrial Ca²⁺ uptake. To test the role of mitochondrial Ca²⁺ buffering at amacrine cell synapses, we record from amacrine cells receiving GABAergic synaptic input. The Ca²⁺ elevations produced by inhibition of mitochondrial Ca²⁺ 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 Ca²⁺ 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 Ca²⁺ channels observed in the absence of mitochondrial Ca²⁺ buffering. These results demonstrate that mitochondrial Ca²⁺ buffering plays a critical role in the function of amacrine cell synapses.