title: Mitochondrial Ca(2+) buffering regulates synaptic transmission between retinal amacrine cells.
abstract: 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.
authors.affiliation: Department of Biological Sciences, Louisiana State University, 202 Life Sciences Building, Baton Rouge, LA 70803, USA.
authors.forename: Kathryn
authors.initials: K
authors.lastname: Medler
authors.forename: Evanna L
authors.initials: EL
authors.lastname: Gleason
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D025042:Amacrine Cells / Q000166:cytology / Q000378:metabolism*; D000818:Animals; D001464:Barium / Q000493:pharmacokinetics; D002118:Calcium / Q000378:metabolism*; D020746:Calcium Channels, L-Type / Q000502:physiology; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone / Q000494:pharmacology; D002478:Cells, Cultured; D002642:Chick Embryo; D005071:Evoked Potentials / Q000502:physiology; D005089:Exocytosis / Q000502:physiology; D006863:Hydrogen-Ion Concentration; D007476:Ionophores / Q000494:pharmacology; D008928:Mitochondria / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000494:pharmacology; D011522:Protons; D012964:Sodium / Q000378:metabolism; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology*
chemical_list: D020746:Calcium Channels, L-Type; D007476:Ionophores; D011522:Protons; D001464:Barium; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone; D000255:Adenosine Triphosphate; D012964:Sodium; D011188:Potassium; D002118:Calcium