title: Accumulation and extrusion of permeant Ca2+ chelators in attenuation of synaptic transmission at hippocampal CA1 neurons.
abstract: The effects of extracellularly applied membrane-permeant Ca2+ chelators on field excitatory postsynaptic potentials were determined in the hippocampal CA1 region of rat brain slices. Field excitatory postsynaptic potentials in slices perfused with 0.05-50 microM bis-(-O-aminophenoxy)-ethane-N,N,N,N,-tetraacetic acid acetoxymethyl (BAPTA-AM) for 15 min were reversibly attenuated by 10-45% in a concentration-dependent manner. Attenuation occurred earlier at higher concentrations of BAPTA-AM, thus indicating that the rate of accumulation of BAPTA salt was concentration dependent. Antidromically evoked responses and presynaptic volleys were unaffected by BAPTA-AM. Attenuation of the field excitatory postsynaptic potentials by BAPTA-AM was temporarily eliminated by repetitive stimulation at 1 Hz, suggesting saturation of the chelator's Ca(2+)-binding capacity. The amplitude of field excitatory postsynaptic potentials was unaffected by similar applications of 5'5-dinitro-BAPTA-AM, a low Ca(2+)-affinity BAPTA analogue, and EGTA-AM (5 or 50 microM), a chelator with slow Ca(2+)-binding kinetics, suggesting a dependence of the BAPTA-AM effect on fast Ca2+ binding and high Ca2+ affinity. BAPTA-AM concentrations as low as 0.05 microM were effective provided application was prolonged to 40 min. Probenecid (1 mM), an anion transport inhibitor, accelerated the onset and significantly enhanced the BAPTA-mediated synaptic attenuation caused by low concentrations of BAPTA-AM. These data show that even very low extracellular concentrations of BAPTA-AM can profoundly affect synaptic transmission provided that sufficient chelator accumulates presynaptically. The effectiveness of BAPTA-AM can be increased by procedures which inhibit chelator extrusion.
authors.affiliation: Playfair Neuroscience Unit, Toronto Hospital Research Institute, University of Toronto, Ontario, Canada.
authors.forename: A
authors.initials: A
authors.lastname: Ouanounou
authors.forename: L
authors.initials: L
authors.lastname: Zhang
authors.forename: M
authors.initials: M
authors.lastname: Tymianski
authors.forename: M P
authors.initials: MP
authors.lastname: Charlton
authors.forename: M C
authors.initials: MC
authors.lastname: Wallace
authors.forename: P L
authors.initials: PL
authors.lastname: Carlen
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002118:Calcium / Q000378:metabolism*; D002463:Cell Membrane Permeability; D002614:Chelating Agents / Q000378:metabolism* / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D006624:Hippocampus / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism; D007700:Kinetics; D008297:Male; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D011339:Probenecid / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology*
chemical_list: C104695:5,5'-dinitro-1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid acetoxymethyl ester; D002614:Chelating Agents; C070379:1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid acetoxymethyl ester; D004533:Egtazic Acid; C046770:EGTA acetoxymethyl ester; D011339:Probenecid; D002118:Calcium