ACCUMULATION AND EXTRUSION OF PERMEANT CA2+ CHELATORS IN ATTENUATION OF SYNAPTIC TRANSMISSION AT HIPPOCAMPAL CA1 NEURONS

The effects of extracellularly applied membrane-permeant Ca2+ chelator s on field excitatory postsynaptic potentials were determined in the h ippocampal CA1 region of rat brain slices. Field excitatory postsynapt ic potentials in slices perfused with 0.05-50 mu M bis-(-O-aminophenox y)-ethane N,N,N,N,-tetraacetic acid acetoxymethyl (BAPTA-AM) for 15 mi n were reversibly attenuated by 10-45% in a concentration-dependent ma nner. Attenuation occurred earlier al higher concentrations of BAPTA-A M, thus indicating that the rate of accumulation of BAPTA salt was con centration dependent. Antidromically evoked responses and presynaptic volleys were unaffected by BAPTA-AM. Attenuation of the field excitato ry postsynaptic potentials by BAPTA-AM was temporarily eliminated by r epetitive stimulation al 1 Hz, suggesting saturation of the chelator's Ca2+-binding capacity. The amplitude of field excitatory postsynaptic potentials was unaffected by similar applications of 5'5-dinitro-BAPT A-AM, a low Ca2+-affinity BAPTA analogue, and EGTA-AM (5 or 50 mu M), a chelator with slow Ca2+-binding kinetics, suggesting a dependence of the BAPTA-AM effect on fast Ca2+ binding and high Ca2+ affinity. BAPT A-AM concentrations as low as 0.05 mu M were effective provided applic ation was prolonged to 40 min. Probenecid (1 mM), an anion transport i nhibitor, accelerated the onset and significantly enhanced the BAPTA-m ediated synaptic attenuation caused by low concentrations of BAPTA-AM. These data show that even very low extracellular concentrations of BA PTA-AM can profoundly affect synaptic transmission provided that suffi cient chelator accumulates presynaptically. The effectiveness of BAPTA -AM can be increased by procedures which inhibit chelator extrusion. C opyright (C) 1996 IBRO.