MODULATION OF HIPPOCAMPAL SYNAPTIC TRANSMISSION BY LOW CONCENTRATIONSOF CELL-PERMEANT CA2- EFFECTS OF CA2+ AFFINITY, CHELATOR STRUCTURE AND BINDING-KINETICS( CHELATORS )

Calcium chelators are commonly used for fluorescence and electrophysio logical studies of neuronal Ca2+ signalling. Recently, they have also been used as neuroprotectants. Since they buffer calcium ions, these a gents also modify the same signals which are being studied. These prop erties may be used to modulate Ca2+ signals such as those involved in synaptic transmission, and may explain their neuroprotective mechanism . To define factors which govern the modulation of synaptic transmissi on by Ca2+ chelators, we examined their actions on synaptic responses evoked in CAI neurons of rat hippocampal slices. We used a spectrum of cell-permeant Ca2+ chelators having different structures, Ca2+-bindin g kinetics and Ca2+ affinities, as well as an impermeant, intracellula rly perfused chelator salt. Application of the cell-permeant -bis(2-am inophenoxy)ethane-N,N,N',N'-tetra-acetate acetoxymethyl ester (50 mu M ) markedly attenuated evoked synaptic responses. This application prod uced an intracellular chelator accumulation of 79-125 mu M, as estimat ed using C-14-labelled chelator. The actions of a Ca2+ chelator on syn aptic responses were dependent on the chelator's Ca2+ affinity, Ca2+-b inding rate and Ca2+ selectivity, because mino-5-nitrophenoxy)ethane-N ,N,N',N'-tetra-acetate acetoxymethyl ester (a low Ca2+ affinity analog ue), ethyleneglycolbis(beta-aminoethyl ether)-N,N,N',N'-tetra-acetate acetoxymethyl ester (a slow buffer with similar Ca2+ affinity to bis(2 -aminophenoxy)ethane-N,N,N',N'-tetra-acetate) and the selective Zn2+ c helator, tetrakis(2-pyridylmethyl)ethylenediamine were ineffective. Th e intrinsic cell membrane properties, including the post-spike train a fterhyperpolarization, were nor significantly affected by any of the C a2+ chelators used in this study. Intracellular perfusion of 100-200 m u M bis-(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetate salt through pa tch pipettes into postsynaptic cells did not affect synaptic potential s, suggesting a presynaptic action of cell-permeant Ca2+ chelators on transmitter release. Other cell-permeant, fast Ca2+-binding chelators reduced synaptic responses according to their Ca2+ affinities, and not their chemical structure: those chelators with K-d values less than o r equal to 25 mu M attenuated synaptic responses, whereas chelators of lesser affinity did not. These data support the ideas that [Ca2+](i) rises to high (micromolar) levels during transmitter release, and that Ca2+ chelators may be used to attenuate excitotoxicity by attenuating excitatory neurotransmission without affecting Ca2+ signalling in the submicromolar [Ca2+](i) range. Copyright (C) 1996 IBRO.