Enhancement of glutamate release uncovers spillover-mediated transmission by N-methyl- d-aspartate receptors in the rat hippocampus

Abstract

Properties of excitatory postsynaptic currents during increased glutamate release were investigated by means of a whole-cell voltage-clamp in CA1 pyramidal neurons of rat hippocampal slices. Enhancement of transmitter release by 50 μM 4-aminopyridine or by elevated extracellular Ca 2+ (up to 5 mM) resulted in a substantial increase in the peak excitatory postsynaptic current amplitude and in the significant stimulus-dependent prolongation of the excitatory postsynaptic current decay. The stronger the stimulus, the slower the excitatory postsynaptic current decay became. The pharmacologically isolated N-methyl- d-aspartate, but not α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid component of the excitatory postsynaptic current exhibited this phenomenon. The possible connection of such behaviour of the N-methyl- d-aspartate component to the loss of voltage control was tested in the following way: the peak of the N-methyl- d-aspartate component was enhanced under 50 μM 4-aminopyridine and then returned back to the control level by a low dose of d-2-amino-5-phosphonopentanoic acid. However, the decay of the decreased N-methyl- d-aspartate component remained slow suggesting another origin of the stimulus-dependent kinetics. Dihydrokainate, a non-competitive inhibitor of glutamate uptake, did not influence the kinetics of the N-methyl- d-aspartate component in control but induced its dramatic stimulus-dependent prolongation when applied on the background of a low dose of 4-aminopyridine (10 μM) which did not affect the decay by itself. We propose that the delayed stimulus-dependent kinetics of the N-methyl- d-aspartate component is due to the saturation of uptake mechanisms and subsequent activation of extrasynaptic N-methyl- d-aspartate receptors. Our present observations therefore support the hypothesis that N-methyl- d-aspartate receptors may play a role in the cross-talk between synapses by means of the transmitter spillover.