1 The presynaptic action of kainate (KA) receptor activation at the mossy fibre‐CA3 synapse was examined using fluorescence measurement of presynaptic Ca2+ influx as well as electrophysiological recordings in mouse hippocampal slices. 2 Bath application of a low concentration (0·2 μM) of KA reversibly increased the amplitude of presynaptic volley evoked by stimulation of mossy fibres to 146 ± 6 % of control (n= 6), whereas it reduced the field excitatory postsynaptic potential (EPSPs) to 30 ± 4 %. 3 The potentiating effect of KA on the presynaptic volleys was also observed in Ca2+‐free solution, and was partly antagonized by (2S,4R)‐4‐methylglutamic acid (SYM 2081, 1 μM), which selectively desensitizes KA receptors. 4 The antidromic population spike of dentate granule cells evoked by stimulation of mossy fibres was increased by application of 0·2 μM KA to 160 ± 10 % of control (n= 6). Whole‐cell current‐clamp recordings revealed that the stimulus threshold for generating antidromic spikes recorded from a single granule cell was lowered by KA application. 5 Application of KA (0·2 μM) suppressed presynaptic Ca2+ influx to 78 ± 4 % of control (n= 6), whereas the amplitude of the presynaptic volley was increased. 6 KA at 0·2 μM reversibly suppressed excitatory postsynaptic currents (EPSCs) evoked by mossy fibre simulation to 38 ± 9 % of control (n= 5). 7 These results suggest that KA receptor activation enhances the excitability of mossy fibres, probably via axonal depolarization, and reduces action potential‐induced Ca2+ influx, thereby inhibiting mossy fibre EPSCs presynaptically. This novel presynaptic inhibitory action of KA at the mossy fibre‐CA3 synapse may regulate the excitability of highly interconnected CA3 networks.
The Journal of Physiology – Wiley
Published: Mar 1, 2000
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