Zinc-induced augmentation of excitatory synaptic currents and glutamate receptor responses in hippocampal CA3 neurons.

Zinc is found throughout the CNS at synapses co-localized with glutamate in presynaptic terminals. In particular, dentate granule cells' (DGC) mossy fiber (MF) axons contain especially high concentrations of zinc co-localized with glutamate within vesicles. To study possible physiological roles of zinc, visualized slice-patch techniques were used to voltage-clamp rat CA3 pyramidal neurons, and miniature excitatory postsynaptic currents (mEPSCs) were isolated. Bath-applied zinc (200 microM) enhanced median mEPSC peak amplitudes to 153.0% of controls, without affecting mEPSC kinetics. To characterize this augmentation further, rapid agonist application was performed on perisomatic outside-out patches to coapply zinc with glutamate extremely rapidly for brief (1 ms) durations, thereby emulating release kinetics of these substances at excitatory synapses. When zinc was coapplied with glutamate, zinc augmented peak glutamate currents (mean +/- SE, 116.6 +/- 2.8% and 143.8 +/- 9.8% of controls at 50 and 200 microM zinc, respectively). This zinc-induced potentiation was concentration dependent, and pharmacological isolation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated currents (AMPAR currents) gave results similar to those observed with glutamate application (mean, 115.0 +/- 5.4% and 132.5 +/- 9.1% of controls at 50 and 200 microM zinc, respectively). Inclusion of the AMPAR desensitization blocker cyclothiazide in the control solution, however, abolished zinc-induced augmentation of glutamate-evoked currents, suggesting that zinc may potentiate AMPAR currents by inhibiting AMPAR desensitization. Based on the results of the present study, we hypothesize that zinc is a powerful modulator of both excitatory synaptic transmission and glutamate-evoked currents at physiologically relevant concentrations. This modulatory role played by zinc may be a significant factor in enhancing excitatory neurotransmission and could significantly regulate function at the mossy fiber-CA3 synapse.