Facilitation of zinc influx via AMPA/kainate receptor activation in the hippocampus

Glutamate and zinc is co-released by excitation of hippocampal mossy fibers and both concentrations are increased in the extracellular compartment. In a novel environment, however, extracellular zinc is persistently decreased in spite of the increase in extracellular glutamate. The mechanism of the decrease in extracellular zinc was studied in the present paper. In rats subjected to the novelty stress under hippocampal perfusion, the differential changes in extracellular glutamate and zinc were abolished in the presence of 1 μM tetrodotoxin (TTX), a sodium channel blocker, which reduced exploratory behavior. When the hippocampus was perfused with corticosterone (50 ng/ml), extracellular zinc was increased. These results suggest that glutamatergic neuron activation elicited by novelty stress is involved in the decrease in extracellular zinc and that glucocorticoid is not a trigger for its decrease. The differential changes in extracellular glutamate and zinc was induced by electrical stimulation to analyze the decrease in extracellular zinc; the differential changes were elicited by delivery of tetanic stimuli (100 Hz for 1 s, 5 min intervals, three times) to the hippocampus instead of the novelty stress, as reported previously. The changes elicited by tetanic stimulation were abolished in the presence of 10 μM CNQX, an AMPA/kainate receptor antagonist. In a hippocampal slice double-labeled with zinc and calcium indicators, furthermore, CNQX inhibited the increase in intracellular zinc levels in mossy fiber synapses after delivery of tetanic stimuli (100 Hz for 5 s) to dentate granule cells. The in vivo and in vitro experiments suggest that AMPA/kainate receptor activation is involved in zinc influx into hippocampal cells, followed by the decrease in extracellular zinc. It is likely that zinc influx is persistently facilitated via stress-induced glutamatergic neuron activation.