Rapid translocation of Zn2+ from presynaptic terminals into postsynaptic hippocampal neurons after physiological stimulation

Zn2+ is found in glutamatergic nerve terminals throughout the mammalian forebrain and has diverse extracellular and intracellular actions. The anatomical location and possible synaptic signaling role for this cation have led to the hypothesis that Zn2+ is released from presynaptic boutons, traverses the synaptic cleft, and enters postsynaptic neurons. However, these events have not been directly observed or characterized. Here we show, using microfluorescence imaging in rat hippocampal slices, that brief trains of electrical stimulation of mossy fibers caused immediate release of Zn2+ from synaptic terminals into the extracellular microenvironment. Release was induced across a broad range of stimulus intensities and frequencies, including those likely to induce long-term potentiation. The amount of Zn2+ release was dependent on stimulation frequency (1-200 Hz) and intensity. Release of Zn2+ required sodium-dependent action potentials and was dependent on extracellular Ca2+. Once released, Zn2+ crosses the synaptic cleft and enters postsynaptic neurons, producing increases in intracellular Zn2+ concentration. These results indicate that, like a neurotransmitter, Zn2+ is stored in synaptic vesicles and is released into the synaptic cleft. However, unlike conventional transmitters, it also enters postsynaptic neurons, where it may have manifold physiological functions as an intracellular second messenger.