Zn(2+) 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 Zn(2+) 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 Zn(2+) 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 Zn(2+) release was dependent on stimulation frequency (1-200 Hz) and intensity. Release of Zn(2+) required sodium-dependent action potentials and was dependent on extracellular Ca(2+). Once released, Zn(2+) crosses the synaptic cleft and enters postsynaptic neurons, producing increases in intracellular Zn(2+) concentration. These results indicate that, like a neurotransmitter, Zn(2+) 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.

Long-term potentiation (LTP) in the hippocampus has attracted attention as a model of neuronal plasticity in the central nervous system. Although accumulating evidence associates protein synthesis with LTP, there is no direct proof that protein synthesis is actually required for the production of LTP. Therefore, we have examined the ability of some inhibitors of protein synthesis to modify LTP in the CA1 region of the rat hippocampal slice. Incubation for 30 min in the presence of emetine, cycloheximide, or puromycin decreased the frequency of occurrence of LTP in field CA1 elicited by repetitive stimulation of the Schaffer collaterals. This blockade was dose dependent and correlated with the ability of individual inhibitors to inhibit incorporation of [3H]valine into proteins. LTP blockade was irreversible for the irreversible inhibitor emetine and was reversible for the reversible inhibitor cycloheximide. Blockade of LTP required a substantial preincubation period to be effective. Even at the highest concentration of emetine used to block LTP, no effect on any intracellularly recorded membrane properties was observed. In contrast, the protein synthesis inhibitor anisomycin was unable to block LTP. Puromycin aminonucleoside, a structural analogue of puromycin which is inactive in inhibiting protein synthesis, was ineffective in blocking LTP. These experiments demonstrate that a variety of protein synthesis inhibitors are able to block the production of LTP in field CA1, suggesting the necessity for a set of newly synthesized or rapidly turned over proteins for hippocampal LTP.

The mammalian CNS contains an abundance of chelatable Zn(2+) sequestered in the vesicles of glutamatergic terminals. These vesicles are particularly numerous in hippocampal mossy fiber synapses of the hilar and CA3 regions. Our recent observation of frequency-dependent Zn(2+) release from mossy fiber synaptic terminals and subsequent entry into postsynaptic neurons has prompted us to investigate the role of synaptically released Zn(2+) in the induction of long-term potentiation (LTP) in field CA3 of the hippocampus. The rapid removal of synaptically released Zn(2+) with the membrane-impermeable Zn(2+) chelator CaEDTA (10 mm) blocked induction of NMDA receptor-independent mossy fiber LTP by high-frequency electrical stimulation (HFS) in rat hippocampal slices. Mimicking Zn(2+) release by bath application of Zn(2+) (50-100 microm) without HFS induced a long-lasting potentiation of synaptic transmission that lasted more than 3 hr. Moreover, our experiments indicate the effects of Zn(2+) were not attributable to its interaction with extracellular membrane proteins but required its entry into presynaptic or postsynaptic neurons. Co-released glutamate is also essential for induction of LTP under physiological conditions, in part because it allows Zn(2+) entry into postsynaptic neurons. These results indicate that synaptically released Zn(2+), acting as a second messenger, is necessary for the induction of LTP at mossy fiber-->CA3 synapses of hippocampus.

No abstract

Long-term potentiation (LTP) in the hippocampus is a longlasting enhancement of synaptic efficacy produced by a brief, high frequency repetitive stimulation of afferents. LTP has generated a great deal of interest as a candidate mechanism in learning and memory. A recent in viva study has shown that depletion of norepinephrine (NE) or serotonin (5-hydroxytryptamine, 5-HT) reduced LTP in the dentate gyrus produced by stimulation of the perforant path. However, it was impossible to tell whether this resulted from depletion in the hippocampus, itself, or was secondary to depletion of other brain areas, and no comparison between hippocampal cell fields was done. Therefore, we have examined the effects of depletion of NE or 5-HT on LTP in the dentate and field CA1 of the isolated in vitro hippocampal slice preparation.We report here that NE depletion markedly reduces the occurrence and amplitude of LTP in the dentate, but not in field CAl. In contrast, depletion of 5-HT does not prevent occurrence of LTP in either area. Furthermore, pharmacologic data indicate that P-receptor stimulation of adenylate cyclase is probably the mechanism of NE's action in the production of LTP in the dentate. These results suggest that endogenous hippocampal NE is more important to LTP in the dentate than is endogenous 5-HT.In the hippocampus, brief, high frequency stimulation of afferents gives rise to a long-lasting increase in the amplitude of the evoked population spike. This enhancement has been termed long-term potentiation (LTP) and has been described for a number of neuronal inputs to the hippocampus (Bliss and Lmmo, 1973; Schwartzkroin and Wester, 1975; Alger and Teyler, 1976). The amplitude of the postsynaptic population spike is up to 10 times greater than the response prior to repetitive stimulation (Schwartzkroin and Wester, 1975). LTP can last for weeks in the intact animal (Bliss and GardnerMedwin, 1973; Douglas and Goddard, 1975) and for the life of the hippocampal slice (10 hr or longer) (Alger and Teyler, 1976; Andersen et al., 1977). The extremely long duration of hippocampal LTP