calyculin-a and 2-(2-3-dicarboxycyclopropyl)glycine

The induction of mossy fiber-CA3 long-term potentiation (LTP) and depression (LTD) has been variously described as being dependent on either pre- or postsynaptic factors. Some of the postsynaptic factors for LTP induction include ephrin-B receptor tyrosine kinases and a rise in postsynaptic Ca2+ ([Ca2+]i). Ca2+ is also believed to be involved in the induction of the various forms of LTD at this synapse. We used photolysis of caged Ca2+ compounds to test whether a postsynaptic rise in [Ca2+]i is sufficient to induce changes in synaptic transmission at mossy fiber synapses onto rat hippocampal CA3 pyramidal neurons. We were able to elevate postsynaptic [Ca2+]i to approximately 1 microm for a few seconds in pyramidal cell somata and dendrites. We estimate that CA3 pyramidal neurons have approximately fivefold greater endogenous Ca2+ buffer capacity than CA1 neurons, limiting the rise in [Ca2+]i achievable by photolysis. This [Ca2+]i rise induced either a potentiation or a depression at mossy fiber synapses in different preparations. Neither the potentiation nor the depression was accompanied by consistent changes in paired-pulse facilitation, suggesting that these forms of plasticity may be distinct from synaptically induced LTP and LTD at this synapse. Our results are consistent with a postsynaptic locus for the induction of at least some forms of synaptic plasticity at mossy fiber synapses.

Topics: Analysis of Variance; Animals; Animals, Newborn; Anticonvulsants; Benzothiazoles; Calcium; Coumarins; Cyclopropanes; Cyclosporine; Electric Stimulation; Enzyme Inhibitors; Excitatory Postsynaptic Potentials; Glycine; Hippocampus; In Vitro Techniques; Long-Term Potentiation; Long-Term Synaptic Depression; Marine Toxins; Mossy Fibers, Hippocampal; Oxazoles; Patch-Clamp Techniques; Photolysis; Pyramidal Cells; Rats; Rats, Sprague-Dawley; Synapses; Synaptic Transmission