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Department of Physiology, Trinity College, Dublin, Ireland.

1. The induction of long-term potentiation (LTP) was investigated in the rat dentate gyrus in the presence of ryanodine, an agent which is known to selectively bind to the ryanodine receptor (RyR) Ca2+ channels which regulate Ca2+ release from intracellular Ca2+ stores. 2. In control media, high frequency stimulation (HFS) induced LTP, and prolonged low frequency stimulation (LFS) induced long-term depression (LTD), of field excitatory postsynaptic potentials (EPSPs) and patch clamped excitatory postsynaptic currents (EPSCs). 3. In the presence of ryanodine, at a threshold concentration of about 1 microM, HFS-induced LTP was inhibited, whereas LFS (5 Hz, 900 pulses) now induced LTP. 4. The N-methyl-D-aspartate receptor (NMDAR) antagonist D-2-amino-phosphonopentanoate (D-AP5), at both 50 and 200 microM, did not prevent the induction of LTP by 5 Hz LFS in the presence of ryanodine. This demonstrates the NMDAR independence of LTP induction in the presence of ryanodine. Furthermore, D AP5 reversed the block of HFS-induced LTP by ryanodine. 5. The induction of LTP by 5 Hz LFS in the presence of ryanodine was blocked by lowering extracellular Ca2+, or by rapidly buffering intracellular Ca2+ to very low levels with BAPTA. 6. The induction of LTP by 5 Hz LFS was inhibited by the L-type voltage-gated Ca2+ channel blocker nifedipine, and also by Ni2+ a commonly used T type voltage-gated Ca2+ channel blocker. 7. The 5 Hz LFS-induced LTP in the presence of ryanodine was inhibited by the metabotropic glutamate receptor (mGluR) antagonist (+)-alpha-methyl 4-carboxyphenylglycine (MCPG). 8. The 5 Hz LFS-induced LTP in the presence of ryanodine was blocked by Ruthenium Red, an agent known to block RyR channel opening, and also by thapsigargin, an agent known to block-ATP-dependent Ca2+ uptake into endoplasmic reticulum. 9. The results of the present studies emphasize the importance of intracellular Ca2+ stores in the induction of LTP.

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