(RS)-ALPHA-METHYL-4-CARBOXYPHENYLGLYCINE (MCPG) DOES NOT BLOCK THETA-BURST-INDUCED LONG-TERM POTENTIATION IN AREA CA1 OF RAT HIPPOCAMPAL SLICES

We have used the selective metabotropic glutamate receptor antagonist (RS)-alpha-methyl-4-carboxyphenylglycine (MCPG) to investigate in the CA1 hippocampal subregion in vitro whether coactivation of N-methyl-D-aspartate (NMDA) and metabotropic glutamate receptors is necessary for the induction of long-term potentiation (LTP) when LTP is induced by theta burst stimulation (TBS). When MCPG (500 mu M) was bath applied 14-30 min prior to a triple high-frequency tetanization (100 Hz, 1 s) and washed out immediately afterwards the potentiation of the extracellularly recorded field potentials decayed gradually to baseline (P < 0.05) over 2-3 h. However, when MCPG was applied in the same manner before a triple TBS (10 bursts at 5 Hz, 100 Hz within the bursts) the resulting potentiation remained stable for at least 4 h. MCPG had no effect on baseline synaptic transmission or post-tetanic potentiation. These results demonstrate a clear difference in the mechanisms underlying these two different forms of LTP.