Evidence for involvement of group II/III metabotropic glutamate receptors in NMDA receptor-independent long-term potentiation in area CA1 of rat hippocampus

Previous studies implicated metabotropic glutamate receptors (mGluRs) in N-methyl-D-aspartate (NMDA) receptor-independent long-term potentiation (LTP) in area CA1 of the rat hippocampus. To learn more about the specific roles played by mGluRs in NMDA receptor-independent LTP, we used whole cell recordings to load individual CA1 pyramidal neurons with a G-protein inhibitor guanosine-5'-O-(2-thiodiphosphate), GDPbetaS]. Although loading postsynaptic CA1 pyramidal neurons with GDPbetaS significantly reduced G-protein dependent postsynaptic potentials, GDPbetaS failed to prevent NMDA receptor- independent LTP, suggesting that postsynaptic G-protein-dependent mGluRs are not required. We also performed a series of extracellular field potential experiments in which we applied group-selective mGluR antagonists. We had previously determined that paired-pulse facilitation (PPF) was decreased during the first 30-45 min of NMDA receptor-independent LTP. To determine if mGluRs might be involved in these PPF changes, we used a twin-pulse stimulation protocol to measure PPF in field potential experiments. NMDA receptor-independent LTP was prevented by a group II mGluR antagonist (2S)-alpha-ethylglutamic acid] and a group III mGluR antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine], but was not prevented by other group II and III mGluR antagonists (RS)-alpha-methylserine-O-phosphate monophenyl ester or (RS)-alpha-methylserine-O-phosphate]. NMDA receptor-independent LTP was not prevented by either of the group I mGluR antagonists we examined, (RS)-1-aminoindan-1,5-dicarboxylic acid and 7-(hydroxyimino)cyclopropab]chromen-1a-carboxylate ethyl ester. The PPF changes which accompany NMDA receptor-independent LTP were not prevented by any of the group-selective mGluR antagonists we examined, even when the LTP itself was blocked. Finally, we found that tetanic stimulation in the presence of group III mGluR antagonists lead to nonspecific potentiation in control (nontetanized) input pathways. Taken together, our results argue against the involvement of postsynaptic group I mGluRs in NMDA receptor-independent LTP. Group II and/or group III mGluRs are required, but the specific details of the roles played by these mGluRs in NMDA receptor-independent LTP are uncertain. Based on the pattern of results we obtained, we suggest that group II mGluRs are required for induction of NMDA receptor-independent LTP, and that group III mGluRs are involved in determining the input specificity of NMDA receptor-independent LTP by suppressing potentiation of nearby, nontetanized synapses.