Metabotropic glutamate receptors are involved in calcium-induced LTP of AMPA and NMDA receptor-mediated responses in the rat hippocampus.

Effects of metabotropic glutamate (mGlu) receptors on calcium-induced long-term potentiation (LTP) of alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated components were investigated in rat hippocampal slices using whole-cell patch-clamp recordings of excitatory postsynaptic currents (EPSCs). Calcium-induced LTP comprises a parallel, long-lasting increase of AMPA and NMDA receptor-mediated components. The calcium-induced LTP of the AMPA receptor-mediated component can be significantly attenuated by the use of a selective NMDA antagonist. (R.S)-alpha-methyl-4-carboxyphenylglycine (MCPG), a selective antagonist of mGlu receptors, abolished the long-lasting increase of both AMPA and NMDA receptor-mediated components observed in calcium-induced LTP. In current clamp mode, the application of a high calcium alone or Schaffer fiber stimulation alone (20 Hz) only generated a short-term increase in the firing rate of evoked action potentials. Conversely, a long-term increase in the firing rate was observed if Schaffer fiber stimulation (20 Hz) accompanied the perfusion of high calcium. These results suggest that calcium-induced LTP involves a parallel, long-lasting enhancement in ionotropic AMPA and NMDA receptor-mediated components. More importantly, the mGlu receptor plays a critical role in the establishment of both AMPA and NMDA receptor-mediated components underlying calcium-induced LTP. In addition, the present study also described an experimental condition in which the coapplication of the high calcium pulse and Schaffer fiber stimulation (20 Hz) can synergistically elicit a long-term increase of neuronal excitability.