Indirect potentiation of synaptic transmission by metabotropic glutamate receptors in the rat hippocampal slice.

The role that the metabotropic glutamate receptor plays in synaptic transmission is complex due to the multiple subtypes involved, which initiate a number of intracellular mechanisms. Here we have investigated the role of the metabotropic glutamate receptor in the induction of long-term potentiation (LTP). We have shown that, providing the CA3 region remains attached to the slice, it is possible to induce potentiation by bath perfusion of the metabotropic receptor agonist (1S,3R) 1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) alone. The extent of the potentiation observed showed a strong negative correlation with the age of the animal from which the slices were prepared. Perfusion of ACPD was associated with an increase in the excitability of antidromically activated CA3 neurones, the appearance of spontaneous burst firing within the CA3 region, and an increased fibre volley recorded in the CA1 region. Blockade of N-methyl-D-aspartate (NMDA) receptors prevented all these effects. We suggest that the ACPD-induced potentiation of CA1 fEPSPs is an indirect effect caused by spontaneous burst firing and/or increased excitatory drive from CA3 neurones.