Metabotropic glutamate receptors inhibiting excitatory synapses in the CA1 area of rat hippocampus

In the CA1 region of hippocampal slices prepared from young adult rats, we studied the ability of several specific agonists of metabotropic glutamate receptors (mGluRs) to depress excitatory synaptic transmission at the CAS-CAI pyramidal cell synapses. Three groups of mGluRs have been described: group 1 (mGluR1 and 5) receptors are positively coupled to phospholipase C whereas group 2 (mGluR2 and 3) and group 3 (mGluR4, 6, 7 and 8) receptors are negatively coupled to adenylate cyclase. We found that the broad-spectrum agonist (1S,3R)-1-aminocyclopentyl-1,3-dicarboxylate and the group 1-specific agonist (R,S)-dihydroxyphenylglycine both reversibly inhibited evoked field excitatory postsynaptic potentials, indicating the involvement of group 1 mGluRs. (R,S)-3,5-dihydroxyphenylglycine presumably inhibited transmission via a presynaptic mechanism, as whole-cell voltage-clamp recordings revealed that inhibition of the synaptic transmission was always accompanied with an increase in paired-pulse facilitation. Treatment with a specific blocker of mGluR1 receptors, the phenylglycine derivative (S)-4-carboxyphenylglycine, was without effect on the (1S,3R)-1-amino-cyclopentyl-1,3-dicarboxylate-induced depression of the field excitatory postsynaptic potentials, strongly suggesting that mGluR5 receptors are responsible for the (1S,3R)-1-aminocydopentyl-1,3-dicarboxylate effect, Two selective agonists of group 2 mGluRs, (2S,1's,2's)-2-(2'-carboxycyclopropyl)glycline and 4-carboxy-3-hydroxyphenylglycine, were totally ineffective in blocking CA3-CA1-evoked synaptic transmission, excluding the involvement of mGluR2/3 subtypes at this developmental stage.