METABOTROPIC GLUTAMATE RECEPTOR-MEDIATED PRESYNAPTIC DEPRESSION AT CORTICOSTRIATAL SYNAPSES INVOLVES MGLUR2 OR MGLUR3

1. The pharmacology of the metabotropic glutamate receptor (mGluR) that mediates synaptic depression at corticostriatal synapses was investigated with the use of field potential and whole cell parch-clamp recording from striatal slices and whole cell recordings from isolated striatal neurons. 2. The mGluR2,3-selective agonists (R,S)-4-carboxy-3-hydroxyphenylglycine (CHPG), (2S, 1'R, 2'R, 3'R)-2-(2,3-dicarboxycyclopropyl) glycine (DCG-IV), and (2S, 3S, 4S)-alpha-(carboxy-cyclopropyl) glycine (L-CCG-I) inhibited the synaptically driven population spike (PS) evoked by afferent stimulation during field potential recording in striatal slices. These agonists also inhibited excitatory postsynaptic potentials (EPSPs) evoked by afferent stimulation during whole cell recordings. The metabotropic receptor antagonist R,S-alpha-methyl-4-carboxyphenylglycine (MCPC) blocked the synaptic depressant actions of DCG-IV and trans-1-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD). 3. The mGluR4,6,7-selective agonist L-serine-O-phosphate (L-SOP) did not alter corticostriatal synaptic transmission, but both this agonist and the mGluR4,6,7 agonist D,L-2-amino-4-phosphonobutyric acid (AP4) reduced the amplitude of the population EPSP and PS evoked in the dentate gyrus (DG) by stimulation of the lateral perforant path (LPP). These data are consistent with earlier observations that AP4 does nor inhibit corticostriatal transmission, but produces presynaptic depression at LPP-DG synapses. 4. Application of mGluR agonists that inhibited transmission did nor alter the input resistance or excitability of striatal neurons and did not inhibit responses evoked by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptor activation. 5. These observations indicate that an mGluR2 or 3 subtype mediates synaptic depression, most likely through a presynaptic inhibitory effect, at corticostriatal synapses.