Either ionotropic or group I metabotropic glutamate receptor activation is required for induction of autaptic LTD. A: time course of EPSC charge (normalized) shows that the AMPA receptor antagonist 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX, 10 μM) in combination with the NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (APV, 25 μM) does not block LTD. B: summary LTD time course for neurons treated with the group I mGluR antagonist (S)4-carboxyphenylglycine (4-CPG, 50 μM) indicates that blocking this class of metabotropic receptor does not prevent autaptic LTD, though there is a trend toward statistical significance (see following text). C: averaged time course of autLTD in presence of CNQX/APV/4-CPG showing that combined antagonism of group I mGluRs and ionotropic glutamate receptors prevents LTD D, Bar graph summarizes results from A–C for LTD inhibition in presence of CNQX/APV (n = 7) or 4-CPG (n = 6) or CNQX/APV/4-CPG (n = 4). φ, P = 0.07, *, P < 0.05 1-way ANOVA, Dunnett's post hoc test vs. control LTD.