Synaptic and extrasynaptic plasticity in glutamatergic circuits

36 37 Chronic global N-methyl-D-aspartate receptor (NMDAR) blockade leads to changes in 38 glutamatergic transmission. The impact of more subunit-selective NMDAR inhibition on glutamatergic 39 circuits remains incomplete. To this end organotypic hippocampal slice cultures were treated for 17-21 40 days with the: high-affinity competitive antagonist D-APV, allosteric GluN2B-selective antagonist 41 Ro25,6981; or newer competitive GluN2A-prefering antagonist, NVP-AAM077. Electrophysiological 42 recordings from dentate granule cells revealed that chronic D-APV treatment increased, while chronic 43 Ro25,6981 reduced epileptiform event-associated sEPSClarge compared to all other treatment groups, 44 consistent with opposite effects on glutamatergic networks. Presynaptically, chronic D-APV or 45 Ro25,6981 increased sEPSCsmall and mEPSCAMPAR frequency. Chronic D-APV or NVP-AAM077, but 46 not Ro25,6981 increased putative vGlut1-positive glutamatergic synapses. Postsynaptically, chronic 47 D-APV dramatically increased mEPSCsAMPAR and profoundly decreased mEPSCNMDAR measures, 48 suggesting increased AMPAR/NMDAR ratio. Ro25,6981 decreased mEPSCsAMPAR charge transfer 49 and modestly decreased mEPSCNMDAR frequency and decay, suggesting downward scaling of AMPAR 50 and NMDAR function without dramatically altering AMPAR/NMDAR ratio. Extrasynaptically, 51 TBOA-enhanced “tonic” NMDAR current amplitude and activated channel number estimates were 52 significantly increased only by chronic Ro25,6981. For intrinsic excitability, action potential threshold 53 was slightly more negative following chronic D-APV or NVP-AAM077. The predominant pro54 excitatory effects of chronic D-APV are consistent with increased glutamatergic transmission and 55 network excitability. The minor effects of chronic NVP-AAM077 on action potential threshold and 56 synapse number are consistent with minimal effects on circuit function. The chronic Ro25,698157 induced downward scaling of synaptic AMPAR and NMDAR function is consistent with decreased 58 postsynaptic glutamate receptors and reduced network excitability. 59