, 2001). Whereas wild-type dentate gyrus granule cells

showed a significant increase in synaptic immunofluorescence for surface GluA1 screening assay after the chemical LTP induction protocol, LRRTM4−/− dentate granule showed only a small increase that did not constitute a significant difference as compared with unstimulated cells. Thus, LRRTM4 not only controls excitatory synapse development but also contributes to activity-regulated synaptic insertion of surface AMPA receptors in dentate gyrus granule cells. To test for changes in excitatory synapse function as a consequence of loss of LRRTM4, we performed whole-cell recordings from dentate gyrus granule cells in hippocampal slices of LRRTM4−/− and wild-type littermate mice ( Figure 8). Miniature excitatory postsynaptic current

(mEPSC) recordings from LRRTM4−/− neurons revealed a 35% reduction in mEPSC frequency as compared to wild-type control neurons (corresponding to an increased interevent interval, Figure 8B). No significant difference in mEPSC amplitude was detected ( Figure 8C). To determine whether changes in mEPSC frequency were specific to dentate gyrus granule cells, we recorded mEPSCs in CA1 pyramidal cells. No significant difference in mEPSC frequency ( Figure 8E) or amplitude ( Figure 8F) BAY 73-4506 order was detected between LRRTM4−/− and wild-type littermate CA1 neurons. Thus, LRRTM4 contributes to development of functional excitatory synapses selectively in dentate gyrus granule neurons. The observed

reduction in mEPSC frequency but not amplitude is consistent with the imaging data, indicating a role for LRRTM4 in controlling excitatory synapse density specifically on dentate gyrus granule neurons. We next assessed inhibitory synapse function but found no difference STK38 in frequency or amplitude of miniature inhibitory postsynaptic currents (mIPSCs) in dentate gyrus granule cells in slices of LRRTM4−/− mice as compared with wild-type mice ( Figure S5). Based on the reduction in excitatory but not inhibitory spontaneous currents, we expected evoked transmission to be reduced in the absence of LRRTM4. To test this prediction, input/output curves were generated by stimulating perforant pathway fibers while recording field excitatory postsynaptic potential (fEPSP) responses from the dentate gyrus molecular layer. Indeed, fEPSP slope was significantly reduced in LRRTM4−/− as compared with wild-type slices ( Figures 8F and 8G), indicating reduced evoked transmission. In contrast, paired-pulse ratio at these synapses showed no difference between genotypes ( Figure 8H), suggesting that LRRTM4 does not affect release probability, which is again consistent with a reduction in synapse number in LRRTM4−/− dentate gyrus. We show using overexpression and genetic knockout approaches that LRRTM4 promotes excitatory synapse development.