1 2 3 4 5 6 Prolonged Synaptic Currents Increase Relay Neuron Firing at the Developing 7 Retinogeniculate Synapse

48 The retinogeniculate synapse, the connection between retinal ganglion cells (RGC) and 49 thalamic relay neurons, undergoes robust changes in connectivity over development. This 50 process of synapse elimination and strengthening of remaining inputs is thought to require 51 synapse specificity. Here we show that glutamate spillover and asynchronous release are 52 prominent features of retinogeniculate synaptic transmission during this period. The immature 53 EPSCs exhibit a slow decay timecourse that is sensitive to low affinity glutamate receptor 54 antagonists and extracellular calcium concentrations, consistent with glutamate spillover. 55 Furthermore, we uncover and characterize a novel, purely spillover-mediated AMPA receptor 56 current from immature relay neurons. The isolation of this current strongly supports the 57 presence of spillover between boutons of different RGCs. In addition, fluorescence 58 measurements of presynaptic calcium transients suggest that prolonged residual calcium 59 contributes to both glutamate spillover and asynchronous release. These data indicate that 60 during development, far more RGCs contribute to relay neuron firing than would be expected 61 based on predictions from anatomy alone. 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81