Synaptogenesis and synaptic protein localization in the postnatal development of rod bipolar cell dendrites in mouse retina.

Retinal responses to photons originate in rod photoreceptors and are transmitted to the ganglion cell output of the retina through the primary rod bipolar pathway. At the first synapse of this pathway, input from multiple rods is pooled into individual rod bipolar cells. This architecture is called convergence. Convergence serves to improve sensitivity of rod vision when photons are sparse. Establishment of convergence depends on the development of a proper complement of dendritic tips and transduction proteins in rod bipolar cells. How the dendrites of rod bipolar cells develop and contact the appropriate number of rods is unknown. To answer this question we visualized individual rod bipolar cells in mouse retina during postnatal development and quantified the number of dendritic tips, as well as the expression of transduction proteins within dendrites. Our findings show that the number of dendritic tips in rod bipolar cells increases monotonically during development. The number of tips at P21, P30, and P82 exceeds the previously reported rod convergence ratios, and the majority of these tips are proximal to a presynaptic rod release site, suggesting more rods provide input to a rod bipolar cell. We also show that dendritic transduction cascade members mGluR6 and TRPM1 appear in tips with different timelines. These finding suggest that (a) rod bipolar cell dendrites elaborate without pruning during development, (b) the convergence ratio between rods and rod bipolar cells may be higher than previously reported, and (c) mGluR6 and TRPM1 are trafficked independently during development.