RIM1/2 in retinal ganglion cells are required for the refinement of ipsilateral axons and eye-specific segregation

Neural activity is crucial for the refinement of neuronal connections during development, but the contribution of synaptic release mechanisms is not known. In the mammalian retina, spontaneous neural activity controls the refinement of retinal projections to the dorsal lateral geniculate nucleus (dLGN) and the superior colliculus (SC) to form appropriate topographic and eye-specific maps. To evaluate the role of synaptic release, the rab-interacting molecules (RIMs), a family of active zone proteins that play a central role in calcium-triggered release, were conditionally ablated in a subset of retinal ganglion cells (RGCs). We found that this deletion is sufficient to reduce presynaptic release probability onto dLGN neurons. Furthermore, eye-specific segregation in the dLGN and topographic refinement of ipsilateral axons in the SC and the dLGN, are impaired in RIM1/2 conditional knock-out (Rim-cDKO) mice. These defects are similar to those found when retinal activity is globally disturbed. However, reduction in synaptic release had no effect on eye-specific lamination in the SC nor on the retinotopic refinement of contralateral axons in the SC. This study highlights a potential distinction between synaptic and non-synaptic roles of neuronal activity for different mapping rules operating in visual system development.