TWO-TIMEFRAME MONOSYNAPTIC RABIES TRACING REVEALS CHANGES IN NEURONAL CONNECTIVITY ASSOCIATED WITH OCULAR DOMINANCE PLASTICITY IN ADULT MICE

Experience-dependent plasticity allows the brain to adapt to changing sensory input. For instance, monocular deprivation (MD), the temporary closure of one eye, shifts neuronal responses in visual cortex towards the open eye. This functional change is accompanied by formation of new dendritic spines. However, the presynaptic partners of these new spines have not been directly identified. Here we introduce a novel approach, two-timeframe monosynaptic rabies virus tracing (TTT), to survey the whole brain for new structural inputs associated with ocular dominance (OD) plasticity.

In a tdTomato reporter mouse we infect ‘starter cells’ in binocular V1, with a rabies virus that expresses eGFP and inducible Cre recombinase, thus labeling their pre-existing inputs. Next, systemically injecting 4OHT induces expression of tdTomato in rabies-infected (traced) neurons. Neurons traced after 4OHT degradation express only eGFP. The system’s read-out therefore distinguishes two sets of input cells: those infected before 4OHT injection (timeframe-1), expressing eGFP and tdTomato, and those infected after 4OHT is metabolized (timeframe-2); the latter ones represent putative new synaptic inputs.

We arranged the approach such that timeframe-1 traced pre-MD inputs, and timeframe-2 captured inputs formed following MD. Overall, we found a significantly larger fraction of putative newly connected neurons in mice that underwent MD (28% increase relative to controls). In particular the dorsal lateral geniculate nucleus (71%) and callosally projecting V1 (48%), which largely represents the eye remaining open during MD, showed substantial increases. These results indicate that TTT can be used to identify changes in synaptic input patterns following experience-dependent plasticity.