microRNA-138-5p suppresses excitatory synaptic strength at the cerebellar input layer

MicroRNAs are small, highly conserved non-coding RNAs that negatively regulate mRNA translation and stability. In the brain, microRNAs contribute to neuronal development, synaptogenesis, and synaptic plasticity. The microRNA 138-5p (miR-138-5p) controls inhibitory synaptic transmission in the hippocampus and is highly expressed in cerebellar excitatory neurons. However, its role in cerebellar synaptic transmission remains unkwown. Here, we investigated excitatory transmission within the cerebellum in mice expressing a sponge construct that sequesters endogenous miR-138-5p. Mossy fiber stimulation-evoked excitatory postsynaptic currents (EPSCs) in granule cells were significantly larger compared with controls. Furthermore, we observed larger miniature EPSC amplitudes, suggesting increased postsynaptic AMPA receptor numbers. High-frequency train stimulations revealed enhanced short-term depression following miR-138-5p downregulation. Together with computational modelling, this suggests a negative regulation of presynaptic release probability. Overall, our results demonstrate that miR-138-5p suppresses synaptic strength through pre- and postsynaptic mechanisms, providing a powerful mechanism for tuning excitatory synaptic input into the cerebellum.