microRNAs are powerful regulators of gene expression that primarily act during development. In this Dissertation I show that the highly-conserved microRNA miR-1 acts in mature muscle to regulate aspects of both pre- and post-synaptic function at the C. elegans neuromuscular junction. miR-1 is expressed in muscle and regulates the sensitivity of muscle to acetylcholine by directly regulating the translation of two subunits of the levamisole-sensitive nicotinic acetylcholine receptor. In addition, miR-1 regulates the magnitude of acetylcholine released though the generation of a retrograde signal from muscle to motor neurons. The retrograde signal initiated by miR-1 requires the activity of the muscle transcription factor MEF-2, which is identified as an additional miR-1 target. Changes in pre-synaptic release of acetylcholine occur through modulation of the synaptic vesicle-associated GTPase RAB-3, which I show to be an effector of retrograde signaling.

Through the identification of these two miR-1 targets, I was able to define a more general pathway for retrograde signaling at the synapse. Both the transcriptional activity of MEF-2 and the generation of a MEF-2-dependent retrograde signal are initiated by acute activation of the levamisole receptor. Further, retrograde signaling in mir-1 mutants is suppressed by removing the levamisole receptor, suggesting that synaptic activity is necessary for MEF-2 to function. I propose a model whereby miR-1 refines synaptic function through coupling changes in muscle activity to changes in pre-synaptic function. Since miR-1 regulates both the levels of MEF-2 as well as its source of activation, this provides a mechanism to adjust the intensity of retrograde signaling.