This proposal focuses on addressing one of the most fundamental questions regarding OL biology: What axonal cues in the CNS microenvironment control OL differentiation and myelination? While it is still yet unclear whether the spatial and temporal patterns of myelination are dependent on inductive or inhibitory cues (or both), we know that exclusively axons – but not all axons – are myelinated by OLs in parallel with neuronal circuit maturation. This suggests that axon-derived signals must be involved in coordinating this process. In this proposal, we have identified a novel axon-derived peptide class, namely dynorphins that promote OL differentiation and myelination. Neuropeptides, have several characteristics that make them an ideal axonal signal to regulate myelination. They are stored in dense core vesicles and released only in response to high levels of neuronal activity, a phenomenon that might signal a form of maturation that qualifies an axon for myelination. Neuropeptides bind to G-protein coupled receptors and have slow-acting effects that may include altering gene expression, providing a mechanism through which they might alter cellular fate. In this proposal we will investigate: 1. Whether OLs and their precursors are influenced by the neuropeptide class, dynorphin, 2. Whether dynorphins are released in response to neuronal activity to regulate myelination and 3. Whether dynorphins influence myelination globally or is restricted only to dynorphin expressing axons. Recent studies demonstrate that biophysical properties of fiber diameter, inhibitory molecules and neuronal activity may all affect OL precursor cell (OPC) proliferation, differentiation, and the selection of axons for myelination (Gibson et al., 2014; Hines et al., 2015; Mensch et al., 2015; Redmond et al., 2016; Mitew et al., 2018; Mayoral et al., 2018). Here, we provide the molecular mechanism and downstream signaling pathways for a specific subset of neurons that may underlie activity dependent differentiation and myelination. Our preliminary data place us in a unique position to determine whether dynorphins are a neuropeptide class that represents an axonal cue to control OL differentiation and myelination. We believe that these findings should impart valuable insight in providing a framework for identifying additional neuropeptides and transmitters that may influence oligodendroglial lineage cells, as well as for profiling inhibitory and inductive cues for myelination.