Many neuronal cytosolic and nuclear proteins are post-translationally modified by the reversible addition of O-linked N-acetylglucosamine (OGlcNAc) on serines and threonines. The cellular functions of O-GlcNAc modifications in neuronal development are not known. We report that O-GlcNAc-modified proteins are distributed nonuniformly throughout cultured primary chicken forebrain neurons, with intense immunostaining of the cell body, punctuate immunostaining in axons and all processes, and localization in filopodia/lamellipodia. Overexpression of OGlcNAcase, the enzyme that removes O-GlcNAc from proteins, increased the percentage of neurons exhibiting axon branching without altering the frequency of axon branches on a per neuron basis and increased the numbers of axonal filopodia. Conversely, pharmacologically increasing O-GlcNAc levels on proteins through specific inhibition of O-GlcNAcase with the inhibitor 9d decreased the numbers of axonal filopodia, but had no effect on axon length or branching. Treatment with an alternative O-GlcNAcase inhibitor, PUGNAc, similarly decreased the number of axonal filopodia. Furthermore, axon branching induced by the adenylyl cyclase activator forskolin was suppressed by pharmacological inhibition of O-GlcNAcase. Western analysis revealed that OGlcNAc levels regulate the phosphorylation of some PKA substrates in response to forskolin. These data provide the first evidence of O-GlcNAc modification-specific influences in neuronal development in primary culture, and indicate specific roles for O-GlcNAc in the regulation of axon morphology. '