Cell adhesion molecules (CAMs) regulate synapse formation and maintenance via transsynaptic contact stabilization involving both extracellular interactions and intracellular postsynaptic scaffold assembly. The CAM neurofascin is localized at the axon initial segment (AIS) of granular cells in rat dentate gyrus, which is mainly targeted by chandelier cells. Lentiviral shRNA-mediated knockdown of neurofascin in adult rat brain indicates that neurofascin regulates the number and size of postsynaptic gephyrin scaffolds, the number of GABAA receptor clusters as well as presynaptic glutamate decarboxylase (GAD65)-positive terminals at the AIS. By contrast, overexpression of neurofascin in hippocampal neurons increases gephyrin cluster size presumably via stimulation of fibroblast growth factor receptor 1 (FGFR1) signaling pathways.