Cupidin (Homer 2/vesl-2) is a post-synaptic adaptor protein that associates with glutamate receptor complexes and the actin cytoskeleton. We analyzed the developmental and activity-dependent localization of Cupidin in mouse cerebellar granule cells. Cupidin is predominantly localized to granule cell post-synapses connecting with mossy fiber terminals in developing post-natal cerebellum, but is diminished in adult cerebellum. In cultured granule cells 7 days in vitro, Cupidin was present as synaptic and extra-synaptic punctate clusters that largely co-localized with the actin-cytoskeletal binding partners F-actin and drebrin, as well as a post-synaptic scaffold protein PSD-95. Upon stimulation with glutamate, Cupidin clusters were rapidly dissociated without protein degradation, and by short-term but not sustained stimulation they were recovered after post-incubation without glutamate. The glutamate-induced declustering of Cupidin preceded that of F-actin and drebrin, was elicited by NMDA receptor-mediated Ca2+ influx, and was followed by a downstream pathway including MAPK/ERK and protein tyrosine kinase. Specific isoforms with post-translational modification were reduced depending on Ca2+-dependent protein phosphatase activity. In cultured hippocampal neurons, Homer family members Homer 1, Cupidin/Homer 2 and Homer 3 showed similar glutamate-induced declustering. We suggest that Cupidin acts as a mobile adaptor protein that changes the distribution states, clustered versus declustered, in response to synaptic activity.