The glutamate receptor adaptor protein Homer is concentrated in the postsynaptic density of excitatory synapses and is critical for normal operation of synaptic transmission. In this study, we investigated the responsiveness of Homer family proteins to dopamine stimulation with the psychostimulant cocaine in rat striatal neurons both in vivo and in vitro. We found that a single dose of cocaine specifically induced a rapid and transient increase in protein levels of the Homer1a, but not Homer1b/c and Homer2a/b, isoforms in the striatum. This selective Homer1a induction was mediated primarily through activation of dopamine D1, but not D2, receptors. Both protein kinase A and Ca 2ϩ /calmodulin-dependent protein kinases are important for mediating the cocaine stimulation of Homer1a expression. At the transcriptional level, cAMP response element-binding protein serves as a prime transcription factor transmitting the signals derived from D1 receptors and associative pathways to the CaCRE sites within the Homer1a promoter. From a functional perspective, non-cross-linking Homer1a, once induced, competed with the cross-linking isoforms of Homer proteins (Homer1b/c and Homer2a/b) to uncouple the connection of group I metabotropic glutamate receptors (mGluRs) with inositol-1,4,5-triphosphate receptors. These results indicate that cocaine possesses the ability to stimulate Homer1a expression in striatal neurons through a specific synapse-to-nucleus pathway. Moreover, inducible Homer1a expression may represent a transcription-dependent mechanism underlying the dynamic regulation of submembranous macromolecular complex formation between group I mGluRs and their anchoring proteins.Synaptic Homer proteins are important for synaptic construction and function Sheng and Kim, 2002). Long-form Homer proteins (Homer1b/c, Homer2a/b, and Homer3) contain the N-terminal EVH1 (Enabled/ VASP homology 1) domain, which binds the C terminus of group I metabotropic glutamate receptors (mGluRs), whereas the C-terminal coiled-coil structure and leucine zipper motifs render a capability for self-assembly (Brakeman et al., 1997;Xiao et al., 1998;Xiao et al., 2000). Thus, as a prominent scaffolding molecule concentrated in the postsynaptic density of excitatory synapses, Homer crosslinks group I mGluRs to other targets in a specific subcellular microdomain to regulate a specific signaling activity. Emerging evidence indicates that Homer proteins play an essential role in the membrane trafficking of mGluR1␣/5 (Ango et al., 2000); the coupling of mGluR1/5 to inositol-1,4,5-triphosphate (IP 3 ) receptors and the cation (Ca 2ϩ or K ϩ ) channel (Yuan et al., 2003); the development of spines, axons, and synapses (Shiraishi et al., 2003); and drug addiction (Swanson et al., 2001;Szumlinski et al., 2004Szumlinski et al., , 2005; also see below).One distinctive member of Homer family is Homer1a (Brakeman et al., 1997;Xiao et al., 1998Xiao et al., , 2000. Unlike the long-form of Homer proteins, this short-form of Homer lacks the C-terminal coiled-co...