Substance use Disorders (SUDs) are characterized by chronic relapse after periods without symptoms. This feature has been hypothesized to stem from persistent alterations in corticolimbic circuit function and structure caused by drug-induced alterations in addiction-related gene (ARG) expression. Transcriptional and epigenetic regulation of ARGs and addiction like behaviors have been well characterized but the role of post- transcriptional regulation is an understudied, yet promising field. One mechanism of post-transcriptional regulation, mRNA stability, can be influenced by direct binding of factors to discrete recognition sites within the 3'UTR. RNA binding proteins (RBPs) are one regulator of mRNA stability. HuD is a neuronal specific RBP that stabilizes mRNAs and is regulated by neuronal activity and cocaine. Another type of post-transcriptional regulator, microRNAs (miRNAs), are non-coding RNAs that target specific mRNAs for degradation or translational repression. Since both RBPs and miRNAs target the 3'UTR, this opens the possibility that these two classes of molecules could compete for access to a specific recognition site. We have found that a specific miRNA, miR-495, and HuD target a set of shared mRNAs via binding to the same GUUUGUUUG sequence. Many of these shared targets, including Bdnf and Camk2a have been implicated in addiction and are considered ARGs. Lentiviral (LV) overexpression (OE) of miR-495 and HuD caused decreased or increased expression, respectively, of target mRNAs within the Nucleus Accumbens Shell (NAcSh), a region associated with addiction-related behaviors. In a preliminary conditioned place preference (CPP) study, we found that cocaine preference appears to be related to lower NAc miR-495 expression. Additionally, a HuD OE mouse line was found to show increased CPP compared to wild type littermates. Since miR-495/HuD and their targets are dendritically localized, we were interested in the role that miR-495 and HuD may play in structural plasticity. Using a diffuse lipophilic dye method (DiOListics), we found that nave HuD OE had distinct structural alterations in spine morphology within the NAcSh. In contrast, LV mediated OE of miR-495 seems to disrupt spine formation or maturation entirely. Combined, this led to my hypothesis that cocaine differentially regulates HuD and miR-495 leading to opposite changes in ARG expression, drug-evoked structural plasticity, and addiction-like behaviors, either facilitating or inhibiting these, respectively. This proposal seeks to test his hypothesis with three specific aims 1) Are HuD, miR-495, and dendritic spine structure within the NAc differentially regulated by cocaine CPP? 2) Do NAC miR-495 and HuD cause opposite changes in dendritic spine morphology? 3) Do miR-495 and HuD have opposite roles in cocaine CPP? Understanding the role of post-transcriptional competition of shared targets on cellular dynamics and behavior may inform new pharmacological treatments that tip this post-transcriptional competition in the favor of remission.