Proteomic Characterization of κOR and Cdk5 Signaling Pathway in Rodent Nucleus Accumbens: Implications in Cocaine Addiction

Addiction is a complex chronic relapsing brain disorder. Addictive drugs such as cocaine target ventral striatal circuitry to induce maladaptive changes in the intracellular signal transduction networks that are dictated by protein translational modifications by kinases and phosphatases. These neuroadaptive alterations eventually contribute to synaptic and behavioral deficits associated with drug addiction. Identifying the protein targets involved in this process will improve our current knowledge of addiction as well as impact our future therapeutic interventions. Kappa opioid receptors (κORs) are G-protein coupled receptors (GPCRs), activated by the endogenous ligand dynorphin (DYN), which elicits anti-reward or aversive-like behaviors in animals and humans. The chronic or repeated intake of cocaine progressively increases DYN levels and activates κORs as a homeostatic mechanism. κOR signaling gradually induces adaptive changes in the meso-corticolimbic circuitry, thereby reducing basal dopaminergic and glutamatergic drive on medium spiny neurons (MSNs) of the nucleus accumbens (NAc) and triggering negative emotional states during drug abstinence. These maladaptive changes caused by increased DYN/κOR signaling are one of the major contributing factors for compulsive drug craving, withdrawal symptoms, and propensity to relapse. The fundamental understanding of κOR biology is essential to unravel the critical mechanisms involved in addiction.Cdk5 is a neuronal kinase activated by its cofactor p35, which plays a prominent role in reward learning by phosphorylating several synaptic substrates. Aberrant activation of Cdk5/p35 is involved in cocaine addiction through its opposing effects on dopamine mediated PKA signaling in the NAc. In studying this phenomenon, we discovered a novel crosstalk between κOR and Cdk5 signaling in the mechanisms of cocaine addiction. Our finding shows that either chronic cocaine exposure or acute κOR agonist U50488 treatment increase the steady-state levels of p35 to cause post-synaptic activation of Cdk5 in the NAc, which is associated with decreased phosphorylation of PKA substrates. Based on these findings, we hypothesize that cocaine addiction is sustained through elevated κOR-Cdk5 signaling in the NAc. In support of this hypothesis our preliminary findings show that systemic administration of Cdk5 inhibitor (25-106) is sufficient to attenuate the cocaine-induced conditional place preference (CPP) in mice, suggesting that Cdk5 inhibition may be a potential strategy to reduce cocaine-mediated addictive behaviors. To broaden our search for identifying candidate protein targets we propose to screen the NAc proteome after chronic cocaine or acute κOR agonist (U50488) administration and compare the effects along with the 25-106 treatment. We will use affinity-based phosphoprotein substrate enrichment strategies, label-free quantitative proteomics coupled with LC/MS-MS to identify the global changes in the NAc proteome after these treatments in mice. These studies will generate novel insights on how κOR-Cdk5 signaling plays a role in cocaine addiction and will identify potential targets for further characterization.