Our overall research question is to understand how chronic exposure to drugs of abuse and stress changes the brain on a molecular level to produce pathological states such as depression and anxiety. Alleviation of negative affective states is thought to be a primary motivation for drug taking and relapse. To address our question, we employ molecular-genetic techniques such as quantitative RT-PCR, Western blotting, and immunohistochemistry to examine the effects of chronic drug exposure and withdrawal on molecular pathways within the mesolimbic dopamine system. One hypothesis we are pursuing is that increased activation of AMPA glutamate receptors in the nucleus accumbens, a critical substrate for reward function, underlies drug withdrawal-induced negative affective states. To test this, we are infusing AMPA receptor antagonists directly into the nucleus accumbens of rats prior to precipitating morphine withdrawal and measuring the effects on withdrawal-induced conditioned place aversions. In addition we are using protein fractionation to measure the effects of morphine withdrawal on intracellular trafficking of AMPA glutamate receptor subunits in the brain. A second hypothesis is that kappa opioid receptors contribute to drug withdrawal-induced depressive-like states through activation of extracellular signal regulated kinase (ERK) activity. To accomplish this, we are measuring the effects of kappa opioid receptor ligands on cocaine-induced changes in intracranial self-stimulation. We are also using viral vectors to manipulate ERK function in the nucleus accumbens and measuring the effects on reward function. Together, these studies may lead to an increased understanding of the mechanisms underlying addiction.