Significant progress has been made by the current R01 grant in identifying adaptations in signal transduction proteins produced in the mesolimbic dopamine system by chronic exposure to drugs of abuse. This neural system, which consists of dopaminergic neurons in the ventral tegmental area (VTA) and their projection regions such as the nucleus accumbens (NAc), is implicated in the reinforcing and locomotor- activating effects of cocaine and other drugs of abuse. We have shown that, in the NAc, chronic cocaine and opiate treatments decrease levels of the G protein subunit, Gialpha and increase levels of adenylyl cyclase and cAMP-dependent protein kinase (PKA). In the VTA, chronic cocaine and opiate treatments increase levels of tyrosine hydroxylase, certain glutamate receptor subunits, and glial fibrillary acidic protein, but decrease levels of neurofilament proteins. These various effects occur with chronic but not acute drug exposure, and are specific to the mesolimbic dopamine system. Moreover, chronic exposure to ethanol, but not to several drugs that lack reinforcing properties (e.g., antidepressant and antipsychotic drugs), produce most of these same effects in the NAc and VTA. Direct evidence for a role of several of these biochemical adaptations in modulating mechanisms of drug reinforcement and locomotor sensitization has been obtained. Our hypothesis is that the observed biochemical adaptations represent part of a common mechanism that underlies some of the long-term actions of drugs of abuse on mesolimbic dopamine function. In this R01 renewal, we will continue our pharmacological characterization of cocaine and opiate regulation of these biochemical adaptations in the NAc and VTA. We will then further characterize these phenomena with respect to their molecular and anatomical properties. We will identify the specific subtypes of each protein that exhibit drug regulation, as well as study their cellular localization within the NAc and VTA. For example, six subunits of PKA are known, and we have demonstrated their differential regulation by chronic cocaine in the NAc. One regulated subunit is highly enriched within the NAc shell, where it appears to be localized to a subpopulation of medium-sized neurons. Finally, we will continue our studies of drug regulation of the Fos and CREB families of transcription factors as possible mechanisms by which long-term exposure to drugs of abuse produces these biochemical adaptations. To date, we have identified novel Fos-like proteins in the NAc induced by chronic cocaine and opiate administration. We also have been able to relate drug regulation of CREB to certain of the adaptations elicited by drug exposure in both the NAc and VTA. Together, our program of research will contribute to an improved understanding of the mechanisms by which cocaine and opiates produce long-term changes in mesolimbic dopamine function.
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