The repeated use of psychostimulants produces adaptations within the CNS that are thought to contribute to persistent and compulsive drug abuse. Our studies have as their goals the delineation of adaptations that occur in the mesocorticolimbic system, a circuit implicated in mediating the behavioral effects of various drugs of abuse, and the identification of homeostatic mechanisms that oppose their development. Included in this goal is the identification of pharmacotherapies that attenuate alterations in behavior and brain chemistry that occur as a consequence of repeated drug use. Our previous studies have shown that kappa opioid receptor(KOR)agonists modulate the activity of the dopamine transporter. The dopamine transporter is a membrane bound protein that regulates extracellular levels of dopamine in the CNS and is the site upon which psychostimulants such as cocaine act to produce their behavioral effects. Our work has shown that KOR agonists not only modulate the activity of the dopamine transporter but prevent alterations in transporter function produced by cocaine. Since KOR agonists also function as cocaine-like antagonists in various paradigms, and are co-localized with dopamine transporters in the mesocorticolimbic system, a major focus of our studies has been identification of the mechanisms by which KOR activation can modulate transporter activity. Using rat synaptosomal preparations and heterologous expression systems, we have examined whether KOR agonists may modulate dopamine transporter function via their ability to activate certain intracellular signalling cascades. Our studies have shown that inhibition of two protein kinases that are activated by KOR decrease transporter activity by causing the re-distribution of transporter from the plasma membrane to the cytosol. In contrast, transient expression of constitutively active forms of these kinases, mimic the effects of acute KOR activation, resulting in increased activity of the transporter and externalization of the transporter. Using confocal microscopy, we have been able to show that these effects reflect alterations in clathrin-mediated transporter internalization. These findings demonstrate that protein kinases can modulate pre- as well as post-synaptic dopamine neurotransmission and provide a mechanism whereby KOR agonists may modulate dopamine transport as well as the interactions of cocaine with this protein. We have also shown that KOR agonists regulate the activity of the D2/D3 dopamine receptor. These receptors, like KOR, are G-protein coupled receptor that stimulate specific protein kinases. Using in vivo (microdialysis) and in vitro (voltammetry) techniques, we have recently shown that their activation not only regulates dopamine release but also transporter function. In future studies,we will examine the mechanism by which D2/D3, KOR and other G-protein coupled receptors modulate transport function and the interplay between these receptors in regulating dopamine transmission. In other studies, we have been examining whether neuropeptides (e.g. tachykinin) that are co-localized and released with endogenous KOR ligands regulate dopamine neurotransmission and whether these peptide systems modulate the behavioral and neurochemical effects of abused drugs.
Showing the most recent 10 out of 49 publications
