Regulation of the dopamine transporter and its role in drug addiction.
The brain neurotransmitter dopamine (DA) plays an important role in many physiology processes – including movement, motivation, reward and affect – and disease states – including Parkinson’s disease, drug addiction and schizophrenia. DA neurotransmission is largely terminated by the DA transporter (DAT), which pumps DA back up into DA neurons. Widely abused psychomotor stimulant drugs like cocaine and amphetamine produce many of their activating and rewarding effects by inhibiting and/or reversing the DAT, thereby elevating levels of extracellular DA in the brain and promoting DA-mediated behaviors.
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Recently, it has been appreciated that DATs are dynamically regulated. Since DAT activity is so critical for “sculpting” DA neurotransmission, it is important to understand how this regulation occurs. Much of my lab’s efforts are focused in this area. Most often, this rapid regulation appears to be due to altered trafficking and cell surface expression of DAT. Specifically, we are investigating regulation induced by DAT substrates (e.g., amphetamine) and inhibitors (e.g., cocaine), growth factor-activated receptor tyrosine kinases and DA D2 autoreceptors. We use several in vitro biochemical approaches to study the cellular and molecular mechanisms involved. In collaboration with Dr. Alexander Sorkin, Department of Pharmacology, UCD, we also use state-of-the-art fluorescent microscopic and RNA interference (RNAi) approaches to study the endocytic mechanisms involved in this rapid regulation in expression systems. We are currently extending our studies to the rodent brain, using in vitro synaptosomal and slice preparations and primary neuronal cultures. We also investigate brain regional differences in DAT regulation by measuring in vivo clearance of locally applied DA with high-speed electrochemical recording.
The other focus of my lab is to understand how DAT regulation helps to explain individual differences in responsiveness to cocaine and how these differences may impact cocaine addiction. We typically observe that outbred male Sprague-Dawley rats injected with a low dose of cocaine exhibit a wide range of open-field behavioral activation. Furthermore, with repeated administration of this dose, rats that were initially less activated develop "behavioral sensitization", i.e. enhanced cocaine-induced locomotor activity, whereas the cocaine-induced activity of rats that were initially more highly activated is unchanged. Our in vivo electrochemical recording showed that a change in the effectiveness of cocaine inhibition of the DAT helps to explain these behavioral differences. Currently, we are using in vitro biochemical assays to explore how cocaine-induced DAT regulation contributes to these differences. We are also using in vivo microdialysis to measure cocaine-induced changes in extracellular DA levels in discrete brain regions while monitoring the rats’ differential behavioral activation. Behavioral sensitization is thought to contribute to drug craving and relapse during abstinence. To determine if differential DAT regulation also impacts cocaine reward, we are collaborating with Dr. Richard Allen, Department of Psychology, UCD, to study cocaine-induced conditioned place preference and self-administration in rats classified as to their initial cocaine locomotor responsiveness. In the future we anticipate extending our studies to the involvement of DA signaling pathways and contributions from glutamate signaling, as well as to other psychomotor stimulants. Our hope is that our work will help us to better understand the biological basis of stimulant drug addiction so that therapeutic preventions and interventions will be more effective.