Projects

Chronic Drug Actions on G-Protein-Coupled Receptor Mechanisms

Many neurotransmitters in brain, as well as several drugs of abuse, act by binding to receptors belonging to the superfamily of G-protein-coupled receptors. These drugs include the opioids, which act directly at their own G-protein-coupled receptors, and cocaine, which increased extrasynaptic dopamine that binds to its own receptor. The goal of this project is to utilize animal models provided by the Center to examine how chronic treatment with opioids and psychostimulants affect coupling of different receptors to G-proteins in brain, and how these changes at the level of the receptor transducer then translate to downstream changes in opioid-mediated second messenger systems. In the first case, the coupling of receptors to G-proteins will be explored by [35S]GTPgS autoradiography, which provides a neuroanatomical localization of the activation of G-protein receptors. Because this technique allows for a number of different receptors to be assayed simultaneously in brain sections from the same animal, this provides an ideal method to efficiently analyze brain tissue from Center-derived animals. This project will follow up on previous studies which showed that chronic morphine treatment produced selective attenuation of mu opioid-activated G-proteins in specific brainstem nuclei. It will also examine the effect of chronic administration of cocaine and other psychotimulants on receptor-coupled G-proteins, with a particular focus on D2 dopamine receptors, 5-HT1A receptors, and opioid receptors. This project will use information obtained from other Center projects in the planning of its studies, and provide information about receptor changes during chronic drug treatment that will be important for studies in other projects.

Epigenetic Imprinting by Chronic Drugs of Abuse

Dr. Kent E. Vrana

This project is conducting experiments to investigate the long-term consequences of cocaine and opiate administration on molecular indices of dopaminergic function. A central problem in substance abuse research is identifying how chronic drug administration changes the brain so as to account for the long-term problems of physical dependence, psychological addiction, and tolerance. One hypothesis is that the drugs create an epigenetic imprint. That is, the chronic administration of drug alters the pattern of gene expression - the levels of RNA and protein - such that they change the state of the brain. The experiments underway in this component of the center are designed to address this issue in a multi-disciplinary way by focusing on a recognized neuroanatomical substrate of substance abuse - the mesolimbic dopamine pathway. Utilizing recombinant DNA research tools (most notably the multiplex hybridization microarray), the hypothesis that chronic cocaine regulates the pattern of gene expression are being tested. Pursuing previous studies from this laboratory, experiments are also examining the long-term epigenetic consequences of administration of tropane analogs with markedly different pharmacokinetics and transporter selectivity than cocaine. The ultimate goal, for many of these studies, is to establish the relationships between epigenetic imprinting by response-dependent (self-administration) vs response-independent cocaine administration in the rodent. These studies will continue the progress we have made in understanding how long-term drug administration affects neuronal genetic set-points and how this might account for long-term drug abuse liabilities.

Neurobiology of Tropane Analogs

Dr. David C.S. Roberts

Project 4 is a coordinated drug discovery program that involves the full range of analytical capabilities represented within the CNIDA . The novel tropane analogs produced by the vinylcarbenoid chemistry of Dr. Huw Davies will be screened for their neurochemical potency, specificity and behavioral actions in an effort to identify compounds with scientifically interesting properties and/or therapeutic value. Compounds will be screened for their specificity and efficacy at monoamine transporters and for their lipophilicity by Drs. Childers and Bennett. Selected compounds will be characterized for their effects on regional glucose metabolism in Dr. Porrino's laboratory; she will also assess the locomotor stimulant effects of these tropanes and determine their time course. Dr. Smith will examine the effects of selected tropanes on extracellular dopamine and serotonin levels through microdialysis studies. Self-administration studies conducted by Dr. Roberts will examine the relative reinforcing efficacy of specific tropane analogues and assess their ability to decrease cocaine self-administration. The aim is to evaluate and develop tropane analogues that will be useful for the analysis of basic neurobiological questions concerning psychostimulant reinforcement and as potential therapeutic agents.

Drug Reinforcement Mechanisms

Dr. James E. Smith

Many drugs that are abused either produce feelings of well being and euphoria or reduce the aversiveness of the perceived environment. Cocaine and heroin, and combinations of these two drugs (speedball), are abused because they are very effective in producing these effects. The basic brain mechanisms underlying these actions are not well understood. This research project directly addresses these issues by studying the brains of animals that self-administer cocaine, heroin and speedball. Laboratory methods are used that permit evaluation of specific brain sites, cell types and neurotransmitters in these animals to define the biological mechanisms of addiction. Understanding these basic mechanisms will help in the identification of more specific and effective treatment of these complex disorders.

Functional Substrates of Cocaine Self Administration

Dr. Linda J. Porrino

The clinical course of cocaine abuse has been characterized as progressing through a number of temporal stages that advance from initial experimentation through casual use and finally to addiction. On a neurobiological level, this clinical course is paralleled by changes in the response to cocaine with repeated exposure, as well as residual changes in brain function and structure that may persist despite prolonged periods of abstinence from drug use. The studies proposed in the present application will focus on the neuroadaptations that accompany long term exposure to cocaine. We will focus on animals self-administration models, as these models provide valid predictions of human cocaine abuse. Furthermore, differences in the behavioral, pharmacological, and neurochemical responses to cocaine have been shown between self-administered cocaine and cocaine administered non-contingently.

In preliminary studies we have observed progressive changes in the patterns of functional activity with continued experience with cocaine self-administration. Structures that form part of the neuroanatomical circuit mediate cocaine self-administration in its initial stages are different from those that appear to be involved later in the course of experience with self-administration. Such neuroadaptations may underlie the overall progression of self-administration as it eventually leads to addiction. The overall goals of this project, then, are (1) to extend these findings to examine more closely the evolution of the functional changes associated with cocaine self-administration; (2) to determine the temporal course of adaptations in dopamine and opioid systems that may underlie these changes in functional activity as they develop over time; and (3) to examine the long term effects of novel tropane analogs synthesized in this Center that have been hypothesized as potential medications for cocaine addiction. It is hoped that this approach will provide important insights into the progressive changes that accompany cocaine abuse and provide a basis for developing pharmacological therapies treatments for addiction.

Neurophysiological Assessment of Cocaine Reinforcement

Dr. Samuel A. Deadwyler

The neurophysiological basis of drug addiction is not well understood. We know for instance that dopaminergic neurons play a role in this process. But it is also quite clear that other neurotransmitters are involved. One of the major objectives of this project is to understand how the underlying neural circuitry in the areas of the brain that recognize and react to rewarding stimuli are altered when people take drugs. This project investigates this issue by recording from neurons in the nucleus accumbens, ventral tegmental area and cerebral cortex during exposure to addicting drugs such as cocaine. We have successfully determined that neural circuits in these areas play a major role in the rewarding component of the addiction process .The project will explore the neurophysiological mechanisms that control drug seeking at the cellular and neural population level but will also investigate the influence of factors such as stimuli that are associated with drug intake and the contexts in which drugs are taken, as of means of understanding human drug seeking activity in its most common circumstances.

The Influence of Dopamine and Serotonin Transporters in Cocaine Abuse

Dr. Michael A. Nader

The goal of this project is to achieve a better understanding of how cocaine works in the brain, in an effort to block its powerful addicting effects. Previous research has shown that cocaine increases the amount of specific chemicals (neurotransmitters) in the brain by blocking the ability of these neurotransmitters to be transported back to the cell. These increases in neurotransmitter levels are believed to produce the cocaine high that results in addiction. The studies proposed in this project are designed to evaluate the role of two neurotransmitters: dopamine (DA) and serotonin (5-HT) in the behavioral sequelae leading to cocaine's high abuse liability. The specific aims of this project are: (1) to investigate the reinforcing strength of drugs that bind with high affinity and selectivity for either DA and/or 5-HT transport; (2) to investigate the ability of drugs with differential selectivity for either DA or 5-HT transporters to selectively decrease cocaine self-administration; (3) to investigate the ability of drugs that inhibit DA or 5-HT transporters to reinstate extinguished cocaine self-administration (an animal model of relapse); and (4) to investigate the role of DA and 5-HT transporters in the discriminative stimulus effects of cocaine (an animal model of subjective drug effects). When completed, this research will provide additional knowledge regarding the role of DA and 5-HT transporters under several models of cocaine abuse. Results from this project may help identify potential drug treatments for cocaine abuse.