Predoctoral and Postdoctoral Training Program"Training in the Molecular Basis of Drug Abuse" T32 DA07278-20
This NIDA grant supports a successful predoctoral and postdoctoral training program designed to provide training in molecular and cellular aspects of drug abuse research. The University of Washington School of Medicine has strong research programs studying molecular aspects of drug receptor signaling mechanisms in several departments, and this training program has dramatically facilitated the coordination of training and collaboration of research effort among the drug abuse researchers at this institution. We expect that the continued application of increasingly sophisticated biochemical and physiological methods will provide important advances in our understanding of the mechanisms by which specific drugs of abuse act. It is the intent of this program to identify and support three predoctoral students and three postdoctoral fellows interested in studying molecular and cellular mechanisms of drug action of specific abused drugs. Beyond the directly beneficial effects on the careers of the trainees, one of the most significant successes of the previously funded program has been its catalytic effect on the research environment at this institution. A funded Drug Abuse Research Training Program will continue to serve as an important catalyst to focus research effort at this institution on the basic neurobiology of a significant health issue.
A list of the principal faculty members involved:
We are interested in the molecular basis of drug addiction. Drugs including heroin, cannabinoids, cocaine, and others produce specific pharmacological effects that ultimately change the functioning of neurons, reorganize plastic neural circuits, and change motivated behavior. In this lab, we take an integrative, neurobiological approach to define these changes at the molecular, cellular, anatomical and behavioral levels. Studies at the molecular level define the structural properties of the receptors and ion channels that mediate the initial actions of opiates and cannabinoids. We are interested in learning how signaling events initiated by receptor activation are regulated, and how receptors, channels, and accessory proteins assemble to form functional macromolecular complexes. At the cellular level, we study how sustained exposure to opiates produces desensitization and how phosphorylation, down regulation, and synaptic plasticity (LTP and LTD) mechanisms contribute to drug tolerance. At the anatomical level, we study the distribution of the key proteins within brain that are important for opiate action and how that distribution helps define the neuronal systems responsible for drug effects. Understanding how that distribution changes following repeated drug exposure helps define the changes underlying drug addiction. Based on these cellular and molecular insights, we generate mice having targeted changes in gene expression of key proteins in specific brain regions, then study the behavioral responses of the animals to opiates and cocaine. The ultimate goal of these studies is to gain a better understanding of the molecular basis of drug addiction, a malleable form of motivated behavior.
The nucleus accumbens had been proposed as an anatomical substrate of limbic-motor integration. It gathers information from midbrain dopaminergic neurons that are activated by salient sensory stimuli and from glutamatergic afferents from the amydala, hippocampus and prefrontal cortex that encode memory of prior responses, the emotional state and the context. This information is processed and, based on previous learning, an appropriate motor response to the stimulus is generated.
We are particularly interested in the dopaminergic input to the nucleus accumbens. Midbrain dopaminergic neurons burst fire in response to natural reinforcers or to stimuli that have been paired to their delivery. This has been presumed to result in a subsecond, transient increase in dopamine in several forebrain structures. Recently, we confirmed this by making direct chemical measurements of such changes during presentation of natural reinforcers, drug reinforcers or their paired cues. Increases in forebrain dopamine typically result in motor activation. In particular, increases in dopamine in the nucleus accumbens have been implicated in goal-directed movements. Using cocaine self-administration as a model of drug abuse, we provided the first direct evidence that phasic dopamine changes are temporally linked to, and can trigger drug-seeking behavior.
While many laboratories have focused on the contribution of dopaminergic neurons to addiction, the serotonin system also appears to play an important role. In particular, cocaine increases synaptic serotonin levels in many brain regions to the same extent as dopamine or norepinephrine. This has led to the hope that producing a better understanding of the effects of abused drugs that are mediated by serotonin may point toward new targets for pharmacological treatments of addiction. My laboratory is focusing on the roles of 5-HT1B and 5-HT6 serotonin receptors in psychostimulant addiction. We use a combination of animal models, neuropharmacological approaches, and molecular biology to address these issues. While evidence from most rat brain studies suggest that activating 5-HT1B receptors in the brain increase the rewarding properties of cocaine, knockout strategies have presented more contradictory evidence. We have taken the strategy of using molecularly and anatomically specific techniques to determine the contribution of 5-HT1B receptors in specific populations of neurons to the locomotor stimulant and rewarding properties of cocaine. We recently addressed this problem by using viral mediated gene transfer directly into nucleus accumbens shell (NAccS) neurons to address the role of 5-HT1B receptors in this important component of the brain reward pathways. These GABAergic neurons project to the ventral tegmental area (VTA), a region that has been previously implicated in 5-HT1B effects on dopaminergic function. It was previously known that GABA release in VTA is regulated by 5-HT1B receptors, but we have now provided direct evidence that the 5-HT1B terminal receptors in NAccS projections to VTA are likely to account for these observations. Increased expression of 5-HT1B in the terminals of these neurons sensitized rats to the locomotor stimulating and rewarding properties of cocaine. We have also found that chronic cocaine exposure increases the expression of 5-HT1B mRNA in NAccS. Our plan is to extend these results by systematically dissecting the role of 5-HT1B receptors in the development of cocaine sensitization, to try to determine whether both rewarding and aversive properties of cocaine are altered by increased 5-HT1B expression in NAccS, and whether these effects generalize to amphetamine. We will examine the regulation of endogenous 5-HT1B mRNA following cocaine administration further, and determine whether increased 5-HT1B expression in serotonergic neurons alters the sensitivity of animals to cocaine. We will then turn our attention to the role of 5-HT1B receptors after acute and chronic discontinuation of cocaine. In the longer term we plan to use self administration procedures to study the role of these receptors in drug and stress-induced reinstatement of cocaine self administration. 5-HT6 receptors are heavily expressed in dorsal and ventral striatum, including NAccS, where they are expressed by medium spiny neurons. Based on their signal transduction mechanisms, these receptors are likely to play a role that generally opposes 5-HT1B receptors in these neurons. We have performed some preliminary experiments that indicate that antagonists of 5-HT6 receptors injected directly into NAccS may also sensitize animals to cocaine's locomotor stimulant effects. We plan to pursue these observations in more detail, and have recently constructed a viral vector with which we can increase 5-HT6 receptor expression in these neurons. Since a number of new and selective 5-HT6 agonists and antagonists are becoming available (and we have several), we will use the complementary strategies of selective drug treatment and selective increases in receptor expression to determine the contribution of 5-HT6 receptors to cocaine's addictive features.
Our lab uses gene knockout techniques in mice to study the role of neurotransmitters in the development and function of the nervous system. Most of the work in the lab centers on the catecholamines norepinephrine and dopamine. For example, we study a line of mice that lack the ability to make dopamine (DA). Considering that enhanced dopamine signaling is implicated in the rewarding properties of virtually all drugs of abuse, the dopamine-deficient mice provide a new model for studying the actions of many different drugs. One would expect that in the absence of dopamine mice would not respond behaviorally to drugs of abuse and maybe not to even to primary rewards such as sweets. In addition to testing this hypothesis in dopamine-deficient mice with a variety of different drugs, we have also perfected means of restoring dopamine to specific brain regions, e.g. just the caudate putamen or just the nucleus accumbens. Thus, if dopamine-deficient mice fail to respond to drugs, we can ask where in the brain dopamine signaling is required for responsiveness. Our experiments so far, have validated this approach. We have shown, for example, that dopamine-deficient mice do not manifest a normal locomotor response to amphetamine, but responsiveness can be restored by viral transduction of the nucleus accumbens. At a more fundamental level, dopamine release in response to rewards (e.g. sweets) is thought to underlie certain forms of associative learning which would be predicted to help animals learn about aspects of their environment that produce a state of well being. As a first step, we are asking whether dopamine-deficient mice show a preference for natural (or artificial) sweets over water, and if so, is dopamine required for associative learning.
While less attention is directed towards norepinephrine in studies on drug abuse, there are numerous indications that it may also be involved. For example, blockade of a1-adrenoreceptors with prazosin inhibits amphetamine-induced locomotion, suggesting that norepinephrine signaling through these receptors is required for dopamine that is released by amphetamine to elicit locomotion. We have initiated studies in this area by asking whether chronic norepinephrine deficiency prevents amphetamine-induced locomotion as would be predicted from the observations described above. We discovered, quite unexpectedly, that norepinephrine-deficient mice show an enhanced locomotor response to amphetamine compared to controls. The locomotor response to amphetamine increases with successive administrations of this drug, a phenomenon referred to as sensitization. Our norepinephrine-deficient mice behave as though they are already sensitized. Another situation where norepinephrine has been implicated involves withdrawal from chronic morphine treatment. The unpleasant behavioral syndrome that is precipitated during withdrawal has been ascribed to norepinephrine release from the locus coeruleus. If this hypothesis is correct, we predict that the behavioral response to withdrawal would be attenuated in mice lacking norepinephrine. Studies exploring this possibility are underway.
My laboratory is interested in how the endogenous cannabinoids (anadamide and 2-AG) are synthesized and act in the brain. In addition, we are studying the molecular mechanisms involved in the interaction between brain tumors and microglial cells mediated by endocannabinoids. Several in vitro and in vivo experiments using rodent models of astrocytomas have shown that cannabinoid compounds, such as D9-tetrahydrocannabinol (THC), inhibit and even reverse tumor growth. To address the question of whether THC directly kills astrocytomas or modulate immune responses of microglial cells, we are investigating the cannabinoid receptors and their endogenous ligands (endocannabinoids), in brain tumors and are testing its role in astrocytomas and microglial cells interaction. Our laboratory uses a variety of models to assess the functionality of the cannabinoid signaling system in brain tumors. Among these are astrocytes and microglia in primary culture, cultured astrocytomas (both primary and transformed cell lines), as well as human biopsy tissue and an animal model of injected astrocytomas. We are also characterizing the anti-tumor properties of various cannabinoid compounds in these models. Our broad, long-term goal is to understand the molecular mechanisms underlying immune surveillance in the brain and the biology of microglial cell activation in the presence of brain tumors. This research should provide novel insight into the pathogenesis of brain tumors and open alternative therapeutic avenues.
The overall goal of my research program is to use a multi-level approach, combining molecular biology, anatomy, genetics and behavioral neuroscience, to understand the role of cortico-basal ganglia circuitry in the development of behaviors that are associated with drug reward and addiction, as well as in the processes that underlie decision-making, motivation and impulsivity. To accomplish these goals, my laboratory employs a novel chemical-genetic approach that uses viral vectors to express artificial, engineered G-protein coupled receptors (known as DREADD receptors) in discrete neuronal cell populations in rodents. Activation of DREADD receptors by the otherwise inert synthetic ligand clozapine-N-oxide will lead to transient alterations in neuronal activity (either increasing or decreasing cell function depending on which G-protein coupled DREADD receptor is expressed) of the targeted cell populations. This neuronal modulation can be paired with specific phases of the behaviors that we study, including psychostimulant-induced behavioral sensitization, drug self-administration and operant learning tasks, in order to parse out the neural circuitry that contributes to behaviors associated with addiction and other neuropsychiatric disorders.
The brain is comprised of numerous discrete nuclei defined by anatomical location, structure, function, and gene expression profiles. Interconnections of the excitatory and inhibitory neurons within and between these structures are the basic components of neural circuits. Electrochemical signals within a circuit generate activity patterns that ultimately provide the substrate upon which the brain performs functional operations.
My lab is particularly interested in the limbic system of the brain. Comprised of multiple interconnected and parallel circuits, the limbic system is critical for determining where we are, where we have been, what we are doing, what we plan to do, and how all of it makes us feel. Perturbations in the functioning of one or multiple components of the limbic system are a major contributor to mental disorders, affecting approximately 1 in 4 adults. My research is directed toward understanding how a small group of neurons located in the ventral midbrain that synthesize and release the neurotransmitter dopamine influence activity patterns within the limbic system to direct behavior.
Dopamine release into target brain regions can either be tonic (low steady levels of neurotransmitter) or phasic (transient high concentrations of neurotransmitter), and these patterns are thought to play distinct roles in modulating the function of the limbic system. The mechanisms responsible for determining these patterns of activity are thought to involve distinct neurotransmitter systems impinging upon dopamine neurons, as well as a complement of neurotransmitter receptors and ion channels. To dissect how selective afferents, neurotransmitter systems, neurotransmitter receptors, and ion channels regulate patterns of dopamine neuron activity, we utilize a multi-disciplinary approach involving conditional gene activation or inactivation and combinatorial viral vector approaches to alter expression of genes at numerous levels within the circuit. Multiunit in vivo electrophysiology and fiber-optic fluorescence microscopy are used to monitor activity patterns and intracellular signaling events within select neural populations. These techniques are integrated with behavioral assays to provide the ultimate link between sensory input, generation of activity within a circuit, and behavioral output.
A list of the trainees supported:
Contact C Chavkin for additional information, email@example.com