Ryan M. Drenan - US grants

DESCRIPTION (provided by applicant): Tobacco addiction is a serious threat to public health in the United States and abroad, and development of new therapeutic approaches is a major priority. Nicotine, the primary psychoactive compound in cigarette smoke, activates and/or desensitizes nicotinic acetylcholine receptors (nAChRs) throughout the brain. nAChRs in the mesolimbic dopamine (DA) pathway are crucial for the rewarding and reinforcing properties of nicotine in rodent models, suggesting that they may be key mediators of nicotine's action in humans. Although it is known that alpha4 and alpha6 nAChR subunits are important components of mesolimbic DA pathway nAChRs, the precise composition and/or stoichiometry of nAChRs in this pathway that are sufficient and/or necessary for nicotine reward remains unknown. Furthermore, the changes in the brain following nicotine exposure that give rise to reward behavior are poorly understood. This proposal will test the hypothesis that activation of alpha4alpha6beta2* (*denotes that other subunits may be present in the nAChR pentamer) nAChRs in the mesolimbic DA pathway is sufficient for nicotine's rewarding effects. Confirming or refuting this hypothesis will result in further identification of the relevant nAChR subtypes in the mesolimbic DA pathway that mediate nicotine's biophysical and/or behavioral effects. Using a combination of behavioral, electrophysiological, and biochemical techniques, we will address our hypothesis with three specific aims. In Specific Aim 1, we will test the idea that activation o alpha4alpha6beta2* nAChRs is sufficient for nicotine reward-like behavior. Using mice which express alpha6* nAChRs that are hypersensitive to nicotine, in parallel with a compound with modest functional selectivity for ?6?2* nAChRs, we will determine whether selective activation of these receptors is sufficient for nicotine CPP. In Specific Aim 2, we will use patch clamp electrophysiology to test the hypothesis that stimulation of alpha4alpha6beta2* nAChRs is sufficient to activate ventral tegmental area (VTA) DA neurons and strengthen glutamatergic input to these cells. In Specific Aim 3, we directly measure changes in nAChR and glutamatergic receptor function in DA neurons from animals exposed to nicotine. The proposed studies will enhance our understanding of the key nAChRs in the mesolimbic DA pathway that are involved in nicotine addiction. Identifying the nAChRs that are sufficient for nicotine reward should lead to a more rational and focused effort to develop new smoking cessation therapeutics.

PROJECT SUMMARY Chronic exposure to nicotine in tobacco products results in numerous health consequences (lung cancer, emphysema, hypertension, etc.) and accounts for over 6 million deaths per year. Relapse rates are high among those who attempt to quit smoking, and pharmacotherapies that seek to foster smoking cessation have limited effectiveness. Thus, there is a significant unmet need for more effective strategies to treat nicotine dependence. Nicotine exposure produces physical dependence, and the physical and/or emotional nicotine withdrawal symptoms ? as compared to the rewarding effects of nicotine ? are often the most important contributors to relapse. Unfortunately, few research studies have probed the important question of physical dependence and nicotine withdrawal mechanisms. Indeed, a critical gap in knowledge exists regarding our understanding of how chronic nicotine exposure establishes physical dependence and therefore makes smokers highly susceptible to relapse. In this project, we will use mouse models to study the medial habenula (MHB), a small brain area in the epithalamic region that has recently been implicated in nicotine withdrawal, and which expresses extraordinarily high levels of several types of nicotinic acetylcholine receptors (nAChRs). nAChRs mediate the psychoactive and addictive action of nicotine, and we intend to identify the relevant nAChRs and MHB circuits involved in nicotine dependence and withdrawal. Three independent and complementary AIMs are proposed, each of which probes a specific mechanistic aspect of the response to nicotine in the MHB. In AIM 1, we will use biophysical and optical techniques to determine through what mechanisms acute nicotine differentially activates specific cell types in MHB. In AIM 2, we will employ similar techniques to determine how chronic nicotine selectively enhances neuronal activity in a specific sub-circuit of the withdrawal pathway. Finally, in AIM 3, we will couple physiology techniques with a novel behavioral/systems approach to identify important MHB circuits involved in generating physical and/or emotional responses during nicotine withdrawal. Together, these AIMs will help us solve the problem of understanding how cessation of nicotine intake causes the brain to generate aversive physical and emotional withdrawal responses that inevitably lead to relapse. Solving this problem could lead to new strategies or drugs to foster smoking cessation.

PROJECT SUMMARY Acetylcholine (ACh) is an important neurotransmitter involved in attention, arousal, visual processing, motor control, and motivated behavior. Cholinergic transmission is perturbed in a number of devastating human disorders/diseases, including Alzheimer's disease, Parkinson's disease, major depression, and drug addiction. Muscarinic ACh receptors are GPCRs, whereas nicotinic ACh receptors (nAChRs; the subject of this proposal) are a family of Cys-loop, ligand-gated cation channels. nAChRs exist either as homopentamers containing 5 a7 subunits, or heteropentamers requiring 2 a subunits, 2 b subunits, and a 5th subunit that may be either an a or a b subunit. Studies aimed at probing nAChR function in the vertebrate brain have relied principally on rodent studies, where mouse genetic techniques have permitted key insights. A key gap in the field is the lack of suitable tools for nAChR gene editing. This R21 project will fill that gap via two independent scientific aims. In Aim 1, we will create ? for each key nAChR subunit gene ? a single vector that will catalyze CRISPR-mediated gene editing that results in a loss-of-function mutation. Guide RNA species will be validated in vitro prior to vector construction. Vectors will be further validated in vivo via gene expression analysis and patch clamp electrophysiology. This set of vectors will be useful for brain-region specific nAChR gene editing in any recipient mouse strain, including selectively-bred strains whose genetics cannot be disturbed by crosses to C57BL/6 or other common backgrounds. In Aim 2, we will create a parallel series of vectors to allow for cell type-specific nAChR gene editing. Vetted gRNAs from Aim 1 will be incorporated into a set of vectors that will be introduced into a specialized set of mouse strains that produce Cas9 nuclease in a Cre-dependent manner. As in Aim 1, we will validate this system in vivo using gene expression analyses and patch clamp electrophysiology. Ultimately, these new vectors will greatly expand our molecular toolbox, allowing for wide control over nAChR gene editing in various genetic backgrounds and in specific circuits.

PROJECT SUMMARY Chronic exposure to nicotine in tobacco products results in numerous health consequences (lung cancer, emphysema, hypertension, etc.) and accounts for over 6 million deaths per year. Relapse rates are high among those who attempt to quit smoking, and pharmacotherapies that seek to foster smoking cessation have moderate effectiveness. Thus, there is a significant unmet need for more effective strategies to treat nicotine dependence. Development of such strategies requires a more detailed understanding of the biological mechanisms leading to nicotine addiction. An essential goal related to mechanistic studies on nAChRs is gaining a better understanding of the location and activity of nAChRs in discrete sites within individual nerve cells. Although some basic research studies have begun to describe nAChR subcellular distribution, there is currently no plausible way to functionally interrogate nAChRs at the subcellular level. This means we are currently unable to determine whether the important nicotine-mediated functional alterations in nAChRs occur in dendrites, axons, presynaptic terminals, or in neuronal somata. Answering this key question is absolutely required for the field to fully understand the molecular and cellular basis for nicotine dependence. Here, we propose a R21/R33 phased innovation project that directly addresses these critical gaps in our ability to study native nAChRs. The R21 phase will develop and characterize a photoactivatable nicotine (PA-Nic) compound for use in nicotine ?uncaging? experiments. R21 Aim 1 focusses on identification of 1 or more suitable compounds using ultraviolet/visible light optical methods, while Aim 2 will characterize promising compounds using 2-photon uncaging techniques, which offer enhanced spatial resolution. In the R33 phase, we propose to employ these innovative compounds and optical methods in discovery experiments designed to uncover new details about nAChR function. R33 Aim 1 will involve functional mapping of nAChRs on key cell types involved in nicotine dependence. R33 Aim 2 will probe how these receptors enable nicotine to participate in circuit-level modulation of neurotransmission in brain's reward system. This project represents a substantial technical advance for the cholinergic biology field, as it will not only produce new tools for widespread use, but will utilize those tools to uncover new mechanistic details about nicotine dependence.