The Role of Neuropeptide Y (NPY) in Uncontrolled Alcohol Drinking and Relapse Behavior Resulting from Exposure to Stressful Events

TECHNICAL ABSTRACT

Background: People who have been exposed to an extremely traumatic event, such as witnessing a death, receiving a death threat, or experiencing a serious injury, may develop a set of symptoms known as posttraumatic stress disorder (PTSD). Events that contribute to the development of PTSD are common to individuals placed in a combat environment. Evidence suggests that there is a high comorbidity between PTSD and alcohol dependence. For example, the rate of alcohol abuse or dependence in the general population is estimated to be between 7% and 9%, while rates in individuals suffering from diagnosed PTSD have been estimated to be anywhere between 16% and 68%. Given the prevalence of PTSD among veterans of war and the increased risk of alcoholism for individuals suffering from PTSD, identifying pharmacological targets with potential therapeutic value in treating PTSD-associated alcoholism may be considered of high relevance to the U.S. military. An interesting candidate is neuropeptide Y (NPY), a neurochemical that is present throughout the central nervous system. NPY is involved with a diverse set of biological functions including the integration of emotional behavior such as anxiety and depression. Interestingly, recent evidence suggests that low NPY levels and deletion of NPY or the NPY Y1 receptor promote high alcohol consumption. Furthermore, combat-related PTSD is associated with decreased plasma levels of NPY, and uncontrolled stress caused by exposure to military survival training results in depletion of plasma NPY levels following extended exposure. Because low NPY levels promote increased alcohol intake, reduced NPY associated with PTSD may be a factor that leaves individuals susceptible to alcoholism.

Objective: The objective of the present research is to test the guiding hypothesis that normal NPY signaling, via the NPY Y1 receptor, protects against uncontrolled alcohol self-administration and relapse caused by exposure to stressful events. To achieve our goals, two rodent models of human alcoholism will be employed, one modeling uncontrolled alcohol drinking and the other modeling relapse.

Specific Aims: Aim 1 of this proposal will determine whether genetically altered mice lacking production of NPY or the NPY Y1 receptor are more susceptible to the effects of stress on uncontrolled alcohol self-administration following a period of alcohol deprivation. Follow-up studies will determine whether stress-induced increases of ethanol self-administration can be rescued by a recombinant adeno-associated virus (rAAV) vector that causes expression and constitutive secretion of NPY (rAAV-FIB-NPY). These studies will establish whether normal NPY signaling protects against the effects of stress on uncontrolled alcohol self-administration. Aim 2 of this proposal will determine whether genetically altered mice lacking NPY or the NPY Y1 receptor display enhanced sensitivity to stress-induced relapse of alcohol-seeking behavior when compared to normal mice. Follow-up studies will determine whether stress-induced relapse behavior by mice can be rescued by the rAAV-FIB-NPY vector.

Study Design: One of the primary criteria of alcohol dependence is loss of control over drinking and relapse following self-imposed abstinence. Procedures have been developed and validated as animal models of uncontrolled drinking. After a period of alcohol deprivation, rodents consume significantly greater amounts of alcohol than they had consumed prior to the deprivation period. This phenomenon has been labeled the alcohol deprivation effect (ADE) and is thought to model compulsive uncontrolled relapse drinking, characteristic of alcohol-dependent humans. Interestingly, exposure to restraint-stress during alcohol deprivation periods significantly enhances uncontrolled drinking when alcohol is returned. We have developed an ADE paradigm in mice using operant self-administration procedures. After responding for alcohol has stabilized, mice are deprived of access to alcohol for several days. Mice show dramatic increases of operant responding for alcohol immediately after the deprivation period. Here, we will assess the effects of shock-induced stress on post-deprivation alcohol responding in mutant mice lacking NPY or the NPY Y1 receptor. A second procedure models stress-induced relapse of alcohol self-administration. To model relapse, mice will first learn to work for alcohol reinforcement by pressing an operant lever. Then, animals will be placed under an extinction protocol where operant responding no longer results in alcohol delivery. Animals quickly learn to discontinue operant responding under these conditions. Importantly, previously extinguished alcohol responding is robustly reinstated if rodents are exposed to a footshock stressor. We will assess the effects of shock-induced relapse responding in mutant mice lacking NPY or the NPY Y1 receptor. Finally, a rAAV-FIB-NPY vector will be transduced into the amygdala of mice in an attempt to prevent the effects of footshock stress.

Relevance: This work will provide important insight into the role of NPY signaling in stress-induced uncontrolled alcohol self-administration and relapse. Such results may lead to the development of compounds useful for treating PTSD-associated alcoholism. Because the risk of PTSD and alcoholism are elevated in individuals placed in a combat environment, this research is highly relevant and should be beneficial to the military.