Cheryl L. Kirstein - US grants

The role of the mesolimbic dopaminergic pathway (specifically the nucleus accumbens septi, NAcc) in reward has been well-documented in ,adult animals. Dnigs of abuse, such as alcohol increase dopamine (DA) levels in the NAcc of adult rats. Similarly, in many studies drug ,expectancy has been shown to increase DA in the adult NAcc. As a result, several studies have implicated this pathway as a potential neural substrate for drug abuse. In humans, drug abuse patterns are often established in adolescence, not adulthood; this is especially true of alcohol. A limited number of studies have examined changes in the NAcc in response to alcohol during development. The establishment of an animal model to study this reward system during early development and adolescence is critical. The data show that drug use begins around adolescence and continues into adulthood. Moreover, development of the brain is still ongoing during this period and this may be critical in elucidating the development of addiction. To this end, we modified and adapted the in vivo micro dialysis procedure to enable us to effectively and reliably recover DA from the NAcc of young rat pups. The dialysis procedure allows measurement of the neurochemical changes resulting from drug administration. The present studies propose to use in vivo micro dialysis to examine: 1. the effects of ethanol on the NAcc in preadolescent rats (postnatal day 25; PND 25) and 2. the function of the mesolimbic pathway in periadolescent (PND 35, 45) and adult animals (PND 60) after repeated ethanol exposure (n=bidaily injections for 4 days at each age) during these different periods of development. The principal goals of these proposed studies are: first, to isolate the dose-response effects of ethanol exposure on the mesolimbic DA pathway in male and female preadolescent rats to determine appropriate low and high doses for use in the repeated administration experiments; second, to examine the acute vs. repeated effects of ethanol on the mesolimbic DA pathway in preadolescent, periadolescent and adult animals; third to see how these processes are altered in periadolescent and adult animals following repeated administration of ethanol during adolescence and adulthood; and fourth to determine the effects of ethanol expectancy on the function of the mesolimbic DA system in preadolescent, pefiadolescent and adult animals. We will examine how this system responds to ethanol administration, how these responses differ between preadolescent, periadolescent and adult animals, and whether drug expectancy following repeated administration is sufficient to elicit the same neurochemical responses (ie., whether there are "expectancy-induced" increases in accumbal DA in response to saline alone after repeated ethanol). These studies will provide insight as to alterations in mesolimbic DA function following repeated exposure during a time when the brain reward system is developing. We have previously reported ethanol-induced increases in DA efflux in the NAcc of preadolescent rats. The proposed studies will allow us to examine the underlying mechanism of ethanol's effects in young animals which is critical in order to understand how these processes control the initiation of drug use. Moreover, we will be able to compare these responses across ages which is critical to understand the mechanisms which may underlie the continued maintenance of ethanol abuse.

DESCRIPTION (provided by applicant): The role of the mesolimbic dopamine (DA) pathway (specifically the nucleus accumbens septi. NAcc) in reward has been well-documented in adult animals. Drugs of abuse, such as cocaine increase DA levels in the NAcc of adult rats. Similarly, in many studies drug "expectancy" or anticipation has been shown to increase accumbal DA in the adult. As a result, several studies have implicated this pathway as a neural substrate mediating drug abuse. In humans, drug abuse is often established in adolescence not adulthood: this is especially true of cocaine, Unfortunately, few studies have examined changes in the NAcc in response to cocaine during adolescence. Studying the functional responsiveness of this reward system and the effects of cocaine during adolescence is critical. The data show that drug use begins around adolescence and continues into adulthood. Moreover, development of the brain is still ongoing during this period, and the DA system may be critically altered by cocaine exposure. To this end, we modified and adapted the in vivo microdialysis procedure to enable us to effectively and reliably recover DA from the NAcc of young rats. The dialysis procedure allows measurement of the neurochemical changes resulting from cocaine. The present studies propose to use microdialysis to examine the effects of acute, repeated or "expected" cocaine on the NAcc in preadolescent (postnatal day 25; PND 25), periadolescent (PND 35, 45) and adult (PND 60) animals. Additionally, behavioral studies will assess responsiveness to cocaine using a conditioned place preference (CPP) paradigm and novelty-seeking behavior to assess age-related vulnerability to cocaine's effects. Finally, we propose to examine the effects of repeated cocaine at these critical ages on functioning of the adult DA system. The principal goals of the proposed studies are to: first, determine the effects of acute, repeated or "expected" cocaine on mesolimbic DA in PND 25, 35, 45 and 60 rats; second, establish CPP across age; third, measure novelty-seeking differences across age and DA activity in high vs. low responders and finally, determine effects of cocaine exposure during critical periods of adolescence on later DA responsiveness. It is our basic hypothesis that periadolescent animals are unique in their response to cocaine and that repeated administration during this time might down-regulate the basal functioning of this developing system but increase responsiveness to subsequent drug administration and therefore drive repeated drug use. The proposed studies will allow us to examine the underlying mechanism of cocaine's effects in adolescence which is critical in order to understand how these processes control the initiation and maintenance of cocaine abuse.

DESCRIPTION (provided by applicant): Elucidating the impact of alcohol on the adolescent brain is of paramount importance not only to understand the nature of the addictive process but also to prevent the progression from casual alcohol use to dependency. The overall goal of this proposal is to test the hypothesis that the adolescent brain is uniquely susceptible to physiological changes induced by ethanol and that these ethanol-induced perturbations during adolescence persist into adulthood rendering the animals more vulnerable to its addicting effects. Specifically, we propose that ethanol exposure during adolescence alters key neuronal circuitry and that these changes persist into adulthood with consequent changes in the normal functions of this pathway that may be responsible for the predisposition of the "ethanol-exposed" brain to ethanol use as compared to the "ethanol-naive" brain. The proposed experiments are designed to determine sex differences in the long-term effects of chronic exposure to experimenter administered ethanol (or saline) during adolescence [postnatal day (PND) 30-50] or adulthood (PND 60-80) in male and female rats. Following ethanol (or saline) exposure, adolescent-exposed and adult- exposed rats will be tested as adults (PND 74 or PND 104, respectively) for the effects of ethanol preexposure on ethanol consumption via voluntary sweetened ethanol intake (Specific Aim 1). For a neurochemical measure, ethanol-induced dopaminergic responses of the nucleus accumbens septi, likely to be affected and involved sequentially in the progression from alcohol use to alcohol dependency, will be measured in response to experimenter-administered ethanol (0.75, 1.75 g/kg/ip ethanol) or saline using in vivo microdialysis (Specific Aim 2). Blood and brain ethanol concentrations will be examined in all proposed experiments to determine possible sex and pretreatment differences in the metabolism of ethanol. Additionally, estrous cycling will be evaluated in all females during adulthood to investigate the effects of changes in the estrous cycle on long term behavioral and neurochemical consequences of chronic ethanol exposure during adolescence or adulthood. Results from these studies will clearly delineate the lasting impact of chronic exposure to ethanol during adolescence as compared to adulthood on the subsequent adult brain and behavior. Information on the alteration of ethanol consumption patterns and neurochemical processes of the brain that render animals susceptible to ethanol addiction is critically important for understanding the addictive process and how it may be prevented or treated. PUBLIC HEALTH RELEVANCE: These studies will examine the long term effects of ethanol on the adult male and female brain and behavior following ethanol exposure during adolescence or adulthood.