The mechanisms that underlie the development and maintenance of ethanol self-administration are not fully understood, but the involvement of dopamine in the mesolimbic system has been hypothesized for some time. Many neurochemical systems are known to regulate the activity of the mesolimbic dopamine system, for example, the opioid peptides and their receptors. Naltrexone, a clinically effective drug used to reduce relapse in alcoholism, is a broad spectrum opioid receptor antagonist, but its mechanism is not known in detail. Mu opioid receptors are anatomically located in the ventral tegmental area (VTA) and are thought to control VTA dopamine neuron activity through the inhibition of GABA interneurons. However, the role of this mechanism in ethanol's regulation of dopamine release, and the role of particular opioid receptor subtypes has not been firmly established. Our preliminary studies show that mu opioid receptors are involved in the mechanism by which ethanol stimulates dopamine release in the shell of the nucleus accumbens in the Long-Evans rat. However, it is still unknown whether mu opioid receptors are involved in the stimulation of mesolimbic dopamine system before or during voluntary ethanol administration. We propose to determine the dose- dependent effects of naltrexone (non-selective opioid receptor antagonist) and beta-funaltrexamine (selective mu opioid receptor antagonist on accumbal dopamine during operant self-administration of ethanol (Aim 1). Finally, the mechanism of the stimulation of mesolimbic dopamine during operant self-administration may involve an increase in the firing rate of dopamine neurons in the ventral tegmental area, but this has not been measured in vivo. We propose to do this in Aim 2. Together, the proposed experiments will help elucidate the role of mu opioid receptors in the modulation of mesolimbic dopamine activity by ethanol in a behaviorally relevant context.
The proposed studies on alcohol drinking behavior are highly relevant to humans. A significant proportion of U.S. citizens suffer from the deleterious effects o excessive alcohol drinking, and this leads to tremendous suffering of their families as well as society. Therefore, understanding the effects of alcohol on basic brain function and behavior and the underlying mechanisms in a rat model can provide information about public policy, prevention, and medical interventions in humans.
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