There is a growing body of evidence that endogenous opioid peptides, including proopiomelanocortin (POMC)- derived b-endorphin, can modulate the neurobiological responses to ethanol and that administration of ethanol alters the expression of POMC and b-endorphin. Given that ethanol has direct effects on POMC activity, it is possible that the other POMC-derived peptides, namely the melanocortins (MCs), are also involved with neurobiological responses to ethanol. MC peptides include a-melanocyte stimulating hormone (a-MSH), which is synthesized in the arcuate nucleus of the hypothalamus and projects to many brain regions of known relevance to alcoholism. Agouti-related protein, synthesized in the arcuate nucleus and secreted in the same terminals as a-MSH, is a natural MC receptor (MCR) antagonist. Consistent with a role in modulating neurobiological responses to ethanol, recent work has shown that MCR agonists reduce, and MCR antagonists increase, ethanol consumption in rodents, and that the MC-4 receptor (MC4R) modulates the effects of MCR compounds on ethanol drinking. Additionally, exposure to ethanol significantly reduces central a-MSH immunoreactivity (IR), and increases central AgRP IR, indicating that these endogenous MCR ligands modulate neurobiological responses to ethanol.
The specific aims proposed below will extend our recent findings by testing the guiding hypothesis that MC4R signaling modulates the reinforcing properties of ethanol and ethanol relapse-like behaviors, in a brain-region-specific and protein kinase A (PKA)- dependent fashion. Specifically, we will determine if operant self-administration of ethanol alters a-MSH, AgRP, MC3R and/or MC4R IR in specific brain regions (Specific Aim 1), if a MC4R agonist modulates ethanol self-administration in brain regions implicated in ethanol reinforcement (Specific Aim 2), if MC4R signaling requires normal PKA activity to modulate operant self-administration of ethanol (Specific Aim 3), and if MC4R signaling modulates relapse-like behaviors (Specific Aim 4). These studies will provide important insight into the mechanisms by which MC4R signaling modulates the reinforcing properties of ethanol and relapse of ethanol-seeking behaviors.
Despite years of research and a wealth of important information, the neurobiological factors that leave some individuals vulnerable to alcohol abuse and alcoholism remain unclear. One hypothesis is that some individuals show increased sensitivity to the reinforcing properties of alcohol, leading to increased early use and a subsequent increased risk of dependence. The work that is outlined in the current proposal will show that melanocortin neuropeptides modulate the reinforcing properties of alcohol and relapse of alcohol-seeking behavior following abstinence.
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