Investigation of non-canonical opioid signaling in the prefrontal cortex of alcohol-dependent rats

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Project Abstract Current treatments of Alcohol Use Disorder (AUD) have shown moderate success in reducing heavy alcohol drinking but are marred with the problem of treatment adherence. In the clinic, opiate receptor pharmaceutics are often combined with cognitive behavioral therapies to improve long-term abstinence. These findings suggest an important relation between endogenous opioid signaling and cognitive factors in the treatment of AUD; however, lapses in treatment can quickly reinstate alcohol drinking, highlighting a knowledge gap in our understanding of how these mechanisms may influence long-term misuse. In this regard, alcohol consumption stimulates the release of endogenous opioids, including enkephalins that bind to mu-type opioid receptors (MOR) commonly found in limbic areas of the brain. The manipulation of MOR signaling alters the rewarding properties of alcohol, with agonists facilitating reward and consumption, and antagonists blocking these responses. The shared relation between opioidergic responses that underlie motivation and cognition are not well understood but present a focal point for addressing complex pathologies that may underlie alcohol-related sensitivities. In this regard, MORs are found in frontal cortical regions that modulate cognitive function, such as the medial prefrontal cortex (mPFC). Preclinical studies in our laboratory demonstrate that alcohol dependence in rats decreases the phosphorylation of MOR in the mPFC, and increases expression of the neuropeptide precursor, proenkephalin (PENK). This pattern of changes overlaps with clinical observations of MOR desensitization in AUD patients. The findings suggest that dependence may dysregulate opioidergic signaling in the mPFC, although the extent to which such conditions surmise changes in the endogenous ligands is unclear. A better understanding is warranted given that adherence to opiate antagonists diminishes over protracted abstinence and may be undermined by molecular intermediates in the processing of small-opioid peptides. Here, we will explore the central hypothesis that non-canonical PENK signaling in the mPFC plays a pivotal role in dysregulating cognitive function during abstinence. Towards this goal, discovery-based and quantitative mass spectrometry approaches will be combined with in vivo microdialysis to broadly capture PENK-mediated signaling in alcohol-dependent rats undergoing abstinence (Aim 1). We will then explore the functional relevance of non-canonical PENK signaling in an operant model of cognitive flexibility, and further determine whether similar dysfunctions in dependence are modulated by mPFC MOR (Aim 2). The results are expected to provide insight into druggable targets that extend beyond conventional opioidergic signaling processes and will lay the foundation for mechanistic studies exploring the role of non-canonical PENK signaling in driving vulnerability to addiction behavior.