Molecular mechanisms underlying alcohol use disorder and major depressive disorder comorbidity

Alcohol Use Disorder (AUD) and Major Depressive Disorder (MDD) are two widespread and debilitating disorders that share a high rate of comorbidity with the presence of either disorder doubling the risk of developing the other. Despite their prevalence, few treatments are available to individuals with comorbid AUD and MDD. Both alcohol and antidepressants promote lasting neuroadaptive changes in synapses and dendrites. With alcohol these changes may provide relief from depressive symptoms, and the initial use of alcohol may be a form of self-medication for individuals with MDD, suggesting ethanol may have antidepressant properties underlying similarities in neurobiological abnormalities. However, the synaptic pathways that are shared by alcohol and antidepressants are unknown. This study aims to identify why acute exposure to ethanol produced lasting antidepressant and anxiolytic behaviors. To understand the functional basis of these behaviors, a molecular pathway activated by rapid antidepressants was investigated. Here ethanol, like rapid antidepressants, altered γ-aminobutyric acid type B receptor (GABA [subscript B] R) expression and signaling, to increase dendritic calcium. New GABA [subscript B] Rs were synthesized in response to ethanol treatment, requiring fragile-X mental retardation protein (FMRP). Ethanol-dependent changes in GABA [subscript B] R expression, dendritic signaling, and antidepressant efficacy were absent in Fmr1-knockout (KO) mice. These findings indicate that FMRP is an important regulator of protein synthesis following acute alcohol exposure, and provided a molecular basis for the antidepressant efficacy of acute ethanol exposure. We identify alterations on a global scale with acute alcohol and antidepressant by sequencing the synaptic transcriptome. We identified parallel alterations in exon usage with acute alcohol and antidepressant treatment. These shared differentially expressed exons may give rise to isoforms and proteins with altered function or localization in the synapse. Some of these differentially expressed exons were identified in genes known to have alternative isoforms with AUD and MDD. These data implicate alternative splicing and isoform expression in the acute antidepressant-like effects of ethanol and the development of comorbid alcohol and depression. Understanding the molecular basis for comorbidity may aid in development of treatment options for afflicted individuals with dual disorders, as well as explore the mechanism for the initiation of addiction with acute exposure to alcohol