Glycogen synthase kinase 3 (GSK-3) is a constitutively active serine-threonine kinase that regulates numerous signaling pathways and has been implicated in neurodegenerative and neuropsychiatric diseases including Alzheimer's, Parkinson's, schizophrenia, and bipolar disorder. Evidence indicates that alcohol exposure increases GSK-3β (ser9) phosphorylation (pGSK-3β); however, few studies have investigated whether GSK-3 regulates the positive reinforcing effects of alcohol, which drive repetitive drug use. This study aimed to address the potential role of GSK-3 in alcohol self-administration by investigating whether direct pharmacological inhibition of GSK-3 alters the positive reinforcing effect of alcohol in mice. Male C57BL/6J mice were trained to lever press on a fixed-ratio 4 schedule of sweetened alcohol or sucrose-only reinforcement in operant conditioning chambers. The GSK-3 inhibitor CHIR 99021 trihydrochloride (0-10 mg/kg, ip) was injected 45-min prior to self-administration sessions in a counterbalanced design. After completion of the self-administration dose-effect curve, potential locomotor effects of the GSK-3 inhibitor were assessed. To determine molecular efficacy, CHIR 99021 (10 mg/kg, ip) was evaluated on pGSK-3β, GSK-3β PICK1, and AMPA receptor GluA2 subunit protein expression in amygdala, nucleus accumbens (NAcb), and frontal cortex in the absence of alcohol. CHIR 99021 (10 mg/kg) dose-dependently increased alcohol reinforced responding with no effect on sucrose self-administration or locomotor activity. CHIR 99021 (10 mg/kg) significantly decreased pGSK-3β expression in all brain regions tested, reduced PICK1 and increased GluA2 total expression only in the NAcb. We conclude that GSK-3 inhibition increased the reinforcing effects of alcohol in mice. This was associated with reduced pGSK-3β and PICK1, and increased GluA2 protein expression. Given prior results showing that AMPA receptor activity regulates alcohol selfadministration, we propose that signaling through the GSK-3 / PICK1 / GluA2 molecular pathway drives the positive reinforcing effects of the drug, which are required for abuse liability.

Protein expression profiling identified an array of proteins and networks in the NAcb, including GSTP1, that are novel molecular targets of chronic alcohol drinking. Pharmacological inhibition of GSTP1 significantly reduced the positive reinforcing effects of alcohol, which regulate repetitive use and abuse liability. The observation that this protein was both upregulated after chronic drinking and that its inhibition could modulate the reinforcing properties of alcohol suggests that it is a key target for alcohol-related pathologies. Proteomic strategies combined with specific preclinical models has potential to identify and validate novel targets of alcohol that may be useful in the medical management of alcohol addiction.