Effects of Glutamate Transporter Modulators in Chronic Ethanol and 6-Methylnicotine Exposure

Abstract

Alcohol use disorder (AUD) and nicotine dependence are among the most prevalent substance use disorders globally. Neurobiologically, ethanol and nicotine exposure share overlapping neuropharmacological mechanisms that involve both glutamatergic and dopaminergic pathways. Chronic exposure to drugs of abuse, including ethanol and nicotine, disrupts glutamate homeostasis in mesocorticolimbic brain regions. Extracellular glutamate uptake is predominantly regulated by the astrocytic glutamate transporter 1 (GLT-1), with the cystine/glutamate antiporter (xCT) contributing to this regulatory effect. Several studies have shown that chronic ethanol consumption downregulates the expression of GLT-1 and xCT in brain regions mediated drug reward and reinforcement, including the nucleus accumbens (NAc) and the medial prefrontal cortex (mPFC). The research presented in this dissertation investigated the effects of two novel beta-lactam compounds, MC-100093 and MC-290401, on ethanol drinking behavior and the expression of these astrocytic glutamate transporters. Specifically, the effects of MC-100093 on glutamate uptake and GLT-1 associated signaling pathways were assessed in male and female alcohol-preferring (P) rats. Additionally, the effects of MC-100093 on ethanol-induced liver injury was examined, as well as effects on the glutamatergic systems, following repeated exposure to nicotine and its synthetic analog, 6-methylnicotine (6-MN). Male and female P rats were given continuous free-choice access to ethanol and water for five weeks, followed by five days of treatment with MC-100093, MC-290401, or saline. Chronic ethanol consumption downregulated GLT-1 and xCT expression in the subregions of the mPFC and NAc, and these effects were different between sexes, indicating possible sex differences within these neurocircuits. MC-100093 treatment reduced ethanol consumption and preference, upregulated GLT-1 and xCT expression in the mPFC and NAc subregions, and increased Na+-dependent and Na+-independent glutamate uptake in the NAc. This upregulation of GLT-1 was associated with increased nuclear NF-κB expression and Akt phosphorylation, suggesting that the Akt/NF-κB signaling pathways may mediate the effect of MC-100093 on GLT-1 expression. Similarly, treatment with the novel beta-lactam MC-290401 reduced ethanol intake in both sexes and reversed ethanol-induced GLT-1 and xCT downregulation in the NAc. In the liver, five weeks of ethanol consumption increased hepatic GLT-1, xCT, triglycerides, and inflammatory cytokines in both sexes. MC-100093 reversed these changes and attenuated ethanol-related liver injury. Repeated exposure to 6-MN in male Wistar rats decreased GLT-1 in the NAc, while nicotine had no effect on GLT-1, xCT, or α7 nAChR expression. MC-100093 treatment restored and increased GLT-1 and xCT expression in both nicotine- and 6-MN-exposed groups. Overall, these findings demonstrate that MC-100093 and MC-290401 attenuated ethanol consumption in male and female P rats, in part, by upregulating of GLT-1 and xCT in the NAc, and that MC-100093 mitigates ethanol-induced hepatic injury in part by modulating hepatic glutamate transporters.