Effects of Cocaine Exposure on Astrocytic Glutamate Transporters and Relapse-Like Ethanol-Drinking Behavior in Male Alcohol-Preferring Rats

Abstract

Aim: Glutamate has been considered as neurotransmitter that is critical in triggering relapse to drugs of abuse, including ethanol and cocaine. Extracellular glutamate concentrations are tightly regulated by several mechanisms, including reuptake through glutamate transporters. Glutamate transporter type 1 (GLT-1) is responsible for clearing the majority of extracellular glutamate. The astrocytic cystine/glutamate antiporter (xCT) regulates also glutamate homeostasis. In this study, we investigated the effects of cocaine exposure and ampicillin/sulbactam (AMP/SUL), a β-lactam antibiotic known to upregulate GLT-1 and xCT, on relapse-like ethanol intake and the expression of astrocytic glutamate transporters in mesocorticolimbic brain regions.

Methods: Male alcohol-preferring (P) rats had free access to ethanol for 5 weeks. On Week 6, rats were exposed to either cocaine (20 mg/kg, i.p.) or saline for 12 consecutive days. Ethanol bottles were then removed for 7 days; during the last 5 days, either AMP/SUL (100 or 200 mg/kg, i.p.) or saline was administered to the P rats. Ethanol bottles were reintroduced, and ethanol intake was measured for 4 days.

Results: Cocaine exposure induced an alcohol deprivation effect (ADE), which was associated in part by a decrease in the expression of GLT-1 and xCT in the nucleus accumbens (NAc) core. AMP/SUL (100 mg/kg, i.p.) attenuated the ADE, while AMP/SUL (200 mg/kg, i.p.) reduced ethanol intake during 4 days of ethanol re-exposure and upregulated GLT-1 and xCT expression in the NAc core, NAc shell and dorsomedial prefrontal cortex (dmPFC).

Conclusion: This study suggests that these astrocytic glutamate transporters might be considered as potential targets for the treatment of polysubstance abuse.

Fig. 1.

Experimental timeline for relapse-like intake.

Fig. 2.

Average daily ethanol consumption (A), ethanol preference (B), water intake (C) and body weight (D) in male P rats during the baseline period (Week 4 and 5) and ethanol re-exposure (Day 1–Day 4) (mean ± SEM). Two-way mixed model ANOVA with repeated measures showed a significant increase in ethanol consumption and preference during the 4 days of ethanol re-exposure in the ethanol-cocaine-saline group as compared to the ethanol-saline group. Rats exposed to cocaine and treated with AMP/SUL (100 mg/kg, i.p.) showed no significant increase in ethanol consumption and preference compared to the ethanol-saline group, while rats exposed to cocaine and treated with AMP/SUL (200 mg/kg, i.p.) revealed a decrease in ethanol intake during the 4 days of ethanol re-exposure compared to the ethanol-saline group. A significant decrease in water intake was observed during the 4 days of ethanol re-exposure in the ethanol-cocaine-saline group as compared to the ethanol-saline group. Rats exposed to cocaine and treated with AMP/SUL (100 mg/kg, i.p.) showed no significant increase in water intake compared to the ethanol-saline group, while rats exposed to cocaine and treated with AMP/SUL (200 mg/kg, i.p.) revealed an increase in water intake during the 4 days of ethanol re-exposure compared to the ethanol-saline group. No significant effect of groups on body weight was revealed (^*P < 0.05, ^**P < 0.01, ^***P < 0.001 and ^****P < 0.00001), (n = 7–10 for each group). Abbreviations: WS, water-saline; ES, ethanol-saline; ECS, ethanol-cocaine-saline; ECA/S-1, ethanol-cocaine-AMP/SUL 100; and ECA/S-2, ethanol-cocaine-AMP/SUL 200.

Fig 3.

The effects of cocaine (20 mg/kg) and AMP/SUL (100 and 200 mg/kg) on GLT-1/β-tubulin (A), xCT/β-tubulin (B) and GLAST/β-tubulin (C) in the NAc core following 4-day ethanol re-exposure (mean ± SEM). One-way ANOVA revealed a significant reduction in GLT-1/β-tubulin and xCT/β-tubulin in the ethanol-cocaine-saline-treated group and an increase in GLT-1/β-tubulin and xCT/β-tubulin in the ethanol-cocaine–AMP/SUL (200 mg/kg) treated group. No significant difference was revealed in GLAST/β-tubulin between groups. (*P < 0.05, **P < 0.01 and ***P < 0.001), (n = 5–7 for each group). Abbreviations: WS, water-saline; ES, ethanol-saline; ECS, ethanol-cocaine-saline; ECA/S-1, ethanol-cocaine-AMP/SUL 100; and ECA/S-2, ethanol-cocaine-AMP/SUL 200.

Fig 4.

The effects of cocaine (20 mg/kg) and AMP/SUL (100 and 200 mg/kg) on GLT-1/β-tubulin (A), xCT/β-tubulin (B) and GLAST/β-tubulin (C) in the NAc shell following 4-day ethanol re-exposure (mean ± SEM). One-way ANOVA revealed an increase in GLT-1/β-tubulin and xCT/β-tubulin in the ethanol-cocaine-AMP/SUL (200 mg/kg) treated group. No significant difference was revealed in GLAST/β-tubulin between groups. (**P < 0.01 and ***P < 0.001), (n = 5–7 for each group). Abbreviations: WS, water-saline; ES, ethanol-saline; ECS, ethanol-cocaine-saline; ECA/S-1, ethanol-cocaine-AMP/SUL 100; and ECA/S-2, ethanol-cocaine-AMP/SUL 200.

Fig 5.

The effects of cocaine (20 mg/kg) and AMP/SUL (100 mg/kg and 200 mg/kg) on GLT-1/β-tubulin (A), xCT/β-tubulin (B) and GLAST/β-tubulin (C) in the dmPFC following 4-day ethanol re-exposure (mean ± SEM). One-way ANOVA revealed increases in GLT-1/β-tubulin and xCT/β-tubulin in the ethanol-cocaine-AMP/SUL (200 mg/kg) treated group. No significant difference was revealed in GLAST/β-tubulin between all groups. (*P < 0.05), (n = 5–7 for each group). Abbreviations: WS, water-saline; ES, ethanol-saline; EDS, ethanol-cocaine-saline; ECA/S-1, ethanol-cocaine-AMP/SUL 100; and ECA/S-2, ethanol-cocaine-AMP/SUL 200.