Upregulation of GLT1 attenuates cue-induced reinstatement of cocaine-seeking behavior in rats

Abstract

Relapse to cocaine-seeking behavior depends on increased glutamate transmission in key regions of the mesocorticolimbic motive circuit, including prefrontal cortex (PFC) and nucleus accumbens (NAcc). Because GLT1 is responsible for the uptake of >or=90% of extracellular glutamate, we tested the hypothesis that increased GLT1 expression attenuates cocaine relapse. Rats were trained to self-administer cocaine (0.125 mg per intravenous infusion) in a lever-pressing task in a daily 2 h session for 10-14 d followed by 5 d of extinction training. Immediately after each extinction session, rats received ceftriaxone (intraperitoneally), a beta-lactam antibiotic believed to increase GLT1 expression, or vehicle. On the following day, presentation of the cue (light and tone) previously associated with cocaine self-administration reinstated lever pressing in rats treated with vehicle, whereas 100 or 200, but not 50 mg/kg ceftriaxone blocked this response. Immunoblotting confirmed that the ceftriaxone-induced blockade of cocaine relapse was associated with an increase in GLT1 expression in both PFC and NAcc. In separate groups of rats, 200 mg/kg ceftriaxone failed to block cue-induced food seeking, arguing against a ceftriaxone-induced effect unique to extinction training or lever pressing. Our results suggest that glutamate plays a key role in cue-induced relapse to cocaine-seeking behavior, implicating GLT1 as a potential therapeutic target for cocaine addiction.

Figure 1.

Active-lever responses during the last 4 d of cocaine SA and during each extinction session. A two-way, repeated-measures ANOVA revealed no significant differences between or within groups during the last 4 d of SA or during all days of extinction training. Note that all groups responded similarly during extinction despite differences in treatment. The legend indicates the treatment each group received after each extinction session. Error bars indicate SEM.

Figure 2.

Effects of treatment with 50 (N = 7), 100 (N = 7), and 200 mg/kg (N = 11) ceftriaxone or saline (N = 5) on cue-induced reinstatement of cocaine seeking. A one-way ANOVA revealed a significant overall difference in active lever presses (F_(3,27) = 9.79; p < 0.001); Bonferroni's post hoc tests showed that treatment with 100 and 200 mg/kg ceftriaxone resulted in significantly lower reinstatement responding in the 100 and 200 mg/kg groups relative to the 50 mg/kg ceftriaxone and saline groups. *p < 0.05. Error bars indicate SEM. SAL, CEF-50, CEF-100, and CEF-200 indicate treatment with saline vehicle, 50, 100, or 200 mg/kg ceftriaxone, respectively. Inset, SAL (N = 4) or CEF-200 (N = 4) treatment on cue-induced reinstatement of food-seeking behavior. An independent-samples t test revealed no significant differences between groups during reinstatement.

Figure 3.

Effects of 50 mg/kg (N = 4) and 200 mg/kg (N = 4) ceftriaxone and saline treatment (N = 4) on GLT1 expression. Each panel presents immunoblots for GAPDH, which was used as a control loading protein, and GLT1. Error bars indicate SEM. Abbreviations are as in Figure 2. A, B, In NAcc, quantitative analysis revealed no difference between SAL and CEF-50, but a significant increase in the ratio of GLT1/GAPDH in the CEF-200 group relative to SAL. *p < 0.05. C, D, In PFC, quantitative analysis revealed no difference between SAL and CEF-50, but a significant increase in the ratio of GLT1/GAPDH in the CEF-200 group relative to SAL. *p < 0.05.

Figure 4.

Effects of food (N = 4) and cocaine SA (N = 4) on GLT1 expression in rats treated with saline. Data are presented as in Figure 3. Quantitative analysis revealed no difference between SAL-food and SAL-cocaine groups in either NAcc (A) or PFC (B).