Blunted cystine-glutamate antiporter function in the nucleus accumbens promotes cocaine-induced drug seeking

Abstract

Repeated cocaine alters glutamate neurotransmission, in part, by reducing cystine-glutamate exchange via system xc-, which maintains glutamate levels and receptor stimulation in the extrasynaptic compartment. In the present study, we undertook two approaches to determine the significance of plasticity involving system xc-. First, we examined whether the cysteine prodrug N-acetylcysteine attenuates cocaine-primed reinstatement by targeting system xc-. Rats were trained to self-administer cocaine (1 mg/kg/200 microl, i.v.) under extended access conditions (6 h/day). After extinction training, cocaine (10 mg/kg, i.p.) primed reinstatement was assessed in rats pretreated with N-acetylcysteine (0-60 mg/kg, i.p.) in the presence or absence of the system xc- inhibitor (S)-4-carboxyphenylglycine (CPG; 0.5 microM; infused into the nucleus accumbens). N-acetylcysteine attenuated cocaine-primed reinstatement, and this effect was reversed by co-administration of CPG. Secondly, we examined whether reduced system xc- activity is necessary for cocaine-primed reinstatement. To do this, we administered N-acetylcysteine (0 or 90 mg/kg, i.p.) prior to 12 daily self-administration sessions (1 mg/kg/200 microl, i.v.; 6 h/day) since this procedure has previously been shown to prevent reduced activity of system xc-. On the reinstatement test day, we then acutely impaired system xc- in some of the rats by infusing CPG (0.5 microM) into the nucleus accumbens. Rats that had received N-acetylcysteine prior to daily self-administration sessions exhibited diminished cocaine-primed reinstatement; this effect was reversed by infusing the cystine-glutamate exchange inhibitor CPG into the nucleus accumbens. Collectively these data establish system xc- in the nucleus accumbens as a key mechanism contributing to cocaine-primed reinstatement.

Figure 1

N-acetylcysteine blocks cocaine-primed reinstatement by targeting system x_c-. The data depict the mean (+ SEM) number of lever presses on the last extinction session or on the reinstatement test day. Drug assignments indicate treatment on only the reinstatement test day (N=6–12/group as indicated in the bar graphs). N-acetylcysteine (0–60 mg/kg, IP) was injected at the time that (S)-4 carboxyphenylglycine (CPG; 0–0.5 µM) was added to the dialysis buffer; rats then received a cocaine injection (10 mg/kg, IP) sixty min later. The operant levers were then extended into the chamber and responding was monitored for 120 min. * indicates a significant difference from extinction responding (t-test, p<.05); # indicates a significant difference from rats treated with cocaine only (Tukey, p<.05); + indicates a significant increase relative to rats treated with cocaine + N-acetylcysteine (60 mg/kg; Tukey, p<.05).

Figure 2

N-acetylcysteine reduces cocaine intake across daily sessions of cocaine self-administration. Data depict the mean (± SEM) number of cocaine infusions (1.0 mg/kg/200 µl, IV) obtained during twelve daily maintenance self-administration sessions (6-hr/day). Thirty min prior to each daily session, rats received either vehicle (1 ml/kg; N=14) or N-acetylcysteine (90 mg/kg, IP; N=12).

Figure 3

N-acetylcysteine pretreatment during self-administration training results in lower levels of extinction responding. Following self-administration training, rats remained in the home cage for 7 days, and then underwent extinction training in which lever presses resulted in a saline infusion. Data depict the mean (± SEM) number of lever presses during the first five extinction sessions (2 hr/day). The first five sessions are depicted because some rats met the extinction criteria prior to the sixth session. Drug assignments indicate treatment during self-administration training (N=12–14/group). * indicates a main effect of drug treatment (ANOVA, p<.05).

Figure 4

Acute disruption of cystine-glutamate exchange reverses the impact of daily N-acetylcysteine pretreatments on cocaine-primed reinstatement. Data depict the mean number of lever presses (± SEM) on the last extinction session or the reinstatement test day. Rats had a history of vehicle or N-acetylcysteine pretreatments prior to each daily session of cocaine self-administration. On the reinstatement test day, rats received intra-accumbens infusions of vehicle or CPG sixty min prior to an injection of cocaine (10 mg/kg, IP). The operant levers were then extended into the chamber and responding was monitored for 120 min.

Figure 5

A schematic illustrating the placement of the 2 mm active membrane portion of the microdialysis probe for the rats included in the microdialysis study. The active regions of the microdialysis probes were primarily located in the nucleus accumbens core, although aspects of nucleus accumbens shell, the striatum dorsal to the nucleus accumbens, and the olfactory tubercles were likely sampled as well.