Repeated N-acetylcysteine administration alters plasticity-dependent effects of cocaine

Abstract

Cocaine produces a persistent reduction in cystine-glutamate exchange via system x(c)- in the nucleus accumbens that may contribute to pathological glutamate signaling linked to addiction. System x(c)- influences glutamate neurotransmission by maintaining basal, extracellular glutamate in the nucleus accumbens, which, in turn, shapes synaptic activity by stimulating group II metabotropic glutamate autoreceptors. In the present study, we tested the hypothesis that a long-term reduction in system x(c)- activity is part of the plasticity produced by repeated cocaine that results in the establishment of compulsive drug seeking. To test this, the cysteine prodrug N-acetylcysteine was administered before daily cocaine to determine the impact of increased cystine-glutamate exchange on the development of plasticity-dependent cocaine seeking. Although N-acetylcysteine administered before cocaine did not alter the acute effects of cocaine on self-administration or locomotor activity, it prevented behaviors produced by repeated cocaine including escalation of drug intake, behavioral sensitization, and cocaine-primed reinstatement. Because sensitization or reinstatement was not evident even 2-3 weeks after the last injection of N-acetylcysteine, we examined whether N-acetylcysteine administered before daily cocaine also prevented the persistent reduction in system x(c)- activity produced by repeated cocaine. Interestingly, N-acetylcysteine pretreatment prevented cocaine-induced changes in [35S]cystine transport via system x(c)-, basal glutamate, and cocaine-evoked glutamate in the nucleus accumbens when assessed at least 3 weeks after the last N-acetylcysteine pretreatment. These findings indicate that N-acetylcysteine selectively alters plasticity-dependent behaviors and that normal system x(c)- activity prevents pathological changes in extracellular glutamate that may be necessary for compulsive drug seeking.

Figures

Figure 1.

N-Acetylcysteine pretreatment prevents escalation of drug intake during daily extended-access cocaine self-administration. a, b, Data depict mean ± SEM daily cocaine intake (mg/kg, i.v.) across 11 maintenance self-administration sessions (a) and difference in intake between days 11 and 1 to illustrate escalation in intake (b). Rats received saline (Sal) or N-acetylcysteine (NAC) (60 mg/kg, i.p.) before daily saline or cocaine (Coc) (1.0 mg/kg, i.v./infusion; 6 h/d) self-administration sessions. The pretreatment/self-administration drug assignments resulted in the following groups: Sal–Sal (N = 19), NAC–Sal (N = 10), Sal–Coc (N = 18), and NAC–Coc (N = 12). Escalation is operationally defined as a significant increase in the number of infusions obtained relative to day 1. *Significant difference from day 1 intake (a) or rats pretreated with saline before saline self-administration training sessions (Sal–Sal) (b); Dunnett's t, p < 0.05. #Significant difference from NAC–Coc rats.

Figure 2.

Daily N-acetylcysteine prevents cocaine-induced behavioral sensitization without altering acute locomotor activity. a–c, Total distance traveled (mean centimeters ± SEM) is depicted in 10 min intervals (a, b) or summed across the 2 h session (c). Group designations refer only to treatments on days 1–7, at which time rats received saline or N-acetylcysteine (0 or 60 mg/kg, i.p.) before each of seven daily injections of saline or cocaine (15 mg/kg, i.p., on days 1 and 7; 30 mg/kg, i.p., on days 2–6). These drug assignments resulted in four groups: Sal–Sal (N = 11), NAC–Sal (N = 11), Sal–Coc (N = 13), and NAC–Coc (N = 15). After a 21 d drug-free period, locomotor activity was measured on day 28 of the experiment after a saline or cocaine (15 mg/kg, i.p.) challenge in the absence of N-acetylcysteine pretreatment. The arrow indicates the timing of the saline or cocaine injection administered on day 1 (a, c), day 7 (c), or day 28 (b, c) of the experiment. a, *Significant difference from interval immediately preceding the saline or cocaine injection regardless of N-acetylcysteine treatment; Dunnett's t, p < 0.05. #Significant difference from rats injected with saline (0 cocaine) regardless of N-acetylcysteine treatment; ANOVA, p < 0.05. b, *Significant difference from the interval immediately preceding the saline or cocaine injection; Dunnett's t, p < 0.05. #Significant difference from every other group; Dunnett's t, p < 0.05. c, *Significant difference from day 1; Dunnett's t, p < 0.05. #Significant difference from every other group; Dunnett's t, p < 0.05.

Figure 3.

Daily N-acetylcysteine prevents cocaine-induced reinstatement without altering cocaine self-administration. a, b, Data are depicted as mean ± SEM number of infusions across daily self-administration sessions (2 h/d) (a) or responses on the active lever during the last three extinction sessions and the test for cocaine-induced reinstatement (b). Group designations refer only to treatments given during self-administration training, at which time rats received saline or N-acetylcysteine (60 mg/kg, i.p.) before self-administering saline or cocaine (0.5 mg/kg, i.v./infusion; 2 h/d). The pretreatment/self-administration drug assignments resulted in four groups: Sal–Sal (N = 14), NAC–Sal (N = 6), Sal–Coc (N = 14), and NAC–Coc (N = 7). Seven days after the last self-administration session, rats underwent daily extinction training in the absence of any drug treatments. This was followed by a test for cocaine-induced (10 mg/kg, i.p.) reinstatement in the absence of N-acetylcysteine treatment. *Significant difference from the last extinction session; Dunnett's t, p < 0.05. #Significant difference from all other groups; Dunnett's t, p < 0.05.

Figure 4.

Daily N-acetylcysteine prevents long-term changes in extracellular glutamate levels in the nucleus accumbens produced by repeated cocaine. a, b, Basal glutamate levels (mean ± SEM) are depicted as picomoles per microliter across 20 min samples (a) or area under the curve calculated using samples collected before the cocaine injection (b). c, d, Cocaine-evoked glutamate is presented as percent change from baseline (c) or area under the curve calculated using samples collected after the cocaine injection (d). Group designations refer only to treatments given during self-administration training, at which time rats received saline or N-acetylcysteine (60 mg/kg, i.p.) before self-administering saline or cocaine (0.5 mg/kg, i.v./infusion; 2 h/d). The pretreatment/self-administration drug assignments resulted in four groups: Sal–Sal (N = 14), NAC–Sal (N = 6), Sal–Coc (N = 14), and NAC–Coc (N = 7). Seven days after the last self-administration session, rats underwent daily extinction training in the absence of any drug treatments. Microdialysis was then used to sample extracellular glutamate before and after an acute cocaine challenge (10 mg/kg, i.p.) in the absence of N-acetylcysteine. *Significant difference from Sal–Sal controls; Dunnett's t, p < 0.05. +Significant difference from the last baseline sample; Dunnett's t, p < 0.05. #Significant difference from NAC–Coc rats (c); Dunnett's t, p < 0.05.

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, although regions of the neostriatum dorsal to the nucleus accumbens as well as the olfactory tubercles were likely sampled as well.

Figure 6.

Daily N-acetylcysteine prevents cocaine-induced blunting of system xc− activity in the nucleus accumbens. Data depict [35S]cystine transport restricted to system xc− activity in nucleus accumbens tissue punches. Group designations refer only to treatments given during self-administration training at which time rats received saline or N-acetylcysteine (60 mg/kg, i.p.) before self-administering saline or cocaine (1.0 mg/kg, i.v./infusion; 6 h/d). The pretreatment/self-administration drug assignments resulted in the following groups: Sal–Sal (N = 12), NAC–Sal (N = 5), Sal–Coc (N = 7), and NAC–Coc (N = 11). After an extended drug-free period (23–34 d), tissue punches were obtained from the nucleus accumbens and used to measure [35S]cystine transport by system xc−. *Significant difference from Sal–Sal controls; Dunnett's t, p < 0.05. #Significant difference from NAC–Coc rats; Dunnett's t, p < 0.05.