Relapse poses a significant barrier to the development of effective treatments for substance use disorder (SUD). Cue-induced drug craving increases during drug abstinence, potentially affecting the efficacy of treatments for SUD. Disruptions in glutamatergic signaling are characteristic of SUDs. Glutamate is maintained by a combination of astrocytic and neuronal transporters within the nucleus accumbens (ACb) and disruptions in this homeostasis engender SUD. One transporter, the cysteine-glutamate transporter (xCT), is primarily localized on astrocytes and helps maintain glutamate concentrations. This process is disrupted by cocaine use, leading to increased cue-induced craving. The therapeutic N-acetylcysteine (NAC) lowers cue-induced relapse to cocaine. However, little research has shown how these effects extend to other psychostimulants, such as amphetamine (AMP). Though both are psychostimulants, AMP and cocaine have distinct mechanisms of action in the brain, particularly with regards to dopamine neurotransmission. In the present study, we assessed disruption of astrocytes and xCT expression following AMP self-administration (SA). We also assessed the degree to which disruptions in astrocyte or xCT expression might explain an increased propensity for relapse when presented with drug cues and the degree to which NAC can attenuate relapse.
78 male Sprague-Dawley rats were surgically implanted with jugular catheter to allow for intravenous SA of AMP. Upon recovery, all rats underwent 14, 2-hr fixed-ratio-1 SA sessions for AMP or the saline (SAL) control. Active lever presses resulted in an infusion of amphetamine (0.1 mg/kg/infusion) which was followed by a 20 s time-out period signaled by the illumination of both cue lights. After 14 SA sessions, all rats entered a forced abstinence period where they were not exposed to the drug or drug-related cues. During abstinence, all rats were given daily injections of NAC (100 mg/mL/ip) or the saline vehicle. Cue-induced relapse tests were conducted at the cessation of the abstinence period. During the relapse test, active lever presses resulted in the illumination of the cue lights present during SA testing, but no delivery of the drug itself. One subset of rats underwent relapse testing after one withdrawal day (WD) 1 and one subset underwent testing on WD 14 to examine any increases in relapse. Following testing, all rats were perfused and brains were extracted for immunofluorescence (IF) analysis of astrocyte and xCT transporter expression within the ACb and medial prefrontal cortex using a simultaneous immunofluorescence (IF) method.
During the cue-induced relapse test, cue-induced responding was significantly higher in AMP-treated rats compared to SAL-treated rats. Importantly, NAC failed to attenuate relapse at either WD 1 or WD 14. We hypothesized that xCT expression would remain unaltered between WD 1 and WD 14 for rats treated with NAC. However, given that NAC failed to lower relapse at either time point, IF results will help determine inform the inefficacy of NAC. For instance, should xCT expression remain unaltered between AMP and SAL-exposed rats, then it would make sense that NAC would likely have little success in lowering relapse. Such a finding would suggest that AMP disrupts glutamate via a different mechanism than cocaine. Thus, results may indicate that the therapeutic efficacy of NAC varies across psychostimulants.
Support or Funding Information
2020 Cornerstone Award from the Histochemical Society
NIDA DA035435
lt;pgt;2020 Cornerstone Award from the Histochemical Societylt;/pgt;lt;pgt;NIDA DA035435lt;/pgt;
Analysis of active responding for amphetamine and saline during FR-1 self-administration indicated that overall active responding was significantly higher for amphetamine when compared to saline, p < .001 .; Analysis of active lever-responding during the cue-induced relapse test revealed that relapse was significantly higher in animals who had previously self-administered amphetamine, p < .001. There were no significant effects of withdrawal period (WD 1 or WD 14), NAC treatment, of their interaction.
