Rats that self-administered nicotine intravenously showed a marked decrease in xCT in the nucleus accumbens and the VTA, and GLT-1 expression in the nucleus accumbens relative to saline controls. As well, xCT levels after nicotine self-administration were lower than in animals administered nicotine passively via continuous subcutaneous osmotic minipump delivery. These findings indicate that phasic administration of nicotine, rather than continuous release via minipumps, is necessary to down-regulate xCT. The reduction in xCT was independent of the development of withdrawal signs since both self-administration and continuous infusion of nicotine led to significant increases in somatic signs of withdrawal. Different adaptations induced by intermittent and continuous nicotine infusion have been observed previously. For example, continuous nicotine upregulates nicotinic acetylcholine receptors to a greater extent than intermittent administration (16
). An alternative explanation is that contingent administration of nicotine may be required to affect xCT and GLT-1. While this possibility cannot be ruled out, at least system xc- is downregulated by either repeated contingent or non-contingent cocaine administration (1
It is interesting that both xCT and GLT-1 are down-regulated in the nucleus accumbens by two different classes of addictive drugs, cocaine (2
) and nicotine (present data). These data support not only the potential relevance of these proteins in the development and/or expression of dependence, but also pose the possibility that systems xc- and XAG may be co-regulated. Both system xc- and glutamate transport via GLT-1 occur predominantly in glia (17
), and previous in vitro
studies provide indications that these proteins may be co-regulated (19
). For example, down-regulating system XAG reduces system xc- activity (20
). However, in the VTA the regulation of the two proteins was dissociated since only xCT was reduced after nicotine self-administration.
The preclinical data presented here suggest that nicotine self-administration decreased xCT levels in brain regions critical to nicotine reinforcement. Because N-acetylcysteine activates system xc- and thereby increases intracellular glutathione synthesis and releases glutamate into the extracellular space (1
), it seems likely that the administration of N-acetylcysteine is also increasing system xc- activity in smokers. Such a restoration of xc- activity may underlie the improved success by the N-acetylcysteine treated subjects in resisting cigarette smoking. In future studies the link between effects of N-acetylcysteine treatment in cigarette smokers and reduced xc- activity will benefit from further experimentation in animal models, akin to what has already been done in animal models of cocaine, and to a lesser extent heroin, relapse. For example, these cocaine and heroin studies have shown that N-acetylcysteine restores xc- activity and that this restoration is associated with reduced drug-seeking (1
Although N-acetylcysteine reduced the number of cigarettes smoked there was no reduction in CO levels. Previous smoking reduction studies found similar decreases in number of cigarettes smoked without corresponding decreases in CO due to smoking fewer cigarettes but with deeper and longer inhalations (23
). Based on the significant decrease in number of cigarettes smoked, N-acetylcysteine may be assisting smokers resist the urge to seek nicotine, but may not reduce nicotine consumption once smoking has begun. Importantly, the daily number of cigarettes smoked co-varied with daily alcohol use, and the effect of N-acetylcysteine was statistically significant only when two individuals were excluded from analysis that had daily alcohol use greater than two standard deviations above the mean of all participants. This highlights the importance of defining populations used in future studies in terms of simultaneous cigarette and alcohol use. Finally, N-acetylcysteine did not affect self-reported withdrawal symptoms, a finding that is in agreement with the lack of relationship between xCT expression and somatic signs of withdrawal in rats trained to self-administer nicotine. Additionally, since the human subjects of this study were not attempting to cease smoking altogether, and continued nicotine consumption throughout the study, there was not an opportunity for them to undergo nicotine withdrawal.
In summary, nicotine self-administration reduces the expression of xCT and GLT-1 in brain structure(s) involved in nicotine reinforcement, suggesting that, akin to cocaine self-administration, the activities of system xc- and GLT-1 are compromised. Moreover, treatment with a pro-cystine drug that increases the activity of system xc- reduced the number of cigarettes smoked on days when alcohol was not consumed. This combination of preclinical and clinical data indicates that N-acetylcysteine may aid in the cessation of cigarette smoking.