UC Irvine

Smoking continues to be one of the leading causes of preventable death in the United States. Although the majority of smokers want to quit, most will not be successful at doing so. Despite extensive research and funding devoted to finding more efficacious pharmacotherapies for smoking cessation, current therapies are not very effective at keeping smokers abstinent for over a year. The goal of my dissertation was to establish a preclinical test that would have better face validity for the development of new pharmacological therapies for smoking cessation. Current preclinical tests of tobacco dependence use nicotine alone. Here I used an aqueous cigarette smoke extract (CSE), which contains nicotine and many other constituents of cigarette smoke, to examine its effects on cue- and drug-induced craving, factors known to influence relapse. The hypothesis that was tested was that CSE would induce greater craving and withdrawal than nicotine alone. Adult male rats that had self-administered CSE were found to reinstate to drug priming alone, unlike animals that had self-administered nicotine, which required the additional presentation of drug-associated cues. AT-1001, a functional α3β4 nicotinic acetylcholine receptor (nAChR) antagonist, attenuated drug-primed reinstatement of CSE- and nicotine-seeking behavior. However, AT-1001 was less potent in blocking drug-primed reinstatement in animals that had self-administered CSE than in those that had self-administered nicotine alone. This was the case even when nicotine alone was used to prime reinstatement in animals that had self-administered CSE, suggesting that prior CSE exposure had altered the functional role of this nAChR.

Another factor known to influence relapse is withdrawal to smoking. Teenagers are especially vulnerable to the effects of withdrawal. However, although adult rats show many of the same signs of withdrawal as humans following chronic nicotine exposure, adolescent rodents do not. In this study, adolescent and adult male rats were exposed for ten days to nicotine or CSE (1.5 mg/kg/day nicotine equivalent) by intravenous injection. Chronic CSE treatment resulted in greater spontaneous somatic and affective withdrawal symptoms in both adolescent and adult rats. Furthermore, adolescents treated chronically with CSE displayed major affective symptoms, as shown by an increase in anxiety-like behavior 30 days after drug withdrawal. Mecamylamine, a non-selective nAChR antagonist, was used to investigate if nAChRs are involved in the enhancement of withdrawal observed following chronic CSE treatment. Whereas mecamylamine precipitated greater somatic withdrawal in animals treated with CSE, it did not precipitate affective withdrawal.

Heavy smokers show an upregulation of nAChR through increased radioligand binding. Similarly, adult rodents show an upregulation of nAChR binding after chronic nicotine, adolescents do not. Since CSE exposure resulted in an increase in precipitated somatic withdrawal in adult and adolescent rats and also augmented nicotine-primed reinstatement, I hypothesized that chronic CSE exposure would result in enhanced nAChR binding in both adult and adolescent rats. Chronic CSE exposure results in enhanced α4β2, α3β4, and α7 nAChR binding regardless of age, in brain areas highly involved in negative aversive states of addiction.

Overall, this work provides evidence that the cigarette smoke constituents influence drug-primed craving, withdrawal, and changes in nAChR properties. Smoke constituents eliminated the protective effects that adolescent rodents display in preclinical tests of nicotine dependence, and enhanced both craving and withdrawal in adults. These findings suggest that use of CSE is a more valid way to study tobacco dependence than nicotine, and should be used in preclinical tests to assess tobacco cessation therapies.