Determinants of nicotine self-administration

Nicotine self-administration, extinction responding and reinstatement in adolescent and adult male rats: evidence against a biological vulnerability to nicotine addiction during adolescence

Initiation of smoking behavior typically occurs during adolescence and rarely occurs during adulthood. Despite this epidemiological evidence, relatively little is known about possible neurobiological differences in the response to nicotine in adolescents that might make them more vulnerable to nicotine addiction. In the current study, we assessed nicotine self-administration under fixed ratio (FR) and progressive ratio (PR) reinforcement schedules in adolescent (postnatal day (P) 33-35) and adult (P91-94) rats. We then assessed extinction and reinstatement of nicotine seeking in adulthood in rats that initiated nicotine self-administration during either adolescence or adulthood. Nicotine self-administration (0.03 mg/kg/infusion, i.v.) was higher in adult rats than in adolescent rats under FR5 and PR reinforcement schedules; no age differences in nicotine self-administration were observed under FR1 or FR2 reinforcement schedules. In contrast, saccharin self-administration under FR5 and PR reinforcement schedules was similar in both age groups, potentially ruling out age differences in general performance. Rats that initiated nicotine self-administration as adults demonstrated a greater resistance to extinction of nicotine taking behavior when saline was substituted for nicotine than rats that initiated self-administration as adolescents. Reinstatement of nicotine seeking following nicotine priming injections (0.075, 0.15, 0.3 mg/kg, s.c.) was independent of the age of onset of nicotine self-administration. The present data from established rat models of drug self-administration and drug relapse suggest that nicotine is less reinforcing in adolescent compared with adult rats and that processes other than the reinforcing effects of nicotine may be involved in the greater susceptibility to smoking during the adolescent developmental stage.

Guidelines on nicotine dose selection for in vivo research

RATIONALE

This review provides insight for the judicious selection of nicotine dose ranges and routes of administration for in vivo studies. The literature is replete with reports in which a dosaging regimen chosen for a specific nicotine-mediated response was suboptimal for the species used. In many cases, such discrepancies could be attributed to the complex variables comprising species-specific in vivo responses to acute or chronic nicotine exposure.

OBJECTIVES

This review capitalizes on the authors' collective decades of in vivo nicotine experimentation to clarify the issues and to identify the variables to be considered in choosing a dosaging regimen. Nicotine dose ranges tolerated by humans and their animal models provide guidelines for experiments intended to extrapolate to human tobacco exposure through cigarette smoking or nicotine replacement therapies. Just as important are the nicotine dosaging regimens used to provide a mechanistic framework for acquisition of drug-taking behavior, dependence, tolerance, or withdrawal in animal models.

RESULTS

Seven species are addressed: humans, nonhuman primates, rats, mice, Drosophila, Caenorhabditis elegans, and zebrafish. After an overview on nicotine metabolism, each section focuses on an individual species, addressing issues related to genetic background, age, acute vs chronic exposure, route of administration, and behavioral responses.

CONCLUSIONS

The selected examples of successful dosaging ranges are provided, while emphasizing the necessity of empirically determined dose-response relationships based on the precise parameters and conditions inherent to a specific hypothesis. This review provides a new, experimentally based compilation of species-specific dose selection for studies on the in vivo effects of nicotine.

Is dopamine important in nicotine dependence?

Nicotine, like other drugs when abused, can produce a wide array of behaviours, some of which collectively propel 'drug-seeking behaviour'. This review focuses on three stimulus properties of nicotine and examines the role of dopamine in mediating each effect with respect to D1 and D2 receptor subtypes. Dopamine appears to be critical in mediating the reinforcing effects of nicotine, which is in line with other commonly abused psychomotor stimulants. However, evidence derived from studies with local microinjections of nicotine suggests that the origin of nicotine action to produce its other stimulus properties may be via multiple neuroanatomical substrates. The aversive stimulus effects are resistant to dopamine receptor antagonists. The discriminative stimulus effects of nicotine, despite showing some modification with dopaminergic compounds, appear not to be solely mediated via the mesolimbic dopamine system. Taken together, the neurobiology of nicotine dependence remains complex. Nonetheless, such dissociation between stimulus properties may permit the development of more effective therapies in combating tobacco dependence.