CHRONIC AND ACUTE TOLERANCE TO SUBJECTIVE, BEHAVIORAL AND CARDIOVASCULAR EFFECTS OF NICOTINE IN HUMANS

Understanding tolerance to effects of nicotine in humans may elucidate processes involved in the onset and maintenance of tobacco dependence. Subjective, behavioral and cardiovascular responses to nicotine were examined as a function of past history of nicotine exposure (i.e., smokers vs. nonsmokers, chronic tolerance) and of immediately preceding nicotine exposure (acute tolerance). Dose-effect relationships between nicotine (0-20 mu g/kg via measured-dose nasal spray) and each response were determined in male and female smokers (n = 17) and nonsmokers (n = 18), with different doses presented on different days. Each day, subjects also received a challenge dose of 20 mu g/kg 30 min after the previous dosing to assess acute tolerance. Plasma nicotine concentrations were 30% lower in nonsmokers compared with smokers and analyses were adjusted to control for this difference. Results showed significant changes in nearly all responses as a function of nicotine dose. Dose-effect curves were shifted to the right or dampened in smokers relative to nonsmokers for most subjective and some behavioral responses, consistent with chronic tolerance, but there was less evidence of chronic tolerance to other behavioral effects or to cardiovascular responses. A pattern of acute tolerance generally similar to that of chronic tolerance was observed across response domains (i.e., clear acute tolerance to subjective measures but less to behavioral or cardiovascular effects). These results support the notions that regular use of nicotine is associated with chronic functional tolerance and that repeated nicotine exposure during a single episode produces acute tolerance. A similar pattern of chronic vs. acute tolerance suggests similarity of mechanisms responsible for both ''types'' of tolerance. However, variability in tolerance magnitude across subjective, behavioral and cardiovascular response domains indicates that different mechanisms may be responsible for these different effects of nicotine.