Baseline-dependency of nicotine effects: a review.

'Rate-dependency', a founding observation of behavioral pharmacology, generally indicates that effects of drugs will be inversely related to the rate of operant responding under control conditions (i.e. decrease in high-rate behaviors and increase in low-rate behaviors). 'Baseline-dependency' encompasses rate-dependency and extends this notion to other drug effects, including those assessed by means other than 'rate' (such as subjective mood self-reports). Although little human research has specifically investigated the effects of nicotine as a function of baseline level of responding, a number of studies suggest baseline-dependent differences in the influence of nicotine on behavioral and cognitive task performance and on subjective mood responses. Results of many animal studies also are very consistent with these observations. Baseline-dependency, perhaps, is most clearly demonstrated in comparisons between groups selected on the basis of specific characteristics (e.g. high vs low 'trait hostility'), or within subjects as a result of acute environmental manipulations (e.g. high vs low 'stress' task). Moreover, baseline-dependency of the effects of nicotine may have broader applicability in explaining individual differences in vulnerability to nicotine dependence, particularly among those with psychiatric disorders (e.g. depression). While individual differences in responses to nicotine may be due to differences in pharmacological sensitivity to nicotine, as commonly assumed, they also may be due partly to individual and situationally determined differences in baseline level of responding on the measure of interest. Consideration of the conditions under which baseline-dependent effects of nicotine are observed may clarify both individual difference and situational influences on responses to nicotine.