Effects of acute nicotine on brain function in healthy smokers and non-smokers: Estimation of inter-individual response heterogeneity

The present study used functional magnetic resonance imaging (fMRI) to investigate the neural mechanisms of nicotine effects on antisaccades (an oculomotor measure of the conflict between a reflexive response and a spatially complex volitional response) and prosaccades (involving reflexive overt attentional shifts). Given the known inter-individual variability in drug response we aimed to identify oculomotor variables and brain areas in which significant inter-individual heterogeneity in response to nicotine is observed. To do so we calculated within-session intraclass correlation (ICC) coefficients over measurements obtained before and after nicotine/placebo administration and reasoned that a significant reduction in ICC with nicotine compared to placebo would reflect the operation of significant inter-individual response heterogeneity. Thirteen light-to-moderate smokers and 11 non-smokers completed fMRI during antisaccades before and after subcutaneous injection of 12 μg/kg nicotine or saline placebo in a double-blind, randomised, cross-over design. All participants were healthy, right-handed males. Nicotine and placebo were given on separate occasions approximately 1 week apart with time of injection kept constant. Nicotine significantly reduced antisaccade latencies in both groups. At the level of brain function, during antisaccades the blood oxygen level dependent (BOLD) response in the left frontal eye field was non-significantly reduced by nicotine while it significantly increased following placebo in non-smokers, but there was no discernible effect in smokers. During prosaccades, it was found that deactivation areas (posterior cingulate gyrus and precuneus; right superior temporal gyrus) showed enhanced deactivations following nicotine administration in both groups. ICC analysis identified significant inter-individual response heterogeneity in antisaccade reflexive errors in smokers, and in a number of brain regions, particularly in non-smokers. These findings suggest that nicotine has beneficial effects at the cognitive level and leads to reductions in task-related activations and further decreases of BOLD in deactivation areas. The comparison of within-session ICCs across drug conditions suggests that the effects of nicotine are subject to inter-individual variability at behavioural and neural levels.