thresholds observed in rats undergoing nicotine withdrawal (Harrison, Liem, & Markou, 2001). Balfour (Balfour & Ridley, 2000) has suggested, however, that nicotine may ultimately decrease 5-HT in the hippocampus (possibly by increasing 5-HT1A receptors), an action that could be interpreted as depressogenic. Thus, it is possible that smokers or a subset of smokers with high 5-HTTLPR expression (i.e., l/l genotpye) and associated low serotonin levels may smoke to enhance serotonergic tone (Staley et al., 2001).
