Analyses of our secondary aim evaluating genetic determinants of subjective rating of mood and behavioral responses to d-amphetamine provided interesting results. Contrary to our hypothesis, the drug did not influence mood differentially in the DRD2 genotypic groups. Results of a previous study indicated that DRD2 availability may be associated with subjective responses to stimulants (Volkow et al., 1999). One possible explanation is that our study may have been underpowered to detect this effect. The finding that genotypic groups of rs12364283 showed a differential pattern of response on the stop task after administration of d-amphetamine is consistent with previous studies. Results of work in animals (Eagle, Tufft, Goodchild, & Robbins, 2007) and humans (de Wit et al., 2000; Mattay et al., 2003) have indicated an interesting pattern of response to psycho-stimulants. Eagle et al. (2007) divided rats into slow and fast responders and found that the administration of methyl-phenidate, a drug that is similar to d-amphetamine, reversed stop RTs only in slow-stopping rats, whereas it worsened stop RTs in fast-stopping rats. A similar pattern of response has been observed in humans
