Amphetamine’s psychomotor and reinforcing effects
Because of the pronounced genotype differences in the psychomotor effects of AMPH, one could argue that locomotor activity had an influence on the observed genotype differences after LgA to AMPH self-administration. However, the lack of genotype differences in AMPH intake under ShA condition and in the number of incorrect responses suggests that the increased AMPH intake under LgA condition in SERT-/- versus SERT+/+ rats did not result from increased AMPH-induced locomotor activity. Interestingly, while AMPH self-administration was only increased in SERT-/- rats under LgA but not ShA conditions, cocaine self-administration was increased in these animals under both conditions (Verheij et al., 2018). While we did not conduct a direct comparison study, this implies that the serotonin influence on AMPH and cocaine self-administration is different. A potential explanation is that AMPH and cocaine have a different affinity for SERT (Howell & Kimmel, 2008), and that serotonin, next to dopamine, contributes to psychostimulant’s reinforcing effects too.