Amphetamine self-administration in SERT−/− and SERT+/+ rats.
As expected, the escalation of AMPH intake over the daily self-administration sessions was higher in LgA than ShA rats (Fig 3A vs Fig 3B: access x self-administration session effect, F(3,73)=3.458, p=0.023), and influenced by SERT genotype (Fig 3A and Fig 3B: genotype x access x self-administration session effect: F(3,172)=3.172, p=0.021). More specifically, SERT-/- rats were found to increase the daily intake of AMPH under LgA (Fig 3B: genotype x self-administration session effect: F(3,83)=3.892, p=0.010), but not under ShA conditions (Fig 3A: genotype x self-administration session effect: not significant: n.s.). No differences between SERT-/- and SERT+/+ rats were found for the number of incorrect lever presses (supplementary Fig 1A and supplementary Fig 1B: genotype (x self-administration session) effect: n.s.) and timeout responses (supplementary Fig 1C and supplementary Fig 1D: genotype (x self-administration session) effect: n.s.).
Effect of ShA and LgA amphetamine self-administration on the glutamate presynaptic terminal in SERT+/+ and SERT-/- rats .
We first focused our attention on the presynaptic terminal by analyzing the protein expression of the vesicular glutamate transporter (vGluT1) that is responsible of glutamate storage in presynaptic vesicles and it is considered an indirect index of glutamate release from the presynaptic neuron to the extracellular space (El Mestikawy et al., 2011). In the cNAc, two-way ANOVA revealed a main effect of AMPH access (F(2,57)=3.46, p=0.038), genotype (F(1,57)=25.57, p<0.0001) and an AMPH access x genotype interaction (F(2,57)=18.20, p<0.0001; Fig 4a). Further intergroup sub-testing revealed increased expression of vGluT1 in SERT-/- rats exposed to either ShA (+58% vs SERT-/--naïve, p<0.0001) or LgA (+46% vs SERT-/--naïve, p=0.0003) with no effects in SERT+/+ rats following the same procedures.
In the sNAc, two-way ANOVA revealed a main effect of AMPH access (F(2,55)=311.50, p<0.0001), genotype (F(1,55)=83.97, p<0.0001) and an AMPH access x genotype interaction (F(2,55)=7.425, p=0.0014; Fig 4b). In this sub region of the NAc, vGluT1 expression was reduced in SERT-/--naïve rats (-41% vs SERT+/+-naïve, p=0.0471). The lack of SERT influenced the response to the different regimens of AMPH exposure. In fact, while vGluT1 protein levels in SERT-/- were increased following ShA (+36% vs SERT-/--naive, p=0.0396) but not LgA to AMPH, in SERT+/+ rats, the expression of vGluT1 was increased independently from the ShA (+41% vs SERT+/+-naive, p=0.0145) or LgA (+61% vs SERT+/+-naive, p<0.0001) procedure.