Effect of repeated exposure to MDMA on the function of the 5-HT transporter as assessed by synaptosomal 5-HT uptake

Abstract

Recent studies have demonstrated that a preconditioning regimen (i.e., repeated low doses) of MDMA provides protection against the reductions in tissue concentrations of 5-HT and 5-HT transporter (SERT) density and/or expression produced by a subsequent binge regimen of MDMA. In the present study, the effects of preconditioning and binge treatment regimens of MDMA on SERT function were assessed by synaptosomal 5-HT uptake. Synaptosomal 5-HT uptake was reduced by 72% 7 days following the binge regimen (10 mg/kg, i.p. every 2 h for a total of 4 injections). In rats exposed to the preconditioning regimen of MDMA (daily treatment with 10 mg/kg for 4 days), the reduction in synaptosomal 5-HT uptake induced by a subsequent binge regimen was significantly less. Treatment with the preconditioning regimen alone resulted in a transient 46% reduction in 5-HT uptake that was evident 1 day, but not 7 days, following the last injection of MDMA. Furthermore, the preconditioning regimen of MDMA did not alter tissue concentrations of 5-HT, whereas the binge regimen of MDMA resulted in a long-term reduction of 40% of tissue 5-HT concentrations. The distribution of SERT immunoreactivity (ir) in membrane and endosomal fractions of the hippocampus also was evaluated following the preconditioning regimen of MDMA. There was no significant difference in the relative distribution of SERTir between these two compartments in control and preconditioned rats. The results demonstrate that SERT function is transiently reduced in response to a preconditioning regimen of MDMA, while long-term reductions in SERT function occur in response to a binge regimen of MDMA. Moreover, a preconditioning regimen of MDMA provides protection against the long-term reductions in SERT function evoked by a subsequent binge regimen of the drug. It is tempting to speculate that the neuroprotective effect of MDMA preconditioning results from a transient down-regulation in SERT function.

Fig. 1. Effect of the preconditioning regimen on the subsequent MDMA-induced reduction in synaptosomal 5-HT uptake

Rats were treated with either the preconditioning regimen of MDMA (10 mg/kg i.p.) or vehicle once daily for four days. On the following day, the rats were injected with either the binge regimen of MDMA (10 mg/kg, i.p. at 2 h intervals for a total of 4 injections) or vehicle and were sacrificed seven days after the last injection by decapitation. The values represent the mean±SE of 5–8 rats. * represents p<0.001 when treatment groups are compared within the factor of prior exposure and # represents p<0.01 when compared to MDMA binge-treated rats previously exposed to vehicle.

Fig. 2. Effect of the preconditioning regimen on synaptosomal uptake of 5-HT at 1 or 7days after the last injection of MDMA

Rats were treated with the preconditioning regimen of MDMA (10 mg/kg i.p.) or vehicle once daily for four days. Rats were sacrificed by decapitation either one or seven days after the last injection. The values represent the mean±SE of 7–11 rats. * represents p<0.01 when compared to the corresponding vehicle-treated group.

Fig. 3. Comparison of the preconditioning and binge regimen of MDMA on tissue 5-HT concentrations in the hippocampus

Rats treated with the preconditioning regimen of MDMA (10 mg/kg i.p., once daily for four days) were sacrificed by decapitation either one (PC1) or seven days (PC7) after the last injection. Rats treated with binge regimen of MDMA (10 mg/kg, i.p. at 2 h intervals for a total of 4 injections) or vehicle were sacrificed seven days after the last injection. The values represent the mean±SE of 5–10 rats. * represents p<0.001 vs. the vehicle-treated group, as well as vs. the PC1 and PC7 groups.

Fig. 4. Effect of the preconditioning regimen of MDMA on hippocampal plasma membrane (a and c) and endosome (b and d) SERT expression

Rats were treated with the preconditioning regimen of MDMA (10 mg/kg i.p.) or vehicle once daily for four days and were sacrificed one day after the last injection by decapitation. The values represent the mean±SE of 5–6 rats.