Effects of MDMA and related analogs on plasma 5-HT: relevance to 5-HT transporters in blood and brain

Abstract

(±)-3,4-Methylenedioxymethamphetamine (MDMA) is an illicit drug that evokes transporter-mediated release of serotonin (5-HT) in the brain. 5-HT transporter (SERT) proteins are also expressed in non-neural tissues (e.g., blood), and evidence suggests that MDMA targets platelet SERT to increase plasma 5-HT. Here we tested two hypotheses related to the effects of MDMA on circulating 5-HT. First, to determine if MDMA metabolites might contribute to actions of the drug in vivo, we used in vitro microdialysis in rat blood specimens to examine the effects of MDMA and its metabolites on plasma 5-HT. Second, to determine whether effects of MDMA on plasma 5-HT might be used as an index of central SERT activity, we carried out in vivo microdialysis in blood and brain after intravenous MDMA administration. The in vitro results show that test drugs evoke dose-related increases in plasma 5-HT ranging from two- to sevenfold above baseline, with MDMA and its metabolite, (±)-3,4-methylenedioxyamphetamine (MDA), producing the largest effects. The ability of MDMA and related analogs to elevate plasma 5-HT is correlated with their potency as SERT substrates in rat brain synaptosomes. The in vivo results reveal that MDMA causes concurrent increases in extracellular 5-HT in blood and brain, but there are substantial individual differences in responsiveness to the drug. Collectively, our findings indicate that MDMA and its metabolites increase plasma 5-HT by a SERT-dependent mechanism, and suggest the possibility that measures of evoked 5-HT release in blood may reflect central SERT activity.

Figure 1

Effects of MDMA, HMMA and methamphetamine on dialysate 5-HT concentrations measured in blood specimens obtained from drug-naïve catheterized rats. Blood specimens were withdrawn and dialyzed in vitro as described under Materials and Methods. Test drugs were dissolved in Ringer's solution to yield the appropriate concentration and added directly to blood specimens via reverse dialysis at 0 min. Top panels show time-course 5-HT data (pg/5 μl) whereas bottom panels depict mean drug effects at each dose (% basal). Data are mean ± S.E.M. for N=5 rats/group. * P < 0.05 compared to 0 dose group for a given drug; # P < 0.05 compared to HMMA and methamphetamine at a given dose.

Figure 2

Effects of MDA, HMA and amphetamine on dialysate 5-HT concentrations measured in blood specimens obtained from drug-naïve catheterized rats. Blood specimens were withdrawn and dialyzed in vitro as described under Materials and Methods. Drugs were dissolved in Ringer's solution to yield the appropriate concentration and added directly to blood specimens via reverse dialysis at 0 min. Top panels show time-course 5-HT data (pg/5 μl) whereas bottom panels depict mean drug effects at each dose (% basal). Data are mean ± S.E.M. for N=5 rats/group. * P < 0.05 compared to 0 dose group for a given drug; # P < 0.05 compared to HMA and amphetamine at a given dose.

Figure 3

Relationship between drug-induced increases in plasma 5-HT and pEC₅₀ values for release of tritiated substrates in rat brain synaptosomes. Mean effects of drugs on plasma 5-HT (i.e., % basal values) were obtained from Figures 2 and 3, whereas pEC₅₀ values were calculated from the molar concentration for each test drug given in Table 1. The mean increase in plasma 5-HT at the 10 μM dose was plotted against the pEC₅₀ value for that drug to release [³H]-5-HT under SERT conditions, and [³H]-MPP+ under NET or DAT conditions. Pearson's correlation coefficient “r” and P value for significance are given.

Figure 4

In vivo effects of MDMA administration on dialysate 5-HT concentrations measured in brain and blood from conscious rats. Dialysate 5-HT in brain is shown in the left panel; dialysate 5-HT in blood is shown in the middle panel; correlation between mean brain and blood measures at each time point is shown in the right panel. MDMA was dissolved in sterile saline and administered by the i.v. route; 0.3 mg/kg was given at 0 min followed by 1.0 mg/kg at 60 min. Dialysates from brain were obtained at 20 min intervals, while blood specimens were withdrawn on the same schedule and immediately dialyzed. 5-HT was assayed by HPLC-ECD as described in Materials and Methods. Data are mean ± S.E.M. for N=6 rats/group, expressed as pg/5 μL samples. * P < 0.05 with respect to preinjection baseline. Pearson's correlation coefficient “r” and P value for significance are given in the right panel.

Figure 5

Correlation between brain and blood 5-HT levels obtained after MDMA administration for each individual rat tested. MDMA was dissolved in sterile saline, and 0.3 mg/kg i.v. was given at 0 min followed by 1.0 mg/kg i.v. at 60 min. Serial samples from blood and brain were withdrawn at 20-min intervals and were immediately dialyzed as described under Materials and Methods. The data show effects determined before and after injections, and are expressed as pg/5 μL values. Pearson's correlation coefficient “r” and P value are given for the data from each individual rat.