In Vivo Imaging of Cerebral Serotonin Transporter and Serotonin 2A Receptor Binding in MDMA and Hallucinogen Users

This positron emission tomography (PET) study (n=45) assessed the differential effects of MDMA and hallucinogen use on cerebral serotonin transporter (SERT) and serotonin 2A receptor binding. The authors found evidence that MDMA, but not hallucinogen, use is associated with changes in the cerebral presynaptic serotonergic transmitter system.

Abstract

Context: Both hallucinogens and 3,4-methylenedioxymethamphetamine (MDMA or “ecstasy”) have direct agonistic effects on postsynaptic serotonin2A receptors, the key site for hallucinogenic actions. In addition, MDMA is a potent releaser and reuptake inhibitor of presynaptic serotonin.

Objective: To assess the differential effects of MDMA and hallucinogen use on cerebral serotonin transporter (SERT) and serotonin2A receptor binding.

Design: A positron emission tomography study of 24 young adult drug users and 21 nonusing control participants performed with carbon 11 (11C)–labeled 3-amino-4-[2-[(di(methyl)amino)methyl]phenyl] sulfanylbenzonitrile (DASB) and fluorine 18 (18F)–labeled altanserin, respectively. Scans were performed in the user group after a minimum drug abstinence period of 11 days, and the group was subdivided into hallucinogen-preferring users (n = 10) and MDMA-preferring users (n = 14).

Participants: Twenty-four young adult users of MDMA and/or hallucinogenic drugs and 21 nonusing controls.

Main Outcome Measures: In vivo cerebral SERT and serotonin2A receptor binding.

Results  Compared with nonusers, MDMA-preferring users showed significant decreases in SERT nondisplaceable binding potential (neocortex, −56%; pallidostriatum, −19%; and amygdala, −32%); no significant changes were seen in hallucinogen-preferring users. Both cortical and pallidostriatal SERT nondisplaceable binding potential was negatively correlated with the number of lifetime MDMA exposures, and the time of abstinence from MDMA was positively correlated with subcortical, but not cortical, SERT binding. A small decrease in neocortical serotonin2A receptor binding in the serotonin2A receptor agonist users (both user groups) was also detected.

Conclusions: We found evidence that MDMA but not hallucinogen use is associated with changes in the cerebral presynaptic serotonergic transmitter system. Because hallucinogenic drugs primarily have serotonin2A receptor agonistic actions, we conclude that the negative association between MDMA use and cerebral SERT binding is mediated through a direct presynaptic MDMA effect rather than by the serotonin2A agonistic effects of MDMA. Our cross-sectional data suggest that subcortical, but not cortical, recovery of SERT binding might take place after several months of MDMA abstinence.”

Authors: David Erritzoe, Vibe G. Frokjaer, Klaus K. Holst, Maria Christoffersen, Sys S. Johansen, Claus Svarer, Jacob Madsen, Peter M. Rasmussen, Thomas Ramsøy, Terry L. Jernigan & Gitte M. Knudsen

Summary

A positron emission tomography study was performed on 24 young adult drug users and 21 nonusing control participants. The participants were divided into hallucinogen-preferring users and MDMA-preferring users.

Results showed that MDMA-preferring users had lower SERT nondisplaceable binding potential in the neocortex, pallidostriatum, and amygdala than nonusers, and that abstinence from MDMA was positively correlated with SERT binding.

We found evidence that MDMA use, but not hallucinogen use, is associated with changes in the cerebral presynaptic serotonergic transmitter system.

Recreational drugs, such as MDMA and LSD, induce states of altered perception, thought, and feelings not normally experienced except perhaps in dreams. MDMA users maintain a sense of contact with reality, although visual hallucinations may occur.

Both hallucinogens and MDMA exert their primary effects through actions on serotonergic neurotransmission. MDMA is a substrate of the serotonin transporter (SERT) and inhibits tryptophan hydroxylase, the rate-limiting enzyme for serotonin synthesis.

Serotonin transporter is crucial for the regulation of serotonin transmission. Long-term exposure to MDMA is associated with a reduction in cerebral serotonin levels and a decreased number of SERT binding sites, but there is no firm evidence of MDMA-induced neurodegeneration in humans.

Several animal studies point to the serotonin2A receptor as a key factor in MDMA-induced effects on neurons, and current MDMA users have decreased cortical serotonin2A receptor binding in contrast to former users, who have increased serotonin2A receptor binding.

In this study, we assessed the effects of MDMA and hallucinogens on cerebral serotonergic markers using positron emission tomography. We found that serotonin2A receptor binding was decreased in both MDMA users and hallucinogen users, but more pronounced in users with a short period of abstinence.

Recruitment and Inclusive

Participants were recruited using fliers and advertisements posted on relevant Web sites, and were also screened for history of alcohol, tobacco, and illegal drug use.

Individuals between 18 and 35 years old with a minimum of 12 lifetime exposures to MDMA or hallucinogens were eligible for inclusion. Control individuals were excluded if they had any history of other illegal drug use.

Participants and Drug Use

Twenty-four young adult users of MDMA and/or hallucinogens and 21 controls (ie, nonusers) were included in the study. The number of consumed ecstasy tablets was calculated by dividing the reported number of milligrams per session of MDMA use in powder form by 60.

Drug Analysis by Urine and Hair

To confirm abstinence from illegal drug intake, 3.5-cm scalp hair segments were analyzed for MDMA concentrations. These measurements were used to confirm reported drug use.

PET Imaging

All participants underwent PET scanning with 11C-DASB and 18F-altanserin on an 18-ring scanner (GE-Advance scanner; GE, Milwaukee, Wisconsin) operating in 3-dimensional Table 1 acquisition mode and producing 35 image sections with an intersection distance of 4.25 mm. The scans were performed in a resting state.

We used a modified reference tissue model designed for quantification of [11C]DASB to calculate the nondisplaceable binding potential (BPND) of [18F]altanserin in the volume of interest (VOI) and the reference region.

Magnetic Resonance Imaging

Magnetic resonance imaging (MR) of the brain was conducted using a Siemens Magnetom Trio 3T MR scanner. T1-weighted, sagittal, magnetization-prepared, rapid gradient-echo scans of the head and T2-weighted scans of the whole brain were performed, and the PET signal was extracted from gray matter voxels.

Volume of Interests

VOIs were automatically delineated on each participant’s transaxial MR image sections, and then transferred onto the PET images.

We computed the average binding potential for neocortex, pallidostriatum, midbrain, amygdala, and thalamus for both markers, and the ratio between gray matter volume and the sum of the white plus gray matter volumes was computed within each high-binding subcortical region.

Statistical Analysis

MDMA and hallucinogens bind to the serotonin2A receptor in the brain. This effect was evaluated using a 2-tailed nonparametric test, and group differences were tested using 1-way analyses of variance and Tukey’s test.

Because of season-dependent changes in cerebral SERT binding and an age-dependent decline in serotonin2A receptor binding, the following factors were tested as covariates in a multiple regression analysis: age, body mass index, daylight minutes on the day of the PET scan, and education score.

To explore whether there were dose-response effects on serotonergic brain markers of accumulated use of MDMA and/or hallucinogens, the following factors were tested separately within the total group of drug users: the logarithmic lifetime number of consumed MDMA tablets and the lifetime exposure to hallucinogens.

We fitted a log-logistic 4-parameter model to the data to quantify the dose-response relationship. This model gives more precise predictions of the response variable at high (or low) doses.

To test whether the serotonergic markers showed any signs of recovery over time, we tested whether the serotonin2A receptor binding was influenced by the extent of MDMA use, and whether the inverted U – shaped relationship between cortical serotonin2A receptor binding and the subcortical SERT binding could be replicated.

The demographic variables, parent socioeconomic level, alcohol and tobacco use variables, and scanning-related variables did not differ significantly between the 3 groups. However, the educational level differed significantly between the groups, and the tobacco smoking differed between the 3 groups.

SERT Binding

There were statistically significant between-group differences in SERT binding in the pallidostriatum, neocortex, and amygdala. MPUs had lower SERT BPND than did HPUs in all 3 regions. The regional SERT binding in MPUs was lower in the MDMA group in the pallidostriatum, amygdala, and neocortex compared with the control group. There was a significant negative correlation between the logarithmic accumulated lifetime intake of MDMA tablets and SERT BPND in all investigated brain regions.

There was no significant relationship between SERT binding and the lifetime use of hallucinogens or the time since last use of hallucinogens. However, there was a positive correlation between SERT binding and the number of days since last use of MDMA in the pallidostriatum.

Serotonin 2A Receptors Binding

When serotonin2A agonist users were compared with controls, serotonin2A agonist users had slightly lower neocortical serotonin2A BPND and 9% lower regional serotonin2A receptor BPP in the neocortex. Two control participants had very high serotonin2A BPP values, but their difference was no longer statistically significant.

There was no significant difference in serotonin2A receptor binding between MPUs, HPUs, and controls, regardless of the accumulated lifetime use of MDMA or hallucinogens or the time since the last use.

After 212 days, the SERT level will return to normal in participants with a median lifetime use of 3,4-methylenedioxymethamphetamine (MDMA), whereas the upper limit estimate is beyond the human lifespan.

Serotonin 2A Receptors vs SERT Binding

In this study, we found that neocortical serotonin2A receptor binding and pallidostriatal SERT binding were significantly decreased in MDMA-preferring individuals, but not in hallucinogen-preferring individuals.

SERT Binding and Use of MDMA

We found that moderate to heavy MDMA users had decreased SERT binding in the pallidostriatum, amygdala, and neocortex, but not in the midbrain, and had only borderline decreased SERT binding in the thalamus. There was a negative correlation between the number of reported lifetime MDMA exposures and SERT binding.

Several studies support the notion that the SERT binding changes in relation to MDMA use are reversible, and that the full recovery of SERT binding takes place approximately 200 days after the last MDMA dose.

Studies in rats and nonhuman primates have pointed to a regionally dependent recovery of SERT binding following MDMA abstinence.

Figure 5 shows the binding potential of specific tracer binding to neocortical serotonin2A receptors in agonist users of 3,4-methylenedioxymethampetamine and hallucinogens.

The most pronounced decrease in SERT binding among MDMA users was detected in the cortex, and no correlation with length of MDMA abstinence was present in the cortex.

We did not find any significant between-group differences in SERT binding in the midbrain, consistent with ex vivo studies using different techniques. However, some early in vivo imaging studies of MDMA users reported decreased SERT binding, but the radioligands used in these studies did not allow for cortical binding assessment.

MDMA users experience a temporary decrease in cerebral SERT binding. It is not entirely clear whether this decrease is an adaptive neural mechanism or reflects toxic exposure of serotonergic neurons with a secondary reinnervation pattern of serotonergic axons. One of the prevailing arguments for MDMA toxicity is the presence of cerebral serotonin depletion, low serotonin uptake, and/or low SERT binding in MDMA-treated animals. However, some studies report decreased SERT binding after nonneurotoxic, pharmacologically induced, chronic serotonin depletion in rats.

Heavy use of MDMA is associated with decreased availability of cerebral SERT binding in humans. It is not fully understood whether this represents axonal toxic effects or rather a temporary downregulation of SERT.

SERT Binding and Use of Hallucinogens

Hallucinogen users did not have signs of serotonergic impairment, and although some degree of SERT reduction could have been expected a priori, hallucinogen treatment of animals did not cause a depletion of cerebral serotonin.

Because hallucinogenic drugs stimulate serotonin2A receptors, the negative association between MDMA intake and cerebral SERT binding may be mediated through a direct presynaptic MDMA effect and a secondary serotonin depletion rather than by the serotonin2A agonistic effects of MDMA.

Serotonin 2A Receptors Binding and Use of MDMA and/or Hallucinogens

Based on the shared stimulatory action on serotonin2A receptors by MDMA and hallucinogenic compounds, serotonin2A receptor binding was the primary analysis investigated within the entire group of MDMA and hallucinogen users. The serotonin2A agonist users had slightly decreased cortical serotonin2A receptor binding compared with the controls.

In this study, serotonin2A receptor binding decreased in agonist users, which could be due to transient receptor down-regulation secondary to stimulation of the receptor, or it could reflect toxic damage to cortical pyramidal serotonin2A receptor – expressing neurons.

In a post hoc analysis, we replicated a significant quadratic relationship between cortical serotonin2A receptor and pallidostriatal SERT binding.

Methodological Considerations

The results should be interpreted in the light of some methodological aspects, such as the use of a reference tissue model without arterial blood sampling, and the high interindividual variability and low signal to noise ratio in cortical areas.

We considered whether the low cerebral SERT binding in MDMA users was a preexisting trait associated with an increased preference for the use of MDMA, but we considered this less likely.

This study was prone to difficulties with obtaining reliable and valid data on drug use from the study participants. However, by measuring drug content in hair samples corresponding to use within the 3 preceding months, we could confirm the self-reported drug use in 13 of the 14 participants.

Between 2003 and 2007, 88% of seized ecstasy tablets contained MDMA as the sole drug, and amphetamines were the most commonly found compound in the seized tablets. 2005 was the year with the highest rate of contaminated tablets. The self-reported use of amphetamine and cocaine was higher in the group of MPUs than among HPUs, but the effect of MDMA use on serotonergic markers was not changed by including recent use of either amphetamine or cocaine in the statistical analysis.

Lifetime and current tobacco smoking were more pronounced among MPUs than among HPUs and controls, but there was no correlation between tobacco smoking and cerebral SERT or serotonin2A receptor binding.

We found that MPUs, but not HPUs, have profound reductions in cerebral SERT binding. Cortical SERT binding might recover after several months of MDMA abstinence.

The authors would like to thank all volunteers for their participation, the staff at Rigshospitalet and Hvidovre Hospital, and the Section of Forensic Chemistry, Faculty of Health Sciences, University of Copenhagen.

MDMA (“Ecstasy”) is a synthetic drug that is used to make ecstasy, a recreational drug. It is a gateway drug to other drugs that can cause addiction and other problems. MDMA (3,4-methylenedioxymethamphetamine) induces serotonin deficits in the rat brain, which are followed by partial recovery over a 52-week period, according to a study by Lew R, Sabol KE, Chou C, Vosmer GL, Richards J, Seiden LS. Axon degeneration is observed in the rat brain after administration of d- and l-methylenedioxyamphetamine. Fischer C, Hatzidimitriou G, Wlos J, Katz J, Ricaurte G, studied the effects of MDMA on 5-HT axons in animals, and McCann UD, Ricaurte GA, studied the effects of MDMA on serotonin innervation patterns. Serotonin transporters in the brain are affected by MDMA (ecstasy) use, and this effect is dependent on dose, sex, and long-term abstention from use. Several studies have shown that ecstasy use decreases serotonin transporter binding in the brain. These studies have been supported by positron emission tomography/[11C]DASB and structural brain imaging studies. Hallucinogens such as MDMA and MDA cause selective ablation of serotonergic axon terminals in forebrain, and can induce apoptosis via 5-HT2A – receptor stimulation in cortical neurons.

done attenuates and reverses hyperthermia induced by 3,4-methylenedioxymethamphetamine (MDMA) in rats and humans, and alters the effects of MDMA on 5-HT2A receptors. Boothman LJ, Allers KA, Rasmussen K, Sharp T, Rattray M, Baldessari S, Gobbi T, Mennini T, Samanin R, Bendotti C, Rothman RB, Jayanthi S, Wang X, Dersch CM, Cadet JL, Prisinzano T, Rice KC, Baumann MH. Skinner HA, Sheu WJ, Wing JK, Babor T, Brugha T, Burke J, Cooper JE, Giel R, Jablenski A, Regier D, Sartorius N. Reliability of alcohol use indices, SCAN, and ecstasy tablet content. Quantitative PET studies of 5-HT2A receptors in the human brain using [18F]altanserin and the bolus/infusion approach are described, as well as linearized reference tissue parametric imaging methods. High familial risk for mood disorder is associated with low dorsolateral prefrontal cortex serotonin transporter binding, and a nonparametric method for automatic correction of intensity nonuniformity in MRI data can be used to delineate volumes of interest in human brain PET images. In vivo molecular imaging studies in humans have shown a nonlinear relationship between cerebral serotonin transporter and 5-HT(2A) receptor binding.

The cerebellum is a SERT-free brain region, and seasonal changes in brain serotonin transporter binding are observed in short serotonin transporter linked polymorphic region-allele carriers but not in long-allele homozygotes. Seasonal variation in human brain serotonin transporter binding, relationship to physiological and demographic variables, and occupancy of serotonin transporters by paroxetine and citalopram during treatment of depression are discussed. Serotonin transporter binding potential measured with [11C]DASB is affected by serotonin transporter genotype. A nonlinear relationship between cerebral serotonin transporter and 5-HT2A receptor binding was found in an in vivo molecular imaging study in humans. A prospective neuroimaging study in novel users of ecstasy showed sustained effects on the human brain. Positron emission tomographic evidence of toxic effect of MDMA on brain serotonin neurons in human beings. Former users of MDMA (ecstasy) showed increased brain serotonin transporter binding in a voxel-based PET study. Ecstasy causes a decrease in serotonin transporter availability in polydrug ecstasy users, but the reduction is reversible. PET imaging of serotonin transporters with [11C]DASB shows that the decrease is reproducible using a multilinear reference tissue parametric imaging method. 3,4-methylenedioxymethamphetamine (MDMA) produces selective serotonergic depletion in the nonhuman primate, and may also cause neurotoxicity. MDMA and other hallucinogenic tryptamines exhibit substrate behavior at the serotonin uptake transporter and the vesicle monoamine transporter, but not at the glial fibrillary acidic protein. The effects of non-medically used psychoactive drugs on monoamine neurotransmission in rat brain have been studied.

MDMA (ecstasy) does not affect cell death or 5-HT2A receptor density in organotypic rat hippocampal cultures, and MDMA causes transient down-regulation of serotonin 5-HT2 receptors. The Department of Forensic Chemistry at Aarhus University, Denmark, reported that ecstasy use was up in 2008 and that narcotics were in the streets in 2008. The department also reported that serotonin transporters were upregulated with chronic cocaine use. Brain serotonin transporter density and aggression are associated with abstinent methamphetamine abusers, and body mass index is inversely related to brain serotonin transporter binding.

Institutes

Institutes associated with this publication

University of Copenhagen
The Neurobiology Research Unit (NRU) at Copenhagen University Hospital have been carrying clinical and preclinical research with psychedelics since 2017.

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