3,4-Methylenedioxymethamphetamine (MDMA) neurotoxicity in rats: a reappraisal of past and present findings

Abstract

Rationale: 3,4-Methylenedioxymethamphetamine (MDMA) is a widely abused illicit drug. In animals, high-dose administration of MDMA produces deficits in serotonin (5-HT) neurons (e.g., depletion of forebrain 5-HT) that have been interpreted as neurotoxicity. Whether such 5-HT deficits reflect neuronal damage is a matter of ongoing debate.

Objective: The present paper reviews four specific issues related to the hypothesis of MDMA neurotoxicity in rats: (1) the effects of MDMA on monoamine neurons, (2) the use of "interspecies scaling" to adjust MDMA doses across species, (3) the effects of MDMA on established markers of neuronal damage, and (4) functional impairments associated with MDMA-induced 5-HT depletions.

Results: MDMA is a substrate for monoamine transporters, and stimulated release of 5-HT, NE, and DA mediates effects of the drug. MDMA produces neurochemical, endocrine, and behavioral actions in rats and humans at equivalent doses (e.g., 1-2 mg/kg), suggesting that there is no reason to adjust doses between these species. Typical doses of MDMA causing long-term 5-HT depletions in rats (e.g., 10-20 mg/kg) do not reliably increase markers of neurotoxic damage such as cell death, silver staining, or reactive gliosis. MDMA-induced 5-HT depletions are accompanied by a number of functional consequences including reductions in evoked 5-HT release and changes in hormone secretion. Perhaps more importantly, administration of MDMA to rats induces persistent anxiety-like behaviors in the absence of measurable 5-HT deficits.

Conclusions: MDMA-induced 5-HT depletions are not necessarily synonymous with neurotoxic damage. However, doses of MDMA which do not cause long-term 5-HT depletions can have protracted effects on behavior, suggesting even moderate doses of the drug may pose risks.

Fig. 1

Chemical structures of MDMA and related compounds

Fig. 2

Effects of (±)-MDMA and (±)-MDA on extracellular levels of 5-HT (top panel) and DA (bottom panel) in rat nucleus accumbens. Male rats undergoing in vivo microdialysis received i.v. injections of saline (0 dose) or drug, and dialysate levels of 5-HT and DA were assayed by HPLC–ECD (Baumann and Rutter 2003). Data are expressed as the percentage of three pre-injection baseline samples; each bar represents the mean±SEM peak effect measured 20 min posttreatment, N=6 rats/group. Baseline levels of 5-HT and DA were 0.17±0.01 and 1.31±0.05 pg/5 μl, respectively. Asterisk denotes significance with respect to zero dose control (P<0.05 Duncan’s)

Fig. 3

Metabolism of MDMA in man. CYP2D6 Cytochrome P450 2D6, CYP3A4 cytochrome P450 3A4, COMT catechol-O-methyltransferase. This is adapted from de la Torre et al. (2004)

Fig. 4

Acute effects of (±)-MDMA on core body temperature in rats. Male rats received three i.p. injections of 1.5 or 7.5 mg/kg MDMA, one dose every 2 h (i.e., injections at 0, 2, and 4 h). Saline was administered on the same schedule. Core temperature was recorded via insertion of a rectal thermometer probe every 2 h. Data are mean±SEM for N=5 rats/group. Asterisk denotes significance with respect to saline-injected control at each time point (P<0.05 Duncan’s)

Fig. 5

Long-term effects of (±)-MDMA on tissue levels of 5-HT (top panel) and DA (bottom panel) in brain regions. Male rats received three i.p. injections of 1.5 or 7.5 mg/kg MDMA, one dose every 2 h. Saline was administered on the same schedule. Rats were killed 2 weeks after injections; brain regions were dissected, and tissue 5-HT and DA were assayed by HPLC–ECD (Baumann et al. 2001). Data are mean±SEM expressed as the percentage of saline-treated control values for each region, N=5 rats/group. Control values of 5-HT and DA were 557±24 and 28±4 pg/mg tissue for frontal cortex (CTX), 429±36 and 10,755±780 pg/mg tissue for striatum (STR), and 1,174±114 and 4,545±426 pg/mg tissue for olfactory tubercle (OT). Asterisk denotes significance compared to saline-injected control for each region (P<0.05 Duncan’s)

Fig. 6

Comparative effects of (±)-MDMA (top panel) and 5,7-DHT (bottom panel) on SERT protein expression in dissected brain regions. One group of rats received three i.p. injections of saline or 7.5 mg/kg MDMA, one dose every 2 h. Another group received single i.c.v. infusions of 5,7-DHT or vehicle. Rats were killed 2 weeks later and brain regions were dissected. Western blot analysis of SERT immunoreactivity in the frontal cortex (CTX), striatum (STR), and hippocampus (HIPP) was carried out as described (Wang et al. 2005). Blots were digitized and quantified using densitometry (NIH IMAGE software). Changes in immunoreactivity are expressed relative to their corresponding control (defined as 100% value). Each value is the mean±SEM for N=4 rats/group. Asteriskdenotes significance with respect to control in each region (P<0.01 Student’s t test). Data taken from Wang et al.

Fig. 7

Comparative effects of (±)-MDMA (top panel) and 5,7-DHT (bottom panel) on GFAP expression in dissected brain regions. One group of rats received three i.p. injections of saline or 7.5 mg/kg MDMA, one dose every 2 h. Another group received single i.c.v. infusions of 5,7-DHT or vehicle. Rats were killed 2 weeks later and brain regions were dissected. Western blot analysis of GFAP immunoreactivity in the frontal cortex (CTX), striatum (STR), and hippocampus (HIPP) was carried out as described (Wang et al. 2005). Blots were digitized and quantified using densitometry (NIH IMAGE software). Changes in immunoreactivity are expressed relative to their corresponding control (defined as 100% value). Each value is the mean±SEM for N=4 rats/group. Asterisk denotes significance with respect to control in each region (P<0.01 Student’s t test). Data were taken from Wang et al.

Fig. 8

Effects of (±)-MDMA pretreatment on secretion of corticosterone (top panel) and prolactin (bottom panel) evoked by acute (±)-MDMA challenge. Male rats received three i.p. injections of 1.5 or 7.5 mg/kg MDMA, one dose every 2 h. Saline was administered on the same schedule. Two weeks later, each rat received a single i.v. injection of 1 mg/kg MDMA at time zero, followed by a single injection of 3 mg/kg MDMA 60 min later. Blood samples were drawn via indwelling catheters immediately before injections and 30 min after each injection; plasma corticosterone and prolactin were measured by RIA (Baumann et al. 1998). Data are mean±SEM expressed as the percentage of baseline hormone levels for N=8 rats/group. Baseline corticosterone and prolactin levels were 73±18 and 2.4±0.6 ng/ml of plasma, respectively. Asterisk denotes significance compared to saline-pretreated control group (P<0.05 Duncan’s)