Methamphetamine and MDMA Neurotoxicity: Biochemical and Molecular Mechanisms

Methamphetamine (METH) and methylenedioxymethamphetamine (MDMA) are psychostimulant drugs that are widely abused. The two drugs can also cause neurotoxic damage and neurodegeneration in several regions of the brains of humans, nonhuman primates, and rodents. METH-induced behavioral and neurotoxic abnormalities occur consequent to alterations of dopamine terminal physiology, which leads to massive release of dopamine (DA) in the synaptic clefts of several brain regions. Increased DA levels in synaptic clefts are further enhanced by METH-induced blockade of DA re-uptake into DA terminals through the DA transporter (DAT). METH toxicity is not only accompanied by terminal dysfunctions/degeneration but also by impairments in complex networks that subserve cognitive and emotional processes. MDMA, a ring-substituted derivative of phenyl-isopropylamine, is structurally similar to METH. MDMA is a substrate of the serotonin transporter (SERT) through which it enters monoaminergic terminals where it triggers release of serotonin (5-HT) from storage vesicles into synaptic clefts of brain regions that receive serotoninergic terminals from the ventral and dorsal midbrain raphe nuclei. In addition, MDMA has been shown to cause selectively neurotoxic damage to serotonergic nerve terminals in rats, guinea pigs, and nonhuman primates even though toxic damage to the human brain has been debatable. There is also evidence that some MDMA users exhibit cognitive deficits. Nevertheless, there is, at present, no firm evidence that links drug-induced DA and/or 5-HT depletion to cognitive impairments. More studies are necessary to clarify the importance of these changes in the development of beneficial therapeutics to the treatment of patients who suffer from METH and MDMA use disorders.