REPEATED ADMINISTRATION OF MDMA CAUSES TRANSIENT DOWN-REGULATION OF SEROTONIN 5-HT2 RECEPTORS

The present study examined short- and long-term effects of MDMA (3,4-methylene-dioxymethamphetamine) on serotonin (5-HT2 and 5-HT1c) receptors in the brain of the rat. N1-Methyl-2-(I-125]lysergic acid diethylamide ([I-125]MIL) was used to label these receptors in vitro and in vivo. The usefulness of [I-125]MIL for in vivo detection of changes in 5-HT2 receptors was confirmed in preliminary experiments in which rats were treated chronically with mianserin (5 mg/kg, once daily for 10 days). Decreases in specific in vivo binding of [I-125]MIL, after treatment with mianserin were found to be of the same magnitude as those determined by others, using in vitro methods. The MDMA (8 doses; 5-20 mg/kg each) was administered to rats over a period of 4 days. At various times after administration of the last dose of MDMA, the binding of [I-125]MIL was measured. Acutely, treatment with MDMA (20 mg/kg) reduced specific in vivo binding of [I-125]MIL in all regions of brain studied. For example, in the frontal cortex, specific binding of [I-125]MIL was decreased by 80% at 6 hr and by 62% at 24 hr, after cessation of treatment with MDMA. Twenty-one days after administration of MDMA however, the number of binding sites for [I-125]MIL was back to control levels. Reductions in in vivo binding of [I-125]MIL in frontal cortex were dependent on the dose of MDMA injected and were associated with decreases in the number of binding sites for [I-125]MIL (B(max) values) in tissue homogenates of the same area. Autoradiographic studies of MDMA-treated rats confirmed the decreased density of 5-HT2 receptors and also suggested that the 5-HT1c receptor of the choroid plexus was not affected. These results indicate that repeated administration of MDMA caused transient down-regulation of 5-HT2 receptors in the brain of the rat. Further, they demonstrated that [I-125]MIL is a suitable radioligand for labeling 5-HT2 receptors, both in vitro and in vivo. Once labeled with an appropriate radionuclide for SPECT (single photon emission computed tomography) or PET (positron emission tomography), MIL should prove useful for monitoring changes in the density of serotonin receptors in the living mammalian brain.