MDMA (ecstasy) activates trace amine-1 receptors (TA1Rs), which normally inhibit dopamine and serotonin release. In mice lacking TA1Rs, MDMA caused only hyperthermia (not the biphasic temperature response seen in normal mice), produced larger increases in dopamine levels in the striatum, frontal cortex, and nucleus accumbens, and led to greater locomotion that was blocked by haloperidol. Serotonin release was also amplified in TA1R-deficient mice. A TA1R agonist reduced the dopamine- and serotonin-releasing effects of another drug in normal mice but not in knockout mice. TA1Rs thus limit MDMA's neurochemical and behavioral effects, suggesting they play a regulatory role in the drug's actions.
The serotonin 5-HT(2A) receptor is a primary target of psychedelic hallucinogens like LSD, mescaline, and psilocybin, which mimic some schizophrenia symptoms. A paradox is that some 5-HT(2A) receptor agonists cause hallucinations while structurally similar ones do not. Comparing the phosphoproteome in HEK-293 cells expressing the 5-HT(2A) receptor, 16 phosphorylation sites differed between the hallucinogen DOI and the nonhallucinogenic agonist lisuride. One site, serine 280 in the receptor's third intracellular loop, was specifically phosphorylated by hallucinogens. In mice, DOI enhanced this phosphorylation in the prefrontal cortex, while lisuride did not. Hallucinogens caused less receptor desensitization than nonhallucinogenic agonists. Mutating serine 280 altered desensitization, revealing biased phosphorylation underlies different desensitization capacities.