MK-212, also known as 6-chloro-2-(1-piperazinyl)pyrazine (CPP), is a serotonin receptor agonist of the arylpiperazine family. It is specifically described as a non-selective serotonin 5-HT<sub>2</sub> receptor agonist or as a "relatively selective serotonin 5-HT<sub>2C</sub> receptor full agonist. The drug promotes the secretion of serum prolactin and cortisol in humans.
MK-212 did not produce hallucinogenic effects in humans at doses of up to 40mg orally. However, in other research, it occasionally produced LSD-like effects in alcoholic patients at a dose of 20mg. In addition, subsequent studies found that MK-212 at 20mg significantly increased ratings of feeling high and feeling strange.
MK-212 is an agonist of the serotonin 5-HT<sub>2</sub> receptors, including the serotonin 5-HT<sub>2C</sub>, 5-HT<sub>2B</sub>, and 5-HT<sub>2A</sub> receptors, in that order of potency. It is a full agonist of the serotonin 5-HT<sub>2C</sub> receptor, a moderate-efficacy partial agonist of the serotonin 5-HT<sub>2B</sub> receptor, and a partial to full agonist of the serotonin 5-HT<sub>2A</sub> receptor. The drug shows similar potency in activating the serotonin 5-HT<sub>2C</sub> and 5-HT<sub>2B</sub> receptors and around 10- to 30-fold lower relative potency in activating the serotonin 5-HT<sub>2A</sub> receptor. It also shows low affinity for the serotonin 5-HT<sub>1A</sub> and 5-HT<sub>1B</sub> receptors. THe comprehensive receptor interactions of MK-212 have been studied.
In a 1977 study by Clineschidt and colleagues, they dosed mice with varying concentrations of MK-212, and observed its effects. The result correlated very well to binding of indolealkylamine receptors, such as the serotonin and tryptamine receptors, which shows four characteristics. Namely, increased frequency of muscle twitching, increased twitching of the head, "an increase in the strength of the crossed extensor reflex in the acutely spinalized rat", and the cause of complex motor syndrome.