5-HT1 and 5-HT2 binding profiles of the serotonergic agents alpha-methylserotonin and 2-methylserotonin.

alpha-Methyl-5-hydroxytryptamine (alpha-Me-5-HT; 2) and 2-methyl-5-hydroxytryptamine (2-Me-5-HT; 3) are considered to be 5-HT2-selective and 5-HT3-selective agents, respectively. These agents were synthesized and examined at serotonin (5-HT) binding sites because there is relatively little documentation as to their selectivity and because they have not been previously examined at the newly discovered 5-HT1D and 5-HT1E sites. As previously reported, 2-Me-5-HT possesses a low affinity (Ki greater than 500 nM) for 5-HT1A, 5-HT1B, 5-HT1C, and 5-HT2 sites; this agent also displays a low affinity for 5-HT1D (Ki = 1220 nM) and 5-HT1E (Ki greater than 10,000 nM) sites. However, alpha-Me-5-HT displays little selectivity for 5-HT1A, 5-HT1B, 5-HT1C, and 5-HT1D sites (Ki = 42, 85, 150, and 150 nM, respectively) and a very low affinity for 5-HT1E (Ki greater than 10,000 nM) sites. Depending upon the radioligand used to label the sites, alpha-Me-5-HT displays either a low affinity (Ki = 880 nM with [3H]ketanserin) or a high affinity (Ki = 3 nM with [3H]DOB) for 5-HT2 sites. These results suggest that alpha-Me-5-HT is not as selective as previously considered and that caution should be used when employing this agent in pharmacological studies because it may act as mixed 5-HT1/5-HT2 agonist.