N1‐Methyl‐2‐¹²⁵I‐Lysergic Acid Diethylamide, a Preferred Ligand for In Vitro and In Vivo Characterization of Serotonin Receptors

Abstract: Methylation of 2‐¹²⁵I‐lysergic acid diethylamide (¹²⁵I‐LSD) at the N1 position produces a new derivative, N1‐methyl‐2‐¹²⁵I‐lysergic acid diethylamide (¹²⁵I‐MIL), with improved selectivity and higher affinity for serotonin 5‐HT₂ receptors. In rat frontal cortex homogenates, specific binding of ¹²⁵I‐MIL represents 80–90% of total binding, and the apparent dissociation constant (K_D) for serotonin 5‐HT₂ receptors is 0.14 nM (using 2 mg of tissue/ml). ¹²⁵I‐MIL also displays a high affinity for serotonin 5‐HT_1C receptors, with an apparent dissociation constant of 0.41 nM at this site. ¹²⁵I‐MIL exhibits at least 60‐fold higher affinity for serotonin 5‐HT₂ receptors than for other classes of neurotransmitter receptors, with the dopamine D₂ receptor as its most potent secondary binding site. Studies of the association and dissociation kinetics of ¹²⁵I‐MIL reveal a strong temperature dependence, with very slow association and dissociation rates at 0°C. Autoradiographic experiments confirm the improved specificity of ¹²⁵I‐MIL. Selective labeling of serotonin receptors was observed in all brain areas examined. In vivo binding studies in mice indicate that ¹²⁵I‐MIL is the best serotonin receptor label yet described, with the highest frontal cortex to cerebellum ratio of any serotonergic radioligand. ¹²⁵I‐MIL is a promising ligand for both in vitro and in vivo labeling of serotonin receptors in the mammalian brain.