Molecular pharmacological differences in the interaction of serotonin with 5-hydroxytryptamine1C and 5-hydroxytryptamine2 receptors.

5-Hydroxytryptamine (5HT)1C and 5HT2 receptors appear to be closely related, from a molecular viewpoint, displaying similar second messenger systems and a high degree of sequence homology. However, there are striking differences in the interactions of 5HT with 5HT1C and 5HT2 receptors; 5HT is generally more potent in stimulating responses mediated through 5HT1C receptors than responses mediated through 5HT2 receptors. Also [3H]5HT labels 5HT1C receptors and not 5HT2 receptors. In order to explore more fully the molecular rationale for these differences, radioligand binding studies were performed in rat, human, and porcine brain and choroid plexus tissues and in mammalian cells transfected with rat 5HT1C or 5HT2 receptors; second messenger studies (inositol phosphate accumulation) were performed in the transfected cells. The second messenger studies confirmed the approximately 10-fold higher potency of 5HT in stimulating intracellular responses through 5HT1C receptors (EC50 = 8.3 nM) than in stimulating intracellular responses through 5HT2 receptors (EC50 = 101 nM). An agonist radioligand selective for the 5HT1C and 5HT2 receptors, 2,5-dimethoxy-(4-[125I]iodo)phenylisopropylamine, was used, as well as [3H]5HT, [3H]mesulergine (antagonist radioligand for 5HT1C receptors), and [3H]ketanserin (antagonist radioligand for 5HT2 receptors). Computer-assisted analyses of the binding data revealed two agonist affinity states for the 5HT1C receptor. The agonist high affinity state of the receptor was modifiable by guanyl nucleotides. The proportion of agonist high affinity states, relative to the total receptor population, was approximately 10% for both receptors. The apparent higher affinity of 5HT for the radiolabeled 5HT1C receptors was due to the higher affinity 5HT displayed for the agonist low affinity state of the 5HT1C receptor, compared with the affinity of 5HT for the agonist low affinity state of the 5HT2 receptor. The correspondence between the higher affinity of 5HT for the agonist low affinity state of the 5HT1C receptor, relative to the 5HT2 receptor, and the higher potency of 5HT in stimulating 5HT1C responses indicates that 5HT interacts with the agonist low affinity state of the 5HT1C and 5HT2 receptors in initiating its biological effects. These observations indicate that guanine nucleotide-binding protein (G protein)-coupled receptors can exhibit high affinity for neurotransmitters in both the free receptor and the G protein-coupled states and that receptors exhibiting this property may represent a novel subfamily of G protein-coupled receptors.