Studies in many rhodopsin-like G-protein-coupled receptors are providing a general scheme of the structural processes underlying receptor activation. Microdomains in several receptors have been identified that appear to function as activation switches. However, evidence is emerging that these receptor proteins exist in multiple conformational states. To study the molecular control of this switching process, we investigated the function of a microdomain involving the conserved helix 7 tyrosine in the serotonin 5HT2C receptor. This tyrosine of the NPXXY motif was substituted for all naturally occurring amino acids. Three distinct constitutively active receptor phenotypes were found: moderate, high, and "locked-on" constitutive activity. In contrast to the activity of the other receptor mutants, the high basal signaling of the locked-on Y7.53N mutant was neither increased by agonists nor decreased by inverse agonists. The Y7.53F mutant was uncoupled. Computational modeling based on the rhodopsin crystal structure suggested that Y7.53 interacts with the conserved aromatic ring at position 7.60 in the recently identified helix 8 domain. This provided a basis for seeking revertant mutations to correct the defective function of the Y7.53F receptor. When the Y7.53F receptor was mutated at position 7.60, the wild-type phenotype was restored. These results suggest that Y7.53 and Y7.60 contribute to a common functional microdomain connecting helices 7 and 8 that influences the switching of the 5HT2C receptor among multiple active and inactive conformations.