Site‐Directed Mutations in the Third Intracellular Loop of the Serotonin 5‐HT1A Receptor Alter G Protein Coupling from Gi to Gs in a Ligand‐Dependent Manner

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Abstract
The effect of mutations (V344E and T343A/V344E) in the third intracellular loop of the serotonin 5‐HT1A receptor expressed transiently in human embryonic kidney 293 cells have been examined in terms of receptor/G protein interaction and signaling. Serotonin, (R)‐8‐hydroxy‐2‐dipropylaminotetralin [(R)‐8‐OH‐DPAT], and buspirone inhibited cyclic AMP production in cells expressing native and mutant 5‐HT1A receptors. Serotonin, however, produced inverse bell‐shaped cyclic AMP concentration‐response curves at native and mutant 5‐HT1A receptors, indicating coupling not only to Gi/Go, but also to Gs. (R)‐8‐OH‐DPAT, however, induced stimulation of cyclic AMP production only after inactivation of Gi/Go proteins by pertussis toxin and only at the mutant receptors. The partial agonist buspirone was unable to induce coupling to Gs at any of the receptors, even after pertussis toxin treatment. The basal activities of native and mutant 5‐HT1A receptors in suppressing cyclic AMP levels were not found to be significantly different. The receptor binding characteristics of the native and mutant receptors were investigated using the novel 5‐HT1A receptor antagonist [3H]NAD‐299. For other receptors, analogous mutations have produced constitutive activation. This does not occur for the 5‐HT1A receptor, and for this receptor the mutations seem to alter receptor/G protein coupling, allowing ligand‐dependent coupling of receptor to Gs in addition to Gi/Go proteins.

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