Stable expression of constitutively activated mutant h5HT6 and h5HT7 serotonin receptors: inverse agonist activity of antipsychotic drugs.

Rationale: In order to determine the possible relationship between antipsychotic drug properties and inverse agonist activity at h5HT6 and h5HT7 receptors, constitutively activated forms of these receptors were created by site-specific mutagenesis. Typical and atypical antipsychotic drugs were assayed for their potencies as inverse agonists at these mutated receptors.

Objectives: Stable cell lines expressing constitutively activated forms of the h5HT6 and h5HT7 receptors were created. Typical and atypical antipsychotic drugs demonstrating high to moderate affinities for the h5HT6 and h5HT7 receptors were assayed for their potencies in reversing the agonist-independent activity (inverse agonist activity).

Results: The E322R h5HT6 mutant and the S267K h5HT7 mutant displayed sufficiently robust agonist-independent activity when expressed in stable cell lines to allow the detailed, concentration-dependent, investigation of the inverse agonist activity of typical and atypical antipsychotic drugs. All the drugs tested displayed inverse agonist activity at both the activated h5HT6 and h5HT7 receptors. Native forms of these receptors did not display any constitutive activity. Interestingly, atypical antipsychotic drugs displayed potent inverse agonist activity, relative to typical antipsychotic drugs, at the h5HT7 receptor. LSD displayed neutral antagonist properties at the mutant h5HT7 receptor.

Conclusions: Site-specific mutations in the third intracellular loop of the G(s)-coupled h5HT6 and h5HT7 receptors produce constitutive activation. Antipsychotic drugs display inverse agonist activity at the activated receptors. The inverse agonist mechanism-of-action of the atypical antipsychotic drugs at the h5HT7 receptors may be different from the typical antipsychotic drugs as these drugs displayed far higher potencies as inverse agonists at the h5HT7 receptor.