We have shown that clozapine and risperidone, atypical antipsychotic drugs, have potent inverse agonist properties at constitutively activated mutant (CAM) forms of the rat 5SHT2A and 5HT2C receptors. Inverse agonist activity may be a significant property of antipsychotic drugs, given the revised ternary complex model of G-protein coupled receptors (GPCR), which predicts a steady-state level of activation of receptors in the absence of ligand stimulation. Further studies of antipsychotic drug actions at CAM forms of clozapine-sensitive human SHT receptors are necessary to determine if inverse agonist activity is a key property of atypical antipsychotic drugs. In order to expand the studies to the human 5HT6 and 5HT7 receptors we have attempted to make CAM forms of these receptors by mutating two well-documented regions of GPCR constitutive activity. Initial experiments involving mutations in these areas have produced forms of the receptor either lacking robust constitutive activity or producing apparently null mutant forms of the receptor (5HT6). While these results have slowed progress on determining the inverse agonist activity of antipsychotic drugs on these receptors they open up interesting avenues of research on the variability in structure within the GPCR family and within 5HT receptors in particular. Therefore we propose to pursue three specific aims: 1) we will continue to test typical and atypical antipsychotic drugs at human CAM forms of the 5HT2A and 5HT2C receptors; 2) we will continue to mutate the human 5HT6 and 5HT7 receptors to produce CAM forms of these receptors and test antipsychotic drugs for inverse agonist activity at these receptors; 3) we will examine effects of constitutive activation on clozapine-sensitive 5HT receptor cellular trafficking, and the effects of inverse agonists on the trafficking of the mutated receptors. The results of these studies should reveal the role inverse agonist activity of antipsychotic drugs plays in the atypical properties of clozapine, and may indicate a major role for one or more of the clozapine-sensitive receptors in the atypical properties of clozapine. This information should be very helpful in designing a new generation of atypical antipsychotic drugs sharing clozapine's unique antipsychotic properties, but lacking its deleterious hematological effects. Information concerning alterations in cellular processing of CAM receptors should also be forthcoming, including information on the molecular domains involved in directing cellular compartmentalization, believed to play a key role in cellular receptor sensitivity states.
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