PROJECT SUMMARY Second generation antipsychotics (SGAs) are widely used clinical tools for the treatment of severe mental illness. However, their utility is highly variable, and they take weeks to become effective. The mechanisms behind this time course are not understood. SGAs are thought of as antagonists at the D2 dopamine receptor. Yet our recent work suggests that some SGAs function as arrestin-biased agonists at the less-characterized D3 dopamine receptor (D3R). Activation of the arrestin-3 pathway leads to degradation of D3R, potentially altering response to these drugs over time. I have generated preliminary data that shows mice grow tolerant to preclinical measures of SGA activity after chronic treatment of an arrestin-biased SGA. I hypothesize that D3R neurons are the locus of tolerance to select SGAs, and this tolerance is driven by decreased D3R membrane expression caused by arrestin-3 recruitment to D3R. I will use in vivo optical methods to assess changes in D3-neuron activity after chronic SGA treatment. I will also measure behavioral tolerance to SGA treatment in transgenic mice with altered abilities to degrade D3Rs and compare D3R levels using saturation binding and PET scans. This project will reveal mechanisms of tolerance to select SGAs at gross anatomy, cell- type, and protein levels.