Project Summary Cognitive impairment is a core feature of many neuropsychiatric disorders that directly correlates with functional outcome for patients. Current treatment options are inadequate and there is a need for novel therapeutic strategies. The dopamine D1-like receptor family (D1 and D5 receptors) has been proposed as a potential target for improving cognitive function. Clinical studies with D1/D5 full agonists have shown promising efficacy, but these compounds suffer from significant drawbacks related to adverse effect profiles. The lack of selective ligands as well as considerable spatial overlap between the D1 and D5 receptors have made it difficult to determine which receptor is responsible for specific efficacy and adverse effect features of the nonselective agonists. There have been efforts to develop selective tools for D1 receptors but, to our knowledge, there have been no formal efforts to identify D5-selective compounds. Recent research from our group and others has shown that the D5 receptor plays a critical role in cognitive function, particularly in cognitive domains significantly disrupted in neuropsychiatric disorders. Additionally, multiple D5-selective signaling pathways relevant for neural function have been identified. Unfortunately, further experimentation to determine the role of the D5 receptor in neurobiology has been hindered by the lack of selective ligands. In this application, we propose to design and synthesize D5-selective chemical probes. Due to the high degree of homology between the D5 and D1 orthosteric binding sites, we will focus on allosteric modulators. Once selectivity for D5 receptors over D1 receptors has been established using cAMP induction assays, potency on cell-based assays of D5-specific signaling cascades will be determined. Compounds shown to be potent and selective will then undergo in vitro screening for metabolic stability and counterscreening against a broad panel of receptors and ion channels. Compounds that are found to be metabolically stable with minimal off-target binding, will be tested in vivo to determine if they have sufficient plasma and brain exposure to engage the D5 receptor and advance to pharmacodynamic assays. Finally, compounds will be tested to determine their ability to induce BDNF expression in the prefrontal cortex, a known effect of signaling through the D5 receptor. Once optimized and characterized, these D5-selective probes will allow us to further characterize the role of the D5 receptor in neural function and determine if it is a viable therapeutic target.