In mammals, G protein coupled receptors (GPCRs) form the largest family of cell surface receptors and are involved in the regulation of every aspect of neurophysiology. GPCRs sense the presence of neurotransmitters, hormones, changes in pH, levels of light, and many other fluctuations in the extracellular environment, and they translate it into intracellular messages leading to appropriate cellular responses. Because of their role as powerful regulators, GPCRs are the most exploited molecular target by currently available drugs. Surprisingly, what activates ~100 members of the GPCR family is still unknown, hence these receptors are named orphans. Nonetheless, human and animal studies revealed relevant physiological roles for many orphan GPCRs, especially in the brain. Orphan GPCRs represent therefore unexploited pharmacological targets for the treatment of a variety of neuropsychiatric disorders. The goal of the proposed study is to build an innovative assay to identify what activates four orphan GPCRs (GPR137b, GPR156, GPR158, and GPR179) that are enriched in the central nervous system and are involved in disorders such as depression and night blindness. This deorphanization process will be completed by screening a library of biologically active molecules whose targets are unknown. With our work, we expect to identify the endogenous ligands activating these orphan GPCRs, or, at least, to find synthetic compounds that are able to modulate their activity. The identification of orphan GPCR ligands will expand our knowledge on the biology of these receptors, and, at the same time, it will boost the development of new therapeutics targeting a variety of diseases including depression, addiction, blindness, and other major neuropsychiatric disorders.