GPR55 is a lysophosphatidylinositol (LPI)-sensitive G-protein coupled receptor (GPCR) that recognizes a sub- set of cannabinoid CB1 and CB2 ligands. The goal of the proposed project is to understand the functional features of GPR55 that may define mechanisms of drug-receptor interactions relevant to physiological and pathophysiological function, including drug abuse. GPR55 has been implicated in inflammatory pain, neuropathic pain, metabolic disorder, bone and neurological development, and cancer, indicating the real potential of GPR55 ligands as therapeutics. Each of these areas alone is medically important and each would benefit by the use of selective agonists and antagonists to further studies. While selective agonists and antagonists for GPR55 from a number of diverse scaffolds have been identified, no low nanomolar potency ligands have been confirmed for this receptor, nor is there a radioligand developed to characterize binding. We have recently identified the first set of GPR55 residues important for agonist signaling and for GPR55 activation. This information should aid in the rational design of next generation GPR55 ligands, using our evolving molecular model. The goal of this proposal is to use structure-based design and cheminformatics tools to develop structure-activity relationships for selected scaffolds, leading to the identification of low nanomolar ligands that retain high receptor selectivity. We aim to discover nanomolar potency GPR55 ligands using a combination of molecular modeling, chemoinformatics, high-throughput screening and site directed mutagenesis. We will dissect the specific signaling pathways governing GPR55 activity as well as investigate ligand bias.