The long-term goal of the proposed research is to understand mechanistically how the neuronal cannabinoid receptor (CB1) becomes activated. Several aspects of this question will be explored. Below is a brief description of my specific aims. (Aim 1) The most widely accepted model for G-protein coupled receptor (GPCR) activation is the two-state model yet recent evidence suggests that this model is too simple to describe GPCR activation. The validity of the two-state model for CB1 activation will be assessed using fluorescence spectroscopy to monitor conformational states of the activated receptor to determine if there is one active state conformation or multiple, distinct conformational states. (Aim 2) Nucleotide binding to docked G-proteins has been shown to affect a GPCR's affinity for its ligands. By exploring whether this is true for the CB1 receptor, I will determine if nucleotide allosteric regulation of ligand binding works through inducing conformational changes in the receptor. (3) The mechanisms by which CB1 is constitutively active will be examined by attempting to make mutations in the receptor that reduce or abolish the constitutive activity. These mutants will then be studied structurally to examine if and how any conformational changes are linked to constitutive activity. Because marijuana is a widely used street drug, and the cannabinoid receptor is a member of the G-protein coupled receptor family (the largest class of drug discovery targets), understanding the mechanism by which the CB1 receptor is modulated is important both clinically and scientifically.