Illuminating tetrahydrocannabinol signaling in the brain’s reward circuitry with in vivo photopharmacology

Oregon Health & Science University, Portland OR

Investigators

Abstract

Project Summary/Abstract 9-tetrahydrocannabinol (THC) is the main psychoactive compound in marijuana, acting on CB1 cannabinoid receptors (CBRs) in the brain. Marijuana use is increasing in the United States, yet its role as an addictive drug is controversial. Excessive use can cause cannabis use disorder, and some research suggests that marijuana is likely to precede the use of other illicit drugs. However, our understanding of the mechanisms by which THC affects the brain is confounded by widespread CB1 receptor expression and the ability of THC to activate other cannabinoid receptor subtypes expressed in overlapping circuits. Much of the field's prior work with THC has relied on systemic injection to rodents or THC wash-on to acute brain slices; these approaches cannot pinpoint the location of CBRs or downstream signaling mechanisms activated by THC. To deconvolute the effects of THC in the brain's reward circuits, there is a critical need for new tools to acutely manipulate THC with enhanced spatiotemporal precision. The applicant's long-term goal is to identify the molecular mechanisms by which THC affects brain reward circuits and illuminate the role of THC in addictive behaviors. To this end, they developed a photo-switchable THC analog containing an azobenzene photoswitch (azo-THC) that can be isomerized between active and inactive configurations using two different colors of light. This allows THC activity to be turned ON and OFF in a reversible manner. To enhance azo-THC's spatial precision in vivo, they also developed THC photoswitch that can be targeted to specific cells or membranes using genetically encoded protein tags (PORTL-THC). Preliminary results demonstrate the feasibility of these tools to reversibly control THC activity with light; however, their application has so far been limited to CBR-overexpressing cell lines. The overall objective of this proposal is to validate the ability of azo-THC and PORTL-THC to reversibly manipulate CBR signaling in cultured neurons and in vivo. The tools will be benchmarked against existing approaches in two interwoven but independent specific aims: 1) Optical control of THC-signaling on cultured neurons and in vivo; 2) Engineer PORTL-THC for cell-specific modulation of THC-activity. These aims will validate the utility of these optical tools to modulate THC activity in neurons, and then map the local effects of THC in the mesolimbic circuit during behavior. This proposal has a strong scientific premise built on rigorous preliminary data, our published studies, and a careful review of the literature. It is innovative because it will generate state-of-the-art tools to target CBRs on specific neuron populations in freely moving mice. The completion of these aims will have significant impact by advancing our knowledge of THC actions in brain reward circuits, and will increase our understanding of the molecular mechanisms that underlie widespread psychiatric disorders, such as addiction.