Embryonic exposure to cannabidiol disrupts active neural circuits, an effect increased by Δ-tetrahydrocannabinol and involving CB1R and CB2R in zebrafish

Considering the wide spread use of cannabis as a recreational and medicinal drug, a knowledge gap exists regarding biological mechanisms and health implication. In the light of legislative changes, delayed effects of cannabis upon brief exposure during embryonic development are of high interest as early pregnancies often go undetected. We hypothesized that brief early cannabinoid exposure impacts neural activity by affecting embryonic brain development through cannabinoid-1 (CB1R) and -2 (CB2R). Zebrafish larvae were exposed to cannabidiol (CBD) and {Delta}{square}-tetrahydrocannabinol (THC) until the end of gastrulation (1-10 hours post-fertilization) and analyzed later in development (4-5 days post-fertilization). In order to measure neural activity, we implemented the fluorescence based calcium detector CaMPARI (Calcium-Modulated Photoactivatable Ratiometric Integrator) and optimized the protocol such that a high number of samples can be economically used in a 96-well format complemented by locomotor analysis. Our results revealed that neural activity was decreased by CBD more than THC. At higher doses both cannabinoids could reduce neural activity close to a level comparable to anesthetized samples and locomotor activity was abolished. Interestingly, the decrease was more pronounced when CBD and THC were combined. At the receptor level, CBD-mediated reduction of locomotor activity was partially prevented using CB1R or CB2R inhibitors. Overall, we provided a mechanistic link to perturbed brain development in vivo, which seems to involve CBD and receptors, CB1R and CB2R. Future studies are warranted to reveal other cannabinoids and receptors involved in this pathway to understand the full health implications of cannabis consumption on fetal development.