Pharmacological characterization of cannabidiol as a negative allosteric modulator of the 5-HT_2A receptor

Promising clinical evidence suggests that psychedelic compounds, like lysergic acid diethylamide (LSD), have therapeutic value for treatment of psychiatric disorders. However, they often produce hallucinations and dissociative states, likely mediated by the serotonin (5-HT) receptor 5-HT_2A, raising challenges regarding therapeutic scalability. Given the reported antipsychotic effects of cannabidiol (CBD) and its promiscuous binding at many receptors, we assessed whether CBD could modulate 5-HT_2A signalling. Activation of the 5-HT_2A intracellular signalling events were assessed using resonance energy transfer- or fluorescence-based biosensors in HEK 293 cells and in rat primary cortical neurons. In 5-HT_2A-transfected HEK 293 T cells, CBD antagonized LSD-mediated Gq activation in a saturable way, while leaving β-arrestin2 recruitment unaffected. CBD decreased Gq activation mediated by the 5-HT_2A-specific agonist DOI as well as LSD-mediated activity in primary rat neonatal cortical neurons. Using Site Identification by Ligand Competitive Saturation (SILCS) simulations, we also predicted that the putative binding site of CBD overlapped with that of oleamide, a positive allosteric modulator of 5-HT_2A, and could displace the binding of orthosteric ligands toward the external binding pocket. Based on these findings, we propose that CBD acts as a negative allosteric modulator of 5-HT_2A.