A Comparative Study of Cannabinoids & CB1 Receptor GI Signaling

The legalization of medical marijuana has highlighted cannabinoids as a potential, opioid-free therapeutic option for pain management; however, the rise in illicit synthetic cannabinoid-induced toxicity has demonstrated the need to outline cannabinoid molecular signaling. The cannabinoid-type 1 (CB1) receptor is an endogenous G protein-gated receptor (GPCR), well-expressed in the central nervous system (CNS) associated with modulating neuronal activity. Cannabinoid agonists bind to the CB1 receptor resulting in the inhibitory G protein (Gi) complex to dissociate into two subunits, Gβγi and Gαi. The Gαi subunit inhibits adenylyl cyclase, leading to a decrease in cyclic adenosine monophosphate (cAMP). The Gβγi subunit activates G protein-gated inwardly-rectifying (GIRK) channels, resulting in the efflux of potassium (K+) ions and the subsequent hyperpolarizing of the neuron. Cannabinoids are a group of compounds with a diverse range of chemical structures. The primary cannabinoid classes are eicosanoid, classical, non-classical, and aminoalkylindole. The aminoalkylindole cannabinoids represent a large portion of illicit cannabinoids, or synthetic cannabinoid receptor agonists (SCRAs), marketed as marijuana alternatives. Contrary to marijuana, intake of SCRAs has toxic and sometimes, lethal consequences. The following studies report: 1) a fluorescent GIRK channel assay sensitive to CB1 receptor-mediated decrease in membrane potential. 2) analysis of the GIRK channel response to cannabinoids representative of the four cannabinoid classes 3) investigation of GIRK channel response to a selection of illicit SCRAs. The cAMP levels were compared for AEA, THC, CP 55, 940, and WIN 55, 212-2, in which all effectively suppressed cAMP. Cannabinoid potency across the primary cannabinoid classes ranked: CP 55, 940 \u3e WIN 55, 212-2 \u3e THC \u3e AEA \u3e THCA-A ≈ CBD. WIN 55, 212-2 (aminoalkylindole) was significantly more effective at activating the GIRK channel response compared to AEA (eicosanoid) and THC (classical). SCRAs had a rank order potency of 5-fluoro MDMB-PICA \u3e 4-fluoro MDMB-BUTINACA \u3e AB-FUBINACA \u3e MDMB-4en-PINACA \u3e JWH-018 \u3e AM1220 \u3e XLR-11 \u3e JWH-122 N-(5-chloropentyl) \u3e WIN 55, 212-2 \u3e UR-144 \u3e AM1248. CBD did not induce a GIRK channel response. Synthetic cannabinoids were more potent and effective at stimulating a GIRK channel response. Indole/Indazole carboxamide substitutions displayed higher potencies. Only 4-fluoro-MDMB-BUTINACA was significantly more efficacious at stimulating a GIRK channel response compared to WIN 55, 212-2. Overall, synthetic cannabinoids have greater GIRK channel potency and efficacy