Synergistic and additive terpene-THC interactions in cannabinoid CB1 and CB2 receptors

Cannabis terpenes are widely recognized for shaping the plant's aromatic and sensory profiles. Yet, their pharmacological role in cannabis is less established. Recent work demonstrated that several cannabis terpenes can activate cannabinoid receptors type 1 and 2 (CB₁R and CB₂R) as low-efficacy agonists, raising the possibility that they may also modulate the activation at these receptors by Δ⁹-tetrahydrocannabinol (THC), through orthosteric or allosteric mechanisms. Here, we systematically investigated the effects of individual terpenes and of terpene mixtures on THC-evoked activation at CB₁R and CB₂R, expressed in Xenopus laevis oocytes. Using G protein-gated inwardly rectifying potassium (GIRK) channel activation as a functional readout, we quantified concentration-response relationships for THC alone and in combination with terpenes, and applied isobolographic analysis to distinguish additive, synergistic, and antagonistic interactions. We found that several terpenes selectively potentiated THC activation at CB₁R and CB₂R. Isobolographic analysis suggested additive THC-terpene interactions for some terpenes, and synergistic interactions for others, including borneol, limonene, sabinene, terpineol, α-pinene and ocimene at CB₁R, and β-caryophyllene and linalool at CB₂R. Terpene mixtures displayed dose-dependent CB₁R activation, and several mixtures synergistically enhanced THC responses. Together, our findings suggest that cannabis terpenes may act as both partial orthosteric agonists and allosteric modulators at CB₁R and CB₂R. These results provide a mechanistic framework for cannabinoid-terpene synergy and highlight the importance of receptor specificity and of formulation design in shaping terpene mediated activation. These findings stress the need for terpenes selection for formulations tailor for specified medicinal needs.