Δ⁹-Tetrahydrocannabinol (THC) and cannabidiol (CBD) are two main cannabinoid constituents of marijuana and hashish. The pharmacology of Δ⁹-THC has been extensively studied, whereas our understanding of the pharmacology of CBD has remained limited, despite excitement in CBD’s potential role in treating certain pediatric epilepsies and its reputation for attenuating some Δ⁹-THC–induced effects. It was established early on that CBD binds poorly to the orthosteric site of CB₁ or CB₂ cannabinoid receptors, and its actions were commonly attributed to other noncannabinoid receptor mechanisms. However, recent evidence suggests that CBD does indeed act at cannabinoid CB₁ receptors as a negative allosteric modulator (NAM) of CB₁ signaling. By altering the orthosteric signaling of a G protein–coupled receptor, allosteric modulators greatly increase the richness of G protein–coupled receptor pharmacology. We have recently surveyed candidate CB₁ NAMs in autaptic hippocampal neurons, a well-characterized neuronal model of endogenous cannabinoid signaling, and have now tested CBD in this model. We find that although CBD has no direct effect on excitatory transmission, it does inhibit two forms of endogenous cannabinoid-mediated retrograde synaptic plasticity: depolarization-induced suppression of excitation and metabotropic suppression of excitation, while not affecting signaling via GABA-B receptors. These results are consistent with the recently described NAM activity of CBD and suggest interesting possible mechanisms for CBD’s therapeutic actions.

Many diseases of the nervous system are accompanied by alterations in synaptic functions. Synaptic plasticity mediated by the endogenous cannabinoid system involves the activation of the cannabinoid receptor 1 (CB1R). The principles of CB1R signaling must be understood in detail for its therapeutic exploration. We detected the Src homology 3-domain growth factor receptor-bound 2-like (endophilin) interacting protein 1 (SGIP1) as a novel CB1R partner. SGIP1 is functionally linked to clathrin-mediated endocytosis and its overexpression in animals leads to an energy regulation imbalance resulting in obesity. We report that SGIP1 prevents the endocytosis of activated CB1R and that it alters signaling via the CB1R in a biased manner. CB1R mediated G-protein activation is selectively influenced by SGIP1, β-arrestin associated signaling is changed profoundly, most likely as a consequence of the prevention of the receptor's internalization elicited by SGIP1.

Background and Purpose: Src homology 3‐domain growth factor receptor‐bound 2‐like endophilin interacting protein 1 (SGIP1) interacts with cannabinoid CB₁ receptors. SGIP1 is abundantly and principally expressed within the nervous system. SGIP1 and CB₁ receptors co‐localize in axons and presynaptic boutons. SGIP1 interferes with the internalization of activated CB₁ receptors in transfected heterologous cells. Consequently, the transient association of CB₁ receptors with β‐arrestin2 is enhanced and prolonged, and CB₁ receptor‐mediated ERK1/2 signalling is decreased. Because of these actions, SGIP1 may modulate affect, anxiety, pain processing, and other physiological processes controlled by the endocannabinoid system (ECS).Experimental Approach: Using a battery of behavioural tests, we investigated the consequences of SGIP1 deletion in tasks regulated by the ECS in SGIP1 constitutive knockout (SGIP1^−/−) mice.Key Results: In SGIP1^−/− mice, sensorimotor gating, exploratory levels, and working memory are unaltered. SGIP1^−/− mice have decreased anxiety‐like behaviours. Fear extinction to tone is facilitated in SGIP1^−/− females. Several cannabinoid tetrad behaviours are altered in the absence of SGIP1. SGIP1^−/− males exhibit abnormal behaviours on Δ⁹‐tetrahydrocannabinol withdrawal. SGIP1 deletion also reduces acute nociception, and SGIP1^−/− mice are more sensitive to analgesics.Conclusion and Implications: SGIP1 was detected as a novel protein associated with CB₁ receptors, and profoundly modified CB₁ receptor signalling. Genetic deletion of SGIP1 particularly affected behavioural tests of mood‐related assessment and the cannabinoid tetrad. SGIP1^−/− mice exhibit decreased nociception and augmented responses to CB₁ receptor agonists and morphine. These in vivo findings suggest that SGIP1 is a novel modulator of CB₁ receptor‐mediated behaviour.

Cannabis contains more than 100 phytocannabinoids. Most of these remain poorly characterized, particularly in neurons. We tested a panel of five phytocannabinoids—cannabichromene (CBC), cannabidiolic acid (CBDA), cannabidivarin (CBDV), cannabidivarinic acid (CBDVA), and Δ⁹-tetrahydrocannabivarin (THCV) in two neuronal models, autaptic hippocampal neurons and dorsal root ganglion (DRG) neurons. Autaptic neurons expressed a form of CB1-dependent retrograde plasticity while DRGs expressed a variety of transient receptor potential (TRP) channels. CBC, CBDA, and CBDVA had little or no effect on neuronal cannabinoid signaling. CBDV and THCV differentially inhibited cannabinoid signaling. THCV inhibited CB1 receptors presynaptically while CBDV acted post-synaptically, perhaps by inhibiting 2-AG production. None of the compounds elicited a consistent DRG response. In summary, we find that two of five ‘minor’ phytocannabinoids tested antagonized CB1-based signaling in a neuronal model, but with very different mechanisms. Our findings highlight the diversity of potential actions of phytocannabinoids and the importance of fully evaluating these compounds in neuronal models.

In addition to phytocannabinoids, cannabis contains terpenoids that are claimed to have a myriad of effects on the body. We tested a panel of five common cannabis terpenoids, myrcene, linalool, limonene, α-pinene and nerolidol, in two neuronal models, autaptic hippocampal neurons and dorsal root ganglion (DRG) neurons. Autaptic neurons express a form of cannabinoid CB1 receptor-dependent retrograde plasticity while DRGs express a variety of transient receptor potential (TRP) channels. Most terpenoids had little or no effect on neuronal cannabinoid signaling. The exception was nerolidol, which inhibited endocannabinoid signaling. Notably, this is not via inhibition of CB1 receptors but by inhibiting some aspect of 2-arachidonoylglycerol (2-AG) production/delivery; the mechanism does not involve reducing the activity of the 2-AG-synthesizing diacylglycerol lipases (DAGLs). Nerolidol was also the only terpenoid that activated a sustained calcium response in a small (7%) subpopulation of DRGs. In summary, we found that only one of five terpenoids tested had notable effects on cannabinoid signaling in two neuronal models. Our results suggest that a few terpenoids may indeed interact with some components of the cannabinoid signaling system and may therefore offer interesting insights upon further study.