Cannabinoid withdrawal in mice: Inverse agonist vs neutral antagonist

Rationale: Previous reports shows rimonabant's inverse properties may be a limiting factor for treating cannabinoid dependence. To overcome this limitation, neutral antagonists were developed, to address mechanisms by which an inverse agonist and neutral antagonist elicit withdrawal. Objective: The objective of this study is to introduce an animal model to study cannabinoid dependence by incorporating traditional methodologies and profiling novel cannabinoid ligands with distinct pharmacological properties/modes of action by evaluating their pharmacological effects on CB1-receptor (CB1R) related physiological/behavioral endpoints. Methods: The cannabinergic AM2389 was acutely characterized in the tetrad (locomotor activity, analgesia, inverted screen/catalepsy bar test, and temperature), with some comparisons made to Δ9-tetrahydrocannabinol (THC). Tolerance was measured in mice repeatedly administered AM2389. Antagonist-precipitated withdrawal was characterized in cannabinoid-adapted mice induced by either centrally acting antagonists, rimonabant and AM4113, or an antagonist with limited brain penetration, AM6545. Results: In the tetrad, AM2389 was more potent and longer acting than THC, suggesting a novel approach for inducing dependence. Repeated administration of AM2389 led to tolerance by attenuating hypothermia that was induced by acute AM2389 administration. Antagonist-precipitated withdrawal signs were induced by rimonabant or AM4113, but not by AM6545. Antagonist-precipitated withdrawal was reversed by reinstating AM2389 or THC. Conclusions: These findings suggest cannabinoid-precipitated withdrawal may not be ascribed to the inverse properties of rimonabant, but rather to rapid competition with the agonist at the CB1R. This withdrawal syndrome is likely centrally mediated, since only the centrally acting CB1R antagonists elicited withdrawal, i.e., such responses were absent after the purported peripherally selective CB1R antagonist AM6545.