Deleting Two Opioid Receptors Altered THC Withdrawal More Than Several Acute Effects in Mice
Mice lacking both mu and delta opioid receptors showed reduced THC withdrawal signs and reward-related behavior, while several acute THC effects remained intact. The genetic experiment separated opioid-system contributions across different cannabinoid outcomes.
Some acute effects persisted
Acute THC-related antinociception and reduced movement were unaffected by the double deletion. The temperature-lowering effect was slightly attenuated. These comparisons show that the mutation did not simply eliminate all responses to THC.
Tolerance also depended on the endpoint
Tolerance to hypothermia developed more slowly in the mutant mice. Tolerance to antinociception and reduced movement developed without the same change. The study therefore does not support a global statement that deleting the receptors prevented cannabinoid tolerance.
Reward and withdrawal showed separate changes
THC-related conditioned place preference was reduced, as were somatic manifestations of withdrawal. Place preference is a learned environmental-choice measure, not a direct report of euphoria. Somatic withdrawal scores likewise capture selected physical signs rather than every aspect of dependence.
The authors interpreted the findings as evidence that mu and delta receptor activity cooperates in the full expression of THC dependence. Lifelong receptor deletion is different from administering a medication to an adult animal or patient. The result identifies a mechanistic research question; it does not establish a human treatment or imply that preserved acute effects make the altered animals unimpaired.
Related: interpreting cannabis tolerance claims.
Research cited
Castañé A; Robledo P; Matifas A; Kieffer BL; Maldonado R. Cannabinoid withdrawal syndrome is reduced in double mu and delta opioid receptor knockout mice. The European journal of neuroscience (2003). doi:10.1046/j.1460-9568.2003.02409.x. Findings discussed from the study abstract.
