Cannabinoid-induced stimulation of motor activity in planaria through an opioid receptor-mediated mechanism.

Planaria, the most primitive example of centralization and cephalization of the nervous system along phylogeny, shows specific stereotyped behavioral patterns following exposure to drugs acting on neural transmission. In this study, the authors investigated the effects of exposure to the synthetic cannabinoid receptor agonist WTN55212.2 on motor activity in planaria. WTN55212.2 produced dose-dependent stimulation of motor behavior. High doses of the drug caused stereotyped activities identical to those seen previously with opioid agonists. These effects were antagonized by coexposure to cannabinoid or opioid receptor antagonists. The results indicate that functional interactions between cannabinoid and opioid systems are highly conserved along phylogeny, at least at the behavioral level.