Functional characterization of the cannabinoid receptors 1 and 2 in zebrafish larvae using behavioral analysis

The endocannabinoid system (ECS) comprises the cannabinoids anandamide and 2-arachidonoylglycerol and the cannabinoid receptors 1 and 2 (Cnr1 and Cnr2). The function of these receptors in relation to zebrafish larval behavior is poorly understood, even though the zebrafish larva has become a versatile animal model in biomedical research.

The objective of the present study is to characterize the function of Cnr1 and Cnr2 in relation to behavior in zebrafish.

Behavioral analysis of zebrafish larvae was performed using a visual motor response (VMR) test, which allows locomotor activity to be determined under basal conditions and upon a dark challenge.

Treatment with the non-specific Cnr agonists WIN55,212-2 and CP55,940 resulted in a decrease in locomotion. This was observed for both basal and challenge-induced locomotion, although the potency for these two effects was different, which suggests different mechanisms of action. In addition, WIN55,212-2 increased the reaction time of the startle response after the dark challenge. Using the Cnr1 antagonist AM251 and a cnr1^-/- mutant line, it was shown that the effects were mediated by Cnr1 and not Cnr2. Interestingly, administration of the antagonist AM251 alone does not have an effect on locomotion, which indicates that endogenous cannabinoid activity does not affect locomotor activity of zebrafish larvae. Upon repeated dark challenges, the WIN55,212-2 effect on the locomotor activity decreased, probably due to desensitization of Cnr1.

Taken together, these results show that Cnr1 activation by exogenous endocannabinoids modulates both basal and challenge-induced locomotor activity in zebrafish larvae and that these behavioral effects can be used as a readout to monitor the Cnr1 responsiveness in the zebrafish larva model system.