Concentration, population, and context-dependent effects of AM251 in zebrafish

Early suppression of the endocannabinoid degrading enzymes FAAH and MAGL alters locomotor development in zebrafish

The fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL) enzymes are the predominant catabolic regulators of the major endocannabinoids (eCBs) anadamide (AEA) and 2-arachidonoylglycerol (2-AG), respectively. The expression and roles of eCBs during early embryogenesis remain to be fully investigated. Here, we inhibited FAAH and MAGL in zebrafish embryos during the first 24 h of life and examined motor neuron and locomotor development at 2 and 5 days post fertilization (dpf). Application of the dual FAAH/MAGL inhibitor, JZL195 (2 µmol l−1), resulted in a reduction in primary and secondary motor neuron axonal branching. JZL195 also reduced nicotinic acetylcholine receptor (nAChR) expression at neuromuscular junctions. Application of URB597 (5 µmol l−1), a specific inhibitor of the FAAH enzyme, also decreased primary motor neuron branching but did not affect secondary motor neuron branching and nAChR expression. Interestingly, JZL184 (5 µmol l−1), a specific inhibitor of MAGL, showed no effects on motor neuron branching or nAChR expression. Co-treatment of the enzyme inhibitors with the CB1R inhibitor AM251 confirmed the involvement of CB1R in motor neuron branching. Disruption of FAAH or MAGL reduced larval swimming activity, and AM251 attenuated the JZL195- and URB597-induced locomotor changes, but not the effects of JZL184. Together, these findings indicate that inhibition of FAAH, or augmentation of AEA acting through CB1R during early development, may be responsible for locomotor deficiencies.

Putative Activation of the CB1 Cannabinoid Receptors Prevents Anxiety-Like Behavior, Oxidative Stress, and GABA Decrease in the Brain of Zebrafish Submitted to Acute Restraint Stress

Anxiety disorder is a well-recognized condition observed in subjects submitted to acute stress. Although the brain mechanisms underlying this disorder remain unclear, the available evidence indicates that oxidative stress and GABAergic dysfunction mediate the generation of stress-induced anxiety. Cannabinoids are known to be efficient modulators of behavior, given that the activation of the cannabinoid receptors type-1 (CB1 receptors) induces anxiolytic-like effects in animal models. In the present study, we aimed to describe the effects of the stimulation of the CB1 receptors on anxiety-like behavior, oxidative stress, and the GABA content of the brains of zebrafish submitted to acute restraint stress (ARS). The animals submitted to the ARS protocol presented evident anxiety-like behavior with increased lipid peroxidation in the brain tissue. The evaluation of the levels of GABA in the zebrafish telencephalon presented decreased levels of GABA in the ARS group in comparison with the control. Treatment with ACEA, a specific CB1 receptor agonist, prevented ARS-induced anxiety-like behavior and oxidative stress in the zebrafish brain. ACEA treatment also prevented a decrease in GABA in the telencephalon of the animals submitted to the ARS protocol. Overall, these preclinical data strongly suggest that the CB1 receptors represent a potential target for the development of the treatment of anxiety disorders elicited by acute stress.

Analyzing cannabinoid-induced abnormal behavior in a zebrafish model

In this study, we investigated locomotor activity and responses to repeated light and dark stimuli to assess cannabinoid-induced abnormal behavior in zebrafish larvae (Danio rerio), as an alternative to standard rodent models. To induce the desired responses, we used cannabidiol and WIN55,212-2, two major cannabinoid components. A repeated light and dark test was used to assess how drug exposure influences locomotory responses. Larvae were examined after moderate cannabidiol and WIN55,212-2 exposure and at 24 h after transfer to untreated water. We found that cannabidiol did not produce a dose-dependent inhibitory effect on locomotor activity, with both 0.5 and 10 μg/mL concentrations reducing movement velocity and the total distance moved. However, 10 μg/mL cannabidiol was observed to attenuate the responses of larvae exposed to darkness. No differences were detected between the control and cannabidiol-treated groups after 24 h in fresh water. Fish treated with WIN55,212-2 at 0.5 and 1 μg/mL showed virtually no activity, even in darkness, whereas a concentration of 10 μg/mL induced mortality. A 24-h period in fresh water had the effect of reversing most of the drug-induced immobilization, even in the WIN55,212-2-treated groups. Larvae were also evaluated for their responses to cannabidiol subsequent to an initial exposure to WIN55,212-2, and it was accordingly found that treatment with cannabidiol could attenuate WIN55,212-2-induced abnormal immobilization, whereas equivalent doses of cannabidiol and WIN55,212-2 produced a mixed response. In conclusion, the behavioral effects of the two cannabinoids cannabidiol and WIN55,212-2 appear to be ratio dependent. Furthermore, the repeated light and dark test could serve as a suitable method for assaying drug-induced behavior.

CB1 and CB2 receptors play differential roles in early zebrafish locomotor development

Endocannabinoids (eCBs) mediate their effects through actions on several receptors, including the cannabinoid receptors CB₁R and CB₂R. The role played by eCBs in the development of locomotor systems is not fully understood. In this study, we investigated the roles of the eCB system in zebrafish development by pharmacologically inhibiting CB₁R and CB₂R (with AM251 and AM630, respectively) in either the first or second day of development. We examined the morphology of motor neurons and we determined neuromuscular outputs by quantifying the amount of swimming in 5 days post-fertilization larvae. Blocking CB₂R during the first day of development resulted in gross morphological deficits and reductions in heart rate that were greater than those following treatment with the CB₁R blocker AM251. Blocking CB₁Rs from 0 to 24 h post-fertilization resulted in an increase in the number of secondary and tertiary branches of primary motor neurons, whereas blocking CB₂Rs had the opposite effect. Both treatments manifested in reduced levels of swimming. Additionally, blocking CB₁Rs resulted in greater instances of non-inflated and partially inflated swim bladders compared with AM630 treatment, suggesting that at least some of the deficits in locomotion may result from an inability to adjust buoyancy. Together, these findings indicate that the eCB system is pivotal to the development of the locomotor system in zebrafish, and that perturbations of the eCB system early in life may have detrimental effects.