Cannabinoids Stimulate the TRP Channel-Dependent Release of Both Serotonin and Dopamine to Modulate Behavior in<i>C. elegans</i>

Abstract

Cannabis sativa alters sensory perception and exhibits potential medicinal benefits. In mammals, cannabinoids activate two canonical receptors, CB 1 /CB 2 , as well additional receptors/ion channels whose overall contributions to cannabinoid signaling have yet to be fully assessed. In Caenorhabditis elegans , the endogenous cannabinoid receptor agonist, 2-arachidonoylglycerol (2-AG) activates a CB 1 ortholog, NPR-19, to modulate behavior (Oakes et al., 2017). In addition, 2-AG stimulates the NPR-19 independent release of both serotonin (5-HT) and dopamine (DA) from subsets of monoaminergic neurons to modulate locomotory behaviors through a complex monoaminergic signaling pathway involving multiple serotonin and dopamine receptors. 2-AG also inhibits locomotion in remodeled monoamine receptor mutant animals designed to measure the acute release of either 5-HT or DA, confirming the direct effects of 2-AG on monoamine release. 2-AG-dependent locomotory inhibition requires the expression of transient receptor potential vanilloid 1 (TRPV1) and TRPN-like channels in the serotonergic or dopaminergic neurons, respectively, and the acute pharmacological inhibition of the TRPV1-like channel abolishes both 2-AG-dependent 5-HT release and locomotory inhibition, suggesting the 2-AG may activate the channel directly. This study highlights the advantages of identifying and assessing both CB 1 /CB 2 -dependent and independent cannabinoid signaling pathways in a genetically tractable, mammalian predictive model, where cannabinoid signaling at the molecular/neuronal levels can be correlated directly with changes in behavior. SIGNIFICANCE STATEMENT This study is focused on assessing CB 1 /CB 2 -independent cannabinoid signaling in a genetically tractable, whole-animal model where cannabinoid signaling at the molecular/neuronal levels can be correlated with behavioral change. Caenorhabditis elegans contains a cannabinoid signaling system mediated by a canonical cannabinoid receptor, NPR-19, with orthology to human CB 1 /CB 2 (Oakes et al., 2017). The present study has characterized an NPR-19-independent signaling pathway that involves the cannabinoid-dependent release of both serotonin and dopamine and the expression of distinct TRP-like channels on the monoaminergic neurons. Our work should be of interest to those studying the complexities of CB 1 /CB 2 -independent cannabinoid signaling, the role of TRP channels in the modulation of monoaminergic signaling, and the cannabinoid-dependent modulation of behavior.