The endocannabinoid system regulates zebrafish GnRH neuronal development

The endocannabinoid system comprises several components, among which specific seven transmembrane-domain receptors (namely CB1 and CB2 cannabinoid receptors), their exogenous (e.g. Δ9-THC) and endogenous ligands (e.g. AEA and 2-AG), and a number of biosynthetic and degradative enzymes. CB1 cannabinoid receptors are widely expressed in the brain and regulate various steps of neuronal development. In the zebrafish embryo, previous data showed that CB1 knockdown causes defects in axonal fasciculation in the anterior commissure. Since this area is particularly rich in GnRH fibers, we assessed whether CB1 receptor could regulate GnRH axonal pathfinding and fasciculation in zebrafish embryos. We therefore performed both antagonistmediated CB1 downregulation and morpholino-mediated CB1 knockdown on GnRH3::GFP zebrafish embryos. We found that CB1 knockdown reduces the number of GnRH3::GFP positive cells in the olfactory epithelium while not changing their position, it reduces the extension of GnRH neuropil, and causes axons misrouting in the anterior commissure. Furthermore, CB1 knockdown downregulates the expression of two genes involved in axonal growth and cell migration, namely Stmn2aand Sez6 Taken together these results indicate that during early zebrafish development the endocannabinoid system is involved in the regulation of GnRH system development.