Cannabinoid Receptor 1 knockdown induces axonal misrouting of the GnRH neurons in zebrafish embryos

The endocannabinoid system is widely conserved in evolution and comprises several components: natural ligands, also known as endocannabinoids, enzymes and receptors. Among the receptors, Cannabinoid Receptor 1 (CB1) is the most expressed in brain and regulates various step of neuronal development. CB1 is heavily expressed in the hypothalamus, where it can negatively affect the secretion of Gonadotropin Releasing Hormone (GnRH). Previous data showed that in zebrafish embryo, CB1 knockdown causes defects in axonal fasciculation in the anterior commissure, which contains GnRH fibers. To attest whether CB1 can regulate GnRH axonal pathfinding and fasciculation in zebrafish embryos, we performed morpholino-mediated CB1 knockdown on GnRH3::GFP zebrafish embryos. We found that CB1 knockdown reduces the number of GnRH::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. Taken together these results indicate that during early zebrafish development, CB1 acts as a regulator of axonal pathfinding on GnRH cells. Future experiments will elucidate if the CB1 miss-regulation also affects GnRH neuron migration from the olfactory placode to the hypothalamus, with consequent effects on sexual maturation and reproduction.