Endocannabinoids (eCBs) are retrograde neurotransmitters that modulate the function of many types of synapses. The presence of eCBs, their CB1 receptor (CB1R), and metabolizing enzymes at embryonic and early postnatal periods have been linked to developmental processes such as neuronal proliferation, differentiation, and migration, axon guidance, and synaptogenesis. Here, we demonstrate the presence of a functional eCB system in the developing visual system and the role of CB1R during axon growth and retinothalamic development. Pharmacological treatment of retinal explants and primary cortical neuron cultures with ACEA, a selective CB1R agonist, induced a collapse of the growth cone (GC). Furthermore the application of AM251, a CB1R inverse agonist, to the neuronal cultures increased the surface area of GC. In vivo, intraocular injection of ACEA diminished retinal projection growth, while AM251 promoted growth and caused aberrant projections. In addition, compared with their wild-type littermates, CB1R-deficient adult mice revealed a lower level of eye-specific segregation of retinal projections in the dorsal lateral geniculate nucleus. Finally, we found that pharmacological modulation of CB1R affected the trafficking of Deleted in colorectal cancer (DCC) receptor to the plasma membrane in a PKAdependent manner. Moreover, pharmacological inhibition or genetic inactivation of DCC abolished the CB1R-induced reorganization of the GC. Overall, these findings establish a mechanism by which the CB1R influences GC behavior and nervous system development in concerted action with DCC.

Navigation of retinal projections towards their targets is regulated by guidance molecules and growth cone transduction mechanisms. Here, we present in vitro and in vivo evidences that the cannabinoid receptor 2 (CB2R) is expressed along the retino-thalamic pathway and exerts a modulatory action on axon guidance. These effects are specific to CB2R since no changes were observed in mice where the gene coding for this receptor was altered (cnr2 −/−). The CB2R induced morphological changes observed at the growth cone are PKA dependent and require the presence of the netrin-1 receptor, Deleted in Colorectal Cancer. Interfering with endogenous CB2R signalling using pharmacological agents increased retinal axon length and induced aberrant projections. Additionally, cnr2 −/− mice showed abnormal eye-specific segregation of retinal projections in the dorsal lateral geniculate nucleus (dLGN) indicating CB2R’s implication in retinothalamic development. Overall, this study demonstrates that the contribution of endocannabinoids to brain development is not solely mediated by CB1R, but also involves CB2R.

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