Cannabinoid 1 Receptor CBIR regulates growth cone filopodia and axon dispersion in the optic tract of Xenopus laevis tadpoles

Evaluating the Role of the Endocannabinoid System in Axon Guidance: A Literature Review

Introduction: The endocannabinoid system (ECS) mediates the actions of cannabis and has been implicated in playing critical roles in key developmental events, including axon guidance. Although several recent studies have demonstrated ECS involvement in neurodevelopment, an emphasis on its putative role in axon guidance has not been reviewed comprehensively. Objective: The purpose of this literature review is to evaluate the interrelationships between the ECS and axon guidance. Methodology: This literature review analyzes existing literature demonstrating the normal role of endocannabinoid (eCB) signaling in axon guidance, with evidence from diverse animal models. Studies were obtained from a search strategy involving terms related to the ECS and axon guidance, and cross-checking cited literature to ensure a complete evaluation. Discussion: Cannabinoid receptors, as well as eCB synthesis and degradation machinery, appear necessary for normal axon guidance during neurodevelopment. Genetic and/or pharmacological disruption of eCB signaling results in axon growth and guidance errors, implying high sensitivity to exogenous cannabinoids. Conclusion: Overall, this review highlights the intricate connections between the ECS and axon guidance in normal neurodevelopment. The mechanistic evidence discussed suggests that alterations of the ECS through genetic and pharmacological interference disrupt its normal functioning and by extension its normal role in regulating neural circuitry formation. A comprehensive understanding of this topic will be valuable in potentially uncovering the mechanisms responsible for the neurodevelopmental defects associated with pre-natal cannabis use.

Cannabinoid receptor type 1 regulates sequential stages of migration and morphogenesis of neural crest cells and derivatives in chicken and frog embryos

The main cannabinoid receptor CB1R first shows expression during early neurula stage in chicken (Gallus gallus) embryos, and at early tailbud stage in the frog (Xenopus laevis) embryos. This raises the question of whether CB1R regulates similar or distinct processes during the embryonic development of these two species. Here, we examined whether CB1R influences the migration and morphogenesis of neural crest cells and derivatives in both chicken and frog embryos. Early neurula stage chicken embryos were exposed to arachidonyl-2'-chloroethylamide (ACEA; a CB1R agonist), N-(Piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251; a CB1R inverse agonist) or Blebbistatin (nonmuscle Myosin II inhibitor) in ovo and examined during migration of neural crest cells and at condensing cranial ganglia stage. Early tailbud stage frog embryos were bathed in ACEA, AM251 or Blebbistatin, and analyzed at late tailbud stage for changes in craniofacial and eye morphogenesis, and in patterning and morphology of melanophores (neural crest-derived pigment cells). In chicken embryos exposed to ACEA and Myosin II inhibitor, cranial neural crest cells migrated erratically from the neural tube, and the right, but not the left, ophthalmic nerve of the trigeminal ganglia was affected in ACEA- and AM251-treated embryos. In frog embryos with inactivation or activation of CB1R, or inhibition of Myosin II, the craniofacial and eye regions were smaller and/or less developed, and the melanophores overlying the posterior midbrain were more dense, and stellate in morphology, than the same tissues and cells in control embryos. This data suggests that despite differences in the time of onset of expression, normal activity of CB1R is required for sequential steps in migration and morphogenesis of neural crest cells and derivatives in both chicken and frog embryos. In addition, CB1R may signal through Myosin II to regulate migration and morphogenesis of neural crest cells and derivatives in chicken and frog embryos.

Cell-autonomous and differential endocannabinoid signaling impacts the development of presynaptic retinal ganglion cell axon connectivity in vivo

Cannabis exposure during gestation evokes significant molecular modifications to neurodevelopmental programs leading to neurophysiological and behavioral abnormalities in humans. The main neuronal receptor for Δ⁹-tetrahydrocannabinol (THC) is the type-1 cannabinoid receptor CB₁R, one of the most abundant G-protein-coupled receptors in the nervous system. While THC is the major psychoactive phytocannabinoid, endocannabinoids (eCBs) are the endogenous ligands of CB₁R and are known to act as retrograde messengers to modulate synaptic plasticity at different time scales in the adult brain. Accumulating evidence indicates that eCB signaling through activation of CB₁R plays a central role in neural development. During development, most CB₁R localized to axons of projection neurons, and in mice eCB signaling impacts axon fasciculation. Understanding of eCB-mediated structural plasticity during development, however, requires the identification of the precise spatial and temporal dynamics of CB₁R-mediated modifications at the level of individual neurons in the intact brain. Here, the cell-autonomous role of CB₁R and the effects of CB₁R-mediated eCB signaling were investigated using targeted single-cell knockdown and pharmacologic treatments in Xenopus. We imaged axonal arbors of retinal ganglion cells (RGCs) in real time following downregulation of CB₁R via morpholino (MO) knockdown. We also analyzed RGC axons with altered eCB signaling following treatment with URB597, a selective inhibitor of the enzyme that degrades Anandamide (AEA), or JZL184, an inhibitor of the enzyme that blocks 2-Arachidonoylglycerol (2-AG) hydrolysis, at two distinct stages of retinotectal development. Our results demonstrate that CB₁R knockdown impacts RGC axon branching at their target and that differential 2-AG and AEA-mediated eCB signaling contributes to presynaptic structural connectivity at the time that axons terminate and when retinotectal synaptic connections are made. Altering CB₁R levels through CB₁R MO knockdown similarly impacted dendritic morphology of tectal neurons, thus supporting both pre- and postsynaptic cell-autonomous roles for CB₁R-mediated eCB signaling.