Obstetricians advise pregnant women to 'just say no' to recreational drugs because research suggests that some of these drugs interfere with normal fetal development. Now Berghuis et al. report that receptors that bind the psychoactive substances in marijuana are important in axon guidance in a recent article in Science.
Marijuana is derived from the Cannabis plant, and its active component tetrahydrocannabinol (THC) acts at cannabinoid receptors in the brain. Postsynaptic neurons in the adult nervous system release the endogenous cannabinoids anandamide and 2-arichidonoylglycerol, which act retrogradely at presynaptic neurons to block neurotransmitter release. In the developing brain, CB1 cannabinoid receptors are expressed in neural precursors and growing axons, and CB1 receptor expression increases during synaptogenesis.
The authors localized CB1 receptors to the developing axons of cortical and hippocampal pyramidal cells in mice at embryonic day 13.5. Similarly, diacylglycerol lipases α and 
(DAGLα/), which are necessary for 2-arichodonoylglycerol production, were present in elongating telencephalic axons, suggesting that endocannabinoid production and signaling may occur in the same neurons during axon outgrowth, according to the authors. Later in gestation (embryonic day 18.5), CB1 receptors localized to the axons and growth cones of GABAergic interneurons. The growth cone particulate fraction of cortical lysates had more CB1 receptor protein relative to total cortical lysates. In contrast, DAGLα/ localized to glutamatergic pyramidal cell dendrites, suggesting that CB1 receptors and endocannabinoids are positioned on opposite sides of pyramidal cell synapses during synaptogenesis.
CB1 receptors localize to the business end of growth cones. In cultures of GABAergic interneurons, CB1 receptors localized to the F-actin–rich filopodial tips of growth cones. Anandamide treatment shifted CB1 receptors into the center of the growth cone. In cultured hippocampal neurons, anandamide treatment induced the retrograde transport of CB1 receptors through the axon, away from the growth cone.
Endocannabinoids are chemorepulsive. The CB1 receptor agonist WIN55,212-2 induced growth cones to turn away from the drug application site and collapse. During growth cone repulsion, RhoA GTPase activates the serine-threonine Rho kinase (ROCK). In contrast to WIN55,212-2 treatment alone, coapplication of WIN55,212-2 with the ROCK inhibitor Y-27632 was chemoattractive, inducing growth cones to turn toward the drug application site, suggesting that CB1 receptor is coupled to RhoA.
GABAergic interneurons express the homeobox genes Dlx5 and Dlx6 during embryonic development and transporters for vesicular GABA (VGAT) and vesicular glutamate 3 (VGLUT3) in adulthood. The authors mated mice with the CB1 receptor gene surrounded by loxP recombination sites with mice expressing Cre recombinase in Dlx5- and Dlx6-expressing cells. Pyramidal cells in the resulting progeny had more VGAT- and VGLUT3-positive synaptic inputs relative to wild-type mice, suggesting that the absence of CB1 receptors impairs axon target selection.
Therefore, marijuana exposure might cause premature growth cone collapse and impaired synaptogenesis in the fetal brain. In adults, cannabinoids localize to dendritic spines in the hippocampus, so marijuana exposure may inhibit synaptic plasticity. Because RhoA inhibits peripheral axon regeneration, the cannabinoid receptor inhibitor AM251 might induce the regrowth of axons following injury.