Cannabinoid receptor 1 is required for neurodevelopment of striosome-dendron bouquets

Cannabinoid receptor 1 (CB1R) has strong effects on neurogenesis and axon pathfinding in the prenatal brain. Endocannabinoids that activate CB1R are abundant in the early postnatal brain and in mothers milk, but few studies have investigated their function in newborns. We examined postnatal CB1R expression in the major striatonigral circuit from striosomes of the striatum to the dopamine-containing neurons of the substantia nigra. CB1R enrichment was first detectable between postnatal days 5 and 7, and this timing coincided with the formation of striosome-dendron bouquets, the elaborate anatomical structures by which striosomal neurons control dopaminergic cell activity through inhibitory synapses. In Cnr1-/- knockouts lacking CB1R expression, striosome-dendron bouquets were markedly disorganized by postnatal day 11 and at adulthood, suggesting a postnatal pathfinding connectivity function for CB1R in connecting striosomal axons and dopaminergic neurons analogous to CB1Rs prenatal function in other brain regions. Our finding that CB1R plays a major role in postnatal wiring of the striatonigral dopamine system, with lasting consequences at least in mice, points to a crucial need to determine whether lactating mothers use of CB1R agonists (e.g., in marijuana) or antagonists (e.g., type 2 diabetes therapies) can disrupt brain development in nursing offspring.