Cannabinoid receptors help the brain’s dopamine system establish key connections after birth, a new mouse study suggests.
Doctors warn that marijuana use during pregnancy may have harmful effects on the development of a fetus, in part because the cannabinoid receptors activated by the drug are known be critical for enabling a developing brain to wire up properly. Now, scientists at MIT’s McGovern Institute for Brain Research have learned that cannabinoid receptors’ critical role in brain development does not end at birth.
In today's online issue of eNeuro, scientists led by McGovern investigator Ann Graybiel report that mice need the cannabinoid receptor CB1R to establish connections within the brain’s dopamine system that take shape soon after birth. The finding raises concern that marijuana use by nursing moms, who pass the CB1R-activating compound THC to their infants when they breastfeed, might interfere with brain development by disrupting cannabinoid signaling.
“This is a real change to one of the truly important systems in the brain — a major controller of our dopamine,” says Graybiel, who is an Institute Professor and a faculty member in the Department of Brain and Cognitive Sciences. Dopamine exerts a powerful influence over our motivations and behavior, and changes to the dopamine system contribute to disorders from Parkinson’s disease to addiction. Thus, the researchers say, it is vital to understand whether postnatal drug exposure might put developing dopamine circuits at risk.
Cannabinoid receptors in the brain are important mediators of mood, memory, and pain. Graybiel’s lab became interested in CB1R due to their dysregulation in Huntington’s and Parkinson’s diseases, both of which impair the brain’s ability to control movement and other functions. While investigating the receptor’s distribution in the brain, they discovered that in the adult mice, CB1R is abundant within small compartments within the striatum called striosomes. The receptor was particularly concentrated within the neurons that connect striosomes to a dopamine-rich area of the brain called the substantia nigra, via structures that Graybiel’s team has dubbed striosome-dendron bouquets.
Striosome-dendron bouquets are easy to overlook within the densely connected network of the brain. But when the cells that make up the bouquets are labeled with a fluorescent protein, the bouquets become visible — and their appearance is striking, says Jill Crittenden, a research scientist in Graybiel’s lab.