Preferential limbic expression of the cannabinoid receptor mRNA in the human fetal brain

The cannabinoid receptor one (CB1) is responsible for the effects of cannabis on motor and cognitive function in the CNS. There is to date very limited information about the CB1 gene expression in the human brain, in particular during fetal development. In the present study, in situ hybridization experiments were used to examine the microscopic and macroscopic organization of the CB1 mRNA expression in normal human fetal (approximately 20 weeks of development) and adult brains. The fetal brain showed a distinct heterogeneous pattern of the CB1 mRNA expression which was low to moderate in many brain areas. The most striking feature of the fetal brain was the intense expression in the hippocampal CA region and basal nuclear group of the amygdaloid complex. Many of the same brain areas that showed positive expression of the CB1 mRNA in the fetal brain also expressed the gene in the adult brain. However, aside from an intense expression in the hippocampus which resembled that in fetal brain, the adult brain showed very high expression throughout the cerebral cortex, caudate nucleus, putamen and cerebellar cortex. These results document a different pattern of the anatomical organization of the CB1 mRNA expression in the mid-gestation fetal and adult human brain. Overall, the high CB1 mRNA expression in the fetal hippocampus and amygdala indicates that these limbic structures might be most vulnerable to prenatal cannabis exposure.