Ultrastructural localization of neuronal brain CB2 cannabinoid receptors

The functional expression of neuronal CB2 cannabinoid receptors (CB2-Rs) in the brain has been controversial. We and others have now demonstrated that CB2-Rs are expressed in neurons and glial cells in the brain. However, the subcellular localization of these receptors has not been characterized. In this study we used immunohistochemical electron microscopy to determine the subcellular distribution of CB2-Rs in two brain regions. Brain sections from the CA1 hippocampal area and substantia nigra were immunostained for CB2-Rs and analyzed by electron microscopy. In each region immunoperoxidase labeling for CB2-Rs was detected in neurons as well as in glial and endothelial cells. In neuronal cells, CB2-R immunoreactivity was observed in somata and large and medium-sized dendrites. In the soma, the CB2-R labeling was mainly associated with the rough endoplasmic reticulum and Golgi apparatus, suggesting its endogenous synthesis. In the dendrites, the CB2-R labeling was observed in the cytoplasm and was associated with the plasma membrane near the area of synaptic contact with axon terminals, indicating a postsynaptic distribution of these receptors. In CB2-Rs in immunoreactive glial and endothelial cells, the labeling was also found to be associated with the plasma membrane. In the substantia nigra, some unmyelinated axons were immunoreactive for CB2-Rs, but we rarely found CB2-R-labeled axon terminals. These results extend our previous detection of postsynaptic cortical CB2-Rs and provide additional ultrastructural evidence that CB2-Rs are mainly postsynaptic in the CA1 area of the hippocampus and substantia nigra. The functional implication of pre- and/or postsynaptic localization of CB2-Rs remains to be determined.