Distribution of CB1 cannabinoid receptors and their relationship with mu-opioid receptors in the rat periaqueductal gray

► CB1 receptor expression is diffuse and extensive in the periaqueductal gray (PAG). ► The mu-opioid peptide (MOP) and CB1 receptors co-localize in many PAG neurons. ► CB1 labeling was mostly somatodendritic, but CB1 axon terminals were also detected. ► The pattern and extent of CB1 labeling was similar in the dlPAG and vlPAG. ► Chronic morphine or THC administration had no effect on CB1 receptor density.