Cannabinoid CB1 receptor and mu-opioid receptor interaction: new insights from conditional knockout mice

Co-administration of cannabinoids and opioids has been shown to enhance analgesic effects. However, the underlying mechanisms remain unclear. One hypothesis suggests that cannabinoid CB1 receptors (CB1R) and mu-opioid receptors (MOR) interact at the cell membrane or intracellular signaling level. This study aimed to test this hypothesis by examining CB1R-MOR colocalization and assessing whether the deletion of one receptor affects the other's response to its ligand. Results from RNAscope in situ hybridization revealed that CB1R and MOR mRNAs exhibit distinct regional distributions in the mouse brain. Colocalization of CB1R and MOR was primarily observed in the paraventricular nucleus of the thalamus (PVT), where ~50% of vesicular glutamate transporter 2 (Vglut2)-positive glutamatergic neurons displayed CB1R-MOR co-expression. In contrast, only a small subset (5%-25%) of Vglut2-positive neurons in pain-related regions, such as the periaqueductal gray (PAG) and the dorsal horn of the spinal cord, or vesicular GABA transporter (Vgat)-positive GABA neurons in reward-related regions, such as the nucleus accumbens, ventral tegmental area, and substantia nigra, exhibited such colocalization. Unexpectedly, the selective deletion of MOR from Vglut2-positive glutamatergic neurons or Vgat-positive GABAergic neurons did not alter the effects of Δ⁹-tetrahydrocannabinol (Δ⁹-THC), including analgesia, hypothermia, catalepsy, rotarod locomotor impairment, or conditioned place aversion. Similarly, CB1R deletion from GABAergic neurons did not affect oxycodone-induced analgesia, hypothermia, or conditioned place preference. These findings do not support the hypothesis of a direct CB1R-MOR interaction. Instead, the enhanced analgesic effects of cannabinoids and opioids may result from the activation of CB1R and MOR in distinct neuronal populations or circuits.