Cannabinoid CB₂ receptors are expressed in glutamate neurons in the red nucleus and functionally modulate motor...

Cannabinoids produce a number of central nervous system effects via the CB₂ receptor (CB₂R), including analgesia, antianxiety, anti-reward, hypoactivity and attenuation of opioid-induced respiratory depression. However, the cellular distributions of the CB₂Rs in the brain remain unclear. We have reported that CB₂Rs are expressed in midbrain dopamine (DA) neurons and functionally regulate DA-mediated behavior(s). Unexpectedly, high densities of CB₂-like signaling were also found in a neighboring motor structure - the red nucleus (RN) of the midbrain. In the present study, we systematically explored CB₂R expression and function in the RN. Immunohistochemistry and in situ hybridization assays showed high densities of CB₂R-immunostaining and mRNA signal in RN magnocellular glutamate neurons in wildtype and CB₁-knockout, but not CB₂-knockout, mice. Ex vivo electrophysiological recordings in midbrain slices demonstrated that CB₂R activation by JWH133 dose-dependently inhibited firing rates of RN magnocellular neurons in wildtype, but not CB₂-knockout, mice, while having no effect on RN GABA neurons in transgenic GAD67-GFP reporter mice, suggesting CB₂-mediated effects on glutamatergic neurons. In addition, microinjection of JWH133 into the RN produced robust ipsilateral rotations in wildtype, but not CB₂-knockout mice, which was blocked by pretreatment with either a CB₂ or DA D1 or D2 receptor antagonist, suggesting a DA-dependent effect. Finally, fluorescent tract tracing revealed glutamatergic projections from the RN to multiple brain areas including the ventral tegmental area, thalamus, and cerebellum. These findings suggest that CB₂Rs in RN glutamate neurons functionally modulate motor activity, and therefore, constitute a new target in cannabis-based medication development for motor disorders.