Papers of the Week

Δ-tetrahydrocannabinol (THC), the major psychoactive ingredient of Cannabis sativa, exerts its actions through the endocannabinoid system by stimulation of the cannabinoid type 1 (CB1) receptor. The widespread distribution of this receptor in different neuronal cell types and the plethora of functions that is modulated by the endocannabinoid system explain the versatility of the effects of THC. However, the cell types involved in the different THC effects are still not fully known. Conditional CB1 receptor knock-out mice were previously used to identify CB1 receptor subpopulations that are "necessary" for the tetrad effects of a high dose of THC: hypothermia, hypolocomotion, catalepsy and analgesia. Here, we used mouse models for conditional CB1 receptor "rescue" in dorsal telencephalic glutamatergic and forebrain GABAergic neurons to determine which CB1 receptor subpopulations are "sufficient" for these tetrad effects. Glutamatergic CB1 receptor was not only necessary but also sufficient for THC-induced hypothermia and hypolocomotion. Analgesic and cataleptic effects of THC are largely independent of glutamatergic and GABAergic CB1 receptors, since no sufficiency was found, in agreement with the previously reported lack of necessity. We also revealed a novel aspect of GABAergic CB1 receptor signaling. In animals with CB1 receptors exclusively in forebrain GABAergic neurons, THC stimulated rather than reduced locomotion. This cell-type selective and hitherto unsuspected hyperlocomotive effect may be occluded in wild-types and conditional knockouts and only be exposed when CB1 signaling is absent in all other cell types, thus underlining the importance of investigating both necessary and sufficient functions to unequivocally unravel cell-type specific actions.