Abstract
The endocannabinoid system (ECS) tightly controls emotional responses to acute aversive stimuli. Repeated stress alters ECS activity but the role played by the ECS in the emotional consequences of repeated stress has not been investigated in detail. This study used social defeat stress, together with pharmacology and genetics to examine the role of cannabinoid type-1 (CB(1)) receptors on repeated stress-induced emotional alterations. Seven daily social defeat sessions increased water (but not food) intake, sucrose preference, anxiety, cued fear expression, and adrenal weight in C57BL/6N mice. The first and the last social stress sessions triggered immediate brain region-dependent changes in the concentrations of the principal endocannabinoids anandamide and 2-arachidonoylglycerol. Pretreatment before each of the seven stress sessions with the CB(1) receptor antagonist rimonabant prolonged freezing responses of stressed mice during cued fear recall tests. Repeated social stress abolished the increased fear expression displayed by constitutive CB(1) receptor-deficient mice. The use of mutant mice lacking CB(1) receptors from cortical glutamatergic neurons or from GABAergic neurons indicated that it is the absence of the former CB(1) receptor population that is responsible for the fear responses in socially stressed CB(1) mutant mice. In addition, stress-induced hypolocomotor reactivity was amplified by the absence of CB(1) receptors from GABAergic neurons. Mutant mice lacking CB(1) receptors from serotonergic neurons displayed a higher anxiety but decreased cued fear expression than their wild-type controls. These mutant mice failed to show social stress-elicited increased sucrose preference. This study shows that (i) release of endocannabinoids during stress exposure impedes stress-elicited amplification of cued fear behavior, (ii) social stress opposes the increased fear expression and delayed between-session extinction because of the absence of CB(1) receptors from cortical glutamatergic neurons, and (iii) CB(1) receptors on central serotonergic neurons are involved in the sweet consumption response to repeated stress.