Relationships between serotonergic and cannabinoid system in depressive-like behavior: a PET study with [11C]-DASB

Chronic stress represents a major environmental risk factor for mood disorders in vulnerable individuals. The neurobiological mechanisms underlying these disorders involve serotonergic and endocannabinoid systems. In this study, we have investigated the relationships between these two neurochemical systems in emotional control using genetic and imaging tools. CB1 cannabinoid receptor knockout mice (KO) and wild-type littermates (WT) were exposed to chronic restraint stress. Depressive-like symptoms (anhedonia and helplessness) were produced by chronic stress exposure in WT mice. CB1 KO mice already showed these depressive-like manifestations in non-stress conditions and the same phenotype was observed after chronic restraint stress. Chronic stress similarly impaired long-term memory in both genotypes. In addition, brain levels of serotonin transporter (5-HTT) were assessed using positron emission tomography. Decreased brain 5-HTT levels were revealed in CB1 KO mice under basal conditions, as well as in WT mice after chronic stress. Our results show that chronic restraint stress induced depressive-like behavioral alterations and brain changes in 5-HTT levels similarly to those revealed in CB1 KO mice in non-stressed conditions. These results underline the relevance of chronic environmental stress on serotonergic and endocannabinoid transmission for the development of depressive symptoms. Chronic restraint stress induces depressive-like behavior and reduced 5-HTT levels in WT mice similar to those revealed in non-stressed CB1-KO mice. Reduced 5-HTT in both genotypes increases synaptic 5-HT concentration. The 5-HT release is modulated through CB1 receptors and the absence of inhibitory CB1 receptor causes decreased inhibition of 5-HT release resulting in high synaptic 5-HT concentration that are not further enhanced by stress.