The Endocannabinoid System in Alzheimer's Disease: A Network Meta-Analysis

The findings concerning the association between endocannabinoid system (ECS) and Alzheimer's disease (AD) exhibited inconsistencies when examining the expression levels of endocannabinoids. This study aimed to provide a comprehensive summary of the studies regarding alterations of the ECS in AD. Six databases were thoroughly searched for literature to select relevant studies investigating the ECS in AD, including changes in cannabinoid receptors (CB1R and CB2R), endocannabinoids (2-AG and AEA), and their associated enzymes (FAAH and MAGL). Traditional meta-analysis evaluated the expression levels of the ECS in AD, and the results showed no significant differences in ECS components between healthy controls and AD patients. However, subgroup analysis revealed significantly lower expression levels of CB1R in AD than in controls, particularly in studies using western blot (SMD = -0.88, p < 0.01) and in studies testing CB1R of frontal cortex (SMD = -1.09, p < 0.01). For studies using HPLC, the subgroup analysis indicated significantly higher 2-AG levels in AD than in controls (SMD = 0.46, p = 0.02). Network meta-analysis examined the rank of ECS alterations in AD compared to controls, and the findings revealed that 2-AG and MAGL exhibited the largest increase and CB1R showed the largest decrease relative to the control group. Based on the findings of traditional meta-analysis and network meta-analysis, we proposed that AD patients may present decreased expression levels of CB1R and increased expression levels of 2-AG and its degrading enzyme MAGL. Our results may contribute to the growing body of research supporting the therapeutic potential of ECS modulation in the management of AD.