Ageing, Neurodegeneration and the Endocannabinoid System.

Numerous studies carried out in the last 30-40 years have strongly demonstrated that the endocannabinoid system exerts important modulatory functions in the central nervous system (CNS). These neuromodulatory functions encompass the whole life of animals, with specific activities during neurodevelopment (prenatal, postnatal and adolescent periods), adulthood and possibly senescence too. However, this is the life stage less investigated in relation with the endocannabinoid system to date. In the aged brain, the activity of this system appears to be altered, which contributes to subtle impairments that typically occur during ageing in learning and memory, motor behaviour, social behaviour and other neurobiological functions. Some of the changes in endocannabinoid activity may represent a process to attenuate ageing-related impairment in the brain function, which is consistent with its role as a pro-homeostatic system. An important observation is that these alterations become extreme when normal brain ageing acquires pathological characteristics, as happens in chronic neurodegenerative disorders. This includes the cannabinoid type-1 (CB₁) receptor downregulation or impairment in its signalling and the increase in endocannabinoid-inactivating enzymes, both hypothesised to contribute to pathogenic events. By contrast, elevated levels of endocannabinoids due to a reduced Fatty acid amide hydrolase (FAAH) and monoacyl glycerol lipase (MAGL) expression and the upregulation of cannabinoid type-2 (CB₂) receptors may in turn serve as endogenous pro-homeostatic adaptations against brain impairment. This review synthesises information on: (i) subtle alterations in the endocannabinoid system in the senescent brain in the absence of pathology, with the purpose of demonstrating that these alterations are representative of the extreme changes experienced by this system in the brain pathological ageing; and (ii) the development of neuroprotective therapies based on the pharmacological management of specific endocannabinoid targets to combat neurodegenerative pathologies. Together, research in this area comes at a critical time as global lifespan is increasing, incidence of age-related neurodegenerative disorders is expanding, and the unmet need for efficacious neuroprotective treatments is a public health necessity.