Cannabinoids, the Endocannabinoid System, and Cognitive Functions: Enemies or Friends?

Cannabinoids are natural compounds found in the hemp (Cannabis sativa). Scientific interest in cannabinoids arose after the discovery of the major psychoactive component in hemp, Δ9-tetrahydrocannabinol. Subsequent studies detected receptors in the brain subject to the actions of this compound, along with ligands for these receptors, i.e., endogenous cannabinoids (EC), which make up, along with the enzymes synthesizing, transporting, and degrading them, the endocannabinoid system (ECS). Interest in EC has consistently increased in recent years, especially after their important role in cognitive functions was discovered. They are regulators of synaptic transmission in the brain, mediate numerous forms of plasticity, and control neuron energy metabolism. EC exert influences using a series of mechanisms and interactions with neuromediators, neurotrophic factors, and neuropeptides. The main functions of EC in the brain are retrograde synaptic signaling and neuromodulation, which maintain cellular homeostasis. Information on the influences of cannabinoid drugs on cognitive functions is very contradictory. The cause of this may be that there are still inadequate strictly scientific data from clinical and sociological studies, while in animal experiments different authors use different methods and approaches for actions on the ECS. Thus, effects can differ depending on the substances used, their doses, and routes of administration, and the tasks and experimental conditions selected for testing. There is an extensive literature on the protective effect of ECS activation in neurodegenerative diseases in humans and models of cognitive deficit in animals. This review addresses data providing evidence of the influences of cannabinoid drugs and activation of the EC system on cognitive functions in the normal brain and in neurodegenerative diseases, Alzheimer’s disease, and temporal epilepsy. The possible causes of contradictions in existing data are also discussed.