According to the United Nations Organization, the derivatives of the plant  Cannabis sativa, cannabis or marijuana, are the most consumed illegal drugs in the world. Thanks to diverse scientific studies, certain dysfunctions caused to the human brain arising from chronic exposure to cannabis have started to become known, although the neurobiological mechanisms involved are not known to date.

It is known that the chronic consumption of cannabis causes a variety of changes to the brain. One recently described alteration is the onset of deficiencies in the coordination of movements that may change such different behaviour as the ability to drive vehicles, for example. The magnitude and possible mechanisms involved in these motor deficiencies induced by cannabis had not been clarified until now.

A team from the  Neuropharmacology Laboratory at Pompeu Fabra University has discovered the brain mechanism responsible for altering the motor coordination associated with chronic exposure to the main psychoactive component of cannabis, delta9-tetrahidrocannabinol (THC).

In this experimental work, published on 24 June in the digital edition of the prestigious journal  The Journal of Clinical Investigation, UPF researchers  Laura Cutando, Arnau Busquets, Emma Puighermanal, Maria Gomis, Rafael Maldonado and  Andrés Ozaita, and researchers of the Pablo de Olavide University in Seville José María Delgado and Agnès Gruart, have shown that the mice exposed to THC develop a deficient functioning of the cerebellum, a critical area of the brain for regulating the coordination of chain and successive movements in motor learning.

This area of the brain showed symptoms of neuroinflammation after exposure to THC, characterized by the appearance of reactive microglia cells. These cells are considered the brain's immune system and react to changes in cerebral homeostasis.

The researchers observed that the microglia, following exposure to THC, showed a morphology typical of localized brain damage in the cerebellum and released inflammatory factors such as different types of cytokines, and that this type of microglia was the cause of problems of motor coordination observed after administering THC. When treatments were used to reduce the reactivity of the microglia, a considerable improvement was achieved in motor coordination and learning, mediated by the cerebellum.

This study also shows that chronic exposure to cannabinoids decreases the number and function of the receptors for cannabinoids in the cerebellum, a reduction which was directly involved in the harmful effects of cannabis. In this regard, the use of genetically modified mice that express less receptors for cannabinoids also showed neuroinflammation in the cerebellum and deficient coordination of movements. These alterations were also avoided by treatment with drugs to prevent neuroinflammation.

The results obtained in laboratory mice have enabled clarifying the mechanisms by which chronic exposure to cannabis would affect the fine coordination of movements and highlight the role of cerebral microglia in functions that had, to date, been attached to the exclusive activity of neuronal cells.

Reference work: Laura Cutando, Arnau Busquets-Garcia, Emma Puighermanal, Maria Gomis-González, José María Delgado-García, Agnès Gruart, Rafael Maldonado and Andrés Ozaita. " Microglial activations underlies cerebellar deficits produced by repeated cannabis exposure".  The Journal of Clinical Investigation. 24/6/2013. doi:10.1172/JCI67569.