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Molecular bases involved in cannabinoid dependence

Final Activity Report Summary - CANSIGWITH (Molecular bases involved in cannabinoid dependence)

Cannabinoids have a long history of consumption both for medical and recreational uses. They are alkaloid derivatives from the plant Cannabis sativa. Nowadays, cannabinoids are the most widely used illicit drugs, what makes them an important target of research. The main psycho-active component has been identified as Delta9-tetrahydrocannabinol (THC), a lipophilic alkaloid that can activate the endogenous cannabinoid system. The administration of THC in rodents, as well as in humans, produces multiple pharmacological responses including locomotor and memory impairment, emotional-like responses, antinociception, hypothermia, rewarding effects and dependence. These effects are mediated by the activation of receptors highly expressed in the central nervous system (CNS), namely CB1 and CB2. Their activation by agonists leads, apart from the inhibition of adenylyl cyclase activity, and modulation of N-type calcium channels and inward rectifier potassium channels, to the activation of mitogen-activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3K) signalling cascades. These features couple the activation of cannabinoid receptors to transcription modulation, and may control the long-term effects of cannabinoids. Most of the signalling characteristics of the cannabinoid receptors have been described in in vitro systems, and therefore there is a lack of information on the intracellular signalling events taking place in vivo in the brain.

This research project addressed the intracellular signalling processes modulated by cannabinoids in vivo, after acute and chronic exposure to THC, and after the pharmacological induction of the withdrawal syndrome in THC tolerant animals. Following the acute protocol of administration we have observed a dose dependent activation of the PI3K/ protein kinase B (PKB/Akt)/ glycogen synthase kinase (GSK-3) pathway in the brain. This effect was mediated by CB1 canabinoid receptors, as it was blocked by the specific antagonist rimonabant.

After a protocol of chronic administration of THC, there was a decrease in the signalling associated to the MAPK cascade in specific regions of the brain, like cerebellum and hippocampus. This decreased activity of MAPK cascade observed in THC chronically exposed animals suffered a rebound during the cannabinoid withdrawal syndrome. Interestingly, the PI3K/Akt/GSK-3 pathway did not show any modification during the withdrawal syndrome in the brain areas specified (cerebellum or hippocampus), suggesting a specific role of the MAPK signalling cascade during cannabinoid physical dependence. Moreover, blocking the signalling through the pre-administration of a specific inhibitor of the MAPK pathway prior to triggering the cannabinoid withdrawal syndrome, there was a significant decrease in the strength of the withdrawal syndrome.

These data suggest a new undisclosed role of MAPK signalling in the physical dependence associated to cannabinoids.