The role of the endocannabinoid system in plasticity and myelination in adolescent female rats

Although scientific evidence suggests that exposure to Cannabis during adolescence increases the risk of mental diseases later in life, this illicit drug remains the most commonly used among adolescents. Adolescence is a sensitive period, characterized by morphological changes such as a loss of grey and an increase in white matter volume. We have previously demonstrated that THC adolescent exposure leads to significant alterations in dynamic changes occurring in the glutamatergic system during the adolescent development of the prefrontal cortex. In order to understand if the harmful effect produced by adolescent exposure to THC could be linked to its interference with the physiological role of the endocannabinoid system, the first aim of our work was to investigate the role that the eCB system plays on the processes of neuronal remodelling taking place in the glutamatergic system during adolescence. To reach our first aim, the adolescent developmental window was divided into five sub-periods of five days each. Sprague-Dawley female rats received daily injections with the selective CB1 receptor antagonist AM251 or its vehicle. At the end of the treatment we studied, through western blot analysis, the effect of this modulation on some markers of plasticity. The results showed that the blockade of the endocannabinoid system negatively impacts on the dynamic changes typically occurring in the adolescent brain. Our second aim was to identify the most sensitive period of the adolescent THC exposure. To this purpose, following the same treatment schedule used before, animals of each period were injected for 5 days with THC or its vehicle. As before, markers of plasticity were monitored at the end of the treatment. Our results showed that the most delicate periods to this psychoactive component exposure were the early and mid-adolescence. Another important process occurring during the adolescent brain development is the white matter increase. Since the involvement of the eCB system in adolescent myelination remains completely unknown, our next aim was to investigate the effect of CB1 receptor blockade on myelination process in adolescent female rats. To reach this objective, female rats were injected with AM251 according to the same treatment protocol previously mentioned. Then, we monitored through western blot analysis MBP and MOG levels, two white matter markers. Since the modulation with the antagonist of CB1 receptors prevented the increase of both these markers we could assert that the eCB tone plays a fundamental role also in the adolescent myelination enhancement. Recent studies have shown that individual variability in myelin growth trajectories, investigated during the transition from adolescence to adulthood, could be linked to the expression of impulsivity traits. On these grounds, we wanted to investigate through the wire-Beam Bridge Test, whether any correlation exists between impulsivity and white matter development in our animal model. To reach this aim we injected adolescent female rats from PND 28 to PND 45 with AM251 and then we tested their impulsivity. As a whole, our results first, confirm that there is an association between the increase of myelin and the reduction of impulsivity in adolescent naïve rats and thereby that the reduction of impulsivity is age-related. Secondly, they suggest that the eCB system blockade during this specific developmental window, preventing the myelin enhancement, leads concurrently to a higher expression of impulsivity. Finally, we tried to identify the cellular mechanism which could be involved in the myelination enhancement induced by the activation of CB1 receptors during this specific developmental window. Our results suggest that CB1 receptor stimulation could induce the activation of the AKT pathway that in turn could inhibit the Hippo signalling pathway, thereby promoting the translocation of YAP into the nucleus, where it could promote myelin gene transcription.