INTERAZIONE TRA SISTEMA ENDOCANNABINOIDE E COCAINA: STUDI PRECLINICI COMPORTAMENTALI E MOLECOLARI IN RATTI ADOLESCENTI E ADULTI

Cannabis is the most commonly used illicit drug, especially among adolescents and in many of them it precedes the use of other drugs such as cocaine (i.e., the so-called ‘gateway hypothesis’) (Kandel, 1975). Since adolescence represent a critical phase of brain development, early exposure to cannabis might dramatically alter brain maturation thus contributing to negative outcomes, such as problematic patterns of polydrug abuse (Iede et al., 2016). Recently, it was demonstrated that the molecular action of a number of drugs of abuse (including alcohol, cocaine, methamphetamine and nicotine) converge on the reduction of p-eIF2a which, in turn, enhances drug sensitivity in adolescence (Huang et al., 2016). Preliminary in vitro data obtained from a collaboration with our laboratory and the Department of Neuroscience, Columbia University, New York, showed that WIN55,212-2 (WIN) causes GADD34-dependent dephosphorylation of eIF2a in neuronal-like model. Based on this finding, the aim of my thesis was to investigate whether in vivo exposure to WIN in adolescent and adult rats might induce the same molecular changes in different corticolimbic areas such as the amygdala (AMYG), the dorsal striatum (DSTR), the hippocampus (HPC), the nucleus accumbens (NAcc) and the prefrontal cortex (PFC). In addition, brain levels of a well-established addiction-related molecule, such ΔFosB and CREB, were also measured. Since specific histone modifications have been found after exposure to drugs of abuse including cannabinoids (Prini et al., 2017), the impact of WIN exposure on histone modifications in the same brain areas was also investigated. To this end, male adolescent and adult rats were treated with increasing doses of WIN (or its vehicle) twice/day for 11 consecutive days. On the last day of treatment, rats from each group were randomly divided into two separate cohorts of animals to carry out molecular analysis. For this purpose brain dissections were performed at two different time points: 24h after last WIN injection (Day 1) and 9 days after the last WIN injection (Day 9). On Day 1, we found that in adolescents WIN led to an increase of ΔFosB in the DSTR and the NAcc, and a reduction in the AMYG. In adolescents, WIN led to a significant decrease in p-eIF2a levels in the NAcc and a trend to decrease in the HPC. In the NAcc of adolescent rats, there was also a significant upregulation of p-ERK1/2, a trend toward upregulation of p-CREB, a significant upregulation in the nuclear/cytoplasmic localization of CBP, and a trend to decreased levels of GADD34. In adult rats, WIN led to an increase in levels of ΔFosB in the DSTR, the NAcc and the PFC. In adult rats, WIN led to no changes in p-eIF2a levels. Moreover we found that WIN was able to affect histone modifications in the brain in a region- and age-specific manner. On the other hand, on Day 9, focusing on the NAcc, p-eIF2a levels did not differ between vehicle and WIN treated rats in both adolescents and adults. On Day 9, WIN affects histone modifications in the brain in a region- and age-specific manner, like in Day 1. Finally, to examine WIN cross-sensitizing behavioral, rats were injected with cocaine (10mg/kg) and activity tests were performed 24 hours and 8 days after the last WIN injection. In response to cocaine, adolescent WIN-pretreated rats showed an enhanced sensitivity compared to vehicles. In contrast, when adult WIN-pretreated rats were compared to vehicles, no enhanced behavioural response to cocaine was found. Taken together, these results reveal that the chronic exposure to cannabinoids in adolescence, but not in adulthood, can lead to epigenetic and neuromolecular modifications, accompanied by behavioural sensitization to the effects of cocaine. In conclusion these picture may reflect an alteration of susceptibility to substance abuse emphasizing the importance of adolescence in drug exposure