Variations in the effect of molecular mechanisms underlying addiction-associated neuroadaptations in several regions of the brain (Adinoff, 2004; Benowitz, 2008; Lupica et al., 2004; Moussas et al., 2009; Nestler, 2005), may also contribute to explain differences in the probability of transition across substances (Renthal and Nestler, 2008). Chronic drug exposure results in the accumulation of ΔFosB, which increases the sensitivity to the reinforcing effects of drugs (Perrotti et al., 2008; Renthal and Nestler, 2008; Wallace et al., 2008). Chronic exposure to Δ9-tetrahydrocannabinol, the active ingredient in cannabis, produces a less dramatic effects on ΔFosB induction in the nucleus accumbens shell and dorsal striatum than the effect observed after chronic exposure to cocaine and alcohol (Perrotti et al., 2008).
