BACKGROUND: Marijuana use by teenagers often predates the use of harder drugs, but the
neurobiological underpinnings of such vulnerability are unknown. Animal studies suggest
enhanced heroin self-administration (SA) and dysregulation of the endogenous opioid
system in the nucleus accumbens shell (NAcsh) of adults following adolescent Δ9-
tetrahydrocannabinol (THC) exposure. However, a causal link between proenkephalin
(Penk) expression and vulnerability to heroin has yet to be established. METHODS: To …