Oh yeah, on a neuroanatomical level, let me assure you, that alcohol is still baffling the neuroscientists, but the opiates and marijuana are definitely linked to producing a dopaminergic induced depression of neuronal firing in the nucleus accumbens via the Ventral Tegmental Area (VTA).  Those seem to be the sites where ALL the action is.   The VTA lies in a neuroanatomical structure known as the midbrain (just below the brain hemispheres, in the brainstem).  The area inputs to the brain cortex in various lobes of the brain itself, especially Frontal and Limbic (mmm...the thrill of the limbic lobes: memory, emotion, sex, drugs, and rock n' roll). 

Gene