What percentage of the U.S. population suffers from drug addiction?
Addiction
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The projections from the ventral tegmental area (VTA) are a network of dopaminergic neurons with co-localized postsynaptic glutamate receptors (AMPAR and NMDAR). These cells respond when stimuli indicative of a reward are present. The VTA supports learning and sensitization development and releases DA into the forebrain.[70] These neurons also project and release DA into the nucleus accumbens,[71] through the mesolimbic pathway. Virtually all drugs causing drug addiction increase the dopamine release in the mesolimbic pathway,[72] in addition to their specific effects.
The nucleus accumbens (NAcc) is one output of the VTA projections.  The nucleus accumbens itself consists mainly of GABAergic medium spiny neurons (MSNs).[73] The NAcc is associated with acquiring and eliciting conditioned behaviors, and is involved in the increased sensitivity to drugs as addiction progresses.[70] Overexpression of ΔFosB in the nucleus accumbens is a necessary common factor in essentially all known forms of addiction;[1] ΔFosB is a strong positive modulator of positively reinforced behaviors.[1]
The prefrontal cortex, including the anterior cingulate and orbitofrontal cortices,[74] is another VTA output in the mesocorticolimbic pathway; it is important for the integration of information which helps determine whether a behavior will be elicited.[75] It is also critical for forming associations between the rewarding experience of drug use and cues in the environment. Importantly, these cues are strong mediators of drug-seeking behavior and can trigger relapse even after months or years of abstinence.[76]