The attenuated (or abolished) DA increases in response to stimulant drugs in cocaine addiction is paradoxical because cocaine-induced DA increases are linked to its rewarding effects, a finding that had led to the hypothesis of a sensitized response to the DA-enhancing effects of drugs (enhanced reward) that drove the exaggerated motivation to procure the drug in addiction. Similarly, the deactivation of the NAc after intravenous cocaine use suggests that these responses are adaptations that follow repeated drug administration (some related to tolerance but others to conditioning) rather than being a result of only the pharmacological effects of the drug.
