The dopamine hypothesis of opiate reward challenged

The dopamine hypothesis of opiate reward was directly tested by blocking dopamine receptor systems with haloperidol. Haloperidol was administered either systemically or locally into different brain areas with dopaminergic terminals in rats allowed to initiate and maintain intravenous heroin self-administration. Haloperidol treatment did not block heroin reward. It is concluded that dopamine is not critically involved in opiate reward and that multiple endogenous reward systems are present in the brain.