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There is increasing evidence that the kappa-opiate receptor, in addition to the mu-opiate receptor, plays an important role in substance use pathophysiology and behavior. As dopamine activity is upregulated through chronic substance use, kappa receptor activity, mediated through the peptide dynorphin, is upregulated in parallel. Dynorphin causes dysphoria and decreased locomotion, and the upregulation of its activity on the kappa receptor likely dampens the excitation caused by increased dopaminergic activity. This feedback mechanism may have significant clinical implications for treating drug dependent patients in various stages of their pathology.