OPRK1 and treatment resistant depression: After an extensive assessment of the opioid interactions of many psychotropic medications with the opioid system in a mouse model of antinociception and the characterization of every one of them regarding agonistic and antagonistic interaction with the different opioid receptor subtypes, we hypothesize the possible use of those medications possessing antagonistic interactions with the kappa opioid receptors for the treatment of treatment-resistant depression (TRD), either as monotherapy or, better yet, as an add-on (augmentation) of the originally prescribed antidepressant.