OPRK1 and respiratory depression: In comparison, kappa opioid receptor (KOR) agonists do not have rewarding effects (Vonvoigtlander et al., 1983), and are not associated with respiratory depression (Freye et al., 1983) or gastrointestinal transit (Porreca et al., 1984), and have potential to treat pain (Beck et al., 2019; Paton et al., 2020a).