Finally, although there has been a focus on the development of KOR antagonists, one biotechnology company is currently developing salvinorin A, a KOR agonist (with other effects on dopaminergic, glutamatergic and cannabinoid systems as highlighted earlier (Taylor and Manzella, 2016)), with plans to examine its safety and efficacy in TRD (Revixia, 2021). Here, OPRK1 is linked to treatment resistant depression.