OPRK1 and Respiratory insufficiency due to muscle weakness: Overall, these results demonstrate that MP1207 and MP1208 produce potent antinociception predominantly mediated by KOR and MOR, yet shows a separation of analgesia from some classic opioid side effects such as respiratory depression, conditioned place preference, and aversion, in contrast with the canonical mu and kappa-opioid receptor selective agonists, morphine and U50,488h.