OPRK1 and respiratory depression: For example, Uprety et al. (2021) and Atigari et al. (2021) provide evidence that the morphinan analogue MP1104, activates KOR preferentially over DOR, with little effect at MOR in vivo. MP1104 produces anti-nociceptive effects in mice and rats without causing respiratory depression, conditioned place preference, or conditioned place aversion, sedation, or cross-tolerance with morphine.