1. Introduction
Clinically, opioids are still the first choice for the treatment of moderate to severe pain, the prescription of opioids for chronic pain has increased in recent years. Despite their high efficacy in the context of acute pain, opioids are associated with several troubling side effects, including respiratory depression, reduced gastric motility, and dependence/addiction. As a result, one of the main goals of pain research is to identify compounds with similar efficacy but fewer side effects.