I have two different papers. One claims that gut motility is reduced by stimulation of the Opioid κ and δ receptors. The receptors are activated by Morphine and certain derivatives, specifically Codeine, Fentanyl, Hydromophone, Methadone, and Oxycodone.
The second paper claims that gut motility is reduced by blocking the 5-HTb3 receptors. Antagonists or competitors to the 5-HTb3 receptors being Morphine and certain derivatives.
Are these two different theories, two different action mechanisms, or am I missing something?