Opiate receptors in GI
The opioid μ-receptor is present on the myenteric neurons. When agonists are present, the activated μ-receptor will inhibit acetylcholine release from the neuron, resulting in decreased GI motility. This effect occurs at dosages lower than that required for analgesia.
The main mechanism of opiates in treating diarrhoea is:
Extra info: Opiates do decrease fluid loss, but this is a minor result of the prolonged transit time in the GI due to opioid effects on the smooth muscle. They do not inhibit reproduction of bacteria, and they do not directly antagonize receptors. Rather, they inhibit the release of acetylcholine, the μ-receptor's agonist.