Mu receptors give analgesia along with the main side effects
Expansion
Mu receptor effects, both wanted and unwanted, since they share a receptor:
- Analgesia (supraspinal and spinal)
- Respiratory depression, by reducing the medullary response to carbon dioxide
- Euphoria and sedation
- Miosis, from Edinger-Westphal stimulation
- Constipation, nausea and vomiting
- Urinary retention, pruritus, bradycardia
- Physical dependence and tolerance
Kappa: analgesia with dysphoria and less respiratory depression. Delta: analgesia, modulation of mood.
Tolerance develops to analgesia, euphoria, sedation, nausea and respiratory depression, but not to constipation or miosis. This is why a long-term user needs escalating doses yet remains constipated, and why pinpoint pupils remain a reliable sign.
Naloxone is a competitive antagonist with a short half-life of about 30 to 60 minutes, much shorter than most opioids, so repeated doses or an infusion are needed after reversing a long-acting opioid.
Respiratory depression is best assessed by respiratory rate and sedation score rather than oxygen saturation, which is maintained by supplemental oxygen while carbon dioxide rises.