Dr. David Lyness
A LOOK AT AGONISTIC DRUGS THAT WORK ON OPIOID RECEPTORS
MOP (mu opioid peptide receptor)
KOP (kappa opioid peptide receptor)
DOP (delta opioid peptide receptor)
NOP (nociceptin orphanin FQ peptide receptor)
CNS – Analgesia, Sedation, Euphoria/Dysphoria, Hallucination (KOP)
Tolerance and Dependence.
CVS – Mild bradycardia due to decreased sympathetic drive & direct effect on the SA node,
Peripheral vasodilation caused by histamine release and reduced sympathetic drive
can cause slight fall in BP. Diamorph/morphine reduces preload to the heart.
RS – MOP receptor will cause respiratory depression via resp centres in brainstem.
RR falls more than tidal volume and brainstem sensitivity to CO2 is reduced.
Hypoxic response is less affected. Cough Suppression.
Halogenated anaesthetics & benzos can cause marked resp depression with opioids.
GI – Stimulation of the CTZ causes N&V. Smooth muscle tone is increased but motility is decreased causing
delayed absorption. Constipation is common. Spasm of the sphincter of Oddi = increased biliary system pressure
Endocrine – INHIBITS the release of ACTH, prolactin and gonadotrophic hormone. INCREASES ADH secretion.
Ocular – MOP & KOP receptors in Erdinger-Westphal nucleus of oculomotor stimulation constricts the pupils.
Histamine Release & Itching – some cause histamine release from mast cells. Most common after IT opioids.
Muscle Rigidity – Large doses can cause this. Especially of thoracic wall, this interfering with ventilation.
Immunity – Depression of immunity can be seen with long term opioid abuse.
Pregnancy/Neonates – All cross the placenta. Can cause neonatal resp depression.
Chronic use = dependence in utero = withdrawl at birth. Not teratogenic.
All the main opioids above have large MOP receptor affinity.
Note Naloxone's effect at most receptor sites except NOP.
Remifentanil is mostly a MOP agonist.