Binding of fentanyl and pethidine to muscarinic receptors in rat brain.

Symptoms similar to the central anticholinergic syndrome are often seen after high dose fentanyl anesthesia. Therefore, the binding of fentanyl, alfentanil, pethidine and morphine to opioid and muscarinic receptors was investigated in rat brain homogenate with [3H]naloxone and [3H]QNB as the radioligands, respectively. Both pethidine and fentanyl inhibited [3H]QNB binding with Ki values in the micromolar range. Alfentanil and morphine had no affinity for muscarinic receptors. The ratio of the Ki values for muscarinic receptors versus opioid receptors was 2.5 for pethidine and 88 for fentanyl. It is concluded that binding of fentanyl to muscarinic receptors is likely to occur during high-dose fentanyl anesthesia.