Morphine Inhibits Calcium Influx and the Response to Acetylcholine in Xenopus Oocytes

: Incubation of intact Xenopus oocytes with the opioid radioligand [3H]diprenorphine (0.5 nM) resulted in specific binding of 1.7 ± 0.3 fmol per oocyte. Morphine (10 μM) inhibited the uptake of 45Ca2+ into the oocyte by 66 ± 9%. The opioid antagonist naltrexone partially blocked this effect of morphine. Preincubation of oocytes with morphine (10 μM, 2 min) partially inhibited the fast and slow responses of the oocyte to acetylcholine by 26 and 52%, respectively. We conclude that native Xenopus oocytes possess opioid receptors that may modulate the muscarinic response by limiting calcium influx into the cell.