Opiates depress ACh and ATP release from cholinergic synaptosomes by blocking calcium uptake

We have studied the effects of opiates on ATP and acetylcholine (ACh) release from cholinergic nerve terminals isolated from the electric organ of Torpedo marmorata. The release of ATP was inhibited by morphine and this action was reversed by naloxone. Morphine, [D-Met2-Pro5]enkephalinamide and [D-Ala2-Leu5]enkephalin also inhibited acetylcholine release. Naloxone prevented these inhibitory effects. The action of enkephalin on ACh release was less effective than that of morphine. The calcium uptake by nerve terminals of Torpedo electric organ was also inhibited by morphine, either under resting or depolarizing conditions, and this effect was reversed by naloxone. Using the quick freeze-fracture method, the structural changes induced by morphine in the presynaptic membrane were also studied. Morphine prevents the rearrangement of intramembrane particles (IMPs) at both freeze-fractured faces of the synaptosomal presynaptic membrane after depolarization. It is concluded that opiates depress the ATP and the ACh releases from cholinergic synaptosomes by inhibiting the calcium uptake by the nerve terminals and the rearrangement of the IMPs after potassium-induced depolarization. Furthermore, ACh release, but not ATP release, seems to be related with the rearrangement of IMPs in the presynaptic membrane.