Effect of morphine on the stimuli-induced calcium uptake into synaptosomes isolated from morphine-tolerant rats.

Abstract

Effects of morphine on Ca uptake into synaptosomes isolated from acutely or chronically morphine-tolerant rat brain was studied. Addition of morphine inhibited the depolarization-stimulated uptake of Ca without affecting uptake under nondepolarizing conditions. This inhibition was prevented by simultaneous addition of naloxone with morphine before Ca uptake was initiated. Acute tolerance to morphine increased depolarization-stimulated synaptosomal Ca uptake. Chronic exposure of rats to morphine to elicit tolerance to and physical dependence on morphine did not influence synaptosomal Ca uptake. These preparations apparently lost the ability of in vitro morphine-inhibition of Ca uptake into the synaptosomes. Adaptive changes of synaptosomal Ca uptake produced by exposure to morphine may be involved in tolerance and physical dependence development, but influence of morphine on Ca uptake by the synaptosomes isolated from the rats acutely tolerant to morphine was different from that of chronic tolerant rats.