Inhibition of synaptic transmission by morphine and its antagonism by Ca++ in the isolated sympathetic ganglion of frog and rat.

Synaptic transmission is inhibited by morphine in isolated sympathetic ganglia of frog and rat. This action of morphine can competitively be antagonized by naloxone, nalorphine as well as by increasing the Ca++ concentration of the medium. The morphine antagonistic action of Ca++ can be suspended by Mg++ applied in an equimolar concentration. These results show that morphine inhibits synaptic transmission by shifting the ionic balance necessary for transmitter release.