Morphine presynaptically inhibits a ganglionic cholinergic synapse.

The effects of morphine on synaptic transmission through the inferior mesenteric ganglion (IMG) were studied using intracellular recording techniques. Morphine produced a reversible, dose-dependent reduction in the quantal content of the synaptic potentials with little or no effect on quantal size or the properties of the ganglion cells. This effect of morphine was antagonized by naloxone suggesting the presence of specific preganglionic opiate receptors.