Effects of morphine and opioid peptides on sensitivity to acetylcholine of dialysed snail neurons

The effects of intracellular and extracellular applications of morphine (in concentrations from 10(-3) to 10(-5) M), leucine-enkephalin and methionine-enkephalin (10(-6) to 10(-8) M) were studied in unidentified acetylcholine-sensitive dialysed neurons of a snail under voltage clamp. Morphine produced inward membrane currents, while enkephalins did not. Both morphine and enkephalins altered the effect of acetylcholine on postsynaptic acetylcholine receptors; intracellular application of these substances being much more effective than extracellular application. This suggested that opioid peptides take part in the regulation of cholinergic synaptic transmission.