On the mechanism and site of action of enkephalin on single myenteric neurons

Intracellular recordings were made from neurons in the isolated myenteric plexus of the guinea-pig ileum. Methionine- and leucine-enkephalin were applied to the neurons either by adding them to the perfusing solution in known concentrations or by ejecting them by microiontophoresis. When applied in the perfusion solution, the enkephalin (1 nM-1 μM) hyperpolarized the soma membrane of less than half the neurons. The hyperpolarization was of rapid onset and reversed fully when the enkephalin was washed out; in a proportion of cells the hyperpolarization was associated with a fall in neuronal input resistance. This hyperpolarization was reversed or prevented by naloxone. When applied by iontophoresis to the soma membrane, enkephalin never hyperpolarized the myenteric neurons. Hyperpolarization in response to iontophoretic application of enkephalin was observed only when the enkephalin was applied to the surface of the ganglion away from the soma membrane; such hyperpolarization was prevented by addition of naloxone to the perfusion solution. The results indicate that the site at which enkephalin causes the naloxone-reversible hyperpolarization is not the soma membrane. The possibility is discussed that enkephalin may cause presynaptic inhibition in this preparation by hyperpolarizing or increasing the conductance of sites on the cellular processes from which acetylcholine is released.