Effect of enkephalins on colonic mechanoreceptor synaptic input to inferior mesenteric ganglion

The effects of leucine-enkephalin (Leu-Enk) on colonic mechanoreceptor input to the inferior mesenteric ganglion (IMG) and on colonic intraluminal pressure of the guinea pig were studied in vitro. Superfusion of the IMG with Leu-Enk decreased colonic, afferent mechanoreceptor synaptic input. In neurons in which mechanoreceptor input caused postsynaptic spikes, Leu-Enk decreased synaptic input and increased the basal intraluminal pressure of the colon. When mechanoreceptor input consisted of singly occurring excitatory postsynaptic potentials (EPSPs), Leu-Enk decreased the frequency of EPSPs but did not cause a change in colonic pressure. The inhibitory effects of Leu-Enk on synaptic transmission were antagonized by naloxone. In the isolated IMG, Leu-Enk converted synchronous action potentials in response to electrical stimulation of intermesenteric nerves to subthreshold EPSPs without a change in the resting membrane potential or input resistance. Action potentials elicited by depolarizing current pulses or by exogenous acetylcholine were unaltered by Leu-Enk. These data suggest that Leu-Enk increased colonic intraluminal pressure by acting on the presynaptic terminals of colonic mechanoreceptive neurons to reduce synaptic input to and output from the inhibitory neurons of the IMG.