AJP - Gastrointestinal and Liver Physiology, Vol 252, Issue 1 128-G135, Copyright © 1987 by American Physiological Society
Effect of enkephalins on colonic mechanoreceptor synaptic input to inferior mesenteric ganglion
H. D. Shu, J. A. Love and J. H. Szurszewski
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.
