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.
D000818:Animals; D003106:Colon / Q000187:drug effects / Q000294:innervation*; D004743:Enkephalin, Leucine / Q000494:pharmacology*; D005071:Evoked Potentials / Q000187:drug effects; D005725:Ganglia, Autonomic / Q000187:drug effects / Q000502:physiology*; D006168:Guinea Pigs; D066298:In Vitro Techniques; D008465:Mechanoreceptors / Q000187:drug effects / Q000502:physiology*; D009270:Naloxone / Q000494:pharmacology; D009474:Neurons / Q000187:drug effects / Q000502:physiology; D013569:Synapses / Q000187:drug effects / Q000502:physiology*
D009270:Naloxone; D004743:Enkephalin, Leucine