Effect of Leu-Enkephalin on Synaptic Transmission in the Inferior Mesenteric Ganglion
Enkephalin immunoreactive fibers are present in the guinea pig inferior mesenteric ganglion (IMG). The purpose of this study was to determine if leu-enkephalin (L-enk) inhibits natural synaptic activity from cholinergic, colonic mechanosensory afferents. Preparations consisted of a segment of the colon attached to the IMG via the lumbar colonic nerve. In a two-compartment organ bath, the IMG was placed in one compartment and the colon in the other. Intraluminal pressure recording techniques were used to study colonic motility and intracellular recording techniques were used to study synaptic transmission in the IMG. Administration of L-enk (10−6 M – 10−5 M) to only the colon decreased colonic motility and decreased mechanoreceptor synaptic input to neurons in the IMG as evidenced by a decrease in the frequency of action potentials and in the frequency and amplitude of e.p.s.p.s. These effects were antagonized by naloxone (10−6 M). When L-enk was added to only the ganglion compartment, the resting membrane potential hyperpolarized from −61.7 ± 3.2 mV to −66.3 ± 3.4 mV (mean ± SEM; p < 0.01). L-enk converted mechanoreceptor, synaptic input from threshold action potentials to subthreshold e.p.s.p.s and decreased the amplitude and frequency of e.p.s.p.s. The inhibitory effect of L-enk was antagonized by naloxone (10−6 M).