Electrical and chemical stimulation of the nucleus raphe magnus inhibits induction of retching by afferent vagal fibers

Like pain sensation, vomiting is induced by C afferent fibers of the vagus nerve. Therefore, it can be assumed to be suppressed by the descending inhibitory system (DIS) of pain. In this study we examined this possibility in immobilized decerebrated dogs. Fictive retching was induced by the stimulation of abdominal afferent vagal fibers (stim-abdVAs) and identified by activity patterns of efferent discharges of the phrenic and abdominal muscle nerves. The induction of retching was suppressed by weak stimulation (20 μA, 20 Hz, 0.5 ms duration) of the pontine area corresponding to the nucleus raphe magnus (NRM). Stronger suppressions were produced by stimulation of the rostral part of the NRM at the pontine level where the abducen nucleus exists. Pontamine sky blue, an excitatory neurotoxin, which was microinjected into the NRM area in 5 dogs, completely suppressed the induction of retching during the injection period. The NRM area at the level of the abducen nucleus was commonly stained by the dye in the 5 dogs. These results suggest that the DIS of pain suppresses the induction of retching by afferent vagal fibers.