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Basic and Applied Aspects olVestibular FUllCtio1l I.C. Hwang, N.G. Daunton and V.I. Wilson (Eds.)© Hong Kong University Press, Hong Kong, 1988 EXPERIMENTAL STUDIES OF GASTRIC DYSFUNCTION IN MOTION SICKNESS: THE EFFECT OF GASTRIC AND VESTIBULAR STIMULATION ON THE VAGAL AND SPLANCHNIC GASTRIC EFFERENTS A. Niijima*, Z.Y. Jiang*, N.G. Daunton** and R.A. Fox*** *Department ofPhysiology, Niigata University School ofMedicine, Niigata 951, Japan; **Neurosciences Branch, National Aeronautics and Space Administration (NASA)Ames Research Center, Moffett Field, CA 94035, U.S.A.; and *** Department ofPsychology, San Jose State University, San Jose, CA 95192, U.sA. Abstract The experiments were conducted in anaesthetized rats. In the first part of the experiments, the effect of CuSO4on the afferent activity in the gastric branch of the vagus nerve was investigated. Gastric perfusionofCuSO4solution (0.04% and. 0.08%) provoked an increase in afferent activity. In the ~econd part of the experiments, the reflex effects of gastric perfusion of CuSO4 solution, repetitive stimulation of the gastric vagus nerve, and caloric stimulation of the right vestibular apparatus (5-18°C water) on gastric autonomic outflow were investigated . The results of these experiments showed that these three different types of stimulation caused an inhibition in efferent activity ofthe gastric vagus nerve and a slight activation ofthe splanchnic gastric efferents. The summation ofthe effect of each stimulation was also observed. These results, therefore, provide evidence for a possible integrative inhibitory function of the vagal gastric center as well as an excitatory function of gastric sympathetic motoneurons in relation to motion sickness. Introduction It has been generally recognized that nausea and emesis with gastricdysfunction are the main symptoms of space and motion sickness. It is assumed that vestibular as well as gastric Keywords: gastric afferents, gastric efferents, vestibulo-vagal reflex, vestibulo-sympathetic reflex, gastrosensory-vestibular-autonomic interactions 134 Niijima et al. stimulation can be the major sources of these symptoms. It is also well known that caloric stimulation ofthe vestibularapparatus can cause emesisand nystagmic responses. Wang and Borrison (1951) reportedthat the intragastric administration ofcoppersulfate inducedemetic responses, and that the surgical interruption of the vagi had a more profound effect on the threshold and latency of vomiting than did sympathectomy, which caused no remarkable changes in these parameters. They stressed that the vagal gastric afferents play a more important role than splanchnic gastric afferents in the mediation of the gastric effects of copper sulfate. The present experiments were designed to study the effects ofindividual and combined vestibular and gastric stimulation on the reflex change in gastric autonomic outflow. Portions of the data describing the effects of copper sulfate on the rate of afferent discharges in the gastric branch ofthe vagus nerve have been reported elsewhere (Niijima et al., 1987). Methods Male Wistar rats weighing 300-400 g were used. Food, but not water, was removed 5 hours before the experiment. Rats were anesthetized with 700 mg/kg of urethane and 50 mg! kg of chloralose, given i.p. A tracheal cannula was inserted. The stomach could be perfused with copper sulfate (CuSO4) or physiological saline through a catheter which was placed in the oesophagus and directed toward the cardiac portion of the stomach. Another catheter was placed in the pyloric portion of the stomach through the duodenum as an outlet for the perfusate. Beforestarting theexperimentalperfusion, the stomach was washedwith isotonic saline. Coppersulfatesolutions (0.04% and 0.08%) and isotonicsaline were used for the experimental perfusions. For each perfusion 4 ml of solution at 38°C were injected by syringe into the stomach over a I-min period. The solution was kept in the stomach for 5-30 min, after which time the stomach was flushed for 1 min with isotonic saline. To stimulate the vestibular apparatus, the right external auditory meatus was irrigated for 3-10 min with cold water (518 °C) and then flushed with warm water (34-35°C). Afferent nerve activity was recorded from a nerve filament isolated from the peripheral cut end of the gastric branch of the vagus nerve, or of the splanchnic nerve. Efferent nerve activity was made from a filament isolated from the central cut end of the ventral gastric branch of the vagus nerve or the gastric branch of the splanchnic nerve. Nerve activity was amplified by means of a condenser-coupled differential amplifier, and stored on magnetic tape. Analysis of the nerve activity was performed after conversion of raw data to standard pulses by a window discriminator that distinguished the nerve discharges from the background noise. To monitor...

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