|M.Sc Student||Sedan Oshra|
|Subject||Vagal Afferents from the Stomach and Pain Perception|
|Department||Department of Medicine||Supervisor||PROF. David Yarnitsky|
In the past two decades evidence has been gathered supporting the involvement of the vagal nerve in the process of pain perception. The first testimonies showed that the activity following activation of spinothalamic neurons’ receptive field in monkeys is reduced by electrical activation of the vagal nerve. Likewise, electrical stimulation of vagal afferent fibers below the diaphragm caused a decrease in the perception of pain in rats. Vagotomy at the sub-diaphragmatic level caused an intensified perception of systemic pain in rats. Research in epileptic patients with transplanted vagal stimulators, has shown that their systemic pain perception is reduced, although contradicting evidence also exists.
The purpose of this research was to broaden the knowledge on the pain inhibiting effect of the vagus in human subjects. We hypothesized that activation of stomach vagal afferents, through drinking 1.5 liters of water within a short period, attenuates systemic pain perception in healthy human subjects. Pain perception was evaluated in 30 healthy subjects, ages 19-30, before and after drinking by measuring: a. Thresholds of heat and mechanical pain. b. Mechanical temporal summation. c. Pain perception during a one minute tonic noxious heat stimulation. In a second session pain perception was evaluated before and after drinking by measuring evoked potentials induced by laser noxious stimulation. Stroop test was performed before and after drinking to evaluate subjects’ state of alertness.
We found: 1. An increase in the heat pain threshold (from 43.26±2.59°C to 44.72±2.24°C, P<0.001). 2. A decrease in the maximal perceived pain (from 56.29±26.18 to 43.74±25.76, P<0.001) and the area under the curve (AUC) (from 1961.72±983.39 to 1410.88±933.94, P<0.001) during tonic noxious heat stimulation. 3. A decrease in the amplitude of the P component (from 13.4 ±4.7µV to 10.9±4.6µV, P=0.003) and in the amplitude of the N component (from 5.7±3.7µV to 4.7±3.0µV, P=0.048) and a decrease in P-N peak to peak difference (from 19.2±1.2µV to 15.6±1.2µV, P=0.005) in the P300 wave registered during laser stimulation. A significant drop in heart rate was registered, as can be expected due to vagal activation caused by increased stomach volume. Drinking did not reduce subjects’ alertness as measured by Stroop test.
In conclusion, the results of this research show that an increase of stomach volume, which in turn causes an elevation of vagal afferent activity, suppresses perception of cutanuous limb pain in healthy subjects.