Vagal stomach afferents inhibit somatic pain perception

Vagal stimulation inhibits systemic pain perception in animals, probably via the nucleus tractus solitarius and its connections with descending nuclei in the brainstem which inhibit pain. Pain-inhibiting effects of such stimulation in humans, obtained from epileptic patients treated by vagal stimulation, are controversial. The aim of our study was to evaluate whether vagal stomach afferent activation inhibits pain perception in healthy humans. Pain thresholds, magnitude of tonic heat pain at 46 °C stimulation, pain temporal summation and laser pain evoked potentials were measured at the hand before and immediately after rapid drinking of 1500 ml water in 31 volunteers. We found an increase in heat pain threshold from 43.3±2.6 to 44.7±2.2 °C, P<0.0001, a decrease of peak pain magnitude to tonic heat from 56.3±26.2 to 43.7±25.8 (on 0-100 VAS), P<0.0001, a lowering of area under the curve during tonic noxious heat stimulus from 1962±984 to 1411±934, P<0.001. Additionally, we observed a decrease in the peak to peak evoked potential amplitude from 19.2 μV±1.2 to 15.6 μV±1.2 (P=0.005) together with a decrease in the estimation of mean laser induced pain from 52.28±18.00 to 48.14±20.18 (P=0.025). Mechanical pain thresholds and temporal summation did not change significantly. We conclude that vagal stomach afferents exert an inhibitory effect on somatic pain perception in humans.