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Effects of vagus nerve stimulation in visceral pain model.

Visceral pain is an important therapeutic problem. A number of studies have established that abdominal vagal afferents modulate somatic pain behavior. Although it is not clear if vagal afferents transmit nociceptive information, a change in their activity can increase or decrease nociceptive transmission in visceral pain. Aims of the present study were to determine whether the subdiaphragmatic vagus nerves play a role in the endogenous pain inhibitory mechanisms in visceral pain model and whether it involves opioidergic pathways. Data obtained in our studies show that vagus nerve plays the direct role in conveying the nociceptive information in the peritonitis model of visceral pain. We have shown, that vagal afferents exhibit an increase in excitability and subdiaphragmatic vagotomy decrease nociceptive behavior in visceral pain in rats. We have also tested two different stimulation parameters of chronic subdiaphragmatic vagal nerve stimulation: VNS1 (high-intensity) and VNS2 (low-intensity) in visceral pain model in rats. Both stimulation parameters increased pain threshold but VNS1 was more effective than VNS2. Naloxone inhibited the antinociceptive effects of VNS, reversing partially increase in the pain threshold in rats and increases number of writhes in visceral pain model. Therefore, our data indicate that this analgesic effect of the VNS is mediated, at least in part, by descending opioidergic pathways. The present study has confirmed the importance of vagal afferents for nociception in general and proven that this role is not limited to somatic pain but also extends to visceral pain.