Subdiaphragmatic vagal afferent innervation in activation of an opioidergic antinociceptive system in response to colorectal distension in rats

In a number of different experimental paradigms of somatic pain, there is evidence for a vagally mediated antinociceptive system. This pathway probably involves opioid mechanisms. However, whether this pathway is activated in visceral pain or if it involves subdiaphragmatic vagal afferents is unclear. The aim of the present study was to determine whether subdiaphragmatic vagal afferents mediate antinociception in response to a visceral stimulus and whether this involves an opioid pathway. Colorectal distension was performed in fasted, conscious male Sprague-Dawley rats using a balloon catheter connected to an electronic distension device. The number of abdominal contractions (visceromotor response) in response to a tonic colorectal distension (60 mmHg for 10 min) was recorded. Experiments were performed in sham or subdiaphragmatically vagotomized, perineural vehicle- or capsaicin-treated rats (to functionally denervate vagal afferents) before and after administration of naloxone (25 mg kg^-1 bodyweight intraperitoneally). Vagotomy, capsaicin and naloxone pretreatments all significantly enhanced the visceromotor response to colorectal distension. The effect of naloxone in capsaicin-treated rats did not appear to be additive. These results suggest that activation of subdiaphragmatic afferents, which can be blocked by capsaicin, may play a role in opioid-dependent antinociceptive pathways activated by a noxious visceral stimulus.