Effects of 5-hydroxytryptamine (5-HT) on the discharge of vagal mechanoreceptors and motility in the upper gastrointestinal tract of the ferret.

This study was undertaken to investigate the sensitivity of vagal mechanoreceptor afferent fibres from the stomach and small intestine to 5-hydroxytryptamine (5-HT, 10 and 50 micrograms), and to relate this to motor activity. The discharge of single afferent fibres was recorded from the cervical vagus of Urethane anaesthetized ferrets. These had receptive fields in the corpus (15 fibres), antrum (11 fibres), duodenum (3 fibres) or jejunum (1 fibre), and their discharge was excited by distension of the segment containing the receptive field, or by mechanical probing. Conduction velocity was calculated in 7 fibres, and always fell in the C-fibre range (0.84 +/- 0.07 m/s). Injection of 5-HT intra-arterially via the coeliac axis evoked both contraction and relaxation with the dominant effect dependent on region and dose of 5-HT. However, irrespective of the motor response, the mechanoreceptor discharge closely mirrored both the magnitude and the time-course of the motor response. The relationship between the changes in discharge of individual mechanoreceptors and change in intraluminal pressure in all regions in response to both doses of 5-HT fitted a linear regression y = 17.1 +/- 1.2x and was highly significant (P < 0.0001), indicating that 5-HT has no modulatory effect on the sensitivity of mechanoreceptors to changes in intraluminal pressure. This study suggests that vagal mechanoreceptors supplying the gastrointestinal tract respond to changes in motor activity evoked by 5-HT, rather than to 5-HT directly.