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 mech anoreceptor afferent fibres from the stomach and small intestine to 5- hydroxytryptamine (5-HT, 10 and 50 mug), and to relate this to motor a ctivity. 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 fib res) or jejunum (1 fibre), and their discharge was excited by distensi on of the segment containing the receptive field, or by mechanical pro bing. 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-arte rially via the coeliac axis evoked both contraction and relaxation wit h the dominant effect dependent on region and dose of 5-HT. However, i rrespective of the motor response, the mechanoreceptor discharge close ly mirrored both the magnitude and the time-course of the motor respon se. The relationship between the changes in discharge of individual me chanoreceptors and change in intraluminal pressure in all regions in r esponse 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 change s in intraluminal pressure. This study suggests that vagal mechanorece ptors supplying the gastroinestinal tract respond to changes in motor activity evoked by 5-HT, rather than to 5-HT directly.