Retracted: Serotonin and cholecystokinin synergistically stimulate rat vagal primary afferent neurones

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Abstract
Recent studies indicate that cholecystokinin (CCK) and serotonin (5‐hydroxytryptamine, 5‐HT) act via vagal afferent fibres to mediate gastrointestinal functions. In the present study, we characterized the interaction between CCK and 5‐HT in the vagal primary afferent neurones. Single neuronal discharges of vagal primary afferent neurones innervating the duodenum were recorded from rat nodose ganglia. Two groups of nodose ganglia neurones were identified: group A neurones responded to intra‐arterial injection of low doses of cholecystokinin octapeptide (CCK‐8; 10–60 pmol); group B neurones responded only to high doses of CCK‐8 (120–240 pmol), and were also activated by duodenal distention. CCK‐JMV‐180, which acts as an agonist in high‐affinity states and as an antagonist in low‐affinity states, dose dependently stimulated group A neurones, but inhibited the effect of the high doses of CCK‐8 on group B neurones. Duodenal perfusion of 5‐HT evoked dose‐dependent increases in nodose neuronal discharges. Some neurones that responded to 5‐HT showed no response to either high or low doses of CCK‐8. A separate group of nodose neurones that possessed high‐affinity CCK type A (CCK‐A) receptors also responded to luminal infusion of 5‐HT. Further, a subthreshold dose of CCK‐8 (i.e. 5 pmol) produced no measurable electrophysiological effects but it augmented the neuronal responses to 5‐HT. This potentiation effect of CCK‐8 was eliminated by CR 1409. From these results we concluded that the vagal nodose ganglion contains neurones that may possess only high‐ or low‐affinity CCK‐A receptors or 5‐HT3 receptors. Some neurones that express high‐affinity CCK‐A receptors also express 5‐HT3 receptors. Pre‐exposure to luminal 5‐HT may augment the subsequent response to a subthreshold dose of CCK.