Sensory Signal Transduction in the Vagal Primary Afferent Neurons
The vagal nerve conveys primary afferent information from the intestinal mucosa to the brain stem. Activation of vagal afferent fibers results in inhibition of food intake, gastric emptying, and stimulation of pancreatic secretion. Afferents nerves terminating near to the mucosa are in a position to monitor the composition of the luminal contents. As afferents do not project directly into the lumen, their activation depends on an intermediary step, i.e. neuronal activation by a secondary substance released from within the mucosal epithelium. This review addresses the role for both cholecytokinin (CCK) and serotonin (5-HT) released from enteroendocrine cells and acting as paracrine agents on the terminals of vagal afferents in responses to a number of luminal signals. CCK acted on both high- and lowaffinity CCK-A receptors present on distinct vagal primary afferent neurons. Neurons of the nodose ganglia respond to intraduodenal perfusions of maltose, glucose, and hypertonic saline. These neurons were also sensitive to exogenous luminally applied 5-HT at concentrations that mimic physiologic levels. Intravenous administration of a 5-HT3 antagonist blocked these responses suggesting that nodose neuronal responses to luminal osmolarity and to the digestion products of carbohydrates are dependent on the release of endogenous 5- HT from the mucosal enterochromaffin (EC) cells, which acts on the 5-HT3 receptors on vagal afferent fibers to stimulate vagal afferent neurons. Double-labeling studies revealed that nodose neurons responded to 5-HT-dependent luminal stimuli contain mainly glutamate and substance P. Over the past year or so it has become clear that there are multiple possible excitatory inputs to a common vagal afferent route with synergistic interactions being common. The nodose ganglion contains neurons that may possess only high- or low-affinity CCK-A receptors or 5-HT3 receptors. Some neurons that express high-affinity CCK-A receptors also express 5-HT3 receptors and (or) secretin receptors. Pre-exposure to luminal 5-HT may augment the subsequent response to a subthreshold dose of CCK. Synergistic interaction between CCK and secretin also occurs at the nodose ganglia; this is mediated by high affinity CCK-A receptor. This may explain the robust postprandial secretion of enzyme, bicarbonate, and fluid despite the modest increase in CCK after a meal. Some neurons that possess low-affinity CCK-A receptor colocalize with leptin receptors (OB-Rs). These neurons also respond to mechanical distention. Interaction between CCK-A receptor and OB-Rs in these neurons likely facilitates leptin mediation of short-term satiety.
Keywords: Sensory transduction, Vagus nerve, Nodose ganglia, Cholecytokinin, Serotonin, Macronutrients, Endocrine and Enterochromaffin cells, Synergistic interactions
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