The mechanism responsible for cholecystokinin (CCK) induced pancreatic enzyme secretion under physiologic conditions remains controversial. Both direct and neural actions of CCK on pancreatic secretion have been proposed. Recently, using anesthetized rats we demonstrated that physiologic doses of CCK-8 act via stimulation of the vagal afferent pathway. This observation now needs to be confirmed in conscious rats. Furthermore we demonstrated that CCK response returns to normal following chronic vagotomy. Based on our preliminary observations we hypothesize that CCK at physiologic levels acts on specific CCK receptors located on vagal afferent branches terminating in the gastroduodenal mucosa. These CCK-sensitive fibers are distinct from the mechanoreceptors located in the muscle wall. Following chronic vagotomy, pancreatic response to CCK normalizes with time and this is mediated by adaptive changes in the enteric neural circuit involving expression of functional CCK receptors in enteric cholinergic neurons which stimulate pancreatic secretion via an intrapancreatic gastrin-releasing peptide (bombesin) neural pathway. To investigate this hypothesis, we plan to examine the effects of perivagal application of capsaicin as well as vagal rootlet section on pancreatic secretion stimulated by exogenous and endogenous CCK in conscious rats. Duodenal mucosal application of capsaicin will be performed to test the hypothesis that CCK-sensitive vagal afferent fibers terminate peripherally in the duodenal mucosa. To characterize the CCK receptors on vagal afferent fibers, receptor autoradiography studies will be performed in both intact as well as perivagal capsaicin-treated rats. Scatchard analysis will be done to define receptor density and affinity states under normal and hypercholecystokininemic states. Combined immunohistochemistry and receptor autoradiography will be used to identify the chemical coding in the nodose ganglia. To provide electrophysiological evidence that CCK stimulates vagal afferent pathway, single afferent fiber recording will be performed to characterize CCK's action. The subclass of vagal afferent fibers sensitive to CCK will be distinguished from those sensitive to mechanical distension. Neurotransmitters in specific CCK sensitive neurons in the nodose will be identified by intracellular recording and labeling techniques. Lastly, we will delineate the adaptive changes in the duodenal enteric neural circuit following chronic vagotomy which mediates CCK's action on pancreatic secretion. Using retrograde tracer studies, electrophysiological recording and neurotransmitter release studies we hope to demonstrate expression of functional CCK receptors in a subpopulation of enteric cholinergic neurons which project to the pancreas and stimulate pancreatic secretion via an intrapancreatic GRP neural pathway. These studies have important physiological ramifications and will revolutionize our current concept regarding how CCK acts to stimulate pancreatic enzyme secretion.
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