Specific uptake of tritiated serotonin in the adult rat pancreas: Evidence for the presence of serotonergic fibers

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Abstract
Fragments of adult rat pancreas were incubated in vitro with tritiated serotonin at concentrations from 10−8 to 10−7 M. The pancreas exhibited an uptake of serotonin which was saturable, with an uptake constant (Km) of 8.75 × 10−7 M, and a Vmax of 873 pmoles per gram. Specificity was determined by the addition of fluoxetine or norepinephrine to the reaction mixture, both at 10−5 M. Fluoxetine significantly reduced the 3H-5HT uptake, whereas norepinephrine was without effect. Metergoline (10−6 M), a specific 5-HT postsynaptic receptor blocker, similarly had no effect on the serotonin uptake in the pancreas. Radioautography of the fragments following uptake of tritiated serotonin (5 × 10−8 M) revealed silver- grain aggregates dispersed along blood vessels in the interstitial spaces of the exocrine and endocrine pancreas, areas known to be traversed by nerve fibers. There were no silver- grain aggregates over the exocrine or islet parenchymal cells. These data support the hypothesis that the pancreas is innervated by serotonergic fibers. Further evidence for this hypothesis was provided by a preliminary study demonstrating the presence of tryptophan hydroxylase in pancreatic homogenates. These serotonergic fibers may be involved in the regulation of pancreatic secretion.