Galanin—Sympathetic Neurotransmitter in Endocrine Pancreas?

The effects of sympathetic neural activation on basal pancreatic hormone secretion cannot be explained solely by the actions of the classic sympathetic neurotransmitter norepinephrine. The nonadrenergic component may be mediated by the 29-amino acid peptide galanin in that this neuropeptide meets several of the criteria necessary to be considered a sympathetic neurotransmitter in the endocrine pancreas. 1) Galanin administration inhibits basal insulin and somatostatin secretion and stimulates basal glucagon secretion from the pancreas, qualitatively reproducing the effects of sympathetic nerve stimulation. These sympathomimetic effects appear to be mediated by direct actions of galanin on the islet. 2) Galanin-like immunoreactivity exists in fibers that innervate pancreatic islets. 3) Galanin is released during electrical stimulation of pancreatic nerves. The quantity released is sufficient to reproduce sympathetic nerve stimulation-induced effects on insulin secretion and to contribute to the neural effects on somatostatin and glucagon release. 4) Whether interference with galanin action or release reduces the islet response to sympathetic nerve stimulation remains to be determined. We hypothesize that galanin and norepinephrine act together to mediate the islet response to sympathetic neural activation. If galanin is a sympathetic neurotransmitter in the endocrine pancreas, it may contribute to the inhibition of insulin secretion that occurs during stress and thereby to the hyperglycemic response. Moreover, the local presence of this potent β-cell inhibitor in the islet leads to speculation on galanin's contribution to the impairment of insulin secretion that occurs in non-insulin-dependent diabetes mellitus and therefore on the potential utility of a galanin antagonist in the treatment of this disease.