The canine sympathetic neuropeptide galanin: A neurotransmitter in pancreas, a neuromodulator in liver

Our laboratory has investigated the role of the neuropeptide galanin in the sympathetic neural control of both the canine endocrine pancreas and liver . Galanin mRNA and peptide were found in the neuronal cell bodies of the ce liac ganglion, which projects fibers to both organs. Galanin fibers formed dense networks around the islets. Galanin was released from these nerves an d the amount released appeared sufficient to markedly inhibit basal insulin secretion. We therefore propose that galanin is a sympathetic neurotransmi tter in canine endocrine pancreas. Galanin was also found in hepatic nerves usually co-localized with tyrosine hydroxylase, a sympathetic marker. Furt her, intraportal administration of the sympathetic neurotoxin, 6-hydroxydop amine, abolished galanin staining in the hepatic parenchyma. We evaluated t he role of galanin in mediating the actions of sympathetic nerves to increa se hepatic glucose production and decrease hepatic arterial conductance. Lo cal infusion of synthetic galanin had little effect by itself, but it did p otentiate the action of norepinephrine to stimulate hepatic glucose product ion, demonstrating a neuromodulatory action. In contrast, galanin had no ef fect on hepatic arterial blood flow. We therefore propose that in the liver galanin functions as a neuromodulator of norepinephrine's metabolic action .