GALANIN IS LOCALIZED IN SYMPATHETIC NEURONS OF THE DOG LIVER

Stimulation of canine hepatic nerves releases the neuropeptide galanin from the liver; therefore, galanin may be a sympathetic neurotransmit ter in the dog liver. To test this hypothesis, we used immunocytochemi stry to determine if galanin is localized in hepatic sympathetic nerve s and we used hepatic sympathetic denervation to verify such localizat ion. Liver sections from dogs were immunostained for both galanin and the sympathetic enzyme marker tyrosine hydroxylase (TH). Galanin-like immunoreactivity (GALIR) was colocalized with TH in many axons of nerv e trunks as well as individual nerve fibers located both in the stroma of hepatic blood vessels and in the liver parenchyma. Neither galanin - nor TH-positive cell bodies were observed. Intraportal 6-hydroxydopa mine (6-OHDA) infusion, a treatment that selectively destroys hepatic adrenergic nerve terminals, abolished the GALIR staining in parenchyma l neurons but only moderately diminished the GALIR staining in the ner ve fibers around blood vessels. To confirm that 6-OHDA pretreatment pr oportionally depleted galanin and norepinephrine in the liver, pre mea sured both the liver content and the hepatic nerve-stimulated spillove r of galanin and norepinephrine from the liver Pretreatment with 6-OHD A reduced the content and spillover of both galanin and norepinephrine by >90%. Together, these results indicate that galanin in dog liver i s primarily colocalized with norepinephrine in sympathetic nerves and may therefore function as a hepatic sympathetic neurotransmitter.