We have explored the innervation of the rainbow trout (O. mykiss) liver using immunohistochemical procedures and light microscopy to detect in situ protein gene product 9.5 and neuronal nitric oxide synthase immunoreactivities (PGP-IR and NOS-IR). The results showed PGP-IR nerve fibres running with the extralobular biliary duct (EBD), hepatic artery (EHA) and portal vein (EPV) that form the hepatic hilum, as well as following the spatial distribution of the intrahepatic blood vessel and biliary channels. These nerve fibres appear as single varicose processes, thin bundles, or thick bundles depending on their diameter and location in the wall of the blood vessel or biliary duct. No PGP-IR fibres were detected in the liver parenchyma. NOS-IR nerve fibres were located only in the vessels and ducts that form the hepatic hilum (EBD, EHA, EPV); in addition, NOS-IR nerve cell bodies were found isolated or forming ganglionated plexuses in the peribiliary fibromuscular tissue of the EBD. No PGP-IR ganglionated plexuses were detected in the EBD. The location of the general (PGP-IR) and nitrergic (nNOS-IR) intrinsic nerves of the trout liver suggest a conserved evolutionary role of the nervous control of hepatic blood flow and hepatobiliary activity.
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