Distribution and possible origin of neuropeptide-containing nerve elements in the mammalian liver

The intrahepatic distribution of nerve fibres is highly species dependent, therefore we searched for a species where the innervation pattern is similar to that of the human liver. Livers of rats, cats, guinea pigs and humans were used. The different nerve elements were identified by ABC immunohistochemistry and analysed semiquantitatively. Large numbers of neuropeptide Y (NPY) and dopamine-beta-hydroxylase immunoreactive (IR) nerve fibres were observed in the human and guinea pig liver, and they were in close contact with portal triads, central veins and ran parallel with liver sinuses. A few substance P, somatostatin and vasoactive intestinal polypeptide IR nerve fibres were also detected intralobularly, while galanin nerve fibres were only observed around portal triads. In the rat liver only a few NPY-positive nerve fibres were found, exclusively in portal tracts. Some nerve cell bodies (IR for NPY and somatostatin) were also found in the liver of guinea pigs, young cats and humans, therefore some of the nerve terminals might originate from these intrinsic ganglia. It can be concluded that the innervation pattern of the guinea pig liver shows the highest similarity to that of the human liver.