Central regulation of hepatic function by neuropeptides

In addition to classical neurotransmitters, such as acetylcholine and norad renaline, neuropeptides have been recognized as new neurotransmitters and n euramodulators. Neuropeptides are widely distributed in the central nervous system as well as in peripheral nerves, and act as neurotransmitters to re gulate various physiological functions. The digestive organs are no excepti on, and several neuropeptides in the central nervous system are shown to ac t in specific brain sites and control gastrointestinal functions, such as g astric acid secretion, and gastrointestinal motility, through the autonomic nervous system. Recently, a relationship between central neuropeptides and hepatic 1 unction through the autonomic nervous system has been revealed i n animal models. Thyrotropin-releasing hormone acts in the medulla, in part icular in the left dorsal vagal complex, to induce stimulation of hepatic b lood flow and hepatic proliferation, and protect against experimental liver injury through vagal and cholinergic pathways. Corticotropin-releasing fac tor injected intracisternally elicits inhibition of the hepatic blood flow and exacerbates experimental liver injury through sympathetic and noradrene rgic pathways. Neuropeptide Y acts in the left dorsal vagal complex, in par ticular in regard to the Y1 receptor subtype, to stimulate bile secretion. Other neuropeptides such as beta -endorghin and bombesin in the brain modul ate hepatic proliferation and bile secretion. Through the use of neuropepti des, new knowledge of the central and peripheral mechanisms underlying brai n regulation of hepatic function will be revealed. Further studies in regar d to the physiological relevance of the central action of neuropeptides on specific brain sites should be performed to unravel the underlying pathways that mediate brain-liver interaction.