Central neuropeptides play a role as physiological regula-noradrenaline, peptides have also been identified as possible neurotransmitters in the central and peripheral nervous sys-tors in the autonomic nervous system. One of these neuropeptides, thyrotropin-releasing hormone (TRH), is distributed tem. 3 These neuropeptides have been shown to act centrally to regulate many physiological functions, including behavior, throughout the central nervous system (CNS) and acts as a neurotransmitter to regulate gastric functions through the va-glucose metabolism, body temperature, and cardiovascular and gastrointestinal functions. [4][5][6][7] In particular, thyrotropin-gus nerve. However, the autonomic nervous system is also involved in hepatic regeneration, but the effect of TRH is releasing hormone (TRH) in the medulla plays a physiological role in the vagal regulation of the digestive system. 6,8-10 unknown. Therefore, the CNS's effect of TRH on hepatic DNA synthesis was studied in rats. Hepatic DNA synthesis was
The liver is richly innervated, 11 and the autonomic nervous system has an important role in the process of hepatic prolif-assessed by [Methyl-3 H]thymidine incorporation 6, 12, 24, 48, and 72 hours after intracisternal injection of the TRH eration after partial hepatectomy and experimental liver necrosis. [12][13][14] Recent experiments have shown that vagal activa-analog, RX 77368 (1, 5, 10, and 100 ng), and by 5-bromo-2deoxyuridine (BrdU) labeling of the liver section. Hepatic tion by lesioning of the ventromedial hypothalamus (VMH) stimulates hepatic proliferation in rats, and that cholinergic DNA synthesis was stimulated by intracisternal TRH analog (10 ng), with a peak response at 24 hours after peptide injec-stimulation results in an increase of hepatic DNA synthesis after carbon tetrachloride-induced hepatic damage. 15,16 tion, and returned to baseline by 72 hours. This stimulatory effect by central TRH analog on hepatic DNA synthesis was These studies have prompted us to examine a possible role for TRH as a centrally acting chemical messenger involved dose-related, ranging from 1 ng to 10 ng (dpm/mg DNA at 24 hours [mean { SE]: saline, 95 { 6; 1 ng, 114 { 14; 5 ng, in the CNS regulation of hepatic proliferation. We report here that TRH acts in the CNS to elicit a vagal-dependent 318 { 57; 10 ng, 693 { 78; 100 ng, 710 { 135). Hepatocytes were randomly labeled by BrdU 24 hours after intracisternal stimulation of hepatic DNA synthesis as a prerequisite to hepatic proliferation. TRH analog (10 ng). Intravenous TRH analog (10 ng) did not influence hepatic DNA synthesis. The stimulatory effect of MATERIALS AND METHODS TRH analog was blocked by hepatic branch vagotomy and atropine, but not by hepatic sympathectomy, 6-hydroxydopa-Animals. Male Wistar rats weighing 200 to 220 g (Charles River mine, insulin antibody, or hypophysectomy. These results Japan Inc., Yokohama, Japan) were used. Rats were housed in group indicate that TRH acts in the CNS to stimulate hepatic DNA cages under controlled conditions of temperature and illumination, and fed ad libitum for at least 7 days before experiments. Protocols synthesis through vagal and cholinergic mechanisms, and that describing the use of rats were approved by the Animal Care Com-TRH may be the chemical messenger involved in brain regulamittee of Asahikawa Medical College, and in accordance with the tion of hepatic proliferation. (HEPATOLOGY 1997;26:1203-National Institutes of Health's Guide for the Care and Use of Labora-
1208.)
tory Animals. All experiments were performed according to the same time schedule, starting at 8 AM in 24-hour food-deprived rats with Although abundant anatomical, physiological, and pharfree access to water up to the beginning of the study. macological evidence suggests that the autonomic nervous Chemicals. The following substances were used: the stable TRH system plays an important role in regulating hepatic funcanalog, RX 77368; p-Glu-His-(3,3-dimethyl)-Pro-NH 2 (Reckitt and Colman, Kingdom-upon-Hill, England); atropine methyl nitrate tions, 1,2 the transmitters in the central nervous system (CNS) (Sigma Chemical, St. Louis, MO); 5-bromo-2-deoxyuridine (BrdU) mediating these effects are not well characterized. In addition (Sigma); 6-hydroxydopamine (Sigma); and anti-insulin antibody to the classical neurotransmitters, such as acetylcholine and (IgG class, Linco Research, Inc., St. Louis, MO). RX 77368 has similar binding characteristics as natural TRH on rat brain membranes but is more stable compared with natural TRH. 17,18 RX 77368 was aliquoted in 0.5% bovine serum albumin and 0.9% saline (pH Abbreviations: CNS, central nervous system; TRH, thyrotropin-releasing hormone; VMH, ventromedial hypothalamus; BrdU, 5-bromo-2-deoxyuridine.