Effect of intracisternal thyrotropin-releasing hormone on hepatic blood flow in rats.

Central neuropeptides play a role in many physiological regulatory processes through the autonomic nervous system. Thyrotropin-releasing hormone (TRH) is distributed in the central nervous system and acts as a neurotransmitter to regulate gastric functions through vagal-muscarinic pathways. The central effect of the TRH analog on hepatic blood flow was investigated in urethan-anesthetized rats. Hepatic blood flow was determined by the hydrogen gas clearance technique. Intracisternal injection of the stable TRH analog RX-77368 (5-100 ng) dose dependently increased hepatic blood flow with peak response at 15 min after the peptide was administered (net change from basal for vehicle and 5, 10, 100, and 500 ng RX-77368 was 2.0 +/- 0.2, 8.9 +/- 0.8, 19.4 +/- 2.6, 32.6 +/- 3.3, and 28.5 +/- 6.8 ml.min-1.100 g-1, respectively), and this stimulatory effect returned to baseline at 90 min. The stimulation of hepatic blood flow by the intracisternally administered TRH analog was abolished by atropine methyl nitrate (0.15 mg/kg ip), indomethacin (5 mg/kg ip), NG-nitro-L-arginine methyl ester (10 mg/kg iv), and hepatic branch vagotomy but not by cervical spinal cord transection (C6 level). Intravenous injection of RX-77368 did not have any effect on hepatic blood flow. These results indicate that TRH acts in the central nervous system to stimulate hepatic blood flow through vagal-muscarinic and indomethacin- and nitric oxide-dependent pathways.