Thyrotropin-releasing hormone in the dorsal vagal complex stimulates hepatic blood flow in rats

Central administration of thyrotropin-releasing hormone (TRH) enhances hepatic blood flow in animal models. TRH nerve fibers and receptors are localized in the dorsal vagal complex (DVC), and retrograde tracing techniques have shown that hepatic vagal nerves arise mainly from the left DVC. However, nothing is known about the central sites of action for TRH to elicit the stimulation of hepatic blood flow. The effect of microinjection of a TRH analogue into the DVC on hepatic blood flow was investigated in urethane-anesthetized rats. After measuring basal flow, a stable TRH analogue (RX-77368) was microinjected into the DVC and hepatic blood flow response was observed for 120 minutes by laser Doppler flowmetry. Either left or right cervical vagotomy or hepatic branch vagotomy was performed 2 hours before the peptide. Microinjection of RX-77368 (0.5-5 ng) into the left DVC dose-dependently increased hepatic blood flow. The stimulation of hepatic blood flow by RX-77368 microinjection into the left DVC was eliminated by left cervical and hepatic branch vagotomy but not by right cervical vagotomy. By contrast, microinjection of RX-77368 into the right DVC did not significantly alter hepatic blood flow. These results suggest that TRH acts in the left DVC to stimulate hepatic blood flow through the left cervical and hepatic vagus, indicating that neuropeptides may act in the specific brain nuclei to regulate hepatic function.