Projection of the hepatic branch of the splanchnic nerve to the brainstem of the rat.

Physiological [2] and behavioral [3,6] studies of the sensory function of liver have suggested that afferents other than those in the hepatic branch of the vagus nerve (HBVN) may be significant in the control of relevant homeostatic processes. In particular, Niijima [2] has provided preliminary evidence for an additional vagal afferent projection from the liver contained within the hepatic branch of the splanchnic nerve (HBSN). This nerve bundle, like the HBVN, terminates within the hilus of the hepatic portal vessel. From this point, however, the nerves project in quite different directions. The HBVN traverses the lesser omentum to connect directly with the anterior subdiaphragmatic vagal trunk [4]. The HBSN leaves the region of the hilum to travel first with the hepatic artery, then the coeliac artery. This nerve branch then penetrates the coeliac ganglion where it joins the dorsal (or accessory [4]) coeliac branch of the vagus nerve [2]. Since these two hepatic vagal nerves project by different visceral paths, [2,4] we thought it possible that the HBSN might terminate in a different region of the solitary nucleus (NST) than the HBVN [1,5]. Given that the HBSN might represent another significant conduit through which the liver exerts its influence over homeostatic processes, we wished to determine the location of those neurons in the NST which receive projections from this newly identified hepatic vagal pathway. Four male Sprague-Dawley rats were anesthetized with urethane (1.5 mg/kg) . Following laparotomy, the stomach was reflected anteriorly to reveal the HBSN and the hepatic artery. The branch point of the hepatic artery with the coeliac artery was exposed by carefully resecting the attachment of the pancreas with the stomach. At this juncture, about a 1-cm length of the HBSN could be dissected free from its