Rat brainstem neurons responsive to changes in portal blood sodium concentration.

Studies were performed to identify the response characteristics of nucleus of the solitary tract neurons receiving afferent projections from the hepatic branch of the vagus nerve. Thirty Sprague-Dawley rats under pentobarbital anesthesia had catheters placed in the ileocolic vein and the inferior vena cava. Neuronal recordings were made in the left medial nucleus of the solitary tract (NST), in the area where hepatic vagal fibers terminate. Sixteen NST cells were identified that responded selectively to the portal infusion of water or hypertonic saline. Two patterns of response were seen: 1) 12 neurons were persistently stimulated by portal hypertonic saline and persistently inhibited by portal water, and 2) four neurons were either transiently excited (n = 3) or transiently inhibited (n = 1) by portal hypertonic saline with no water effect. All units recorded responded to changes of 1% or less in portal blood sodium concentration. Hypertonic mannitol was an ineffective stimulus but choline chloride was as effective as sodium chloride. This suggests that the hepatic receptors utilize an Na+-K+-ATPase electrogenic pump in the transduction process.