Effects of portal infusion of hypotonic- and hypertonic solutions on neuronal activity in the rat dorsal motor nucleus of the vagus.

The present experiment was designed to elucidate the characteristics of the response of neurons in the dorsal motor nucleus of the vagus (DMV) to stimulation of the hepatoportal area by hypotonic as well as hypertonic solutions. Responses of 81 neurons that exhibited an antidromic response to electrical stimulation of the ventral gastric vagus were recorded in the left DMV in urethane-chloralose anesthetized rats. The effects on these 81 neurons of portal infusion of hypertonic saline (3.6% NaCl) and of pure water were examined. The discharge rates of 16 neurons increased in response both to portal infusion of hypertonic saline and to that of water. Portal infusion of 0.9% NaCl produced no changes in firing rates. Their discharge rates of seven neurons increased in response to portal infusion of hypertonic saline but not to that of water. The other 58 neurons did not respond to these stimuli. Jugular infusion of water produced no response. Therefore, the responses to portal infusion of water appear to be derived from activation of the hepatoportal receptors. These results indicate that a certain fraction of DMV neurons respond similarly to portal infusions of hypertonic and hypotonic saline. It is possible that there exist some reflex arcs that mediate a similar response to both an increase and a decrease in portal blood osmolarity (or Na+ concentration), namely, a suppression of absorption.