Convergence of hepatic osmoreceptive inputs on sodium-responsive units within the nucleus of the solitary tract of the rat.

Single-shock electrical stimulation of the hepatic branch of the vagus induced both facilitation and suppression in units within the nucleus of the solitary tract (NTS). Some units that were facilitated by electrical stimulation also increased their discharge rates during topical iontophoretic application of Na+, as well as portal infusion of hypertonic saline. Other facilitatory units produced opposite responses; their discharge rates decreased during topical iontophoretic application of Na+ and portal infusion of hypertonic saline. Some units that were suppressed by electrical stimulation also responded to the topical application of Na+ and portal infusion of hypertonic saline. The responses to these two different stimuli were positively correlated in some units but not in others. The evidence suggests that the units within the NTS that are responsive to hepatic osmosensitive primary afferents may serve a Na+-responsive function and may be important in integration within the NTS for isosmotic or isovolemic homeostasis.