FOS INDUCTION IN RAT-BRAIN NEURONS AFTER STIMULATION OF THE HEPATOPORTAL NA-SENSITIVE MECHANISM

Responses of hepatic afferent nerves to intraportal bolus injection of hypertonic solutions were examined in anesthetized rats. Hepatic affe rent nerve activity increased in response to an intraportal injection of 0.75 M NaCl or NaHCO3 but did not respond to a similar injection of 1.5 M mannitol, 0.75 NI LiCl, or 0.15 M NaCl, implying that nerves in the hepatoportal area are sensitive to increases in Na concentrations and that this leads to stimulation of hepatic afferent nerve activity . To study central activation in response to stimulation of the hepati c Na-sensitive mechanism, c-fos induction was monitored. After electri cal stimulation of hepatic afferent nerves, neurons containing Fos-lik e immunoreactivity (Fos-li) were found in the area postrema, nucleus o f the solitary tract, paraventricular hypothalamic nucleus, and suprao ptic nucleus at 90 min after stimulation. Induction of Fos-li was also studied after simultaneous infusion of 0.45 M NaCl into the portal ve in and distilled water into the inferior vena cava in conscious rats s o as to keep the total amount of solution introduced into the systemic circulation isotonic, thus avoiding changes in mean arterial pressure , plasma osmolality, and plasma NaCl concentrations. Fos-li-containing neurons were found in the same regions in which they were found after electrical stimulation. However, few if any, Fos-li-containing cells were found if the rats were hepatically denervated or if they received an intraportal infusion of hypertonic LiCl or mannitol. These data pr ovide evidence for involvement of the brain stem and forebrain structu res in NaCl regulatory functions induced by stimulation of the hepatop ortal Na-sensitive mechanism. However, stimulation of the hepatoportal osmosensitive mechanism does not activate these central structures.