c-fos-like immunoreactivity in the subfornical organ and nucleus of the solitary tract following salt intake by sodium-depleted rats.

Acute sodium depletion by furosemide induces a robust salt appetite in the rat which is satiated rapidly by ingestion of sodium chloride (salt) solutions. To identify neuronal populations activated by sodium depletion and by salt intake, we quantified c-fos-like immunoreactivity (c-FLI) in the subfornical organ (SFO) and nucleus of the solitary tract (NTS) after sodium depletion and at time intervals from 30 min to 12 h after 1 h of access to 0.3 M NaCl. Rats drank 10+/-1.6 mL over 1 h, with most of the intake occurring by 30 min. Increased numbers of c-FLI-positive cells were observed in the SFO 24 h after sodium depletion; c-FLI remained elevated for 90 min after 0.3 M NaCl intake and then declined until the number of c-FLI-positive cells at 12 h was not significantly different from mock-depleted levels. Sodium depletion alone did not significantly elevate c-FLI in the NTS, but the number of c-FLI-positive nuclei in the NTS was significantly increased after 0.3 M NaCl intake. The cellular location and temporal pattern of c-FLI expression are consistent with activation of neural circuitry sensitive to humoral, gustatory, and postingestive stimuli accompanying sodium depletion and 0.3 M NaCl ingestion. c-FLI in the SFO and NTS may serve as quantifiable markers in the central nervous system of the state of sodium depletion and of ingestive (orosensory and gastrointestinal) sensory stimulation, respectively.