Serotonin release in the subfornical organ area induced by sodium and water intake in the rat

Previous studies have shown that the subfornical organ (SFO) is involved in the modulation of sodium intake in rats. To clarify whether serotonergic mechanisms in the SFO participate in the modulatory system, the present study was carried out to examine the effects of sodium and water intake on serotonin (5-hydroxytryptamine, 5-HT) release in the subfornical organ (SFO) in freely moving rats. The ingestion of 0.3 M NaCl and water was induced by subcutaneous injections of the diuretic furosemide (FURO, 10 mg/kg) and the angiotensin converting enzyme inhibitor captopril (CAP, 5 mg/kg), and extracellular concentrations of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the region of the SFO were measured using intracerebral microdialysis techniques. The combined treatment with FURO and CAP elicited significant decreases in the 5-HT and 5-HIAA levels in the SFO area. The 0.3 M NaCl and water intake caused by the treatment significantly enhanced the 5-HT and 5-HIAA concentrations in the SFO area compared with the basal levels. No significant changes in the 5-HT and 5-HIAA levels caused by either the FURO and CAP treatment or water and NaCl intake were observed in the sites away from the SFO. These results suggest that the serotonergic mechanism in the SFO may be important for the control of sodium appetite and thirst.