Effects of SFO lesions on salt appetite during multiple sodium depletions

A depletion of sodium may increase sodium intake by increasing the synthesis of angiotensin (ANG) II in the blood and by stimulating ANG II receptors in the subfornical organ (SFO) of the rat. Lesions of SFO reportedly reduce these intakes. The present experiments tested the effects of SFO lesions on salt appetite after three successive depletions. After a furosemide-induced natriuresis, Long-Evans rats had free access to water- and sodium-deficient diet for 22 h. Water and 0.3 M NaCl were given for 2 h, and then the rats received regular chow, water, and 0.3 M NaCl until the next injection 5 or 7 days later. SFO lesions reduced water intake 1-2 h after each furosemide injection but not during the overnight periods. The lesions did not affect salt appetite the next day, 24-26 h after furosemide, but they did prevent the expected increase in the chronic daily 0.3 M NaCl intake after repeated depletions. The second experiment was similar to the first except that three subcutaneous injections of 100 mg/kg captopril were given at 1, 18, and 20 h after furosemide for the second depletion only. After the first depletion, the results were similar to those of the same condition of the first experiment. After the second depletion, captopril greatly reduced water intake and salt appetite in all rats including those with SFO lesions. Thus, we found that the lesion reduced chronic intake, but we did not replicate results showing large effects of SFO lesions on acute salt appetite. This residual acute appetite after SFO lesion remains dependent on the synthesis of ANG II.