Characteristics of salt appetite in chronically sodium-depleted rats using a progressive ratio schedule of procurement

Satiation of sodium appetite of rats was studied using a protocol of sodium loss induced by chronic diuretic treatment and daily salt consumption sessions. In the first experiment in which NaCl was freely available from a drinking spout, rats consumed substantially more hypertonic NaCI than their estimated physiological deficit. In the second and third experiments, a simulated foraging cost was imposed on obtaining NaCl. In these experiments, rats were required to perform a progressive ratio lever press task to procure aliquots of hypertonic NaCl, and under these conditions they showed a break point or satiation for NaCl when they had consumed an amount close to their estimated deficit. In the fourth experiment, we found that the chronic diuretic regimen produced hypovolernia along with large increases in plasma aldosterone concentration and renin activity, and that all of these parameters were reversed toward non-depleted control values at the time of behavioral satiation in the progressive ratio protocol. Examination of natriuresis revealed that excessive intake of NaCl had no lasting benefit in this protocol: NaCl consumed in excess of need on 1 day was completely lost at the subsequent diuretic treatment. In the final two experiments, we showed that oral preloads of NaCl had no significant effect on progressive ratio procurement of NaCl, but that oral preloads reduced subsequent oral drinking. The data are discussed in terms of definitions of satiation of appetite in regard to the tightly regulated domain of body fluid homeostasis, and by extension, their relevance to systems such as energy homeostasis where set points appear to be less tightly regulated. (c) 2006 Elsevier Inc. All rights reserved.