Voluntary sodium ingestion in wild-type and oxytocin knockout mice

Oxytocin knockout (OT KO) mice acutely consume inappropriate amounts of sodium following overnight water deprivation suggesting that oxytocinergic neurons inhibit excessive sodium ingestion (Amico JA, Morris M, Vollmer RR. Mice deficient in oxytocin manifest increased saline consumption following overnight fluid deprivation. Am J Physiol - Regul Integr Comp Physiol 2001; 281:R1368-R1373). This study sought to determine whether oxytocin (OT) provides long-term regulation of voluntary sodium ingestion. Wild-type (WT) and oxytocin knockout male mice were provided choices between diets or drinking solutions that differed in their sodium content. Mice were given access for 1 week to two diets, one containing low sodium (0.01% sodium chloride [NaCl]) content and a second containing a normal sodium (1.0% NaCl) content. During the second week, the animals were given a choice between a low sodium diet and a high sodium (8.0% NaCl) diet. In the second week, mice consumed 4 times more sodium; however, there were no differences between WT and OT KO mice. In a second experiment, mice had access to a two-bottle choice of tap water and a 0.5 M NaCl solution made palatable by the addition of a 4.1% Intralipid emulsion. Both genotypes consumed large, but equivalent, volumes of the Intralipid/sodium solution. The ingestion of this sodium-rich solution stimulated thirst and enhanced the intake of water. Thus, the availability of palatable sodium-rich food or solutions can lead to excessive voluntary sodium ingestion. Compared with oxytocin knockout mice, enhanced voluntary ingestion of sodium-rich solid and liquid diets proceeded unimpeded in WT mice. Therefore, OT pathways may not be essential for regulating solute intake in this setting.