Increased aversion to bitter-tasting fluids in the Brattleboro rat

Abstract The large daily water intake of Brattleboro rats with diabetes insipidus (BDI) is believed to be due to the volume of depletion caused by their lack of circulating anti-diuretic hormone. However an increased fluid consumption cannot necessarily be interpreted as an increased behavioural drive to drink. In order to examine these rats' motivation to drink, quinine at various concentrations was given in the drinking water as sole fluid source to BDI rats and rats of the parent Long Evans (LE) strain, and their fluid balance after twenty-four hours was determined. Brattleboro rats reduced their fluid intakes significantly more than the control LE rats at the highest concentration of drug used (100 mg/l), suggesting a decreased motivation to drink. A two bottle preference test for quinine showed that the BDI rats actually have a higher taste threshold for the drug in solution in drinking water than the control LE rats. This suggests that the increased aversion to quinine in solution when it is the sole drinking source cannot be due to the BDI rats tasting the quinine at a lower concentration than the LE animals. Nor are greater toxic effects of quinine in BDI rats likely to account for the different drinking responses when presented with quinine solutions, since intragastric administration of the drug had no effect on water consumption in these animals. When BDI rats were given bitter-tasting sucrose octa-acetate solutions as their sole drinking source, drinking was again drastically reduced. However LE rats actually increased their fluid consumption when given sucrose octa-acetate showing a preference for this substance in solution to tap water. Therefore the two strains of rat may perceive these two bitter-tasting substances differently. The aversive effect of quinine solutions is reduced on second presentation in both BDI and LE rats.