Mechanisms of umami taste preference and aversion in rats

The influence of glutamate intake on growth and appetite, and the mechanisms of preference and aversion for monosodium L-glutamate (MSG) solutions were investigated in rats. Food intake, but not weight gain, was reduced significantly in rats fed a glutamate + glutamine (Glx)-deficient diet compared with those fed a control diet. Increase in the voluntary intake of Glx solutions was more rapid in rats fed the Glx-deficient diet. The preference and aversion for MSG solutions were distinctly different in 14 rat strains tested. Brown-Norway rats showed a strong preference for 60 mmol/L MSG and did not show aversive behavior toward solutions containing up to 600 mmol/L MSG. Sprague-Dawley (SD) rats showed a moderate preference for 60 mmol/L MSG and a weak aversion for MSG concentrations higher than 240 mmol/L; Long-Evans Agouti rats showed a moderate preference for 60 mmol/L MSG and a marked aversion for MSG concentrations higher than 120 mmol/L. Aversion was not due to nonspecific hyperosmotic effects. After section of gastric branches of the vagus nerve, MSG became aversive to SD rats. Aversion to 240 mmol/L MSG was reduced by 23-39% when combined with proline, alanine, glycine and glucose. These results show that the preference and aversion for MSG are determined by genetic factors, as well as vagus nerve function, and that the aversion to high MSG concentrations is reduced by the presence of other glucogenic amino acids and sugars.