Taste preference synergy between glutamate receptor agonists and inosine monophosphate in rats

Monosodium glutamate (MSG) elicits a taste called umami and interacts syner gistically with nucleotide monophosphates such as 5'-inosine monophosphate (IMP) to potentiate this taste intensity. Indeed, the synergistic interacti on of nucleotide monophosphates and MSG is a hallmark of umami. We examined interactions between MSG and other taste stimuli, including IMP, by measur ing the lick rates of non-deprived rats during 30 s trials. To control for non-linear psychophysical functions, the concentration of one taste stimulu s in a binary mixture was systematically increased while the concentration of the second taste stimulus was decreased (stimulus substitution method). Synergy between two stimuli was detected if the lick rate for a binary mixt ure exceeded that expected from the sum of the lick rates for each stimulus alone. In initial experiments, taste synergy was observed when rats were p resented with mixtures of MSG and IMP but not with mixtures of MSG and sucr ose. In subsequent experiments, glutamate receptor agonists other than MSG were presented with IMP to lest for taste synergy. No evidence of synergy w as seen when rats were presented with mixtures of IMP and kainic acid or IM P and N-methyl-D-aspartate. However, taste synergy between IMP and L-AP4, a potent agonist at mGluR4 receptors, was observed. These results suggest th at a metabotropic glutamate receptor similar to mGluR4 may be involved in t he taste synergy that characterizes umami.