Behavioral taste similarities and differences among monosodium L-glutamateand glutamate receptor agonists in C57BL mice

Monosodium L-glutamate (MSG) and 5'-ribonucleotides elicit umami taste in h umans and probably in some species of animals, Previous studies suggest tha t taste-mGluR4 and NMDA receptor may be involved in taste transduction for umami, but behavioral responses in rats do not support the involvement of N MDA receptor. In the present study, behavioral similarities and differences among MSG, mGluR4 agonist L(+)-2-amino-4-phosphonobutyrate (L-AP4), and NM DA receptor agonist N-methyl-D-aspartate (NMDA) were compared in C57BL mice by using a conditioned taste aversion paradigm. Mice conditioned to avoid either MSG or 10 mM L-AP4 appeared to avoid MSG, disodium 5'-inosinate (IMP ), a mixture of MSG and IMP, and L-AP4, but not NMDA. Aversive conditioning to either sucrose or NMDA was generalized only to a mixture of MSG+IMP or NaCl. However, aversive conditioning to L-AP4 at 1 mM was generalized to NM DA and the umami substances. Licit rates for L-AP4 increased by mixing with (RS)-alpha -cycloprophy-4-phosphonophenylglycine (mGluR4 antagonist) when animals were conditioned to avoid MSG or L-AP4. Lick rates for NMDA also ei ther decreased or increased by mixing with glycine (NMDA receptor coagonist ) or D(-)-2-amino-5-phosphonopentanoic acid (NMDA receptor antagonist) when animals were conditioned to avoid L-AP4 or NMDA. In sucrose-conditioned mi ce, gurmarin (a sweet inhibiting peptide) suppressed the avoidance of sucro se and a mixture of MSG and IMP, but not L-AP4 and NMDA. The results sugges t the possibility that to C57BL mice MSG may taste similar to L-AP4 but dif ferent from NMDA, although both types of glutamate receptors as well as gur marin-sensitive sweet receptor may be involved in perception of umami taste .