Behavioral responses to glutamate receptor agonists and antagonists implicate the involvement of brain-expressed mGluR4 and mGluR1 in taste transduction for umami in mice

Recent molecular studies have identified many candidate receptors for umami, typically the taste of monosodium glutamate (MSG). The candidate receptors, including taste-mGluR4, T1R1+T1R3, and truncated mGluR1, respond to MSG in the millimolar concentration range. Expression of brain-expressed mGluR4 and mGluR1 with much higher sensitivities to glutamate has also been reported in taste papillae. To test the involvement of brain-expressed mGluRs in umami taste, we tested glutamate agonists and antagonists at concentration ranges relevant to both types of the receptors using a combination of a detection threshold and conditioned taste aversion (CIA) methods in mice. The detection threshold experiment showed that mice could detect the group III mGluR agonist L(+)-2-amino-4-phosphonobutyrate (L-AP4) taste thresholds at 0.0009-0.0019 mM. Mice conditioned using CIA methods to avoid either MSG or MPG showed aversive responses to MSG with and without amiloride or to MPG, respectively, at concentrations of 0.0001 mM and above. A CIA to L-AP4 or MSG showed comparable concentration-response ranges for L-AP4 and MSG. The Group III mGluR antagonist, (RS)-alpha-cyclopropyl-4-phosphonophenylglycine (CPPG), and the mGluR1 antagonist, 1-aminoindan-1,5-dicarboxylic acid (AIDA), suppressed aversive responses to glutamate agonists at concentrations between 0.0001 and 100 mM in the cm experiments. Our results suggest the possibility that brain-expressed mGluR4 and mGluR1 may contribute to umami taste in mice. (C) 2011 Elsevier Inc. All rights reserved.