Taste preference and nerve response to 5'-inosine monophosphate are enhanced by glutathione in mice.

Previous human sensory evaluation studies have shown that glutathione (GSH) enhances deliciousness, accompanied by thickness, mouthfulness, and continuity feeling, which is known as "kokumi" in Japanese, in an umami solution containing monosodium glutamate and 5'-inosine monophosphate (IMP). We conducted behavioral and electrophysiological experiments to explore possible interactions of taste effectiveness between GSH and umami substances in mice. The 2-bottle preference test revealed that the mice preferred GSH at concentrations ranging from 1 to 10 mM. When GSH was added to IMP or a mixture of IMP and monopotassium glutamate (MPG), the mice showed increased preference for these solutions over the individual IMP or the binary mixture of IMP and MPG in both short-term and long-term tests. The addition of GSH to MPG, however, did not increase preference. Neural responses of the chorda tympani and glossopharyngeal nerves to the mixture of IMP and GSH showed synergism, whereas synergism was not observed in the mixture of MPG and GSH in either taste nerve. Another behavioral study with the use of the conditioned taste aversion paradigm showed that aversions to MPG generalized moderately to GSH, but aversions to GSH did not generalize to MPG. The present study suggests that GSH enhances preference for umami solutions containing 5'-ribonucleotide rather than glutamate. On the basis of these results, we discuss possible receptors involved for the action of GSH.

Altered taste function in mice deficient in the 65-kDa isoform of glutamate decarboxylase.

The 65-kDa isoform of glutamate decarboxylase (GAD65) is considered to play an important role for GABA synthesis in the central nervous system. Using mice with targeted ablation of the GAD65 gene (GAD65(-/-) mice) we investigated a possible involvement of GABAergic neurotransmission in several taste functions. Preference/aversion responses to four basic tastes were not different between GAD65(-/-) and wild-type mice during a 48-h two-bottle choice test. GAD65(-/-) mice consumed less sucrose-quinine mixtures than did wild-type mice. The injection of midazolam (5 mg/kg), a benzodiazepine agonist, significantly increased the consumption of 100 mM sucrose in the wild-type mice. The same injection, however, failed to increase intake of the 100 mM sucrose in GAD65(-/-) mice. These results suggest that GAD65-generated GABA is not implicated in basic taste functions such as simple detection and discrimination. Rather, more complex processing of taste information including taste mixtures and palatability may be finely tuned by GAD65-mediated GABA synthesis.

A comparison of voluntary salt-intake behavior in Nax-gene deficient and wild-type mice with reference to peripheral taste inputs.

The Na(x) channel, a subfamily of voltage-gated sodium channels, is thought to be a specific sodium receptor in the central nervous system. Our previous study revealed that Na(x)-gene-deficient mice consumed excessive amounts of NaCl even under water-deprived conditions. In the present study, to investigate whether the peripheral taste inputs are involved in the abnormal intake of salt in Na(x)-deficient mice (homo), voluntary intake of various taste solutions in homo and wild-type mice (wild) was examined under non-deprived conditions. Homo showed a higher preference for 0.15 M NaCl solution than wild. Preference ratios for other basic tastants were identical between groups. Transection of the chorda tympani (CT) or the glossopharyngeal (GP) nerve had little effect on salt-intake behavior in homo and wild. Although combined transection of the superior laryngeal (SL) and GP nerves decreased NaCl intake in homo but not in wild, there were no differences in preference ratios for NaCl in homo before and after SL+GP transection. On the other hand, preference ratios for NaCl in wild tended to increase after combined SL and GP transection. Consequently, preference ratios for NaCl after SL+GP transection were no different between homo and wild. While electrophysiological responses of the CT and the GP to various taste solutions were indistinguishable between homo and wild, those of the SL to NaCl in homo were smaller than those in wild only at lower concentrations (0.01 and 0.03 M). Thus, chemosensory inputs from the oro-pharyngeal regions had little effect on abnormal salt intake in homo, if any. From these results, it is suggested that the higher preference for NaCl in homo is mainly due to the lack of Na(x) channels in the central nervous system.