The Molecular Biology of Glutamate Receptors in Rat Taste Buds

Receptor proteins for photoreception have been studied for several decades. More recently, putative receptors for olfaction have been isolated and characterized. In contrast, no receptors for taste have been identified yet by molecular cloning. This report describes experiments aimed at identifying a receptor responsible for the taste of monosodium glutamate (MSG). Using reverse transcriptase (IV)-PCR, we found that several ionotropic glutamate receptors are present in rat lingual tissues. However, these receptors also could be detected in lingual tissue devoid of taste buds. On the other hand, RT-PCR and RNase protection assays indicated that a G-protein-coupled metabotropic glutamate receptor, mGluR4, also is expressed in lingual tissues and is limited only to taste buds. In situ hybridization demonstrated that mGluR4 is detectable in 40-70% of vallate and foliate taste buds but not in surrounding nonsensory epithelium, confirming the localization of this metabotropic receptor to gustatory cells. Expression of mGluR4 in taste buds is higher in preweaning rats compared with adult rats. This may correspond to the known higher sensitivity to the taste of MSG in juvenile rodents. Finally, behavioral studies have indicated that MSG and L-2-amino-4-phosphonobutyrate (L-AP4), a ligand for mGluR4, elicit similar tastes in rats. We conclude that mGluR4 may be a chemosensory receptor responsible, in part, for the taste of MSG.

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“…Some of these results have been reported previously in preliminary form (Cartford et al, 1994;Chaudhari et al, 1994).…”

supporting

confidence: 75%

The Taste of Monosodium Glutamate: Membrane Receptors in Taste Buds

et al. 1996

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“…These experiments suggest that taste receptors for L-glutamate may be L-glutamate-gated cation channels. However, recent molecular biological and behavioral evidence from rat suggests that L-glutamate taste transduction is mediated by a metabotropic receptor displaying high sequence homology with a metabotropic L-glutamate receptor from brain named mGluR4 (Chaudhari et al, 1994). Nevertheless, little is known about the response of intact taste buds or isolated taste cells to L-glutamate.…”

Section: Resultsmentioning

confidence: 99%

“…In this paper we report a method that uses the calciumsensitive dye fura-2 (Grynkiewicz et al, 1985) and the fast voltage-sensitive dye di-8-ANEPPS (Bedlack et al, 1992;Loew, 1993;Rohr and Salzberg, 1994) to perform rapid alternate measurement of membrane potential and [Ca2+]i in cell ensembles. We show that this method is amenable for use with isolated mouse taste buds, and we employ it to determine whether mouse taste cells are responsive to metabotropic and ionotropic glutamate receptor agonists, as suggested by previous molecular biological (Chaudhari et al, 1994) and electrophysiological (Teeter et al, 1992) studies.…”

Section: Introductionmentioning

confidence: 87%

Measurement of membrane potential and [Ca2+]i in cell ensembles: application to the study of glutamate taste in mice

Hayashi

Zviman

Brand

et al. 1996

Biophysical Journal

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We have studied the spectral properties of the voltage-sensitive dye, 1-(3-sulfonatopropyl)-4-[beta [2-(di-n-octylamino)-6-naphtyl]vinyl] pyridinium betaine (di-8-ANEPPS), and the Ca(2+)-sensitive dye, fura-2, in azolectin liposomes and in isolated taste buds from mouse. We find that the fluorescence excitation spectra of di-8-ANEPPS and fura-2 are largely nonoverlapping, allowing alternate ratio measurements of membrane potential and intracellular calcium ([Ca2+]i). There is a small spillover of di-8-ANEPPS fluorescence at the excitation wavelengths used for fura-2 (340 and 360 nm). However, voltage-induced changes in the fluorescence of di-8-ANEPPS, excited at the fura-2 wavelengths, are small. In addition, di-8-ANEPPS fluorescence is localized to the membrane, whereas fura-2 fluorescence is distributed throughout the cytoplasm. Because of this, the effect of spillover of di-8-ANEPPS fluorescence in the [Ca2+]i estimate is < 1%, under the appropriate conditions. We have applied this method to study of the responses of multiple taste cells within isolated taste buds. We show that membrane potential and [Ca2+]i can be measured alternately in isolated taste buds from mouse. Stimulation with glutamate and glutamate analogs indicates that taste cells express both metabotropic and ionotropic receptors. The data suggest that the receptors responding to 2-amino-4-phosphonobutyrate (L-AP4), presumably metabotropic L-glutamate receptors, do not mediate excitatory glutamate taste responses.

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