The Taste of Monosodium Glutamate: Membrane Receptors in Taste Buds

Abstract

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.

Immunohistochemical Studies on Protein Gene Product 9.5, Serotonin and Neuropeptides in Vallate Taste Buds and Related Nerves of the Guinea Pig.

“…The sensory innervation of the tongue is particularly interesting because it plays a major role not only in taste perceptions of sweet, sour, salt, bitter or umami (ASTBACK et al, 1995;CHAUDHARI et al, 1996), but also in information about the temperature and consistency of the food or drink. The PGP 9.5 immunoreactive nerve fibers supplying taste buds demonstrated in the present study would generally seem to represent the afferent innervation for those sensations.…”

Signal transduction and information processing in mammalian taste buds

“…Chaudhari et al [93,94] identified a truncated N-terminal version of mGluR4 in rat taste buds and showed that Chinese hamster ovary (CHO) cells expressing this receptor responded to L-glutamate in a concentration range appropriate for umami taste. Yang et al [95] showed by ISH that mGluR4 was expressed in a subset of rat taste bud cells.…”

A Review of Glutamate Receptors I: Current Understanding of Their Biology

“…Many tissues demonstrate Glu, GluR and GluTPs, including but not limited to, CNS 122 , autonomic and sensory ganglia 123 , peripheral nerves-myelinated and unmyelinated 124,125 , vagus and other cholinergic nerves 124 , tachykinin containing sensory nerves and vestibular tissues 1 2 6 , 1 2 7 retinal photoreceptors 1 2 8 -1 3 0 , Muller cells 1 2 8 , 1 3 1 , adrenal medulla 123,133 , pituitary 134 , pineal gland 135 , endocrine pancreas [136][137][138][139][140][141][142][143][144][145][146][147] , immune system 148 , taste buds 149,150 , esophagus 151 , gut 70,100,[152][153][154] , ileal longitudinal muscle [155][156][157] , h e pa to c yt e s 6 8 , 1 5 8 , k i d n e y , s p l e e n s , o v a r i e s 4 8 , 1 5 9 , lungs 60,99,145,148,154,[158][159][160][161][162][163][164][165] , heart …”