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The Journal of Neuroscience, May 23, 2007, 27(21):5584-5592; doi:10.1523/JNEUROSCI.0651-07.2007
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Cellular/Molecular
Characterization of Ligands for Fish Taste Receptors
Hideaki Oike,1 Toshitada Nagai,1 Akira Furuyama,2 Shinji Okada,1 Yoshiko Aihara,1 Yoshiro Ishimaru,1 Takayuki Marui,2 Ichiro Matsumoto,1 Takumi Misaka,1 andKeiko Abe1 1Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Bunkyo-ku, Tokyo 113-8657, Japan, and 2Department of Oral Function and Molecular Biology, Ohu University School of Dentistry, Tomita-machi, Koriyama, Fukushima 963-8611, Japan
Correspondence should be addressed to Keiko Abe, Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan. Email: aka7308{at}mail.ecc.u-tokyo.ac.jp
Recent progress in the molecular biology of taste reception has revealed that in mammals, the heteromeric receptors T1R1/3 and T1R2/3 respond to amino acids and sweeteners, respectively, whereas T2Rs are receptors for bitter tastants. Similar taste receptors have also been characterized in fish, but their ligands have not been identified yet. In the present study, we conducted a series of experiments to identify the fish taste receptor ligands. Facial nerve recordings in zebrafish (Danio rerio) demonstrated that the fish perceived amino acids and even denatonium, which is a representative of aversive bitter compounds for mammals and Drosophila. Calcium imaging analysis of T1Rs in zebrafish and medaka fish (Oryzias latipes) using an HEK293T heterologous expression system revealed that both T1R1/3 and a series of T1R2/3 responded to amino acids but not to sugars. A triple-labeling, in situ hybridization analysis demonstrated that cells expressing T1R1/3 and T1R2/3s exist in PLCß2-expressing taste bud cells of medaka fish. Functional analysis using T2Rs showed that zfT2R5 and mfT2R1 responded to denatonium. Behavior observations confirmed that zebrafish prefer amino acids and avoid denatonium. These results suggest that, although there may be some fish-specific way of discriminating ligands, vertebrates could have a conserved gustatory mechanism by which T1Rs and T2Rs respond to attractive and aversive tastants, respectively.
Key words: taste; receptor; T1R; T2R; amino acid; fish
Received Feb. 13, 2007; revised April 6, 2007; accepted April 6, 2007.
Correspondence should be addressed to Keiko Abe, Department of Applied Biological Chemistry, Graduate School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan. Email: aka7308{at}mail.ecc.u-tokyo.ac.jp
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