Characterization of Ligands for Fish Taste Receptors

Abstract

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 PLC␤2-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.

The taste system of small fish species

“…2,5) The functional analysis of fish T1Rs and T2Rs was performed using a heterologous expression in cultured cells. 6,7) The mammalian T1R1/T1R3 heteromer detects umami substances (amino acids) and the mammalian T1R2/T1R3 heteromer detects sweet substances. Fish T1R1/T1R3 heteromer-expressing cells responded to some L-amino acids, such as L-Arg and L-Ser in a dose-dependent manner.…”

Ligand-specific induction of endocytosis in taste receptor cells

“…The fact that the majority of these cells were found in different taste buds, might be related to the species and sets of substances that we used. Ishimaru et al and Oike et al examined the types of taste receptors (T1R and T2R) expressed in the taste buds of zebrafish and medaka (Ishimaru et al, 2005;Oike et al, 2007). They suggested that, among vertebrates, attractive tastants are recognized by T1Rs and aversive tastants by T2Rs.…”

Identification of RL-TGR, a coreceptor involved in aversive chemical signaling

“…Genetic analysis of the fish gustatory system has shown that fish have receptor molecules homologous to those of the mammalian system (8), suggesting that the receptors are conserved as a mechanism through which an organism can distinguish between foods that are nutritious and those that are potentially harmful. Some organisms have other mechanisms for identifying harmful substances, such as the nociceptor pathway; for example, binding of capsaicin (a compound found in chili peppers) to its receptor results in the perception of heat and intense pain in mammals (9).…”