Taste buds: cells, signals and synapses

Abstract

The past decade has witnessed a consolidation and refinement of the extraordinary progress made in taste research. This Review describes recent advances in our understanding of taste receptors, taste buds, and the connections between taste buds and sensory afferent fibres. The article discusses new findings regarding the cellular mechanisms for detecting tastes, new data on the transmitters involved in taste processing and new studies that address longstanding arguments about taste coding.

Figure 1. Membrane proteins that transduce taste

Type 2 taste receptors (T2Rs; bitter-taste receptors) are G protein-coupled receptors (GPCRs) that have short amino termini and may function as monomers (not shown) or dimers. T1Rs (sweet-taste and umami receptors) are also GPCRs, but they have long N termini that contain bilobed (venus flytrap) domains and function as dimers that use T1R3 as an obligate subunit. T1R1–T1R3 is an umami receptor, and T1R2–T1R3 is a sweet-taste receptor. All these taste GPCRs use a common transduction pathway that includes a Gβγ-activated phospholipase C (PLCβ2) and transient receptor potential cation channel subfamily M member 5 (TRPM5). The epithelial Na channel (ENaC) has three subunits and is thought to transduce salty taste in rodents. Glucose transporter type 4 (GLUT4) — which has 12 membrane-spanning segments — transports glucose by facilitative diffusion, whereas sodium/glucose cotransporter 1 (SGLT1) is Na dependent. One or both of these transporters are hypothesized to be part of an alternative glucose-sensing pathway that is similar to the one used in pancreatic β cells.

Figure 2. The combinatorial model of taste coding

Individual type II taste bud cells are mostly tuned to one taste quality (for example, bitter, sweet or salty): that is, they are ‘specialists’ (umami has been omitted for clarity). The type III cells sense sour tastes and also respond secondarily to other taste stimuli via cell-to-cell (paracrine) communication within the taste bud (represented by the arrows between the taste bud cells). Thus, type III cells can be termed ‘generalists’. Some afferent ganglion neurons receive input from taste cells that respond to a single taste quality and hence would be specialist neurons. Other afferent ganglion neurons receive input from many taste cells or from type III cells and thus are multiply sensitive ‘generalist’ neurons. Moving to the CNS, sensory ganglion cells converge on hindbrain neurons in the nucleus of the solitary tract.