FIGURE 10.1

(see facing page) (See color insert following page 140.) Illustration of a taste bud, taste receptor cell, and associated neurons. A. Diagram of a TRC and respective synapse with a primary gustatory neuron. Several receptors and transduction pathways are drawn in a single model taste receptor cell (TRC). The apical membrane of the cell contains receptors for tastants dissolved in the saliva. GPCRs (G-protein-coupled receptors) for amino acid (T1R1/T1R3), sweet (T1R2/T1R3) or bitter (T2R) tastants activate intracellular signal transduction cascades involving PLCβ2 (a phospholipase C). PLCβ2 degrades PIP₂ (phosphatidylinositol-4,5-biphosphate) to produce DAG (diacylglycerol) and IP₃ (inositol-1,4,5-triphosphate). Calcium release from the endoplasmic reticulum, mediated by IP₃ binding to IP₃R3 receptors, can activate TRPM5, a transient receptor potential ion channel, on the basolateral membrane of the TRC. Ion channels involved in salt (ENaCs) and sour (PKD2L1) tastant detection are also shown in the apical cell membrane. Other GPCRs and ion channels, shown on the TRC baso-lateral membrane, are responsive to peptides, hormones, and neurotransmitters that modulate responses to tastants. TRC activation culminates in the release of neurotransmitters, namely ATP, from intracellular vesicles to synapses with primary gustatory nerves, shown here with a postsynaptic purinergic P2X receptor. B. Depiction of a taste bud embedded in epithelial tissue. Note that taste receptors and signal transduction molecules described earlier (A) are not necessarily expressed in the same TRC (see text for details). Here, TRCs and associated gustatory neurons are color coded according to “best-response” to a specific tastant quality. Primary gustatory neurons project ipsilaterally to the rostral nucleus tractus solitarius (rNTS). (Adapted from Simon, S.A. and de Araujo, I.E. et al. (2006). The neural mechanisms of gustation: a distributed processing code. Nat Rev Neurosci 7(11): 890–901)