**2. Mammalian taste system**

*Animal Models in Medicine and Biology*

of molecular- genetics analyses with the availability of transgenic and mutant flies makes fly a unique system to study taste. In addition, flies share the same molecular

*Taste system of adult Drosophila. (A) Taste neurons are located on different body parts of fly namely labellum, pharynx, legs, wings and ovipositor as shown with red and green dots. (B) Three different types of taste sensillum (Large-L, Intermediate-I and small-S) present on the labellum (mouth part of a fly). (C) Taste sensillum structure showing pore at the tip and different types of taste neurons in a taste sensillum namely sweet (green), bitter (magenta), salt (yellow) and water (sky blue) neurons together with one mechanosensory* 

Different members of gustatory receptor (GRs) genes expressed in gustatory neurons mediate the detection of taste compounds such as sugars and bitter compounds [3–7]. Expression patterns of taste receptors is based largely on transgenic GAL4 expression studies and suggest that different GRs are expressed in overlapping but non-identical subsets of sugar- and bitter-sensing neurons [6–8]. In addition, electrophysiological studies from taste neurons suggest heterogeneity among the responses of individual sugar- or bitter-sensing cells [9–11] suggesting diversity among the peripheral cell types that detect sugars or bitter compounds in *Drosophila.* This organization provides the potential for different taste cell types to be activated by different compounds within a taste modality and the possibility for

This chapter will present a research progress made in the field of taste perception in the fruit fly and will describe the anatomical properties of the *fly* gustatory system. We shall then review taste perception mainly from a molecular genetic perspective that includes the results from behavioral, electrophysiological and imaging analyses. The parallels between the flies and human taste system will provide insight into how the detection of taste compounds regulates feeding decisions.

**130**

logic of taste as mammals.

*neuron (black) surrounded by support cells (gray).*

**Figure 1.**

intra modality discrimination.

In humans, taste receptors cells (TRC's) helps in the detection of taste stimuli. TRC's are present in taste buds and palate epithelium at the back and sides of the tongue (circumvallate and foliate papillae). The taste buds called fungiform are scattered across the front of the tongue and on the palate. Three morphologically distinct cell types (I, II and III) are present in a taste bud and constitute five functional classes of sensory cells, each specialized to detect one of the five basic taste qualities (bitter, sweet, umami, sour and salty). TRCs are epithelial cells that extend a process to the apical surface of the epithelium, where a taste pore allows direct contact with chemicals in the environment. The life of taste cells is short and they replenish from proliferative basal keratinocytes [12]. TRCs can relay information of taste quality independent of cells relaying other taste qualities [13]. Neurotransmitter receptors are present on taste cells. TRC's release various neurotransmitters to communicate among cells in the taste bud to shape the output of the bud [14]. Vertebrate TRCs do not possess an axon, and instead are innervated by pseudo unipolar neurons whose cell bodies reside in the petrosal and geniculate ganglia. The chorda tympani nerve that (innervates the anterior tongue) contain fungiform papillae and the glossopharyngeal nerve, (innervates the posterior tongue and most of the palate) carry most of the taste information. Neurons from taste ganglia project to the nucleus of the solitary tract, and from there information is relayed to the gustatory cortex [15].
