Gustatory receptor cells

Lepidopterous larvae bear on their mouthparts two pair of styloconic sensilla (see Figure 1). The papilla of each sensillum possesses one terminal pore which gives entrance to the dendritic regions of four gustatory receptor cells. Besides, a fifth cell in each sensillum acts as a mechanoreceptor in detecting positional changes of the papilla (7, ) ... 

Table I. Response spectra of gustatory receptor cells in Pieris brassicae larvae (7,8).

In biting their food and by means of a relatively small number of gustatory receptor cells, the larvae are informed about the composition of nutrients and secondary plant substances. Taste perception, the integration of sensory information in the insect s central nervous system, is not merely a process of summation. Synergistic as well as antagonistic effects between individual compounds can be observed in the food uptake of larvae on artificial diets. 

Different insect species posses different gustatory receptor cells, their response spectra being adapted to the perception of chemical components distributed in their host plant species (9 ). Taste perception in P. brassicae larvae forms a representative example for phytophagous insects, which are able to discriminate a number of compounds like sugars, amino acids, salts, and secondary plant substances acting as feeding inhibitors or feeding incitants (3,6,, 10). 

Toyono T, Kataoka S, Seta Y, Shigemoto R, Toyoshima K. Expression of group II metabotropic glutamate receptors in rat gustatory papillae. Cell Tissue Res. 2007 328 57-63. 

Toyono T, Seta Y, Kataoka S, Kawano S, Shigemoto R, Toyoshima K. Expression of metabotropic glutamate receptor group I in rat gustatory papillae. Cell Tissue Res. 2003 313 29-35. Yang H, Wanner IB, Roper SD, Chaudhari N. An optimized method for in situ hybridization with signal amplification that allows the detection of rare mRNAs. J. Histochem. Cytochem. 1999 47 431-446. 

At present, the functional architecture of gustatory receptor gene expression, in particular for sweet and bitter, is being unravelled however, the functional configuration of these receptors as monomer, dimers or oligomers is not clear, and neither are their downstream intracellular signalling cascades, the transmitter used by their host sensory neurons and the precise connectivity of these cells to second-order intemeurons. 

Inoshita T, Tanimura T (2006) Cellular identification of water gustatory receptor neurons and their central projection pattern in Drosophila. Proc Natl Acad Sci USA 103 1094-1099 Ishimoto H, Tanimura T (2004) Molecular neurophysiology of taste in Drosophila. Cell Mol Life Sci 61 10-18... 

Behrens M, Foerster S, Staehler F, Raguse JD, Meyerhof W (2007) Gustatory expression pattern of the human TAS2R bitter receptor gene family reveals a heterogenous population of bitter responsive taste receptor cells. J Neurosci 27 12630-12640 Behrens M, Meyerhof W (2006) Bitter taste receptors and human bitter taste perception. Cell Mol Life Sci 63 1501-1509... 

Schematic diagram of the gustatory pathway in rodents. Taste receptor cells are innervated by one of three cranial nerves (VII, IX, or X), which project topographically into the rostral portion of nucleus of the solitary tract (NST). Cells within the NST send projections into the reticular formation (RF), through which connections are made to oral motor nuclei V, VII, and XII and the nucleus ambiguous (NA). Ascending fibers connect to the parabrachial nuclei (PbN) of the pons, from which two parallel pathways emerge. One pathway carries taste information to the insular cortex (IC) via the ventral posterior medial nucleus, parvicellularis (VPMpc), of the thalamus. The other pathway projects into areas of the limbic forebrain involved in food and water regulation, reinforcement, reward, and stress, including the lateral hypothalamus (LH), the central nucleus ofthe amygdala (CeA), and the bed nucleus of the stria terminalis (BST). These areas and the IC are interconnected and send descending projections back to both the PbN and NST...

Taste begins with transduction events in receptor cells of the taste buds. There has been a lot of recent progress in our understanding of gustatory receptor mechanisms and transduction. The challenge is to relate molecular mechanisms that are evident at the receptor level to the extensive literature on gustatory neurophysiology and taste-mediated behavior. 

Saltiness is sensed by taste receptor cells that respond primarily to sodinm chloride. The proteins in the cell membranes involved in transforming the presence of salt into nervons signals are epithelial sodium channels that allow the sodium ions to enter the cells, initiate the release of chemical neurotransmitters, and stimulate adjacent gustatory afferent axons (nerve cells that carry taste information to the brain). Sourness (hydrochloric acid, citric acid, or acetic acid) is fikewise sensed by taste receptor cells in ways that directly affect ion channels. Protons either enter via the epithelial sodinm channels or block epithelial potassinm channels to initiate the cellular response. The bitterness of quinine and calcium is also sensed by blocking potassium channels in taste receptor cell membranes. 

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