, 2009 and Han and Luo, 2010) In support of this model, carbonic

, 2009 and Han and Luo, 2010). In support of this model, carbonic anhydrase inhibitors block CO2 cellular responses and car2 mutants do not show behavioral responses to CO2 ( Hu et al., 2007). In addition, although the biochemical mechanism of activation has not been established, it has been shown that bicarbonate can activate cGMP production PD0325901 mouse when GC-D is expressed in heterologous cells ( Guo et al., 2009 and Sun et al., 2009). Moreover,

cellular and behavioral CO2 responses are absent in animals lacking the CNGA3 channel ( Han and Luo, 2010). However, many aspects of this model remain to be tested; for example, the requirement for CAII or GC-D for cellular activation has not been established. Other studies of GC-D olfactory neurons have shown that they respond to

the small peptides guanylin and uroguanylin (Leinders-Zufall et al., 2007) and carbon disulfide (CS2) (Munger et al., 2010). Guanylin and uroguanylin detection requires GC-D but not CAII, whereas CS2 detection is absent in car2 mutants and reduced in gc-d mutants ( Munger et al., 2010). The responses to CS2 or peptides were reported to be about 10,000-fold more sensitive than the responses to CO2 ( Munger et al., 2010). These results call into question the natural ligand for these cells. One interpretation GSK J4 order is that the CO2-sensing neurons may be multimodal neurons that integrate detection of multiple cues. Second-order neurons that synapse onto necklace glomeruli, the sites where GC-D neurons project, also respond to multiple cues. Ten percent of mitral/tufted cells in proximity of necklace glomeruli respond to CO2 and are activated or inhibited by a small number of other odors ( Gao et al., 2010). Together, these findings suggest that CO2 is not processed by a dedicated olfactory channel. Instead, CO2 signals may be integrated with other cues very early on in the olfactory pathway. One way that an animal could glean information from emission of a generic molecule like

CO2 would be to couple its detection to that of other odors or peptides. Whereas the also olfactory system mediates long-range detection of volatile CO2, the gustatory system mediates short-range detection. Humans obviously appreciate carbonated beverages but the taste of carbonation does not clearly fall within the classic taste modalities of sweet, bitter, sour, salt, or umami. Only recently have there been studies to examine the molecular basis for the taste of carbonation. Taste cells on the mammalian tongue respond to different taste modalities: sugar, bitter, sour, and salt-sensing cells have been identified (Yarmolinsky et al., 2009). Sour-sensing cells express a membrane-tethered extracellular carbonic anhydrase (CAR4) (Chandrashekar et al., 2009) in addition to an ion channel PKD2L1/PKD1L3 that can be activated in response to acidic solutions (Huang et al., 2006, Ishimaru et al., 2006 and Inada et al., 2008).

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