, 2004). For C. elegans and Drosophila O2-sensing, ancient heme-based sensors were co-opted by sensory cells to transform detection into a change in neural activity in the brain and animal behavior. In the case of CO2 detection, sensors have been identified in the mammalian gustatory and olfactory systems and Drosophila olfaction. In mammalian detection, carbonic anhydrases play a central role. These enzymes are found in bacteria and algae and participate in fundamental processes such as photosynthesis, respiration, and acid-base homeostasis ( Tashian, 1989). Carbonic anhydrases (CAs) catalyze the reaction of CO2 and water into the intermediate

carbonic acid, which http://www.selleckchem.com/products/BKM-120.html is instantaneously Nintedanib solubility dmso converted to bicarbonate ions and protons. Different products of CA can act as messengers for signaling: bicarbonate is proposed to activate a receptor guanylate cyclase in mammalian olfactory neurons and protons are proposed to gate a pH-sensitive channel in gustatory neurons. Thus, these cells have also adopted existing strategies for detection and coupled them to brain and behavior. Similarly, chemoreceptors on fish gills and plant stomatal guard cells both sense CO2 in the

environment and require carbonic anhydrases for detection ( Hu et al., 2010 and Qin et al., 2010). Does sensory detection occur without CA involvement? Drosophila olfactory neurons detect CO2 with two gustatory receptor genes, gr21a and gr63a. GRs are multipass transmembrane domain proteins most similar to Drosophila odorant receptors ( Robertson et al., 2003). As Drosophila odorant receptors have recently been proposed to function as ligand-gated ion channels with some capacity to activate G proteins ( Sato et al., 2008 and Wicher et al., 2008), this may also be the case for GRs. CO2 may directly activate GRs, as misexpressing the receptors

in heterologous olfactory neurons confers CO2 responses ( Jones et al., 2007 and Kwon Bay 11-7085 et al., 2007). In this scenario, the function of Gr21a/Gr63a may be akin to Rhesus proteins (Rh), which act as ion channels/transporters directly gated by CO2 ( Kustu and Inwood, 2006). Alternatively, it is possible that CAs act upstream of Gr21a/Gr63a and that these receptors detect a reaction product, similar to the mechanism thought to underly mammalian taste. Understanding CO2 detection in additional sensory systems may shed more light on the diversity of CO2 sensors. The ability to extract information about subtle changes in O2 levels, or CO2 on the tongue or in the air, affords an unanticipated flexibility in behavior toward these essential and prevalent gases. Sensory neurons, for the most part, capitalize on long-standing cellular strategies for detection, such as soluble guanylate cyclases and carbonic anhydrases.