Tasting toxicants as bitter: phytoecdysteroids
Taste is often seen as one key mechanism to detect potentially toxic chemicals. However, designing taste receptors which detect toxicity seems to be an impossible challenge. Toxic substances encompass a bewildering array of chemical structures and although most toxic substances taste bitter, the reverse is not true (Glendinning, 1994). Mammals, insects and worms have developed bitter-sensitive receptors tuned to detect potentially toxic molecules. In humans, 30 to 50 receptors are believed to detect bitter substances, more than those devoted to the detection of sugars and food stimulants (Mueller et al., 2005; Reed et al., 2006). Insects are probably in the same situation although direct proof is still lacking. In Drosophila, bitter-sensitive taste neurones co-express several taste receptors including Gr66a, while sugar-sensing cells co-express Gr5a (Thorne et al., 2004; Wang et al., 2004). This suggests a partition between taste receptor genes detecting bitter molecules and those tuned to other modalities. From this perspective, it seems that there is no relationship between bitterness and toxicity. Nevertheless, some authors have suggested a more causal relationship between bitterness and toxicity, for example using a GABA receptor as a sensor (Mullin et al., 1994) or by suggesting that tastants would interfere directly with transduction pathways (Naim et al., 2002; Peri et al., 2000; Zubare-Samuelov et al., 2003, 2005).