, 2005, Bretscher et al., 2008, Hallem and Sternberg, 2008 and Zimmer et al., 2009). Even animals that live in enclosed spaces may monitor ambient concentrations. When CO2 levels

in the hive increase by ∼1%–2%, honeybees exhibit fanning behavior to ventilate the nest in order to maintain a low CO2 environment ( Seeley, 1974). CO2 emitted during respiration may also serve as a secreted chemical signal that other animals detect. In this way, CO2 may act as a chemosensory signal that alerts animals to potential food, predators, or danger. Blood-feeding insects such as mosquitoes, black flies, and tsetse flies are attracted to CO2 and use this signal to hone in on their human hosts (Gibson and Torr, 1999). The hawkmoth, Manduca Sexta, prefers flowers that emit a high level of CO2, suggesting that CO2 acts as a proximal signal BIBF 1120 chemical structure for nectar ( Guerenstein et al., 2004 and Thom et al., 2004). CO2 increases can also signal avoidance, as CO2 emitted by Drosophila upon stress acts as a signal for other Drosophila to flee ( Suh et al., 2004). How do animals detect and respond to varying concentrations of O2 and CO2 in their environment? Ribociclib cell line Recent studies of the model

organisms C. elegans, Drosophila melanogaster and mice have begun to elucidate the neural and molecular bases of detection. In all cases, detection occurs in specialized sensory cells; in Drosophila and mice, subsets of olfactory and gustatory neurons respond specifically to CO2. In most cases, these neurons respond to discrete features in their environment, such as increases or decreases in O2 or short-range or long-range cues. Detection can lead to attraction or avoidance behavior, and these behaviors are plastic. Plasticity may be especially important to allow animals to interpret the rather nonspecific signals of O2 and CO2 in the context of their complex sensory world. The molecular underpinnings 4-Aminobutyrate aminotransferase of detection are beginning to be elucidated, highlighting similarities across organisms and commonalities with ancient cellular mechanisms of detection. The nematode C. elegans lives in the soil. O2 levels in this environment vary from 1%–21%, depending on depth from

the surface as well as soil properties such as compaction, aeration, and drainage ( Anderson and Ultsch, 1987). C. elegans show a behavioral preference for 5%–10% O2 levels and avoid higher and lower concentrations ( Gray et al., 2004). This preferred O2 setpoint may reflect a compromise between the metabolic needs of the animal (favoring high O2) and oxidative stress (favoring low O2) ( Lee and Atkinson, 1977). The study of C. elegans O2 sensation has provided a framework for understanding how animals monitor gas levels to select a preferred environment. Recent progress has been made elucidating the neural and molecular bases for hyperoxia avoidance. Two pairs of neurons, URX and BAG, play critical roles in sensing O2 (Zimmer et al., 2009) (Figure 1).