They also raise the possibility that other C. elegans neurons HER2 inhibitor and tissues respond to CO2. To investigate how CO2 sensors contribute to avoidance in spatial

gradients, we genetically ablated neurons. We focused on AFD and BAG neurons because the Ca2+ responses of ASE to CO2 stimuli were slow, and those of AQR, PQR, and URX, weak. Specification of the AFD neurons requires the otd/Otx homeodomain transcription factor ttx-1, which is expressed only in AFD ( Satterlee et al., 2001). ttx-1 mutants show thermotactic defects equivalent to those of animals in which AFD has been removed by laser ablation ( Mori and Ohshima, 1995). ttx-1 mutants had a strong CO2 avoidance defect off food, and a weaker defect on food ( Figure 5G). Wild-type avoidance was restored to ttx-1 mutants by a transgene containing ttx-1 genomic DNA ( Figure 5G). These data suggest that the AFD neurons promote CO2 avoidance in spatial CO2 gradients. To ablate BAG we expressed the egl-1 programmed cell death activator from a BAG-specific gcy-33 promoter ( Conradt and Horvitz, 1998 and Yu et al., 1997) (we thank M. Beverly and P. Sengupta for this line). Both BAGL and BAGR neurons were absent in greater than 90% of animals bearing

this transgene ( Table ISRIB mouse S1 available online). Surprisingly, the CO2 avoidance of BAG-ablated animals was not significantly different from wild-type, both on and off food ( Figure 5G). We asked if combined genetic ablation of AFD and BAG causes a synthetic CO2 avoidance phenotype. Ablating the BAG neurons disrupted the residual CO2 avoidance of ttx-1(p767) mutants on food ( Figure 5G).

However, in the absence of food, ttx-1(p767); pgcy-33::egl-1 animals showed no greater defect than ttx-1(p767) single mutants ( Figure 5G). These data show that AFD and BAG promote CO2 avoidance in spatial gradients on food, and that AFD and at least one other neuron that is not BAG promote avoidance when food is absent. Isotretinoin Thus, the importance of different sensory neurons for CO2 avoidance in spatial gradients depends on context. In 5%-0% CO2 spatial gradients (Figure 1), a C. elegans moving at ∼0.3 mm/s experiences a change of 0%-0.05% CO2/s, depending on bearing relative to the gradient. In our Ca2+-imaging experiments, immobilized animals experienced much sharper temporal gradients of ∼1% CO2/s. In the wild, animals are likely to encounter a variety of CO2 gradients. To analyze behavioral responses to sharp CO2 gradients, we designed a square-shaped microfluidic chamber that enables CO2 levels over freely moving animals to be switched rapidly ( Movie S1 available online). We recorded responses and used custom software to extract instantaneous speed, reversal rate, and rate of omega turns, turns in which an animal’s head and tail touch to form an “Ω” shape (N2, Figure 6B). In the absence of food, a rise in CO2 from 0% to 5% elicited a brief slowing followed by a transient increase in reversals and omega turns ( Figure 6B).