Discussion

Brains organize behavior and physiology to optimize the response to threats or opportunities. I will discuss in molecular and circuitry terms how ambient levels of O2 and CO2 reprogram C. elegans’ global state. C. elegans uses sharply tuned circuits to recognize 21% O2. Our data suggest these circuits enable this nematode to recognise and escape from, or failing that, adapt to, exposure at the surface. The circuits drive sustained locomotory arousal, alter sensory perception, and change metabolism. Remarkably, C. elegans has a whole array of sensory neurons that respond to CO2, including olfactory, gustatory and thermosensory neurons. We suggest that this distributed network helps the animal integrate information about CO2 levels with information about other sensory cues. CO2 gradients are ubiquitous, and the ecologically relevant information this respiratory gas communicates will depend on context and the dynamics of the CO2 stimulus. Our work in C. elegans may provide insights into how parasites use ambient O2 and CO2 levels to change their behavior and find their hosts.