Decision making in C. elegans chemotaxis to alkaline pH (612.1)

Monitoring of environmental and tissue pH is crucial for animal’s survival behavior. The nematode C. elegans is an excellent model organism for analysis of neural circuits that regulate animal behavior. The animal is attracted to mildly alkaline pH, but avoids strongly alkaline pH. Here we analyzed the chemotactic behavior at molecular and cellular levels. Genetic dissection and Ca 2+ imaging demonstrated that ASEL and ASH are the major sensory neurons responsible for attraction to mildly alkaline pH and repulsion from strongly alkaline pH, respectively. In ASEL, A transmembrane guanylyl cyclase, GCY‐14, is activated by mildly alkaline pH, and in turn opens a cGMP‐gated cation channels consisting of TAX‐2 and TAX‐4. It has also been shown that a specific histidine residue plays an essential role in the extracellular alkaline pH. In ASH, TRPV channels consisting of OSM‐9 and OCR‐2 are activated by strongly alkaline pH for Ca 2+ influx into the neuron. A G‐protein α subunit, GOA‐1, is also required for its avoidance behavior perhaps through regulating synaptic vesicle exocytosis in ASH. By analyzing chemotactic behavior of various mutants, we also found that activities of ASEL and ASH compete each other for decision making or behavior switching in C. elegans chemotaxis to alkaline pH. While mildly alkaline pH preferentially activates ASEL, strongly alkaline pH activates both ASEL and ASH, and ASH activity overrides the activity of ASEL. Neural circuits responsible for this decision making will be discussed.