Electrophysiological and Behavioral Mechanisms of Caenorhabditis Elegans Feeding
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The nematode (roundworm) Caenorhabditis elegans lives in soil and eats bacteria. Its feeding organ is a neuromuscular pump called the pharynx. First, I developed a voltage clamp preparation for the pharynx and recorded native ionic current in the pharyngeal muscle. I showed that a T-type Ca channel CCA-1, an L-type Ca channel EGL-19 and a potassium channel EXP-2 shape pharyngeal action potentials. CCA-1 works in the pharyngeal muscle to boost its response to neurotransmission from the MC pharyngeal neuron. Next, EXP-2 is not an inward rectifier in the pharynx; it generates large currents upon hyperpolarization and has nearly linear voltage dependence. Finally, the pharynx adapts to the loss of MC excitatory inputs by raising its resting membrane potential, which makes it more excitable. Second, I studied food seeking and food preference behaviors in C.elegans. In the laboratory, C.elegans is routinely kept on plates seeded with E.coli, and it is not known how worms behave in an environment where diverse food is available. I identified additional food sources, such as Bacillus megaterium, Comamonas sp., and Bacillus simplex, and showed that bacterial food varies in quality. C.elegans hunts for the food of higher quality, the one that better supports growth. This seeking activity is further enhanced in animals that have already experienced good food. Next, the food regulates C.elegans locomotion, particularly the equilibrium between two locomotion modes, known as roaming and dwelling. On good food, dwelling is more common, on poor food, roaming is predominant. The normal balance between these states is essential for the food seeking behavior. In ttx-3 and osm-6 mutants the food-dependent equilibrium between locomotion states is impaired: worms tend to spend less time roaming on poor food. ttx-3 defects are partially reproduced by laser ablation of AIY interneurons, suggesting that AIY functions to inhibit the roaming-to-dwelling transition and to extend food-seeking periods. On the other hand, tax-6 mutants show increased roaming even on high quality food. tax-6, osm-6 and ttx-3 mutants are defective in food choice behavior. C.elegans may serve as a new system to uncover mechanisms that enable animals to find high quality food in diverse environments.
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