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Riddle DL, Blumenthal T, Meyer BJ, et al., editors. C. elegans II. 2nd edition. Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 1997.

Cover of C. elegans II

C. elegans II. 2nd edition.

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Section VIRegulation of Chemotaxis and Thermotaxis by Experience

C. elegans displays flexibility in many of its behavioral responses, and both chemotaxis and thermotaxis behaviors can be modified by the experience of the animal (see Jorgensen and Rankin, this volume). Thermotaxis intrinsically contains an experience-dependent component, since the preferred temperature to which C. elegans will thermotax depends on the temperature at which it was raised (Hedgecock and Russell 1975). Animals shifted from one temperature to another shift their preference to the new temperature over approximately 4 hours (Hedgecock and Russell 1975; I. Mori and Y. Ohshima, unpubl.).

Starvation is an important modulator of thermotaxis (Hedgecock and Russell 1975). Animals that have been starved for 2−4 hours avoid their cultivation temperature instead of approaching it (I. Mori and Y. Ohshima, unpubl.). These animals might have learned to avoid that adverse temperature, or they might have a general suppression of thermotaxis. To examine this response more closely, animals were cultivated at one temperature, starved for 2 hours either at the old temperature or at a new temperature, and then cultivated with food at the new temperature (I. Mori and Y. Ohshima, unpubl.). Regardless of starvation, animals that had sensed the new temperature 2 hours longer acquired a preference for the new temperature more rapidly. This result implies that acclimation to the exposed temperature occurs under all conditions, whereas starvation inhibits thermotaxis, which is the behavioral expression of the acclimation. The mechanism by which sensation of food is transmitted to the behavioral expression of temperature acclimation is unknown. Starvation also regulates chemotaxis responses. Starved animals appear to be suppressed in their responses to water-soluble attractants and some repellents but are enhanced in their attraction to some volatile attractants (H. Colbert et al., unpubl.). This regulation may allow starved animals to seek out distant food sources that release airborne chemicals.

A specific behavioral modification can occur after prolonged exposure to odorants. Animals exposed to high concentrations of an attractive odorant in the absence of food slowly lose their sensitivity to that attractant over a few hours (Colbert and Bargmann 1995). These adapted animals still respond normally to other chemical attractants, including those that are detected by the same chemosensory neurons as the adapting attractant. If the adapting odorant is removed, animals recover their sensitivity to the odorant over the course of a few hours. The mechanisms of adaptation and recovery are not well understood, but their odorant selectivity indicates that they are not due to silencing of the sensory neurons. Rather, some specific change must block the ability of the sensory neuron to direct chemotaxis to a subset of its normal ligands.

Mutations in two genes, adp-1 and osm-9 , lead to defects in adaptation to specific volatile odorants (Colbert and Bargmann 1995). Like some of the olfactory mutants described above, these genes have odorant-specific functions. Both benzaldehyde and isoamyl alcohol are sensed by the AWC olfactory neurons, but adp-1 affects only adaptation to benzaldehyde, and osm-9 affects only adaptation to isoamyl alcohol. Like olfaction itself, olfactory adaptation appears to be a complex process that is regulated separately for different olfactory molecules.

Copyright © 1997, Cold Spring Harbor Laboratory Press.
Bookshelf ID: NBK20179
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