Learning to Avoid Bad Food

Science's STKE  15 Nov 2005:
Vol. 2005, Issue 310, pp. tw402
DOI: 10.1126/stke.3102005tw402

People who become nauseated after eating a particular food often associate that food with the subsequent discomfort; indeed, individuals about to undergo chemotherapy are advised to avoid their favorite foods so that their ability to enjoy those foods will not be destroyed. Zhang et al. used the nematode Caenorhabditis elegans, which ingests pathogenic and nonpathogenic bacteria, to investigate the molecular bases of this sort of aversive olfactory learning. C. elegans raised in the presence of the control bacterium Escherichia coli OP50 (OP50) showed no migratory preference for OP50 over pathogenic bacteria. However, worms grown in the presence of both OP50 and a pathogen--or exposed to a pathogen for 4 hours as an adult--strongly preferred OP50 to that pathogen. The authors used a four-choice maze assay, in which worms could choose between familiar and unfamiliar pathogens and nonpathogens, to show that aversive changes in migratory behavior appeared more rapidly than did attractive ones. Worms that lacked tryptophan hydroxylase (an enzyme required for serotonin synthesis) did not learn to avoid pathogens or to prefer nonpathogens, and worms lacking one of 12 possible serotonin receptors (MOD-1, a serotonin-gated chloride channel found in sensory interneurons) showed defects in aversive learning. Exposure to pathogens stimulated an increase in serotonin immunoreactivity in ADF chemosensory neurons, and expression of tryptophan hydroxylase in these neurons rescued aversive learning. Moreover, exposure to serotonin facilitated acquisition of the aversive response. Thus, an increase in serotonin appears to act through MOD-1 to promote aversive olfactory learning.

Y. Zhang, H. Lu, C. Bargmann, Pathogenic bacteria induce aversive olfactory learning in Caenorhabditis elegans. Nature 438, 179-184 (2005). [PubMed]