Butanone: the memory of a scent.

Editor's Note: These short reviews of a recent paper in the Journal, written exclusively by graduate students or postdoctoral fellows, are intended to mimic the journal clubs that exist in your own departments or institutions. For more information on the format and purpose of the Journal Club, please see Review of Torayama et al. The correlation of genes with animal behavior has always been a complicated and arguable issue. The matter is simplified in Caenorhabditis elegans with the unlimited population of clones, a fully sequenced ge-nome, a repertoire of logical and predictable behaviors, and only 302 neurons with well mapped morphologies and connections. In addition to its simplicity, this small nematode responds to a number of stimuli (temperature, smell, taste, touch) with evidence of learned behavior. Learning has been historically divided into associative and nonassociative types (Carew and Sahley, 1986). Habituation, a nonassociative paradigm, refers to a dec-rement in response after repeated presentations of a stimulus. The best example of this simple behavior in C. elegans is the " tap response. " Worms usually react to touch by changing their locomotion direction , but after repeated mechanosen-sory stimulation, the response decreases. Associative learning implies a relationship between two or more events. Stimuli sensed by C. elegans (conditioned cue), when coupled to bacteria as a source of food or to a harmful incentive (modula-tory input), gives rise to a pattern comparable to associative learning. This behavior has been observed in several situations Torayama et al. (2007) describe a novel behavioral associative plasticity, " bu-tanone enhancement. " Worms learned and remembered a spatial association of food with the volatile attractant butanone. This increased the animal attraction toward butanone above that of naive worms [Torayama et al. Ten mutants defective in this behavior were newly isolated and two of them further characterized. Olfaction in C. elegans is mediated by five pairs of ciliated amphid (head) neu-rons (Bargmann, 2006). Each expresses multiple chemosensory receptors. Two pairs of sensory neurons, AWA and AWC, are involved in attractive odor responses whereas the other three pairs mediate repulsive responses. Butanone is exclusively sensed by AWC. The AWC neuron pair differs in olfactory properties. The G-protein-coupled predicted olfactory receptor STR-2 is stochastically and asymmetrically expressed in only one of the two AWC neurons, designated as AWC ON and AWC OFF (Troemel et al., 1999). The first mutant with defects in bu-tanone enhancement, olrn-1, had a mutation in the …