09-23_Innate versus learned odour processing in the mouse olfactory bulb

09-23_Innate versus learned odour processing in the mouse olfactory bulb

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ARTICLES Innate versus learned odour processing in the mouse olfactory bulb Ko Kobayakawa 1 * , Reiko Kobayakawa 1 * , Hideyuki Matsumoto 2 , Yuichiro Oka 1 , Takeshi Imai 1 , Masahito Ikawa 3 , Masaru Okabe 3 , Toshio Ikeda 4 , Shigeyoshi Itohara 4 , Takefumi Kikusui 5 , Kensaku Mori 2 & Hitoshi Sakano 1 The mammalian olfactory system mediates various responses, including aversive behaviours to spoiled foods and fear responses to predator odours. In the olfactory bulb, each glomerulus represents a single species of odorant receptor. Because a single odorant can interact with several different receptor species, the odour information received in the olfactory epithelium is converted to a topographical map of multiple glomeruli activated in distinct areas in the olfactory bulb. To study how the odour map is interpreted in the brain, we generated mutant mice in which olfactory sensory neurons in a specific area of the olfactory epithelium are ablated by targeted expression of the diphtheria toxin gene. Here we show that, in dorsal-zone-depleted mice, the dorsal domain of the olfactory bulb was devoid of glomerular structures, although second-order neurons were present in the vacant areas. The mutant mice lacked innate responses to aversive odorants, even though they were capable of detecting them and could be conditioned for aversion with the remaining glomeruli. These results indicate that, in mice, aversive information is received in the olfactory bulb by separate sets of glomeruli, those dedicated for innate and those for learned responses. The mouse olfactory system can detect and discriminate diverse odorants using a repertoire of about 1,000 odorant receptor genes 1 . Each olfactory sensory neuron (OSN) expresses only one member of the odorant receptor gene family in a monoallelic manner 2 . Furthermore, OSNs expressing the same odorant receptor converge their axons to a specific set of glomeruli in the olfactory bulb 3 . Thus, odorous information received in the olfactory epithelium is con- verted to topographical maps of activated glomeruli. On the basis of the expression patterns of zone-specific markers 4,5 , the olfactory epithelium can be divided into two, non-overlapping areas: a dorsal zone (D zone) and a ventral zone (V zone). Vertebrate odorant receptor genes arephylogeneticallydividedinto two distinct classes 6 : class I and class II. Class I odorant receptors are expressed exclusively in the D zone of the olfactory epithelium, and OSNs expressing them project their axons to the most antero-dorsal area in the olfactory bulb 7,8 . In addition to the class I odorant recep- tors, , 300 class II odorant receptors are also expressed in the D zone, buttheircorresponding glomeruliresideon theperipheryoftheclassI area in the olfactory bulb. The remaining class II odorant receptors are expressed in the V zone and their glomeruli are found in the ventro- lateral area in the olfactory bulb 4,9 . Thus, the glomerular map seems to besubdividedintothreecompartmentsalongthedorso-ventralaxisin
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09-23_Innate versus learned odour processing in the mouse olfactory bulb

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