Making Scents Out of Spatial and Temporal Codes in Specialist and Generalist Olfactory Networks

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Abstract
The first-order olfactory centers in the brains of vertebrates and invertebrates are characterized by arrays of morphologically discrete glomeruli, and cross-phyletic comparisons have repeatedly found striking similarities in glomerular organization across evolutionarily remote animals. A growing list of studies shows that the vertebrate olfactory bulb (OB) and insect antennal lobe (AL) are organized chemotopically, and an individual glomerulus reflects the odor-response profile of the olfactory receptor neurons (ORNs) that converge on it (Bozza and Kauer, 1998; Belluscio et al., 2002; Ng et al., 2002; Wang et al., 2003). While these different studies using a wide range of methods have supported glomerular chemotopy in diverse species, many fundamental questions about the neural circuitry underlying these activity patterns remain. Evidence is increasing, in fact, that the molecular receptive range (MRR) or ‘odor tuning’ of a glomerulus is also shaped by interglomerular—and particularly inhibitory—interactions (Christensen et al., 1998; Lei et al., 2002; Sachse and Galizia, 2003; Aungst et al., 2003; Nagayama et al., 2004), but the cellular and synaptic mechanisms underlying this modulation remain poorly understood.