Spatial representations of odorants in olfactory bulbs of rats and mice: Similarities and differences in chemotopic organization

Abstract

In previous studies, we mapped glomerular layer 2-deoxyglucose uptake evoked by hundreds of both systematically related and chemically distinct odorants in rat olfactory bulbs. To determine which principles of chemotopic organization revealed in these studies may be more fundamental and which may be more species-typical, we now have characterized patterns of responses to 30 of these odorants in mice. We found that only a few odorants evoked their multiple foci of peak activity in exactly the same locations in the two species. In mice, as in rats, odorants that shared molecular features evoked overlapping patterns, but the locations of the feature-responsive domains often differed in rats and mice. In rats, increasing carbon number within a homologous series of aliphatic odorants is generally associated with rostral and ventral progressions of activity within domains responding to odorant functional group and/or hydrocarbon backbone. Such chemotopic progressions were not obvious in mice, which instead showed more abrupt differences in activated glomeruli within the domains for odorants differing by a single methylene group. Despite the differences, quantitative relationships between overall uptake patterns exhibited a similar organization with respect to odorant chemistry for the two species, probably due to partial overlaps of peak domains and more extensive overlaps in large, low-activity areas for rats and mice. We conclude that clustering responses to shared odorant features may be a general strategy for odor coding, but that the specific locations of high-activity domains may be unique to a species.

Differential Associative Training Enhances Olfactory Acuity inDrosophila melanogaster

“…This is of importance as the enhanced discriminability of the two odors might have resulted from potential backward conditioning caused by the CS Ϫ after the CS ϩ /unconditioned stimulus (US) stimulation. Indeed, inhibitory learning through backward US-CS paring has been described in Drosophila and honeybees (Hellstern et al, 1998;Tanimoto et al, 2004). However, when preference indices of the two groups of animals were separated post hoc, we did not find any significant difference between those animals that had perceived the CS ϩ first and the CS Ϫ afterward and those animals that had received CS ϩ and CS Ϫ in the reverse order (1-Oct differential training CS ϩ vs CS Ϫ : t (14) ϭ 1.1, p ϭ 0.3; 1-Oct absolute training CS ϩ vs CS Ϫ : t (14) ϭ Ϫ1.6, p ϭ 0.13; 3-Oct absolute training CS ϩ vs CS Ϫ : t (14) ϭ Ϫ1.6, p ϭ 0.13; 3-Oct differential training CS ϩ vs CS Ϫ : t (14) ϭ Ϫ0.9, p ϭ 0.38; two-sample t test) (Fig.…”

“…In some instances, it is advantageous for the animal to learn to differentiate precisely a relevant stimulus from a physically similar, but irrelevant, one. Enhanced behaviorally expressed discriminability between similar odors, which is referred to as olfactory acuity (Wilson and Stevenson, 2006), can be induced by differential associative learning in the course of which a conditioned stimulus (CS ϩ ) is reinforced by punishment or reward; a second, similar stimulus (CS Ϫ ) is explicitly not reinforced (Pavlov, 1927;Hanson, 1959;Giurfa, 2004;Mishra et al, 2010). The ability to shift olfactory discrimination from generalizing across to differentiating between similar odors has been demonstrated in many species (Bitterman et al, 1983;Cleland et al, 2002;Fletcher and Wilson, 2002;Mishra et al, 2010;Chapuis and Wilson, 2011;Chen CF et al, 2011).…”

Early transformations in odor representation

“…Coarser-scale studies may utilize experimental techniques that do not resolve individual glomeruli, or may simply analyze their data broadly, e.g., by identifying regions of the MOB that tend to respond more prominently to a certain group of odorants, despite the additional presence of glomeruli within each region that exhibit dissimilar receptive fields [16, 27]. This latter approach can reveal interesting nonuniformities in the distribution of glomerular receptive fields, notably the tendency in rats (but perhaps not mice) for heavier molecules to be mapped more ventrally in the bulb [29, 30], likely owing to the physics of odorant deposition in the rat's intricate airway [31]. However, coarse-scale mapping also effectively masks the presence of inactive glomeruli in favor of nearby active glomeruli, resulting in overestimation of the breadth and clustering of odor-evoked activity.…”

Olfactory coding in five moth species from two families

“…Recent comparative studies revealed OSNs with mainly conserved receptive ranges or conserved representation patterns of odorants in the first olfactory neuropil across species, with only little impact of speciesspecific life histories. In these studies, however, only species belonging to the same family [Nymphalidae (Carlsson et al, 2011;Ômura and Honda, 2009)], subfamily [Heliothinae (Rostelien et al, 2005;Stranden et al, 2003); Murinae (Johnson et al, 2009;Soucy et al, 2009)], or genus [Drosophila (de Bruyne et al, 2010;Stensmyr et al, 2003)] were investigated. Remarkable similarities in olfactory coding were also found across the ant Camponotus fellah, the bee Apis mellifera and the rat Rattus norvegicus, i.e.…”