Uncoupling stimulus specificity and glomerular position in the mouse olfactory system

Abstract

Sensory information is often mapped systematically in the brain with neighboring neurons responding to similar stimulus features. The olfactory system represents chemical information as spatial and temporal activity patterns across glomeruli in the olfactory bulb. However, the degree to which chemical features are mapped systematically in the glomerular array has remained controversial. Here, we test the hypothesis that the dual roles of odorant receptors, in axon guidance and odor detection, can serve as a mechanism to map olfactory inputs with respect to their function. We compared the relationship between response specificity and glomerular position in genetically-defined olfactory sensory neurons expressing variant odorant receptors. We find that sensory neurons with the same odor response profile can be mapped to different regions of the bulb, and that neurons with different response profiles can be mapped to the same glomeruli. Our data demonstrate that the two functions of odorant receptors can be uncoupled, indicating that the mechanisms that map olfactory sensory inputs to glomeruli do so without regard to stimulus specificity.

Introduction

In many sensory systems, information is mapped in neural space with neighboring neurons responding to similar stimulus features. Such sensory maps typically form during development in an activity dependent way, such that neuronal circuits wire with respect to their functional properties (Chklovskii and Koulakov, 2004, Knudsen et al., 1987, Luo and Flanagan, 2007). The olfactory system represents information about chemical structure. Volatile chemicals are transduced by olfactory sensory neurons (OSNs) in the nasal cavity. Each OSN expresses one member of a large family of odorant receptor (OR) genes—there are over 1000 genes in mice. OSNs that express a defined OR send axonal projections to specific glomeruli in the olfactory bulb (Mombaerts, 2006), and each glomerulus is thought to get input exclusively from OSNs expressing the same OR protein (Bozza et al., 2002, Jourdan et al., 1980, Lancet et al., 1982, Treloar et al., 2002, Wachowiak et al., 2004). The spatial organization of glomeruli in the olfactory bulb is thought to play a role in odor coding (Hildebrand and Shepherd, 1997, Mori et al., 2006, Murthy, 2011, Wilson and Mainen, 2006).

There is a long-standing controversy about whether glomeruli are organized “chemotopically” with respect to physicochemical properties of odorants (Bozza et al., 2004, Farahbod et al., 2006, Johnson et al., 2002, Ma et al., 2012, Meister and Bonhoeffer, 2001, Mori et al., 2006, Soucy et al., 2009, Takahashi et al., 2004, Wachowiak and Cohen, 2001). To complicate matters, even defining chemotopy is problematic given the diversity of chemical structures and potential features that could be mapped (Murthy, 2011). Given these difficulties, we have taken an alternate approach to understand olfactory mapping by asking whether a mechanism exists to systematically map glomeruli based on stimulus sensitivity.

In addition to their sensory function, ORs are involved in axon guidance and influence glomerular position (Mombaerts et al., 1996). As new OR genes are formed during evolution, and nascent OR coding sequences diverge by random mutation, OSNs may develop novel odorant response profiles and/or glomerular positions. Thus, the degree to which odorant sensitivity is mapped by this mechanism is dictated by the correlation between the sensory and axon guidance functions of ORs. Moreover, glomerular position is further influenced by which OSNs express a given OR (Bozza et al., 2009). The olfactory epithelium contains intermingled populations of OSNs (OSN-types) that are restricted to project to separate domains in the olfactory bulb. Random mutations in OR gene regulatory elements that shift OR expression to a new OSN-type would be expected to shift glomerular position as is seen with OR transgenes and targeted alleles (Rothman et al., 2005). Thus, it is critical to understand how changes in OR gene function correlate with resulting changes in axon guidance.

We have asked whether mutations in ORs that change response specificity necessarily change axon guidance and vice versa. We used mice in which OSNs expressing the highly homologous ORs M71 or M72 are labeled (Feinstein and Mombaerts, 2004). M71- and M72-expressing OSNs project to distinct glomeruli in the dorsal bulb, while M71/M72 hybrid mutations (in which the M71 OR is mutated to encode amino acids from M72) exhibit a wide variety of axon guidance phenotypes (Feinstein and Mombaerts, 2004). For some of the hybrids, OSNs expressing ORs with different primary amino acid sequences project to the same glomeruli (Feinstein and Mombaerts, 2004; See also Ishii et al., 2001). These observations prompted us to examine the relationship between odorant responsiveness and glomerular formation in these mutants.

Here we define the odorant response profiles of the model ORs M71, M72, and M71/M72 hybrid ORs using patch clamp recordings from genetically defined OSNs in gene targeted mice. We show for the first time that M71 and M72 have distinct odorant response profiles. A mutation in M71 that redirects axons to a new glomerulus also dramatically changes the response profile of the OR. More surprisingly, a mutation in M71 that changes the response profile does not change glomerular targeting, resulting in a glomerulus that receives functionally distinguishable inputs. Finally, we demonstrate that the same OR can be mapped to different positions in the olfactory bulb despite having the same response specificity. The data indicate that the olfactory system does not have acute mechanisms for ensuring that glomeruli are organized strictly according to response specificity.