Olfactory sensory neurons extend their axons solely to the olfactory bulb, which is dedicated to odor information processing. The olfactory bulb is divided into multiple layers, with different types of neurons found in each of the layers. Therefore, neurons in the olfactory bulb have conventionally been categorized based on the layers in which their cell bodies are found; namely, juxtaglomerular cells in the glomerular layer, tufted cells in the external plexiform layer, mitral cells in the mitral cell layer, and granule cells in the granule cell layer. More recently, numerous studies have revealed the heterogeneous nature of each of these cell types, allowing them to be further divided into subclasses based on differences in morphological, molecular, and electrophysiological properties. In addition, technical developments and advances have resulted in an increasing number of studies regarding cell types other than the conventionally categorized ones described above, including short-axon cells and adult-generated interneurons. Thus, the expanding diversity of cells in the olfactory bulb is now being acknowledged. However, our current understanding of olfactory bulb neuronal circuits is mostly based on the conventional and simplest classification of cell types. Few studies have taken neuronal diversity into account for understanding the function of the neuronal circuits in this region of the brain. This oversight may contribute to the roadblocks in developing more precise and accurate models of olfactory neuronal networks. The purpose of this review is therefore to discuss the expanse of existing work on neuronal diversity in the olfactory bulb up to this point, so as to provide an overall picture of the olfactory bulb circuit.

Odorant receptors (OR) are strongly implicated in coalescence of olfactory sensory neuron (OSN) axons and the formation of olfactory bulb (OB) glomeruli. However, when ORs are first expressed relative to basal cell division and OSN axon extension is unknown. We developed an in vivo fate-mapping strategy that enabled us to follow OSN maturation and axon extension beginning at basal cell division. In parallel, we mapped the molecular development of OSNs beginning at basal cell division, including the onset of OR expression. Our data show that ORs are first expressed around 4 d following basal cell division, 24 h after OSN axons have reached the OB. Over the next 6+ days the OSN axons navigate the OB nerve layer and ultimately coalesce in glomeruli. These data provide a previously unidentified perspective on the role of ORs in homophilic OSN axon adhesion and lead us to propose a new model dividing axon extension into two phases. Phase I is OR-independent and accounts for up to 50% of the time during which axons approach the OB and begin navigating the olfactory nerve layer. Phase II is OR-dependent and concludes as OSN axons coalesce in glomeruli.olfactory epithelium | axon guidance | tamoxifen | olfactory marker protein | Ascl1 I n the mouse olfactory system, olfactory sensory neurons (OSNs) extend their axons from the olfactory epithelium (OE) to the olfactory bulb (OB), where they converge to form glomeruli. Each OSN expresses only 1 of ∼2,400 candidate odorant receptor (OR) alleles. OSNs expressing the same OR can be widely dispersed in the OE, yet their axons converge in only two to three molecularly specific glomeruli of a possible 3,700 (1). It was first recognized almost 20 y ago that substituting an OR-coding region with that of a different OR resulted in the glomerular convergence of axons at an ectopic location relative to that of the native ORs (2). This led to the suggestion that ORs have an instructive role in the extension and glomerular coalescence of OSN axons, most likely mediated by homophilic fasciculation (3-5).Postnatal OSNs are derived from self-renewing precursors located proximal to the deep basal lamina of the OE (6). Following the division of globose basal stem cells, OSN neuroblasts transiently express Achaete-scute homolog 1 (Ascl1) followed by two phases of differentiation (6-8). Initially, they express growthassociated protein-43 (GAP-43), a marker of immature cells. Subsequently, the OSNs down-regulate GAP-43 and express olfactory marker protein (OMP), a universal marker of mature OSNs.Although there is a consensus on the involvement of ORs in OSN axon glomerular convergence, when ORs exert that influence following basal cell division or axon extension is not known. Moreover, the developmental progression of GAP-43 to OMP, as a measure of OSN differentiation and maturation, or of adenylate cyclase 3 (AC3), a downstream signaling molecule also implicated in axon extension, has not been considered in the context of OR expression or OSN dynamics. Here, we determined when ORs exert thei...

With advancing age, the ability of humans to detect and discriminate odors declines. In light of the rapid progress in analyzing molecular and structural correlates of developing and adult olfactory systems, the paucity of information available on the aged olfactory system is startling. A rich literature documents the decline of olfactory acuity in aged humans, but the underlying cellular and molecular mechanisms are largely unknown. Using animal models, preliminary work is beginning to uncover differences between young and aged rodents that may help address the deficits seen in humans, but many questions remain unanswered. Recent studies of odorant receptor (OR) expression, synaptic organization, adult neurogenesis, and the contribution of cortical representation during aging suggest possible underlying mechanisms and new research directions.

An odorant receptor map in mammals, that is constructed by the glomerular coalescence of sensory neuron axons in the olfactory bulb, is essential for proper odor information processing. However, how this map is linked with olfactory cortex is unknown. Here, we use a battery of methods, including various markers of cell division in combination with tracers of neuronal connections and time-lapse live imaging, to show that early- and late-generated mouse mitral cells become differentially distributed within the dorsal and ventral subdivisions of the odorant receptor map. In addition, we demonstrate that the late-generated mitral cells extend significantly stronger projections to the olfactory tubercle than the early-generated. Together, these data indicate that the odorant receptor map is developmentally linked to the olfactory cortices in part by the birthdate of mitral cells. This endows different olfactory cortical regions a role to process information from distinct regions of odorant receptor map.

Olfactory glomeruli are the loci where the first odor-representation map emerges. The glomerular layer comprises exquisite local synaptic circuits for the processing of olfactory coding patterns immediately after their emergence. To understand how an odor map is transferred from afferent terminals to postsynaptic dendrites, it is essential to directly monitor the odor-evoked glomerular postsynaptic activity patterns. Here we report the use of a transgenic mouse expressing a Ca(2+)-sensitive green fluorescence protein (GCaMP2) under a Kv3.1 potassium-channel promoter. Immunostaining revealed that GCaMP2 was specifically expressed in mitral and tufted cells and a subpopulation of juxtaglomerular cells but not in olfactory nerve terminals. Both in vitro and in vivo imaging combined with glutamate receptor pharmacology confirmed that odor maps reported by GCaMP2 were of a postsynaptic origin. These mice thus provided an unprecedented opportunity to analyze the spatial activity pattern reflecting purely postsynaptic olfactory codes. The odor-evoked GCaMP2 signal had both focal and diffuse spatial components. The focalized hot spots corresponded to individually activated glomeruli. In GCaMP2-reported postsynaptic odor maps, different odorants activated distinct but overlapping sets of glomeruli. Increasing odor concentration increased both individual glomerular response amplitude and the total number of activated glomeruli. Furthermore, the GCaMP2 response displayed a fast time course that enabled us to analyze the temporal dynamics of odor maps over consecutive sniff cycles. In summary, with cell-specific targeting of a genetically encoded Ca(2+) indicator, we have successfully isolated and characterized an intermediate level of odor representation between olfactory nerve input and principal mitral/tufted cell output.

Segregation of neuron-type-specific synaptic connections in different strata is a characteristic feature shared by the olfactory bulb (OB) and retina. In the mammalian OB, mitral cells form dendrodendritic synapses with granule cells (GCs) in the deep stratum of the external plexiform layer (EPL), whereas tufted cells form dendrodendritic synapses in the superficial stratum. In the search for membrane proteins with strata-specific expression patterns, we found that a leucine-rich repeat membrane protein (5T4 oncofetal trophoblast glycoprotein) was expressed selectively by a subset of superficial GCs. The somata of 5T4-positive GCs were localized in or near the mitral cell layer, and their apical dendrites ramified preferentially in the superficial stratum of the EPL, where tufted cell dendrites ramified. Strata-specific expression of 5T4 was found also in the retina: 5T4 was expressed selectively by rod-bipolar cells and a subset of amacrine cells whose dendrites ramified in a specific sublamina of the inner plexiform layer. During the perinatal and postnatal development of the OB, 5T4 expression paralleled in time the formation of dendrodendritic synapses in the EPL. Odor deprivation during the first postnatal month selectively reduced the thickness of the superficial stratum of the EPL and the number of 5T4-positive GCs. Because 5T4 is known to interact with actin cytoskeleton, these observations suggest that 5T4 is involved in the formation or maintenance of strata-specific dendritic ramification or synaptic connection of subsets of local interneurons.

The olfactory mucosa (OM) is exposed to environmental agents and therefore vulnerable to inflammation. To examine the effects of environmental toxin-initiated OM inflammation on the olfactory bulb (OB), we induced persistent rhinitis in mice and analyzed the spatial and temporal patterns of histopathological changes in the OM and OB. Mice received unilateral intranasal administration of lipopolysaccharide (LPS) or saline three times per week, and were immunohistologically analyzed at 1, 3, 7, 14 and 21 days after the first administration. LPS administration induced an inflammatory response in the OM, including the infiltration of Ly-6G-, CD11b-, Iba-1- and CD3-positive cells, the production of interleukin-1β by CD11b- and Iba-1-positive cells, and loss of olfactory sensory neurons (OSNs). In the OB, we observed activation of microglia and astrocytes and decreased expression of tyrosine hydroxylase in periglomerular cells, vesicular glutamate transporter 1, a presynaptic protein, in mitral and tufted projection neurons, and 5T4 in granule cells. Thus, the OM inflammation exerted a detrimental effect, not only on OSNs, but also on OB neurons, which might lead to neurodegeneration.

Odour information induces various innate responses that are critical to the survival of the individual and for the species. An axon guidance molecule, Neuropilin 2 (Nrp2), is known to mediate targeting of olfactory sensory neurons (primary neurons), to the posteroventral main olfactory bulb (PV MOB) in mice. Here we report that Nrp2-positive (Nrp2+) mitral cells (MCs, second-order neurons) play crucial roles in transmitting attractive social signals from the PV MOB to the anterior part of medial amygdala (MeA). Semaphorin 3F, a repulsive ligand to Nrp2, regulates both migration of Nrp2+ MCs to the PV MOB and their axonal projection to the anterior MeA. In the MC-specific Nrp2 knockout mice, circuit formation of Nrp2+ MCs and odour-induced attractive social responses are impaired. In utero, electroporation demonstrates that activation of the Nrp2 gene in MCs is sufficient to instruct their circuit formation from the PV MOB to the anterior MeA.