Target specific functions of EPL interneurons in olfactory circuits

Orbitofrontal control of the olfactory cortex regulates olfactory discrimination learning

Serving as an integral node for cognitive processing and value-based decision-making, the orbitofrontal cortex (OFC) plays a multifaceted role in associative learning and reward-driven behaviours through its widespread synaptic integration with both subcortical structures and sensory cortices. Despite the OFC's robust innervation of the olfactory cortex, the functional implications and underlying mechanisms of this top-down influence remain largely unexplored. In this study, we demonstrated that the OFC formed both direct excitatory and indirect inhibitory synaptic connections with pyramidal neurons in the anterior piriform cortex (aPC). OFC projection predominantly regulated spontaneous and odour-evoked excitatory activity in the aPC of awake mice. Importantly, suppression of this OFC-aPC projection disrupted olfactory discrimination learning, potentially due to a consequent decrease in the excitability of aPC principal output neurons following inhibition of this projection. Whole-cell recordings revealed that olfactory learning increased the intrinsic excitability of aPC neurons while concurrently decreasing OFC input to these neurons. These findings underscore the pivotal influence of orbitofrontal modulation over the olfactory cortex in the context of olfactory learning and provide insight into the associated neurophysiological mechanisms. KEY POINTS: The orbitofrontal cortex (OFC) densely innervates the anterior piriform cortex (aPC) through direct excitatory synaptic connections. The OFC regulates both spontaneous and odour-evoked excitatory activities in the aPC of awake mice. Inhibition of OFC projections disrupts olfactory discrimination learning, probably due to reduced excitability of aPC main output neurons. Following olfactory learning, the intrinsic excitability of aPC neurons increases while the OFC-aPC input decreases, highlighting the importance of adaptable OFC input for olfactory learning. These results provide new perspectives on how the OFC's top-down control modulates sensory integration and associative learning.

ErbB4 deficiency exacerbates olfactory dysfunction in an early-stage Alzheimer's disease mouse model

Olfactory dysfunction is increasingly recognized as an early indicator of Alzheimer's disease (AD). Aberrations in GABAergic function and the excitatory/inhibitory (E/I) balance within the olfactory bulb (OB) have been implicated in olfactory impairment during the initial stages of AD. While the neuregulin 1 (NRG1)/ErbB4 signaling pathway is known to regulate GABAergic transmission in the brain and is associated with various neuropsychiatric disorders, its specific role in early AD-related olfactory impairment remains incompletely understood. This study demonstrated that olfactory dysfunction preceded cognitive decline in young adult APP/PS1 mice and was characterized by reduced levels of NRG1 and ErbB4 in the OB. Further investigation revealed that deletion of ErbB4 in parvalbumin interneurons reduced GABAergic transmission and increased hyperexcitability in mitral and tufted cells (M/Ts) in the OB, thereby accelerating olfactory dysfunction in young adult APP/PS1 mice. Additionally, ErbB4 deficiency was associated with increased accumulation of Aβ and BACE1-mediated cleavage of APP, along with enhanced CDK5 signaling in the OB. NRG1 infusion into the OB was found to enhance GABAergic transmission in M/Ts and alleviate olfactory dysfunction in young adult APP/PS1 mice. These findings underscore the critical role of NRG1/ErbB4 signaling in regulating GABAergic transmission and E/I balance within the OB, contributing to olfactory impairment in young adult APP/PS1 mice, and provide novel insights for early intervention strategies in AD. This work has shown that ErbB4 deficiency increased the burden of Aβ, impaired GABAergic transmission, and disrupted the E/I balance of mitral and tufted cells (M/Ts) in the OB, ultimately resulting in olfactory dysfunction in young adult APP/PS1 mice. NRG1 could enhance GABAergic transmission, rescue E/I imbalance in M/Ts, and alleviate olfactory dysfunction in young adult APP/PS1 mice. OB: olfactory bulb, E/I: excitation/inhibition, Pr: probability of release, PV: parvalbumin interneurons, Aβ: β-amyloid, GABA: gamma-aminobutyric acid.

Decorated-Induced Oxygen Vacancy Engineering for Ultra-Low Concentration Nonanal Sensing: A Case Study of La-Decorated Bi2O2CO3

La-decorated Bi2O2CO3 (BCO-La) microspheres are synthesized using a facile wet chemical strategy for sensing low-concentration nonanal (C9H18O) at room temperature. These BCO-La gas sensors are applied to evaluate agricultural product quality, specifically for cooked rice. The sensitivity of the BCO-6La sensor significantly surpassed that of the pure BCO sensor, achieving a response value of 174.6 when detecting 30 ppm nonanal gas. Notably, the BCO-6La sensor demonstrated a faster response time (36 s) when exposed to 18 ppm of nonanal. Additionally, the selectivity toward nonanal gas detection is higher (approximately 4-24 times) compared to interfering gases (1-octanol, geranyl acetone, linalool, hexanal, 2-pentyfuran, and 1-octen-3-ol) during cooked rice quality detection. The gas sensing mechanism and the factors contributing to the enhanced sensing performance of the BCO-La microspheres are demonstrated through in situ FT-IR spectra and DFT analysis while the realistic detection scenario is carried out. In a broader context, the reported sensors here represent a novel platform for the detection and monitoring of gases released by agricultural products during storage.

Beyond anosmia: olfactory dysfunction as a common denominator in neurodegenerative and neurodevelopmental disorders

Olfactory dysfunction has emerged as a hallmark feature shared among several neurological conditions, including both neurodevelopmental and neurodegenerative disorders. While diseases of both categories have been extensively studied for decades, their association with olfaction has only recently gained attention. Olfactory deficits often manifest already during prodromal stages of these diseases, yet it remains unclear whether common pathophysiological changes along olfactory pathways cause such impairments. Here we probe into the intricate relationship between olfactory dysfunction and neurodegenerative and neurodevelopmental disorders, shedding light on their commonalities and underlying mechanisms. We begin by providing a brief overview of the olfactory circuit and its connections to higher-associated brain areas. Additionally, we discuss olfactory deficits in these disorders, focusing on potential common mechanisms that may contribute to olfactory dysfunction across both types of disorders. We further debate whether olfactory deficits contribute to the disease propagation or are simply an epiphenomenon. We conclude by emphasizing the significance of olfactory function as a potential pre-clinical diagnostic tool to identify individuals with neurological disorders that offers the opportunity for preventive intervention before other symptoms manifest.

Dense and Persistent Odor Representations in the Olfactory Bulb of Awake Mice

Recording and analysis of neural activity are often biased toward detecting sparse subsets of highly active neurons, masking important signals carried in low-magnitude and variable responses. To investigate the contribution of seemingly noisy activity to odor encoding, we used mesoscale calcium imaging from mice of both sexes to record odor responses from the dorsal surface of bilateral olfactory bulbs (OBs). The outer layer of the mouse OB is comprised of dendrites organized into discrete "glomeruli," which are defined by odor receptor-specific sensory neuron input. We extracted activity from a large population of glomeruli and used logistic regression to classify odors from individual trials with high accuracy. We then used add-in and dropout analyses to determine subsets of glomeruli necessary and sufficient for odor classification. Classifiers successfully predicted odor identity even after excluding sparse, highly active glomeruli, indicating that odor information is redundantly represented across a large population of glomeruli. Additionally, we found that random forest (RF) feature selection informed by Gini inequality (RF Gini impurity, RFGI) reliably ranked glomeruli by their contribution to overall odor classification. RFGI provided a measure of "feature importance" for each glomerulus that correlated with intuitive features like response magnitude. Finally, in agreement with previous work, we found that odor information persists in glomerular activity after the odor offset. Together, our findings support a model of OB odor coding where sparse activity is sufficient for odor identification, but information is widely, redundantly available across a large population of glomeruli, with each glomerulus representing information about more than one odor.

Room-Temperature Sensing Mechanism of GQDs/BiSbO4 Nanorod Clusters: Experimental and Density Functional Theory Study

Creating high-performance gas sensors for heptanal detection at room temperature demands the development of sensing materials that incorporate distinct spatial configurations, functional components, and active surfaces. In this study, we employed a straightforward method combining hydrothermal strategy with ultrasonic processing to produce mesoporous graphene quantum dots/bismuth antimonate (GQDs/BiSbO4) with nanorod cluster forms. The BiSbO4 was incorporated with appropriate contents of GQDs resulting in significantly improved attributes such as heightened sensitivity (59.6@30 ppm), a lower threshold for detection (356 ppb), and quicker period for response (40 s). A synergistic mechanism that leverages the inherent advantages of BiSbO4 was proposed, while its distinctive mesoporous hollow cubic structure, the presence of oxygen vacancies, and the catalytic enhancement provided by GQDs lead to a marked improvement in heptanal detection. This work introduces a straightforward and effective method for crafting sophisticated micro-nanostructures that optimize spatial design, functionality, and active mesoporous surfaces, showing great promise for heptanal sensing applications.

Co-transmitting interneurons in the mouse olfactory bulb regulate olfactory detection and discrimination

Co-transmission of multiple neurotransmitters from a single neuron increases the complexity of signaling information within defined neuronal circuits. Superficial short-axon cells in the olfactory bulb release both dopamine and γ-aminobutyric acid (GABA), yet the specific targets of these neurotransmitters and their respective roles in olfaction have remained unknown. Here, we implement intersectional genetics in mice to selectively block GABA or dopamine release from superficial short-axon cells to identify their distinct cellular targets, impact on circuit function, and behavioral contribution of each neurotransmitter toward olfactory behaviors. We provide functional and anatomical evidence for divergent superficial short-axon cell signaling onto downstream neurons to shape patterns of mitral cell firing that contribute to olfactory-related behaviors.

An ErbB4-Positive Neuronal Network in the Olfactory Bulb for Olfaction

Olfactory information is relayed and processed in the olfactory bulb (OB). Mitral cells, the principal output excitatory neurons of the OB, are controlled by multiple types of interneurons. However, mechanisms that regulate the activity of OB interneurons are not well understood. We provide evidence that the transmembrane tyrosine kinase ErbB4 is selectively expressed in subsets of OB inhibitory neurons in both male and female mice. ErbB4-positive (ErbB4+) neurons are mainly located in the glomerular layer (GL) and granule cell layer (GCL) and do not express previously defined markers. Optogenetic activation of GL-ErbB4+ neurons promotes theta oscillation, whereas activation of those in the GCL generates γ oscillations. Stimulation of OB slices with NRG1, a ligand that activates ErbB4, increases GABA transmission onto mitral cells, suggesting a role of OB NRG1-ErbB4 signaling in olfaction. In accord, ErbB4 mutant mice or acute inhibition of ErbB4 by a chemical genetic approach diminishes GABA transmission, reduces bulbar local field potential power, increases the threshold of olfactory sensitivity, and impairs odor discrimination. Together, these results identified a bulbar inhibitory network of ErbB4+ neurons for olfaction. Considering that both Nrg1 and Erbb4 are susceptibility genes for neuropsychiatric disorders, our study provides insight into pathologic mechanisms of olfactory malfunctions in these disorders.SIGNIFICANCE STATEMENT This study demonstrates that ErbB4+ neurons are a new subset of olfactory bulb inhibitory neurons in the glomerular layer and granule cell layer that innervate mitral cells and ErbB4- cells. They regulate olfaction by controlling local synchrony and distinct oscillations. ErbB4 inhibition diminishes GABA transmission, reduces bulbar local field potential power, increases the threshold of olfactory sensitivity, and impairs odor discrimination. Our results provide insight into pathophysiological mechanism of olfaction deficits in brain disorders associated with Nrg1 or Erbb4 mutations.

Anatomical and Functional Connectivity at the Dendrodendritic Reciprocal Mitral Cell-Granule Cell Synapse: Impact on Recurrent and Lateral Inhibition

In the vertebrate olfactory bulb, reciprocal dendrodendritic interactions between its principal neurons, the mitral and tufted cells, and inhibitory interneurons in the external plexiform layer mediate both recurrent and lateral inhibition, with the most numerous of these interneurons being granule cells. Here, we used recently established anatomical parameters and functional data on unitary synaptic transmission to simulate the strength of recurrent inhibition of mitral cells specifically from the reciprocal spines of rat olfactory bulb granule cells in a quantitative manner. Our functional data allowed us to derive a unitary synaptic conductance on the order of 0.2 nS. The simulations predicted that somatic voltage deflections by even proximal individual granule cell inputs are below the detection threshold and that attenuation with distance is roughly linear, with a passive length constant of 650 μm. However, since recurrent inhibition in the wake of a mitral cell action potential will originate from hundreds of reciprocal spines, the summated recurrent IPSP will be much larger, even though there will be substantial mutual shunting across the many inputs. Next, we updated and refined a preexisting model of connectivity within the entire rat olfactory bulb, first between pairs of mitral and granule cells, to estimate the likelihood and impact of recurrent inhibition depending on the distance between cells. Moreover, to characterize the substrate of lateral inhibition, we estimated the connectivity via granule cells between any two mitral cells or all the mitral cells that belong to a functional glomerular ensemble (i.e., which receive their input from the same glomerulus), again as a function of the distance between mitral cells and/or entire glomerular mitral cell ensembles. Our results predict the extent of the three regimes of anatomical connectivity between glomerular ensembles: high connectivity within a glomerular ensemble and across the first four rings of adjacent glomeruli, substantial connectivity to up to eleven glomeruli away, and negligible connectivity beyond. Finally, in a first attempt to estimate the functional strength of granule-cell mediated lateral inhibition, we combined this anatomical estimate with our above simulation results on attenuation with distance, resulting in slightly narrowed regimes of a functional impact compared to the anatomical connectivity.

Pathological consequences of chronic olfactory inflammation on neurite morphology of olfactory bulb projection neurons

Chronic olfactory inflammation (COI) in conditions such as chronic rhinosinusitis significantly impairs the functional and anatomical components of the olfactory system. COI induced by intranasal administration of lipopolysaccharide (LPS) results in atrophy, gliosis, and pro-inflammatory cytokine production in the olfactory bulb (OB). Although chronic rhinosinusitis patients have smaller OBs, the consequences of olfactory inflammation on OB neurons are largely unknown. In this study, we investigated the neurological consequences of COI on OB projection neurons, mitral cells (MCs) and tufted cells (TCs). To induce COI, we performed unilateral intranasal administration of LPS to mice for 4 and 10 weeks. Effects of COI on the OB were examined using RNA-sequencing approaches and immunohistochemical analyses. We found that repeated LPS administration upregulated immune-related biological pathways in the OB after 4 weeks. We also determined that the length of TC lateral dendrites in the OB significantly decreased after 10 weeks of COI. The axon initial segment of TCs decreased in number and in length after 10 weeks of COI. The lateral dendrites and axon initial segments of MCs, however, were largely unaffected. In addition, dendritic arborization and AIS reconstruction both took place following a 10-week recovery period. Our findings suggest that olfactory inflammation specifically affects TCs and their integrated circuitry, whereas MCs are potentially protected from this condition. This data demonstrates unique characteristics of the OBs ability to undergo neuroplastic changes in response to stress.

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Early-stage chronic olfactory inflammation activates the ​interferon-γ-driven inflammatory pathways in the olfactory bulb.

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Tufted cells undergo neurite dysregulation in response to chronic olfactory inflammation.

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Mitral cells and interneurons in the external plexiform layer are largely unaffected by chronic olfactory inflammation.

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Tufted cells experience complete recovery from neurite dysregulation following a period of ceased inflammation

VIP interneurons regulate olfactory bulb output and contribute to odor detection and discrimination

In the olfactory bulb (OB), olfactory information represented by mitral/tufted cells (M/Ts) is extensively modulated by local inhibitory interneurons before being transmitted to the olfactory cortex. While the crucial roles of cortical vasoactive-intestinal-peptide-expressing (VIP) interneurons have been extensively studied, their precise function in the OB remains elusive. Here, we identify the synaptic connectivity of VIP interneurons onto mitral cells (MCs) and demonstrate their important role in olfactory behaviors. Optogenetic activation of VIP interneurons reduced both spontaneous and odor-evoked activity of M/Ts in awake mice. Whole-cell recordings revealed that VIP interneurons decrease MC firing through direct inhibitory synaptic connections with MCs. Furthermore, inactivation of VIP interneurons leads to increased MC firing and impaired olfactory detection and odor discrimination. Therefore, our results demonstrate that VIP interneurons control OB output and play critical roles in odor processing and olfactory behaviors.

Cell and circuit origins of fast network oscillations in the mammalian main olfactory bulb

Neural synchrony generates fast network oscillations throughout the brain, including the main olfactory bulb (MOB), the first processing station of the olfactory system. Identifying the mechanisms synchronizing neurons in the MOB will be key to understanding how network oscillations support the coding of a high-dimensional sensory space. Here, using paired recordings and optogenetic activation of glomerular sensory inputs in MOB slices, we uncovered profound differences in principal mitral cell (MC) vs. tufted cell (TC) spike-time synchrony: TCs robustly synchronized across fast- and slow-gamma frequencies, while MC synchrony was weaker and concentrated in slow-gamma frequencies. Synchrony among both cell types was enhanced by shared glomerular input but was independent of intraglomerular lateral excitation. Cell-type differences in synchrony could also not be traced to any difference in the synchronization of synaptic inhibition. Instead, greater TC than MC synchrony paralleled the more periodic firing among resonant TCs than MCs and emerged in patterns consistent with densely synchronous network oscillations. Collectively, our results thus reveal a mechanism for parallel processing of sensory information in the MOB via differential TC vs. MC synchrony, and further contrast mechanisms driving fast network oscillations in the MOB from those driving the sparse synchronization of irregularly firing principal cells throughout cortex.

Olfactory Optogenetics: Light Illuminates the Chemical Sensing Mechanisms of Biological Olfactory Systems

The mammalian olfactory system has an amazing ability to distinguish thousands of odorant molecules at the trace level. Scientists have made great achievements on revealing the olfactory sensing mechanisms in decades; even though many issues need addressing. Optogenetics provides a novel technical approach to solve this dilemma by utilizing light to illuminate specific part of the olfactory system; which can be used in all corners of the olfactory system for revealing the olfactory mechanism. This article reviews the most recent advances in olfactory optogenetics devoted to elucidate the mechanisms of chemical sensing. It thus attempts to introduce olfactory optogenetics according to the structure of the olfactory system. It mainly includes the following aspects: the sensory input from the olfactory epithelium to the olfactory bulb; the influences of the olfactory bulb (OB) neuron activity patterns on olfactory perception; the regulation between the olfactory cortex and the olfactory bulb; and the neuromodulation participating in odor coding by dominating the olfactory bulb. Finally; current challenges and future development trends of olfactory optogenetics are proposed and discussed.

Timely Inhibitory Circuit Formation Controlled by Abl1 Regulates Innate Olfactory Behaviors in Mouse

More than one-half of the interneurons in a mouse olfactory bulb (OB) develop during the first week after birth and predominantly connect to excitatory tufted cells near the superficial granule cell layer (sGCL), unlike late-born interneurons. However, the molecular mechanisms underlying the temporal specification are yet to be identified. In this study, we determined the role of Abelson tyrosine-protein kinase 1 (Abl1) in the temporal development of early-born OB interneurons. Lentiviral knockdown of Abl1 disrupts the sGCL circuit of early-born interneurons through defects in function and circuit integration, resulting in olfactory hyper-sensitivity. We show that doublecortin (Dcx) is phosphorylated by Abl1, which contributes to the stabilization of Dcx, thereby regulating microtubule dynamics. Finally, Dcx overexpression rescues Abl1 knockdown-induced anatomic or functional defects. In summary, specific signaling by Abl1-Dcx in early-born interneurons facilitates the temporal development of the sGCL circuit to regulate innate olfactory functions, such as detection and sensitivity.

Olfaction in Lamprey Pallium Revisited-Dual Projections of Mitral and Tufted Cells

The presence of two separate afferent channels from the olfactory glomeruli to different targets in the brain is unravelled in the lamprey. The mitral-like cells send axonal projections directly to the piriform cortex in the ventral part of pallium, whereas the smaller tufted-like cells project separately and exclusively to a relay nucleus called the dorsomedial telencephalic nucleus (dmtn). This nucleus, located at the interface between the olfactory bulb and pallium, in turn projects to a circumscribed area in the anteromedial, ventral part of pallium. The tufted-like cells are activated with short latency from the olfactory nerve and terminate with mossy fibers on the dmtn cells, wherein they elicit large unitary excitatory postsynaptic potentials (EPSPs). In all synapses along this tufted-like cell pathway, there is no concurrent inhibition, in contrast to the mitral-like cell pathway. This is similar to recent findings in rodents establishing two separate exclusive projection patterns, suggesting an evolutionarily conserved organization.

Subpopulations of Projection Neurons in the Olfactory Bulb

Generation of neuronal diversity is a biological strategy widely used in the brain to process complex information. The olfactory bulb is the first relay station of olfactory information in the vertebrate central nervous system. In the olfactory bulb, axons of the olfactory sensory neurons form synapses with dendrites of projection neurons that transmit the olfactory information to the olfactory cortex. Historically, the olfactory bulb projection neurons have been classified into two populations, mitral cells and tufted cells. The somata of these cells are distinctly segregated within the layers of the olfactory bulb; the mitral cells are located in the mitral cell layer while the tufted cells are found in the external plexiform layer. Although mitral and tufted cells share many morphological, biophysical, and molecular characteristics, they differ in soma size, projection patterns of their dendrites and axons, and odor responses. In addition, tufted cells are further subclassified based on the relative depth of their somata location in the external plexiform layer. Evidence suggests that different types of tufted cells have distinct cellular properties and play different roles in olfactory information processing. Therefore, mitral and different types of tufted cells are considered as starting points for parallel pathways of olfactory information processing in the brain. Moreover, recent studies suggest that mitral cells also consist of heterogeneous subpopulations with different cellular properties despite the fact that the mitral cell layer is a single-cell layer. In this review, we first compare the morphology of projection neurons in the olfactory bulb of different vertebrate species. Next, we explore the similarities and differences among subpopulations of projection neurons in the rodent olfactory bulb. We also discuss the timing of neurogenesis as a factor for the generation of projection neuron heterogeneity in the olfactory bulb. Knowledge about the subpopulations of olfactory bulb projection neurons will contribute to a better understanding of the complex olfactory information processing in higher brain regions.

Paradoxically Sparse Chemosensory Tuning in Broadly Integrating External Granule Cells in the Mouse Accessory Olfactory Bulb

The accessory olfactory bulb (AOB), the first neural circuit in the mouse accessory olfactory system, is critical for interpreting social chemosignals. Despite its importance, AOB information processing is poorly understood compared with the main olfactory bulb (MOB). Here, we sought to fill gaps in the understanding of AOB interneuron function. We used 2-photon GCaMP6f Ca2+ imaging in an ex vivo preparation to study chemosensory tuning in AOB external granule cells (EGCs), interneurons hypothesized to broadly inhibit activity in excitatory mitral cells (MCs). In ex vivo preparations from mice of both sexes, we measured MC and EGC tuning to natural chemosignal blends and monomolecular ligands, finding that EGC tuning was sparser, not broader, than upstream MCs. Simultaneous electrophysiological recording and Ca2+ imaging showed no differences in GCaMP6f-to-spiking relationships in these cell types during simulated sensory stimulation, suggesting that measured EGC sparseness was not due to cell type-dependent variability in GCaMP6f performance. Ex vivo patch-clamp recordings revealed that EGC subthreshold responsivity was far broader than indicated by GCaMP6f Ca2+ imaging, and that monomolecular ligands rarely elicited EGC spiking. These results indicate that EGCs are selectively engaged by chemosensory blends, suggesting different roles for EGCs than analogous interneurons in the MOB.SIGNIFICANCE STATEMENT The mouse accessory olfactory system (AOS) interprets social chemosignals, but we poorly understand AOS information processing. Here, we investigate the functional properties of external granule cells (EGCs), a major class of interneurons in the accessory olfactory bulb (AOB). We hypothesized that EGCs, which are densely innervated by excitatory mitral cells (MCs), would show broad chemosensory tuning, suggesting a role in divisive normalization. Using ex vivo GCaMP6f imaging, we found that EGCs were instead more sparsely tuned than MCs. This was not due to weaker GCaMP6f signaling in EGCs than in MCs. Instead, we found that many MC-activating chemosignals caused only subthreshold EGC responses. This indicates a different role for AOB EGCs compared with analogous cells in the main olfactory bulb.

Distinct Developmental Features of Olfactory Bulb Interneurons

The olfactory bulb (OB) has an extremely higher proportionof interneurons innervating excitatory neurons than otherbrain regions, which is evolutionally conserved across species.Despite the abundance of OB interneurons, little is knownabout the diversification and physiological functions ofOB interneurons compared to cortical interneurons. In thisreview, an overview of the general developmental processof interneurons from the angles of the spatial and temporalspecifications was presented. Then, the distinct featuresshown exclusively in OB interneurons development andmolecular machinery recently identified were discussed.Finally, we proposed an evolutionary meaning for thediversity of OB interneurons.