Action of the noradrenergic system on adult-born cells is required for olfactory learning in mice

Linking Adult Olfactory Neurogenesis to Social Reproductive Stimuli: Mechanisms and Functions

Over the last three decades, adult neurogenesis in mammals has been a central focus of neurobiological research, providing insights into brain plasticity and function. However, interest in this field has recently waned due to challenges in translating findings into regenerative applications and the ongoing debate about the persistence of this phenomenon in the adult human brain. Despite these hurdles, significant progress has been made in understanding how adult neurogenesis plays a critical role in the adaptation of brain circuits to environmental stimuli regulating key brain functions. This review focuses on the role of olfactory neurogenesis in the brain's response to social reproductive cues in rodents, highlighting its influence on animal behaviors critical for survival. We also address open questions and propose future directions to advance our understanding of the relationship between adult neurogenesis and reproductive function regulation.

Olfactory neurogenesis plays different parts at successive stages of life, implications for mental health

The olfactory bulb is a unique site of continuous neurogenesis, primarily generating inhibitory interneurons, a process that begins at birth and extends through infancy and adulthood. This review examines the characteristics of olfactory bulb neurogenesis, focusing on granule cells, the most numerous interneurons, and how their age and maturation affect their function. Adult-born granule cells, while immature, contribute to the experience-dependent plasticity of the olfactory circuit by enabling structural and functional synaptic changes. In contrast, granule cells born early in life form the foundational elements of the olfactory bulb circuit, potentially facilitating innate olfactory information processing. The implications of these neonatal cells on early life olfactory memory and their impact on adult perception, particularly in response to aversive events and susceptibility to emotional disorders, warrant further investigation.

Age-related differences in perception and coding of attractive odorants in mice

Hedonic perception deeply changes with aging, significantly impacting health and quality of life in elderly. In young adult mice, an odor hedonic signature is represented along the antero-posterior axis of olfactory bulb, and transferred to the olfactory tubercle and ventral tegmental area, promoting approach behavior. Here, we show that while the perception of unattractive odorants was unchanged in older mice (22 months), the appreciation of some but not all attractive odorants declined. Neural activity in the olfactory bulb and tubercle of older mice was consistently altered when attraction to pleasant odorants was impaired while maintained when the odorants kept their attractivity. Finally, in a self-stimulation paradigm, optogenetic stimulation of the olfactory bulb remained rewarding in older mice even without ventral tegmental area's response to the stimulation. Aging degrades behavioral and neural responses to some pleasant odorants but rewarding properties of olfactory bulb stimulation persisted, providing new insights into developing novel olfactory training strategies to elicit motivation even when the dopaminergic system is altered as observed in normal and/or neurodegenerative aging.

Long-term olfactory enrichment promotes non-olfactory cognition, noradrenergic plasticity and remodeling of brain functional connectivity in older mice

Brain functional and structural changes lead to cognitive decline during aging, but a high level of cognitive stimulation during life can improve cognitive performances in the older adults, forming the cognitive reserve. Noradrenaline has been proposed as a molecular link between environmental stimulation and constitution of the cognitive reserve. Taking advantage of the ability of olfactory stimulation to activate noradrenergic neurons of the locus coeruleus, we used repeated olfactory enrichment sessions over the mouse lifespan to enable the cognitive reserve buildup. Mice submitted to olfactory enrichment, whether started in early or late adulthood, displayed improved olfactory discrimination at late ages and interestingly, improved spatial memory and cognitive flexibility. Moreover, olfactory and non-olfactory cognitive performances correlated with increased noradrenergic innervation in the olfactory bulb and dorsal hippocampus. Finally, c-Fos mapping and connectivity analysis revealed task-specific remodeling of functional neural networks in enriched older mice. Long-term olfactory enrichment thus triggers structural noradrenergic plasticity and network remodeling associated with better cognitive aging and thereby forms a promising mouse model of the cognitive reserve buildup.

α2-Adrenergic modulation of Ih in adult-born granule cells in the olfactory bulb

In the olfactory bulb (OB), a large population of axon-less inhibitory interneurons, the granule cells (GCs), coordinate network activity and tune the output of principal neurons, the mitral and tufted cells (MCs), through dendrodendritic interactions. Furthermore, GCs undergo neurogenesis throughout life, providing a source of plasticity to the neural network of the OB. The function and integration of GCs in the OB are regulated by several afferent neuromodulatory signals, including noradrenaline (NA), a state-dependent neuromodulator that plays a crucial role in the regulation of cortical function and task-specific decision processes. However, the mechanisms by which NA regulates GC function are not fully understood. Here, we show that NA modulates hyperpolarization-activated currents (I_h) via the activation of α₂-adrenergic receptors (ARs) in adult-born GCs (abGCs), thus directly acting on channels that play essential roles in regulating neuronal excitability and network oscillations in the brain. This modulation affects the dendrodendritic output of GCs leading to an enhancement of lateral inhibition onto the MCs. Furthermore, we show that NA modulates subthreshold resonance in GCs, which could affect the temporal integration of abGCs. Together, these results provide a novel mechanism by which a state-dependent neuromodulator acting on I_h can regulate GC function in the OB.

Why Would the Brain Need Dormant Neuronal Precursors?

Dormant non-proliferative neuronal precursors (dormant precursors) are a unique type of undifferentiated neuron, found in the adult brain of several mammalian species, including humans. Dormant precursors are fundamentally different from canonical neurogenic-niche progenitors as they are generated exquisitely during the embryonic development and maintain a state of protracted postmitotic immaturity lasting up to several decades after birth. Thus, dormant precursors are not pluripotent progenitors, but to all effects extremely immature neurons. Recently, transgenic models allowed to reveal that with age virtually all dormant precursors progressively awaken, abandon the immature state, and become fully functional neurons. Despite the limited common awareness about these cells, the deep implications of recent discoveries will likely lead to revisit our understanding of the adult brain. Thus, it is timely to revisit and critically assess the essential evidences that help pondering on the possible role(s) of these cells in relation to cognition, aging, and pathology. By highlighting pivoting findings as well as controversies and open questions, we offer an exciting perspective over the field of research that studies these mysterious cells and suggest the next steps toward the answer of a crucial question: why does the brain need dormant neuronal precursors?

The Onset of Maternal Behavior in Sheep and Goats: Endocrine, Sensory, Neural, and Experiential Mechanisms

In sheep and goats, the onset of maternal behavior at parturition is characterized by a first phase called maternal responsiveness during which the mother is attracted to any newborn. In a second phase, called maternal selectivity, the mother establishes a selective bond with her young so that she only accepts it at suckling. After a description of the behavioral expression of both phases, this chapter reviews the physiological, sensory, and neural mechanisms involved. These two behavioral processes are synchronized with parturition by the vaginocervical stimulation induced by the expulsion of the newborn. Olfactory cues provided by the neonate are involved in maternal responsiveness and selectivity. Oxytocin supported by estrogens is the key factor for maternal responsiveness. The neural network involved in maternal responsiveness is mainly hypothalamic and is different from the circuitry involved in selectivity, which mainly concerns olfactory processing regions. Visual and auditory cues are necessary for offspring recognition at a distance. This multisensory recognition suggests that mothers form a mental image of their young. Maternal experience renders mothers more responsive to maternally relevant physiology and to young-related sensory inputs.

Role of Adult-Born Versus Preexisting Neurons Born at P0 in Olfactory Perception in a Complex Olfactory Environment in Mice

Olfactory perceptual learning is defined as an improvement in the discrimination of perceptually close odorants after passive exposure to these odorants. In mice, simple olfactory perceptual learning involving the discrimination of two odorants depends on an increased number of adult-born neurons in the olfactory bulb, which refines the bulbar output. However, the olfactory environment is complex, raising the question of the adjustment of the bulbar network to multiple discrimination challenges. Perceptual learning of 1 to 6 pairs of similar odorants led to discrimination of all learned odor pairs. Increasing complexity did not increase adult-born neuron survival but enhanced the number of adult-born neurons responding to learned odorants and their spine density. Moreover, only complex learning induced morphological changes in neurons of the granule cell layer born during the first day of life (P0). Selective optogenetic inactivation of either population confirmed functional involvement of adult-born neurons regardless of the enrichment complexity, while preexisting neurons were required for complex discrimination only.

Neural processing of the reward value of pleasant odorants

Pleasant odorants are represented in the posterior olfactory bulb (pOB) in mice. How does this hedonic information generate odor-motivated behaviors? Using optogenetics, we report here that stimulating the representation of pleasant odorants in a sensory structure, the pOB, can be rewarding, self-motivating, and is accompanied by ventral tegmental area activation. To explore the underlying neural circuitry downstream of the olfactory bulb (OB), we use 3D high-resolution imaging and optogenetics and determine that the pOB preferentially projects to the olfactory tubercle, whose increased activity is related to odorant attraction. We further show that attractive odorants act as reinforcers in dopamine-dependent place preference learning. Finally, we extend those findings to humans, who exhibit place preference learning and an increase BOLD signal in the olfactory tubercle in response to attractive odorants. Thus, strong and persistent attraction induced by some odorants is due to a direct gateway from the pOB to the reward system.

HPG-Dependent Peri-Pubertal Regulation of Adult Neurogenesis in Mice

Adult neurogenesis, a striking form of neural plasticity, is involved in the modulation of social stimuli driving reproduction. Previous studies on adult neurogenesis have shown that this process is significantly modulated around puberty in female mice. Puberty is a critical developmental period triggered by increased secretion of the gonadotropin releasing hormone (GnRH), which controls the activity of the hypothalamic-pituitary-gonadal axis (HPG). Secretion of HPG-axis factors at puberty participates to the refinement of neural circuits that govern reproduction. Here, by exploiting a transgenic GnRH deficient mouse model, that progressively loses GnRH expression during postnatal development (GnRH::Cre;Dicer ^loxP/loxP mice), we found that a postnatally-acquired dysfunction in the GnRH system affects adult neurogenesis selectively in the subventricular-zone neurogenic niche in a sexually dimorphic way. Moreover, by examining adult females ovariectomized before the onset of puberty, we provide important evidence that, among the HPG-axis secreting factors, the circulating levels of gonadal hormones during pre-/peri-pubertal life contribute to set-up the proper adult subventricular zone-olfactory bulb neurogenic system.

Noradrenergic Activity in the Olfactory Bulb Is a Key Element for the Stability of Olfactory Memory

Memory stability is essential for animal survival when environment and behavioral state change over short or long time spans. The stability of a memory can be expressed by its duration, its perseverance when conditions change as well as its specificity to the learned stimulus. Using optogenetic and pharmacological manipulations in male mice, we show that the presence of noradrenaline in the olfactory bulb during acquisition renders olfactory memories more stable. We show that while inhibition of noradrenaline transmission during an odor-reward acquisition has no acute effects, it alters perseverance, duration, and specificity of the memory. We use a computational approach to propose a proof of concept model showing that a single, simple network effect of noradrenaline on olfactory bulb dynamics can underlie these seemingly different behavioral effects. Our results show that acute changes in network dynamics can have long-term effects that extend beyond the network that was manipulated.SIGNIFICANCE STATEMENT Olfaction guides the behavior of animals. For successful survival, animals have to remember previously learned information and at the same time be able to acquire new memories. We show here that noradrenaline in the olfactory bulb, the first cortical relay of the olfactory information, is important for creating stable and specific olfactory memories. Memory stability, as expressed in perseverance, duration and specificity of the memory, is enhanced when noradrenergic inputs to the olfactory bulb are unaltered. We show that, computationally, our diverse behavioral results can be ascribed to noradrenaline-driven changes in neural dynamics. These results shed light on how very temporary changes in neuromodulation can have a variety of long-lasting effects on neural processing and behavior.

Rapid online learning and robust recall in a neuromorphic olfactory circuit

We present a neural algorithm for the rapid online learning and identification of odourant samples under noise, based on the architecture of the mammalian olfactory bulb and implemented on the Intel Loihi neuromorphic system. As with biological olfaction, the spike timing-based algorithm utilizes distributed, event-driven computations and rapid (one-shot) online learning. Spike timing-dependent plasticity rules operate iteratively over sequential gamma-frequency packets to construct odour representations from the activity of chemosensor arrays mounted in a wind tunnel. Learned odourants then are reliably identified despite strong destructive interference. Noise resistance is further enhanced by neuromodulation and contextual priming. Lifelong learning capabilities are enabled by adult neurogenesis. The algorithm is applicable to any signal identification problem in which high-dimensional signals are embedded in unknown backgrounds.

Plasticity in olfactory bulb circuits

Olfaction is crucial for animal survival and human well-being. The olfactory bulb is the obligatory input station for olfactory information. In contrast to the traditional view as a static relay station, recent evidence indicates that the olfactory bulb dynamically processes olfactory information in an experience-dependent and context-dependent manner. Here, we review recent studies on experience-dependent plasticity of the main circuit components within the olfactory bulb of rodents. We argue that the olfactory bulb plasticity allows optimal representations of behaviorally-relevant odors in the continuously changing olfactory environment.

Short-term availability of adult-born neurons for memory encoding

Adult olfactory neurogenesis provides waves of new neurons involved in memory encoding. However, how the olfactory bulb deals with neuronal renewal to ensure the persistence of pertinent memories and the flexibility to integrate new events remains unanswered. To address this issue, mice performed two successive olfactory discrimination learning tasks with varying times between tasks. We show that with a short time between tasks, adult-born neurons supporting the first learning task appear to be highly sensitive to interference. Furthermore, targeting these neurons using selective light-induced inhibition altered memory of this first task without affecting that of the second, suggesting that neurons in their critical period of integration may only support one memory trace. A longer period between the two tasks allowed for an increased resilience to interference. Hence, newly formed adult-born neurons regulate the transience or persistence of a memory as a function of information relevance and retrograde interference.

Olfactory bulb neurogenesis raises the question of how persistent olfactory memories are retained while remaining flexible to encode new memories. Here, the authors show that new neurons can only support a single odor memory within their critical period of integration into the circuit.

Acetylcholine Regulates Olfactory Perceptual Learning through Effects on Adult Neurogenesis

Learning to perceptually discriminate between chemical signals in the environment (olfactory perceptual learning [OPL]) is critical for survival. Multiple mechanisms have been implicated in OPL, including modulation of neurogenesis and manipulation of cholinergic activity. However, whether these represent distinct processes regulating OPL or if cholinergic effects on OPL depend upon neurogenesis has remained an unresolved question. Using a combination of pharmacological and optogenetic approaches, cholinergic activity was shown to be both necessary and sufficient to drive OPL, and this process was dependent on the presence of newly born cells in the olfactory bulb (OB). This study is the first to directly demonstrate that cholinergic effects on OPL require adult OB neurogenesis.

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Acetylcholine modulates olfactory perceptual learning

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Cholinergic modulation alters olfactory bulb neurogenesis

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Cholinergic effects on olfactory perceptual learning require adult neurogenesis

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Cholinergic excitation does not alter the phenotype of newborn olfactory bulb cells

Kinase activity in the olfactory bulb is required for odor memory consolidation

Long-term fear memory formation in the hippocampus and neocortex depends upon brain-derived neurotrophic factor (BDNF) signaling after acquisition. Incremental, appetitive odor discrimination learning is thought to depend substantially on the differentiation of adult-born neurons within the olfactory bulb (OB)—a process that is closely associated with BDNF signaling. We sought to elucidate the role of neurotrophin signaling within the OB on odor memory consolidation. Male mice were trained on odor–reward associative discriminations after bilateral infusion of the kinase inhibitor K252a, or vehicle control, into the OB. K252a is a partially selective inhibitor of tyrosine kinase (Trk) receptors, including the TrkB receptor for BDNF, though it also inhibits other plasticity-related kinases such as PKC and CaMKII/IV. K252a infusion into the OB did not impair odor acquisition or short-term (2 h) memory for the learned discriminations, but significantly impaired long-term (48 h) odor memory (LTM). This LTM deficit also was associated with reduced selectivity for the conditioned odorant in a reward-seeking digging task. Infusions of K252a immediately prior to testing did not impair LTM recall. These results indicate that kinase activation in the OB is required for the consolidation of odor memory of incrementally acquired information.

Noradrenergic effects on olfactory perception and learning

We here review modulation of olfactory guided behavioral tasks by noradrenaline. In this review we focus on modulation of the main olfactory system in adult rodents. We detail behavioral paradigms commonly used and discuss how sensory perception and learning can be measured using these paradigms. We then describe neuromodulatory effects on several aspects of olfactory processing, including detection and encoding. We describe how memory duration, specificity and duration are affected by noradrenergic modulation.

α2-Adrenergic receptor activation promotes long-term potentiation at excitatory synapses in the mouse accessory olfactory bulb

The formation of mate recognition memory in mice is associated with neural changes at the reciprocal dendrodendritic synapses between glutamatergic mitral cell (MC) projection neurons and GABAergic granule cell (GC) interneurons in the accessory olfactory bulb (AOB). Although noradrenaline (NA) plays a critical role in the formation of the memory, the mechanism by which it exerts this effect remains unclear. Here we used extracellular field potential and whole-cell patch-clamp recordings to assess the actions of bath-applied NA (10 µM) on the glutamatergic transmission and its plasticity at the MC-to-GC synapse in the AOB. Stimulation (400 stimuli) of MC axons at 10 Hz but not at 100 Hz effectively induced N-methyl-d-aspartate (NMDA) receptor-dependent long-term potentiation (LTP), which exhibited reversibility. NA paired with subthreshold 10-Hz stimulation (200 stimuli) facilitated the induction of NMDA receptor-dependent LTP via the activation of α₂-adrenergic receptors (ARs). We next examined how NA, acting at α₂-ARs, facilitates LTP induction. In terms of acute actions, NA suppressed GC excitatory postsynaptic current (EPSC) responses to single pulse stimulation of MC axons by reducing glutamate release from MCs via G-protein coupled inhibition of calcium channels. Consequently, NA reduced recurrent inhibition of MCs, resulting in the enhancement of evoked EPSCs and spike fidelity in GCs during the 10-Hz stimulation used to induce LTP. These results suggest that NA, acting at α₂-ARs, facilitates the induction of NMDA receptor-dependent LTP at the MC-to-GC synapse by shifting its threshold through disinhibition of MCs.

Adult-born neurons facilitate olfactory bulb pattern separation during task engagement

The rodent olfactory bulb incorporates thousands of newly generated inhibitory neurons daily throughout adulthood, but the role of adult neurogenesis in olfactory processing is not fully understood. Here we adopted a genetic method to inducibly suppress adult neurogenesis and investigated its effect on behavior and bulbar activity. Mice without young adult-born neurons (ABNs) showed normal ability in discriminating very different odorants but were impaired in fine discrimination. Furthermore, two-photon calcium imaging of mitral cells (MCs) revealed that the ensemble odor representations of similar odorants were more ambiguous in the ablation animals. This increased ambiguity was primarily due to a decrease in MC suppressive responses. Intriguingly, these deficits in MC encoding were only observed during task engagement but not passive exposure. Our results indicate that young olfactory ABNs are essential for the enhancement of MC pattern separation in a task engagement-dependent manner, potentially functioning as a gateway for top-down modulation.

Opposite regulation of inhibition by adult-born granule cells during implicit versus explicit olfactory learning

Both passive exposure and active learning through reinforcement enhance fine sensory discrimination abilities. In the olfactory system, this enhancement is thought to occur partially through the integration of adult-born inhibitory interneurons resulting in a refinement of the representation of overlapping odorants. Here, we identify in mice a novel and unexpected dissociation between passive and active learning at the level of adult-born granule cells. Specifically, while both passive and active learning processes augment neurogenesis, adult-born cells differ in their morphology, functional coupling and thus their impact on olfactory bulb output. Morphological analysis, optogenetic stimulation of adult-born neurons and mitral cell recordings revealed that passive learning induces increased inhibitory action by adult-born neurons, probably resulting in more sparse and thus less overlapping odor representations. Conversely, after active learning inhibitory action is found to be diminished due to reduced connectivity. In this case, strengthened odor response might underlie enhanced discriminability.

Generative Biophysical Modeling of Dynamical Networks in the Olfactory System

Generative models are computational models designed to generate appropriate values for all of their embedded variables, thereby simulating the response properties of a complex system based on the coordinated interactions of a multitude of physical mechanisms. In systems neuroscience, generative models are generally biophysically based compartmental models of neurons and networks that are explicitly multiscale, being constrained by experimental data at multiple levels of organization from cellular membrane properties to large-scale network dynamics. As such, they are able to explain the origins of emergent properties in complex systems, and serve as tests of sufficiency and as quantitative instantiations of working hypotheses that may be too complex to simply intuit. Moreover, when adequately constrained, generative biophysical models are able to predict novel experimental outcomes, and consequently are powerful tools for experimental design. We here outline a general strategy for the iterative design and implementation of generative, multiscale biophysical models of neural systems. We illustrate this process using our ongoing, iteratively developing model of the mammalian olfactory bulb. Because the olfactory bulb exhibits diverse and interesting properties at multiple scales of organization, it is an attractive system in which to illustrate the value of generative modeling across scales.

Persistent, generalized hypersensitivity of olfactory bulb interneurons after olfactory fear generalization

Generalization of fear from previously threatening stimuli to novel but related stimuli can be beneficial, but if fear overgeneralizes to inappropriate situations it can produce maladaptive behaviors and contribute to pathological anxiety. Appropriate fear learning can selectively facilitate early sensory processing of threat-predictive stimuli, but it is unknown if fear generalization has similarly generalized neurosensory consequences. We performed in vivo optical neurophysiology to visualize odor-evoked neural activity in populations of periglomerular interneurons in the olfactory bulb 1 day before, 1 day after, and 1 month after each mouse underwent an olfactory fear conditioning paradigm designed to promote generalized fear of odors. Behavioral and neurophysiological changes were assessed in response to a panel of odors that varied in similarity to the threat-predictive odor at each time point. After conditioning, all odors evoked similar levels of freezing behavior, regardless of similarity to the threat-predictive odor. Freezing significantly correlated with large changes in odor-evoked periglomerular cell activity, including a robust, generalized facilitation of the response to all odors, broadened odor tuning, and increased neural responses to lower odor concentrations. These generalized effects occurred within 24 h of a single conditioning session, persisted for at least 1 month, and were detectable even in the first moments of the brain's response to odors. The finding that generalized fear includes altered early sensory processing of not only the threat-predictive stimulus but also novel though categorically-similar stimuli may have important implications for the etiology and treatment of anxiety disorders with sensory sequelae.

Development and Refinement of Functional Properties of Adult-Born Neurons

New neurons appear only in a few regions of the adult mammalian brain and become integrated into existing circuits. Little is known about the functional development of individual neurons in vivo. We examined the functional life history of adult-born granule cells (abGCs) in the olfactory bulb using multiphoton imaging in awake and anesthetized mice. We found that abGCs can become responsive to odorants soon after they arrive in the olfactory bulb. Tracking identified abGCs over weeks revealed that the robust and broadly tuned responses of most newly arrived abGCs gradually become more selective over a period of ∼3 weeks, but a small fraction achieves broader tuning with maturation. Enriching the olfactory environment of mice prolonged the period over which abGCs were strongly and broadly responsive to odorants. Our data offer direct support for rapid integration of adult-born neurons into existing circuits, followed by experience-dependent refinement of their functional connectivity.

Measuring Olfactory Processes in Mus musculus

This paper briefly reviews the literature that describes olfactory acuity and odour discrimination learning. The results of current studies that examined the role of the neurotransmitters noradrenalin and acetylcholine in odour discrimination learning are discussed as are those that investigated pattern recognition and models of human disease. The methodology associated with such work is also described and its role in creating disparate results assessed. Recommendations for increasing the reliability and validity of experiments so as to further our understanding of olfactory processes in both healthy mice and those modelling human disease are made throughout the paper.

The role of sleep in the plasticity of the olfactory system

The central olfactory system mediates a variety of odor-guided behaviors crucial for maintenance of animal life. The olfactory neural circuit must be highly plastic to ensure that it responds appropriately to changing odor circumstances. Recent studies have revealed that the processing of odor information changes drastically during waking and sleep and that neural activity during sleep plays pivotal roles in the structural reorganization and functional plasticity of the olfactory system. While olfactory information from the external world is efficiently transferred to the olfactory cortex (OC) via the olfactory bulb (OB) during waking, this information flow is attenuated during slow-wave sleep: during slow-wave sleep, the OC neurons exhibit synchronous discharges without odor input under the entrainment of sharp waves in the local field potential recording. Top-down transfer of sharp-wave activity to the OB during slow-wave sleep promotes structural reorganization of the OB neural circuit. Further, the activity of the OC during sleep is affected by the olfactory experience during prior waking period, and perturbation of the sleep activity disrupts proper olfactory memory. Thus, as is seen also in the hippocampus and neocortex, the neural activities of the olfactory system during sleep likely play essential roles in circuit reorganization and memory consolidation.

Topographical representation of odor hedonics in the olfactory bulb

Hedonic value is a dominant aspect of olfactory perception. Using optogenetic manipulation in freely behaving mice paired with immediate early gene mapping, we demonstrate that hedonic information is represented along the antero-posterior axis of the ventral olfactory bulb. Using this representation, we show that the degree of attractiveness of odors can be bidirectionally modulated by local manipulation of the olfactory bulb's neural networks in freely behaving mice.

Changes in Olfactory Sensory Neuron Physiology and Olfactory Perceptual Learning After Odorant Exposure in Adult Mice

The adult olfactory system undergoes experience-dependent plasticity to adapt to the olfactory environment. This plasticity may be accompanied by perceptual changes, including improved olfactory discrimination. Here, we assessed experience-dependent changes in the perception of a homologous aldehyde pair by testing mice in a cross-habituation/dishabituation behavioral paradigm before and after a week-long ester-odorant exposure protocol. In a parallel experiment, we used optical neurophysiology to observe neurotransmitter release from olfactory sensory neuron (OSN) terminals in vivo, and thus compared primary sensory representations of the aldehydes before and after the week-long ester-odorant exposure in individual animals. Mice could not discriminate between the aldehydes during pre-exposure testing, but ester-exposed subjects spontaneously discriminated between the homologous pair after exposure, whereas home cage control mice cross-habituated. Ester exposure did not alter the spatial pattern, peak magnitude, or odorant-selectivity of aldehyde-evoked OSN input to olfactory bulb glomeruli, but did alter the temporal dynamics of that input to make the time course of OSN input more dissimilar between odorants. Together, these findings demonstrate that odor exposure can induce both physiological and perceptual changes in odor processing, and suggest that changes in the temporal patterns of OSN input to olfactory bulb glomeruli could induce differences in odor quality.

Circadian Clock Genes Are Essential for Normal Adult Neurogenesis, Differentiation, and Fate Determination

Adult neurogenesis creates new neurons and glia from stem cells in the human brain throughout life. It is best understood in the dentate gyrus (DG) of the hippocampus and the subventricular zone (SVZ). Circadian rhythms have been identified in the hippocampus, but the role of any endogenous circadian oscillator cells in hippocampal neurogenesis and their importance in learning or memory remains unclear. Any study of stem cell regulation by intrinsic circadian timing within the DG is complicated by modulation from circadian clocks elsewhere in the brain. To examine circadian oscillators in greater isolation, neurosphere cultures were prepared from the DG of two knockout mouse lines that lack a functional circadian clock and from mPer1::luc mice to identify circadian oscillations in gene expression. Circadian mPer1 gene activity rhythms were recorded in neurospheres maintained in a culture medium that induces neurogenesis but not in one that maintains the stem cell state. Although the differentiating neural stem progenitor cells of spheres were rhythmic, evidence of any mature neurons was extremely sparse. The circadian timing signal originated in undifferentiated cells within the neurosphere. This conclusion was supported by immunocytochemistry for mPER1 protein that was localized to the inner, more stem cell-like neurosphere core. To test for effects of the circadian clock on neurogenesis, media conditions were altered to induce neurospheres from BMAL1 knockout mice to differentiate. These cultures displayed unusually high differentiation into glia rather than neurons according to GFAP and NeuN expression, respectively, and very few BetaIII tubulin-positive, immature neurons were observed. The knockout neurospheres also displayed areas visibly devoid of cells and had overall higher cell death. Neurospheres from arrhythmic mice lacking two other core clock genes, Cry1 and Cry2, showed significantly reduced growth and increased astrocyte proliferation during differentiation, but they generated normal percentages of neuronal cells. Neuronal fate commitment therefore appears to be controlled through a non-clock function of BMAL1. This study provides insight into how cell autonomous circadian clocks and clock genes regulate adult neural stem cells with implications for treating neurodegenerative disorders and impaired brain functions by manipulating neurogenesis.

Statins Promote Long-Term Recovery after Ischemic Stroke by Reconnecting Noradrenergic Neuronal Circuitry

Inhibitors of HMG-CoA reductase (statins), widely used to lower cholesterol in coronary heart and vascular disease, are effective drugs in reducing the risk of stroke and improving its outcome in the long term. After ischemic stroke, cardiac autonomic dysfunction and psychological problems are common complications related to deficits in the noradrenergic (NA) system. This study investigated the effects of statins on the recovery of NA neuron circuitry and its function after transient focal cerebral ischemia (tFCI). Using the wheat germ agglutinin (WGA) transgene technique combined with the recombinant adenoviral vector system, NA-specific neuronal pathways were labeled, and were identified in the locus coeruleus (LC), where NA neurons originate. NA circuitry in the atorvastatin-treated group recovered faster than in the vehicle-treated group. The damaged NA circuitry was partly reorganized with the gradual recovery of autonomic dysfunction and neurobehavioral deficit. Newly proliferated cells might contribute to reorganizing NA neurons and lead anatomic and functional recovery of NA neurons. Statins may be implicated to play facilitating roles in the recovery of the NA neuron and its function.

Endogenous neurogenic cell response in the mature mammalian brain following traumatic injury

In the mature mammalian brain, new neurons are generated throughout life in the neurogenic regions of the subventricular zone (SVZ) and the dentate gyrus (DG) of the hippocampus. Over the past two decades, extensive studies have examined the extent of adult neurogenesis in the SVZ and DG, the role of the adult generated new neurons in normal brain function and the underlying mechanisms regulating the process of adult neurogenesis. The extent and the function of adult neurogenesis under neuropathological conditions have also been explored in varying types of disease models in animals. Increasing evidence has indicated that these endogenous neural stem/progenitor cells may play regenerative and reparative roles in response to CNS injuries or diseases. This review will discuss the potential functions of adult neurogenesis in the injured brain and will describe the recent development of strategies aimed at harnessing this neurogenic capacity in order to repopulate and repair the injured brain following trauma.

Olfactory perceptual learning requires action of noradrenaline in the olfactory bulb: comparison with olfactory associative learning

Noradrenaline contributes to olfactory-guided behaviors but its role in olfactory learning during adulthood is poorly documented. We investigated its implication in olfactory associative and perceptual learning using local infusion of mixed α1-β adrenergic receptor antagonist (labetalol) in the adult mouse olfactory bulb. We reported that associative learning, as opposed to perceptual learning, was not affected by labetalol infusions in the olfactory bulb. Accordingly, this treatment during associative learning did not affect the survival of bulbar adult-born neurons. Altogether, our results suggest that the noradrenergic system plays different parts in specific olfactory learning tasks and their neurogenic correlates.

The interplay between reproductive social stimuli and adult olfactory bulb neurogenesis

Adult neurogenesis is a striking form of structural plasticity that adapts the brain to the changing world. Accordingly, new neuron production is involved in cognitive functions, such as memory, learning, and pattern separation. Recent data in rodents indicate a close link between adult neurogenesis and reproductive social behavior. This provides a key to unravel the functional meaning of adult neurogenesis in biological relevant contexts and, in parallel, opens new perspectives to explore the way the brain is processing social stimuli. In this paper we will summarize some of the major achievements on cues and mechanisms modulating adult neurogenesis during social behaviors related to reproduction and possible role/s played by olfactory newborn neurons in this context. We will point out that newborn interneurons in the accessory olfactory bulb (AOB) represent a privileged cellular target for social stimuli that elicit reproductive behaviors and that such cues modulate adult neurogenesis at two different levels increasing both proliferation of neuronal progenitors in the germinative regions and integration of newborn neurons into functional circuits. This dual mechanism provides fresh neurons that can be involved in critical activities for the individual fitness, that is, the processing of social stimuli driving the parental behavior and partner recognition.

Properties and mechanisms of olfactory learning and memory

Memories are dynamic physical phenomena with psychometric forms as well as characteristic timescales. Most of our understanding of the cellular mechanisms underlying the neurophysiology of memory, however, derives from one-trial learning paradigms that, while powerful, do not fully embody the gradual, representational, and statistical aspects of cumulative learning. The early olfactory system—particularly olfactory bulb—comprises a reasonably well-understood and experimentally accessible neuronal network with intrinsic plasticity that underlies both one-trial (adult aversive, neonatal) and cumulative (adult appetitive) odor learning. These olfactory circuits employ many of the same molecular and structural mechanisms of memory as, for example, hippocampal circuits following inhibitory avoidance conditioning, but the temporal sequences of post-conditioning molecular events are likely to differ owing to the need to incorporate new information from ongoing learning events into the evolving memory trace. Moreover, the shapes of acquired odor representations, and their gradual transformation over the course of cumulative learning, also can be directly measured, adding an additional representational dimension to the traditional metrics of memory strength and persistence. In this review, we describe some established molecular and structural mechanisms of memory with a focus on the timecourses of post-conditioning molecular processes. We describe the properties of odor learning intrinsic to the olfactory bulb and review the utility of the olfactory system of adult rodents as a memory system in which to study the cellular mechanisms of cumulative learning.

The transcription factor Zif268/Egr1, brain plasticity, and memory

The capacity to remember our past experiences and organize our future draws on a number of cognitive processes that allow our brain to form and store neural representations that can be recalled and updated at will. In the brain, these processes require mechanisms of neural plasticity in the activated circuits, brought about by cellular and molecular changes within the neurons activated during learning. At the cellular level, a wealth of experimental data accumulated in recent years provides evidence that signaling from synapses to nucleus and the rapid regulation of the expression of immediate early genes encoding inducible, regulatory transcription factors is a key step in the mechanisms underlying synaptic plasticity and the modification of neural networks required for the laying down of memories. In the activated neurons, these transcriptional events are thought to mediate the activation of selective gene programs and subsequent synthesis of proteins, leading to stable functional and structural remodeling of the activated networks, so that the memory can later be reactivated upon recall. Over the past few decades, novel insights have been gained in identifying key transcriptional regulators that can control the genomic response of synaptically activated neurons. Here, as an example of this approach, we focus on one such activity-dependent transcription factor, Zif268, known to be implicated in neuronal plasticity and memory formation. We summarize current knowledge about the regulation and function of Zif268 in different types of brain plasticity and memory processes.

Adult neurogenesis in the olfactory system shapes odor memory and perception

The olfactory system is a dynamic place. In mammals, not only are sensory neurons located in the sensory organ renewed through adult life, but also its first central relay is reconstructed by continuous neuronal recruitment. Despite these numerous morphological and physiological changes, olfaction is a unique sensory modality endowed with a privileged link to memory. This raises a clear conundrum; how does the olfactory system balance its neuronal turnover with its participation in long-term memory? This review concentrates on the functional aspects of adult neurogenesis, addressing how the integration of late-born neurons participates in olfactory perception and memory. After outlining the properties of adult neurogenesis in the olfactory system, and after describing their regulation by internal and environmental factors, we ask how the process of odorant perception can be influenced by constant neuronal turnover. We then explore the possible functional roles that newborn neurons might have for olfactory memory. Throughout this review, and as we concentrate almost exclusively on mammalian models, we stress the idea that adult neurogenesis is yet another form of plasticity used by the brain to copes with a constantly changing olfactory world.

Alteration of olfactory perceptual learning and its cellular basis in aged mice

Olfactory perceptual learning reflects an ongoing process by which animals learn to discriminate odorants thanks to repeated stimulations by these odorants. Adult neurogenesis is required for this learning to occur in young adults. The experiments reported here showed that olfactory perceptual learning is impaired with aging and that this impairment is associated with a reduction of neurogenesis and a decrease in granule cell responsiveness to the learned odorant in the olfactory bulb. Interestingly, we showed that the pharmacological stimulation of the noradrenergic system using dexefaroxan mimics olfactory perceptual learning in old mice, which is accompanied by an increase of granule cell responsiveness in response to the learned odorant without any improvement in neurogenesis. We provide the first published evidence that, in contrast to young adult mice, the improvement of olfactory performances in old mice is independent of the overall level of neurogenesis. In addition, restoring behavioral performances in old mice by stimulation of the noradrenergic system underlies the importance of this neuromodulatory system in regulating bulbar network plasticity.

Reorganization of neuronal circuits of the central olfactory system during postprandial sleep

Plastic changes in neuronal circuits often occur in association with specific behavioral states. In this review, we focus on an emerging view that neuronal circuits in the olfactory system are reorganized along the wake-sleep cycle. Olfaction is crucial to sustaining the animals' life, and odor-guided behaviors have to be newly acquired or updated to successfully cope with a changing odor world. It is therefore likely that neuronal circuits in the olfactory system are highly plastic and undergo repeated reorganization in daily life. A remarkably plastic feature of the olfactory system is that newly generated neurons are continually integrated into neuronal circuits of the olfactory bulb (OB) throughout life. New neurons in the OB undergo an extensive selection process, during which many are eliminated by apoptosis for the fine tuning of neuronal circuits. The life and death decision of new neurons occurs extensively during a short time window of sleep after food consumption (postprandial sleep), a typical daily olfactory behavior. We review recent studies that explain how olfactory information is transferred between the OB and the olfactory cortex (OC) along the course of the wake-sleep cycle. Olfactory sensory input is effectively transferred from the OB to the OC during waking, while synchronized top-down inputs from the OC to the OB are promoted during the slow-wave sleep. We discuss possible neuronal circuit mechanisms for the selection of new neurons in the OB, which involves the encoding of olfactory sensory inputs and memory trace formation during waking and internally generated activities in the OC and OB during subsequent sleep. The plastic changes in the OB and OC are well coordinated along the course of olfactory behavior during wakefulness and postbehavioral rest and sleep. We therefore propose that the olfactory system provides an excellent model in which to understand behavioral state-dependent plastic mechanisms of the neuronal circuits in the brain.

Odor-specific, olfactory marker protein-mediated sparsening of primary olfactory input to the brain after odor exposure

Long-term plasticity in sensory systems is usually conceptualized as changing the interpretation of the brain of sensory information, not an alteration of how the sensor itself responds to external stimuli. However, here we demonstrate that, in the adult mouse olfactory system, a 1-week-long exposure to an artificially odorized environment narrows the range of odorants that can induce neurotransmitter release from olfactory sensory neurons (OSNs) and reduces the total transmitter release from responsive neurons. In animals heterozygous for the olfactory marker protein (OMP), this adaptive plasticity was strongest in the populations of OSNs that originally responded to the exposure odorant (an ester) and also observed in the responses to a similar odorant (another ester) but had no effect on the responses to odorants dissimilar to the exposure odorant (a ketone and an aldehyde). In contrast, in OMP knock-out mice, odorant exposure reduced the number and amplitude of OSN responses evoked by all four types of odorants equally. The effect of this plasticity is to preferentially sparsen the primary neural representations of common olfactory stimuli, which has the computational benefit of increasing the number of distinct sensory patterns that could be represented in the circuit and might thus underlie the improvements in olfactory discrimination often observed after odorant exposure (Mandairon et al., 2006a). The absence of odorant specificity in this adaptive plasticity in OMP knock-out mice suggests a potential role for this protein in adaptively reshaping OSN responses to function in different environments.

Age-dependent regional changes in the rostral migratory stream

Throughout life the subventricular zone (SVZ) is a source of new olfactory bulb (OB) interneurons. From the SVZ, neuroblasts migrate tangentially through the rostral migratory stream (RMS), a restricted route approximately 5 mm long in mice, reaching the OB within 10-14 days. Within the OB, neuroblasts migrate radially to the granule and glomerular layers where they differentiate into granule and periglomerular (PG) cells and integrate into existing synaptic circuits. SVZ neurogenesis decreases with age, and might be a factor in age-related olfactory deficits. However, the effect of aging on the RMS and on the differentiation of interneuron subpopulations remains poorly understood. Here, we examine RMS cytoarchitecture, neuroblast proliferation and clearance from the RMS, and PG cell subpopulations at 6, 12, 18, and 23 months of age. We find that aging affects the area occupied by newly generated cells within the RMS and regional proliferation, and the clearance of neuroblasts from the RMS and PG cell subpopulations and distribution remain stable.

The impact of adult neurogenesis on olfactory bulb circuits and computations

Modern neuroscience has demonstrated how the adult brain has the ability to profoundly remodel its neurons in response to changes in external stimuli or internal states. However, adult brain plasticity, although possible throughout life, remains restricted mostly to subcellular levels rather than affecting the entire cell. New neurons are continuously generated in only a few areas of the adult brain-the olfactory bulb and the dentate gyrus-where they integrate into already functioning circuitry. In these regions, adult neurogenesis adds another dimension of plasticity that either complements or is redundant to the classical molecular and cellular mechanisms of plasticity. This review extracts clues regarding the contribution of adult-born neurons to the different circuits of the olfactory bulb and specifically how new neurons participate in existing computations and enable new computational functions.

Adult-born neurons in the olfactory bulb: integration and functional consequences

The generation of new neurons is sustained throughout life in the olfactory system. In recent years, tremendous progress has been made toward understanding the proliferation, differentiation, migration, and integration of newborn neurons in the olfactory bulb. Here, we discuss recent findings that shed light on different aspects of the integration of adult-born neurons into olfactory circuitry and its significance for behavior.