Nathalie Mandairon

Olfactory perceptual learning requires adult neurogenesis

Perceptual learning is required for olfactory function to adapt appropriately to changing odor environments. We here show that newborn neurons in the olfactory bulb are not only involved in, but necessary for, olfactory perceptual learning. First, the discrimination of perceptually similar odorants improves in mice after repeated exposure to the odorants. Second, this improved discrimination is accompanied by an elevated survival rate of newborn inhibitory neurons, preferentially involved in processing of the learned odor, within the olfactory bulb. Finally, blocking neurogenesis before and during the odorant exposure period prevents this learned improvement in discrimination. Olfactory perceptual learning is thus mediated by the reinforcement of functional inhibition in the olfactory bulb by adult neurogenesis.

Variant Brain-Derived Neurotrophic Factor (Val66Met) Alters Adult Olfactory Bulb Neurogenesis and Spontaneous Olfactory Discrimination

Neurogenesis, the division, migration, and differentiation of new neurons, occurs throughout life. Brain derived neurotrophic factor (BDNF) has been identified as a potential signaling molecule regulating neurogenesis in the subventricular zone (SVZ), but its functional consequences in vivo have not been well defined. We report marked and unexpected deficits in survival but not proliferation of newly born cells of adult knock-in mice containing a variant form of BDNF [a valine (Val) to methionine (Met) substitution at position 66 in the prodomain of BDNF (Val66Met)], a genetic mutation shown to lead to a selective impairment in activity-dependent BDNF secretion. Utilizing knock-out mouse lines, we identified BDNF and tyrosine receptor kinase B (TrkB) as the critical molecules for the observed impairments in neurogenesis, with p75 knock-out mice showing no effect on cell proliferation or survival. We then localized the activated form of TrkB to a discrete population of cells, type A migrating neuroblasts, and demonstrate a decrease in TrkB phosphorylation in the SVZ of Val66Met mutant mice. With these findings, we identify TrkB signaling, potentially through activity dependent release of BDNF, as a critical step in the survival of migrating neuroblasts. Utilizing a behavioral task shown to be sensitive to disruptions in olfactory bulb neurogenesis, we identified specific impairments in spontaneous olfactory discrimination, but not general olfactory sensitivity or habituation to olfactory stimuli in BDNF mutant mice. Through these observations, we have identified novel links between genetic variant BDNF and adult neurogenesis in vivo, which may contribute to significant impairments in olfactory function.

DEPRIVATION OF SENSORY INPUTS TO THE OLFACTORY BULB UP- REGULATES CELL DEATH AND PROLIFERATION IN THE SUBVENTRICULAR ZONE OF ADULT MICE

The main olfactory bulb (MOB) is the first relay on the olfactory sensory pathway and the target of the neural progenitor cells generated in the subventricular zone (SVZ) lining the lateral ventricles and which migrate along the rostral extension of the SVZ, also called the rostral migratory stream (RMS). Within the MOB, the neuroblasts differentiate into granular and periglomerular interneurons. A reduction in the number of granule cells during sensory deprivation suggests that neurogenesis may be influenced by afferent activity. Here, we show that unilateral sensory deafferentation of the MOB by axotomy of the olfactory receptor neurons increases apoptotic cell death in the SVZ and along the rostro-caudal extent of the RMS. The vast majority of dying cells in the RMS are migrating neuroblasts as indicated by double Terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick-end labeling/PSA-NCAM labeling. Counting bromodeoxyuridine-labeled cells in animals killed immediately or 4 days after tracer administration showed a bilateral increase in proliferation in the SVZ and RMS which was balanced by cell death on the operated side. These data suggest that olfactory inputs are required for the survival of newborn neural progenitors. The greatest enhancement in proliferation occurred in the extension of the RMS located in the MOB, revealing a population of local precursors mitotically stimulated following axotomy. Together, these findings indicate that olfactory inputs may strongly modulate the balance between neurogenesis and apoptosis in the SVZ and RMS and provide a model for further investigation of the underlying molecular mechanisms of this activity-dependent neuronal plasticity.

Long-term fate and distribution of newborn cells in the adult mouse olfactory bulb: Influences of olfactory deprivation

The adult subventricular zone produces neuroblasts that migrate to the main olfactory bulb, where they differentiate into interneurons in the glomerular and granular layers. Using bromodeoxyuridine labeling, the survival of newborn cells was assessed in these two layers of the MOB in control mice and in mice unilaterally deprived from sensory input by naris occlusion. In control main olfactory bulbs, bromodeoxyuridine-positive cell density decreased about 70% between 15 and 180 days post-bromodeoxyuridine administration but earlier in the glomerular layer than in the granular layer. At all time points examined, newborn cell density was higher in the deep granular layer than in the superficial granular layer. Occlusion started at the age of 2 months and lasted for 15, 30, 45, 60 or 180 days. The newborn cell survival was similarly reduced in both layers by occlusion, during a critical period 15 and 45 days post-occlusion. Interestingly, olfactory deprivation decreased bromodeoxyuridine-positive cell density in the deep granular layer only, indicating a greater dependence of cell fate on sensory input in this sub-layer. Neuronal differentiation was assessed in the granular layer and glomerular layer by multiple double-labeling 45 days post-bromodeoxyuridine-injections, the time point at which the proportion of bromodeoxyuridine-positive cells expressing a neuronal marker reached approximately 85% in the granular layer and approximately 50% in the glomerular layer. Naris occlusion did not significantly affect these proportions. Taken together, our results reveal that the survival of newborn cells has a different time course in the glomerular layer and in the granular layer, but is similarly decreased in each layer by olfactory deprivation. In addition, our data suggest a functional heterogeneity of neurogenesis within the granular layer.

Odor Perception and Olfactory Bulb Plasticity in Adult Mammals

The adult mammalian olfactory bulb (OB) is unique in that olfactory sensory neurons project directly, without prior thalamic relay, to the OB. This review discusses evidence for the direct involvement of the OB in odor perception and its modulation by olfactory experience. We first discuss recent data showing that the OB exhibits a high level of plasticity in response to olfactory experience including exposure, enrichment, and learning. We next review evidence showing that, in return, experimental manipulation of the OB neural network changes how odorants are processed and perceived. We finally review in more detail a few experiments showing a tight correlation between the modulation of OB neural processing and odor perception. We argue that the OB has evolved to be an adapting network, allowing animals to adjust olfactory computations to changing environments.

Noradrenergic modulation in the olfactory bulb influences spontaneous and reward-motivated discrimination, but not the formation of habituation memory

The mammalian main olfactory bulb receives a significant noradrenergic input from the locus coeruleus. Norepinephrine is involved in acquisition of conditioned odor preferences in neonatal animals and in some species‐specific odor dependent behaviors. Thus far, the role of norepinephrine in odor processing in adult rats remains less studied. We tested the role of noradrenergic modulation in the olfactory bulb of cannulated rats by bilateral injections of vehicle (6 µL saline), the α noradrenergic receptor antagonist phentolamine (3.15 or 10 mm), the β noradrenergic receptor antagonist alprenolol (12 or 120 mm), the α1 noradrenergic receptor antagonist prazosin (1 or 10−2 mm) and the α2 noradrenergic receptor antagonist yohimbine (2 or 0.02 mm) 20 min before two different behavioral tasks. We found that local blockade of noradrenergic receptors in the olfactory bulb did not affect the formation of habituation memory to an odorant over sequential presentations separated by 5‐min intertrial intervals. However, spontaneous discrimination between chemically related odorants was impaired when noradrenergic receptors, and in particular α1 receptors, were blocked by local antagonist infusion into the olfactory bulb. By contrast, discrimination was improved when β receptors were blocked. These results show that although the formation of a habituation memory to odorants is not affected by noradrenergic modulation, the specificity of this memory is affected. In contrast, reward‐motivated discrimination learning was not impaired, but slowed down in rats in which both α and β receptors had been blocked.

Early locus coeruleus degeneration and olfactory dysfunctions in Tg2576 mice

Olfactory deficiency has been reported in the early stages of Alzheimers disease (AD) in humans but is very poorly understood due to the lack of investigations in animal models of AD. Recent studies point to the noradrenergic system as an important target of the AD pathological process. In addition, noradrenalin has been shown to influence adult neurogenesis which is implicated in cognitive functions. We have therefore investigated the olfactory neurogenesis and cognitive performances in young transgenic Tg2576 mice in relation with the status of the noradrenergic and the cholinergic systems. Tg2576 showed a deficit in neurogenesis in the olfactory bulb evidenced by an increased death of newborn cells and a reduced expression of PSA-NCAM. The locus coeruleus degenerated in Tg2576 between the age of 6.5 and 8 months. These changes were associated with olfactory memory impairments. Our findings indicate that a noradrenergic deficiency could play a role in the early stages of the pathological process in this transgenic model and induce olfactory cognitive impairments through an alteration of olfactory neurogenesis.

Humans and mice express similar olfactory preferences.

In humans, the pleasantness of odors is a major contributor to social relationships and food intake. Smells evoke attraction and repulsion responses, reflecting the hedonic value of the odorant. While olfactory preferences are known to be strongly modulated by experience and learning, it has been recently suggested that, in humans, the pleasantness of odors may be partly explained by the physicochemical properties of the odorant molecules themselves. If odor hedonic value is indeed predetermined by odorant structure, then it could be hypothesized that other species will show similar odor preferences to humans. Combining behavioral and psychophysical approaches, we here show that odorants rated as pleasant by humans were also those which, behaviorally, mice investigated longer and human subjects sniffed longer, thereby revealing for the first time a component of olfactory hedonic perception conserved across species. Consistent with this, we further show that odor pleasantness rating in humans and investigation time in mice were both correlated with the physicochemical properties of the molecules, suggesting that olfactory preferences are indeed partly engraved in the physicochemical structure of the odorant. That odor preferences are shared between mammal species and are guided by physicochemical features of odorant stimuli strengthens the view that odor preference is partially predetermined. These findings open up new perspectives for the study of the neural mechanisms of hedonic perception.

Olfactory enrichment improves the recognition of individual components in mixtures

Odor mixtures can exhibit synthetic (the mixture is qualitatively different from its components) or elemental properties (the components are recognizable). We tested how prior olfactory enrichment affects the recognition of individual components in binary mixtures. Experimental rats were exposed to pairs of similar odors for one-hour periods twice daily over 20 days. Spontaneous discrimination between the binary mixture and the individual components of similar pairs of odors was tested before and after the odor enrichment. We found that, after the enrichment period, rats could discriminate components in binary mixtures that had not been discriminated prior to the enrichment period, and that this increase in discrimination capability was not always specific to the odorants used during the enrichment period.

Action of the Noradrenergic System on Adult-Born Cells Is Required for Olfactory Learning in Mice

We have previously shown that an experience-driven improvement in olfactory discrimination (perceptual learning) requires the addition of newborn neurons in the olfactory bulb (OB). Despite this advance, the mechanisms which govern the selective survival of newborn OB neurons following learning remain largely unknown. We propose that activity of the noradrenergic system is a critical mediator providing a top-down signal to control the selective survival of newly born cells and support perceptual learning. In adult mice, we used pharmacological means to manipulate the noradrenergic system and neurogenesis and to assess their individual and additive effects on behavioral performance on a perceptual learning task. We then looked at the effects of these manipulations on regional survival of adult-born cells in the OB. Finally, using confocal imaging and electrophysiology, we investigated potential mechanisms by which noradrenaline could directly influence the survival of adult-born cells. Consistent with our hypotheses, direct manipulation of noradrenergic transmission significantly effect on adult-born cell survival and perceptual learning. Specifically, learning required both the presence of adult-born cell and noradrenaline. Finally, we provide a mechanistic link between these effects by showing that adult-born neurons receive noradrenergic projections and are responsive to noradrenaline. Based upon these data we argue that noradrenergic transmission is a key mechanism selecting adult-born neurons during learning and demonstrate that top-down neuromodulation acts on adult-born neuron survival to modulate learning performance.