Auswahl der wissenschaftlichen Literatur zum Thema „Circuit olfactif“

Feeding can be regulated by a variety of external sensory stimuli such as olfaction and gustation, as well as by systemic internal signals of feeding status and metabolic needs. Faced with a major health epidemic in eating-related conditions, such as obesity and diabetes, there is an ever increasing need to dissect and understand the complex regulatory network underlying the multiple aspects of feeding behavior. In this minireview, we highlight the use of Drosophila in studying the neural circuits that control the feeding behavior in response to external and internal signals. In particular, we outline the work on the neuroanatomical and functional characterization of the newly identified hugin neuronal circuit. We focus on the pivotal role of the central nervous system in integrating external and internal feeding-relevant information, thus enabling the organism to make one of the most basic decisions – to eat or not to eat.

Olfaction in mammals is a dynamic process driven by the inhalation of air through the nasal cavity. Inhalation determines the temporal structure of sensory neuron responses and shapes the neural dynamics underlying central olfactory processing. Inhalation-linked bursts of activity among olfactory bulb (OB) output neurons [mitral/tufted cells (MCs)] are temporally transformed relative to those of sensory neurons. We investigated how OB circuits shape inhalation-driven dynamics in MCs using a modeling approach that was highly constrained by experimental results. First, we constructed models of canonical OB circuits that included mono- and disynaptic feedforward excitation, recurrent inhibition and feedforward inhibition of the MC. We then used experimental data to drive inputs to the models and to tune parameters; inputs were derived from sensory neuron responses during natural odorant sampling (sniffing) in awake rats, and model output was compared with recordings of MC responses to odorants sampled with the same sniff waveforms. This approach allowed us to identify OB circuit features underlying the temporal transformation of sensory inputs into inhalation-linked patterns of MC spike output. We found that realistic input-output transformations can be achieved independently by multiple circuits, including feedforward inhibition with slow onset and decay kinetics and parallel feedforward MC excitation mediated by external tufted cells. We also found that recurrent and feedforward inhibition had differential impacts on MC firing rates and on inhalation-linked response dynamics. These results highlight the importance of investigating neural circuits in a naturalistic context and provide a framework for further explorations of signal processing by OB networks.

Olfaction is an important neural system for survival and fundamental behaviors such as predator avoidance, food finding, memory formation, reproduction, and social communication. However, the neural circuits and pathways associated with the olfactory system in various behaviors are not fully understood. Recent advances in optogenetics, high-resolution in vivo imaging, and reconstructions of neuronal circuits have created new opportunities to understand such neural circuits. Here, we generated a transgenic zebrafish to manipulate olfactory signal optically, expressing the Channelrhodopsin (ChR2) under the control of the olfactory specific promoter, omp. We observed light-induced neuronal activity of olfactory system in the transgenic fish by examining c-fos expression, and a calcium indicator suggesting that blue light stimulation caused activation of olfactory neurons in a non-invasive manner. To examine whether the photo-activation of olfactory sensory neurons affect behavior of zebrafish larvae, we devised a behavioral choice paradigm and tested how zebrafish larvae choose between two conflicting sensory cues, an aversive odor or the naturally preferred phototaxis. We found that when the conflicting cues (the preferred light and aversive odor) were presented together simultaneously, zebrafish larvae swam away from the aversive odor. However, the transgenic fish with photo-activation were insensitive to the aversive odor and exhibited olfactory desensitization upon optical stimulation of ChR2. These results show that an aversive olfactory stimulus can override phototaxis, and that olfaction is important in decision making in zebrafish. This new transgenic model will be useful for the analysis of olfaction related behaviors and for the dissection of underlying neural circuits.

Olfaction and satiety status influence each other: cues from the olfactory system modulate eating behavior, and satiety affects olfactory abilities. However, the neural mechanisms governing the interactions between olfaction and satiety are unknown. Here, we investigate how an animal’s nutritional state modulates neural activity and odor representation in the mitral/tufted cells of the olfactory bulb, a key olfactory center that plays important roles in odor processing and representation. At the single-cell level, we found that the spontaneous firing rate of mitral/tufted cells and the number of cells showing an excitatory response both increased when mice were in a fasted state. However, the neural discrimination of odors slightly decreased. Although ongoing baseline and odor-evoked beta oscillations in the local field potential in the olfactory bulb were unchanged with fasting, the amplitude of odor-evoked gamma oscillations significantly decreased in a fasted state. These neural changes in the olfactory bulb were independent of the sniffing pattern, since both sniffing frequency and mean inhalation duration did not change with fasting. These results provide new information toward understanding the neural circuit mechanisms by which olfaction is modulated by nutritional status.

Abstract With less than a million neurons, the western honeybee Apis mellifera is capable of complex olfactory behaviors and provides an ideal model for investigating the neurophysiology of the olfactory circuit and the basis of olfactory perception and learning. Here, we review the most fundamental aspects of honeybee’s olfaction: first, we discuss which odorants dominate its environment, and how bees use them to communicate and regulate colony homeostasis; then, we describe the neuroanatomy and the neurophysiology of the olfactory circuit; finally, we explore the cellular and molecular mechanisms leading to olfactory memory formation. The vastity of histological, neurophysiological, and behavioral data collected during the last century, together with new technological advancements, including genetic tools, confirm the honeybee as an attractive research model for understanding olfactory coding and learning.

All an animal can do to infer the state of its environment is to observe the sensory-evoked activity of its own neurons. These inferences about the presence, quality, or similarity of objects are probabilistic and inform behavioral decisions that are often made in close to real time. Neural systems employ several strategies to facilitate sensory discrimination: Biophysical mechanisms separate the neuronal response distributions in coding space, compress their variances, and combine information from sequential observations. We review how these strategies are implemented in the olfactory system of the fruit fly. The emerging principles of odor discrimination likely apply to other neural circuits of similar architecture.

Animals rely on olfaction to find food, attract mates, and avoid predators. To support these behaviors, they must be able to identify odors across different odorant concentrations. The neural circuit operations that implement this concentration invariance remain unclear. We found that despite concentration-dependence in the olfactory bulb (OB), representations of odor identity were preserved downstream, in the piriform cortex (PCx). The OB cells responding earliest after inhalation drove robust responses in sparse subsets of PCx neurons. Recurrent collateral connections broadcast their activation across the PCx, recruiting global feedback inhibition that rapidly truncated and suppressed cortical activity for the remainder of the sniff, discounting the impact of slower, concentration-dependent OB inputs. Eliminating recurrent collateral output amplified PCx odor responses rendered the cortex steeply concentration-dependent and abolished concentration-invariant identity decoding.

Olfaction has a direct influence on behavior and cognitive processes. There are different neuromodulatory systems in olfactory circuits that control the sensory information flowing through the rest of the brain. The presence of the cannabinoid type-1 (CB1) receptor, (the main cannabinoid receptor in the brain), has been shown for more than 20 years in different brain olfactory areas. However, only over the last decade have we started to know the specific cellular mechanisms that link cannabinoid signaling to olfactory processing and the control of behavior. In this review, we aim to summarize and discuss our current knowledge about the presence of CB1 receptors, and the function of the endocannabinoid system in the regulation of different olfactory brain circuits and related behaviors.

Alors que les signaux biochimiques impliqués dans la croissance axonale et la migration neuronale sont largement étudiés, la contribution des signaux mécaniques dans la formation des circuits neuronaux reste peu explorée in vivo. Nous cherchons à étudier comment les forces mécaniques contribuent à la formation du circuit olfactif du poisson-zèbre. Ce circuit se développe durant la morphogénèse de la placode olfactive (PO), par le mouvement passif des corps cellulaires qui s’éloignent de l’extrémité de leurs axones. Mes travaux de thèse s’intéressent à la contribution mécanique de l’œil, qui se forme sous la PO par des mouvements d’évagination et d’invagination, à cette migration passive des neurones et à l’extension de leurs axones. L'analyse quantitative des mouvements cellulaires a tout d’abord révélé que les mouvements des cellules de la PO et de l’œil sont corrélés. Chez des embryons dans lesquels l’œil ne se développe pas, les mouvements des cellules de la PO sont affectés, ce qui produit des PO plus fines et des axones plus courts, et la tension mécanique dans la direction d’élongation des axones dans la PO est réduite. Enfin, la matrice extracellulaire s’accumule à l’interface oeil/PO et sa dégradation enzymatique réduit la corrélation entre les mouvements des cellules de la PO et de l’œil. Ces résultats suggèrent que l’œil en formation exerce des forces de traction sur la PO, transmises par la matrice, entrainant le mouvement des neurones et l’extension des axones. Ce travail apporte un éclairage nouveau sur le rôle des forces mécaniques échangées entre les neurones en développement et les tissus environnants dans la formation des circuits neuronaux in vivo
Whereas the biochemical signals guiding axon growth and neuronal migration are extensively studied, the contribution of mechanical cues in neuronal circuit formation is still poorly explored in vivo. We aim at investigating how mechanical forces influence the construction of the zebrafish olfactory circuit. This circuit forms during the morphogenesis of the olfactory placode (OP) by the passive displacement of neuronal cell bodies away from the tip of their axons. My PhD work focuses on the mechanical contribution of the adjacent eye tissue, which develops underneath the OP through extensive evagination and invagination movements, to this passive neuronal migration and to their associated axon elongation. Quantitative live cell imaging analysis during OP morphogenesis first revealed that OP and eye cells undergo correlated movements. In embryos lacking eyes, the movements of OP cell bodies are affected, resulting in thinner placodes and shorter axons, and the mechanical stress along the direction of axon elongation within the OP is reduced. Finally, extracellular matrix was observed to accumulate at the eye/OP interface, and its enzymatic degradation decreased the correlation between OP and eye cell movements. Altogether, these results suggest that the developing eye exerts traction forces on the OP through extracellular matrix, mediating proper neuronal movements and axon extension. This work sheds new light on the role of mechanical forces exchanged between developing neurons and surrounding tissues in the sculpting of neuronal circuits in vivo

Comment les odeurs contrôlent-elles le comportement animal ? Dans ma thèse, j'ai utilisé des manipulations optogénétiques et chimiogénétiques in vivo de l'activité neurale combinées à des analyses comportementales pour explorer l'organisation de circuits cérébraux impliqués dans des comportements olfactifs chez la souris. J'ai mis au point un test de conditionnement aversif olfactif indépendant de l'intensité des odeurs. J'ai démontré que les souris pouvaient généraliser une réponse aversive en présentant différentes concentrations d'odeurs. J’ai ensuite testé si les souris pouvaient apprendre cette tâche en inactivant les interneurones exprimant la parvalbumine dans le cortex olfactif (piriforme). J'ai trouvé que l’inactivation des cellules PV, n'était pas suffisante pour abolir l'aversion aux odeurs acquise, ce qui suggère que des composants de circuits neuronaux supplémentaires contribuent à la perception de l'odeur indépendamment de sa concentration. Ensuite, j'ai tenté de comprendre la constitution relative des différentes voies neurales du piriforme dans ce comportement d’aversion apprise. À l'aide d'outils génétiques et viraux, j'ai ciblé des sous-populations distinctes de neurones piriformes, et j'ai constaté que l'activité neurale induite par la lumière dans les cellules du piriforme projetant vers le bulbe olfactif et vers le cortex préfrontal, mais pas dans les cellules du piriforme projetant vers l’amygdale corticale et vers le cortex entorhinal latéral était suffisante pour supporter le conditionnement aversif. Ces résultats contribuent à mieux comprendre les propriétés fonctionnelles des circuits neuronaux corticaux pour l'olfaction
How do odors control animal behavior ? In my thesis, I have used in vivo optogenetic and chemogenetic manipulations of neural activity combined with behavioral analyses to explore the organization of brain circuits involved in olfactory behaviors in mice. In the first part of the thesis, I established an odor intensity-independent olfactory conditioning task. I demonstrated that mice were able to generalize a learned escape behavior across a range of different odor concentrations. I then tested if by silencing Parvalbumin-expressing interneurons in the olfactory (piriform) cortex, a candidate cell population for mediating odor concentration invariance, mice would fail to learn the task. I found that silencing PV cells was not sufficient to abolish learned aversion, suggesting that additional neural circuit components contribute to concentration-invariant odor perception. Next, I asked whether different piriform neural output pathways differed in their ability to support learned aversion. Using viral-genetic tools, I targeted distinct subpopulations of piriform neurons and I found that light-induced neural activity in only piriform principle cells could drive a behavioral response. Furthermore, I tested the sufficiency of subpopulations of piriform projection neurons to drive learned aversion. I found that photostimulation of olfactory bulb- and prefrontal cortex-projecting piriform neurons was sufficient to support aversive conditioning, but not the photostimulation of cortical amygdala- and lateral entorhinal cortex-projecting piriform neurons. Together, these results provide new insights into the functional properties of cortical neural circuits for olfaction

Les cellules periglomerulaires du bulbe olfactif conforment une population hétérogène avec des propriétés moléculaires, synaptiques, morphologiques et biophysiques diverses toujours étudiés de façon indépendante. Toutefois, cette diversité suggère que des groupes différents des cellules periglomerulaires pourraient avoir des rôles différents. Dans la première partie de ma thèse je cherche à associer, pour la première fois, différents marqueurs de la diversité des neurones periglomerulaires de façon à aider à comprendre les potentielles implications fonctionnelles que les cellules periglomerulaires pourraient avoir dans le traitement de l’information olfactive. Les cellules periglomerulaires reçoivent des courants inhibiteurs postsynaptiques mais les circuits responsables de cette inhibition restent méconnus. À l’aide des enregistrements électrophysiologiques dans des tranches aiguës horizontales de bulbe olfactif de souris et des techniques d’optogénétique je montre que des projections centrifuges GABAergiques en provenance du télencéphale basal modulent fortement l’inhibition des cellules periglomerulaires de type 2 ainsi que des cellules granulaires et des cellules à axone courtes
In the olfactory bulb periglomerular cells form a heterogeneous population with diverse molecular, synaptic, morphological and biophysical properties that have always been considered independently and never explored together. However, such diversity suggests different functional implications. On the first part of this thesis, I aim to associate, for the first time, different markers of periglomerular diversity together to put in perspective the functional implications that differebt subgroups of these cells could exert in odor processing. Periglomerular cells receive inhibitory postsynaptic currents but the circuits mediating this inhibition remain poorly understood. Using a combination of patch-clamp recordings in mouse horizontal olfactory bulb slices and optogenetics I demonstrate that centrifugal GABAergic projections from the basal forebrain strongly mediate inhibition of type 2 periglomerular cells but also granule cells and deep short axon cells

Drosophila is a simple and genetically tractable model system for studying neural circuits. This dissertation consists of two studies, with the broad goal of understanding sensory processing in neural circuits using Drosophila as a model system.

L'objectif de cette thèse est de décrire le réseau cérébral et la dynamique neuronale qui pourraient servir de support aux aversions alimentaires de type olfactives. Nous avons réalisé des enregistrements multisites de potentiel de champ locaux chez le rat vigile engagé dans cet apprentissage, en proposant deux modes de présentation de l'indice olfactif : à proximité de l'eau de boisson (distal) ou ingéré (distal-proximal). Après apprentissage, la présentation du seul indice distal induit l'émergence d'une activité oscillatoire de forte amplitude dans la bande de fréquence beta (15-40 Hz). Finement corrélée au comportement d'aversion de l'animal, cette activité est proposée comme la signature du réseau de structures fonctionnellement impliquées dans la reconnaissance de l'odeur en tant que signal. Nous montrons que ce réseau peut être plus ou moins étendu selon la façon dont le stimulus a été perçu lors du conditionnement: dans certaines aires (bulbe olfactif, cortex piriforme, amygdale basolatérale, cortex orbitofrontal) la modulation en puissance de l'activité beta se fait indépendamment du mode de conditionnement; dans d'autres aires (cortex insulaire, cortex infralimbique) ces changements ont lieu si et seulement si l'odeur a été ingérée. Complétés par l'étude des interactions fonctionnelles entre ces différentes structures dans la bande de fréquence considérée, ces résultats nous permettent de mieux comprendre comment un stimulus peut être représenté en mémoire dans un réseau cérébral en fonction de l'expérience que l'animal en a fait.

La formation de la mémoire repose sur la modification des connexions neuronales, définissant ainsi une " trace nmésique ". En particulier, alors qu'un apprentissage intense ou répété peut conduire à la formation d'une mémoire à long terme (MLT), après une période de consolidation, lors de laquelle la trace mnésique passe d'un état labile à un état stable. L'étude des mécanismes moléculaires de la consolidation dans différents paradigmes d'apprentissage a montré que ceux-ci sont fortement conservés entre les espèces. La consolidation dépend de la synthèse protéique (transcription et traduction), ainsi que de cascades de signalisation spécifiques comme la signalisation basée sur l'activation du facteur de transcription CREB (cAMP responses element binding protein). Nous nous sommes intéressés aux bases moléculaires et cellulaires de la consolidation d'une mémoire olfactive associative chez l'abeille domestique (Apis mellifera). Pour cela, nous avons utilisé le paradigme du conditionnement du réflexe d'extension du proboscis (REP), qui permet chez cette espèce modèle l'apprentissage et la mémorisation d'une association entre une odeur et une récompense sucrée. Notre étude porte plus précisément sur les voies de signalisation et les processus de réorganisation structurale du réseau neuronal associés à la formation de la forme la plus stable de la MLT, dépendante de la transcription : la mémoire à long terme tardive (MLT-t). Les objectifs étaient (i) de préciser les caractéristiques et les mécanismes moléculaires impliqués dans la formation de cette MLT-t et (ii) d'étudier l'implication de ces mécanismes dans la réorganisation structurale de la voie olfactive de l'abeille, associée à la formation de cette MLT-t. En focalisant notre analyse sur la part de mémoire spécifique à l'association odeursucre, nous avons montré tout d'abord que la MLT-t est induite spécifiquement par un conditionnement en plusieurs essais suffisamment espacés dans le temps, et qu'elle requiert deux vagues successives de transcription, précoce et tardive. Nous avons alors fait l'hypothèse que la vague précoce de transcription était induite, au moins en partie, par l'action de la protéine CREB. Nous avons donc évalué les conséquences de l'inhibition de CREB sur la MLT-t : nos données suggèrent que ce facteur de transcription pourrait jouer un rôle, non exclusif et apparemment localisé aux lobes antennaires (premier relais de la voie olfactive), dans la consolidation, de la MLT-t. Nous avons également mis à jour une modulation de la MLT-t par une voie de signalisation impliquant sans doute l'arginine, dont les substrats moléculaires précis restent à identifier. Par la suite, nous avons étudié l'implication éventuelle de la signalisation par le monoxyde d'azote et le calcium, dans l'établissement des modifications neurales associées à cette forme de mémoire. En effet, ces deux voies de signalisation ont été identifiées précédemment comme étant importantes pour la mémorisation à long terme. Nous avons donc cherché à mettre en évidence des modifications de changements structuraux associés à la MLT-t après perturbation de l'une ou l'autre voie (par traitement pharmacologique). Nous nous sommes intéressés pour cela à des changements de volume et de densité des unités neuropilaires fonctionnelles de la voie olfactive, respectivement les glomérules des lobes antennaires et les microglomérules des corps pédonculés. Si le manque de reproductibilité des données nous a empêché de conclure quant à l'implication du monoxyde d'azote, en revanche nos résultats indiquent une implication du calcium dans la réorganisation du circuit olfactif de l'abeille au niveau des corps pédonculés, lors de la formation d'une MLT-t. Dans l'ensemble, ces résultats ont permis de mieux caractériser les mécanismes de la mémorisation à long terme chez l'abeille, et en montrent les points communs avec d'autres espèces. Ils ouvrent des perspectives pour l'étude des mécanismes fins de plasticité synaptique associés à la mémoire dans ce modèle
Memory formation is based on the modification of neuronal connections, which thus define a "memory trace". In particular, an intense or repeated learning process may lead to the formation of a long-term memory (LTM) after a period of consolidation, during which the memory trace changes from a labile to a stable state. Studies of the molecular mechanisms of consolidation in different learning paradigms have shown that these are highly conserved between species. Consolidation depends on protein synthesis (transcription and translation), as well as specific signaling cascades such as signaling based on the activation of the transcription factor CREB (cAMP responses element binding protein). We were interested in the molecular and cellular bases of the consolidation of olfactory associative memory in the honeybee (Apis mellifera). For this, we used the proboscis extension reflex (PER) conditioning paradigm, which allows this model species to learn and remember an association between an odor and a sucrose reward. Our study focuses specifically on the signaling pathways and processes of structural reorganization of the neural network associated with the formation of the most stable form of transcription-dependent LTM: the late long-term memory (l-LTM). The objectives were (i) to clarify the characteristics and molecular mechanisms involved in the formation of the l-LTM and (ii) to study the involvement of these mechanisms in the structural reorganization of the bee olfactory pathway, associated with l-LTM formation. By focusing our analysis on the part of memory specific of the odor-sugar association, we showed first that l-LTM is induced specifically by conditioning using several trials separated by enough time, and requires two successive waves of transcription, an earlier one and a later one. We then hypothesized that the early wave of transcription was induced, at least in part, after CREB activation. We thus evaluated the effects of CREB inhibition on l-LTM: our data suggest that this transcription factor could play a non-exclusive role, apparently localized in the antennal lobes (first olfactory centers), in LTM consolidation. We have also discovered a modulation of l-LTM by a signaling pathway involving probably L-arginine, whose precise molecular substrates remain to be identified. Subsequently, we investigated the possible involvement of nitric oxide and calcium, in the formation of neural changes associated with this form of memory. Indeed, these two signaling pathways have been identified previously as being important for long-term memory. We therefore sought to highlight changes in structural changes associated with l-LTM after interfering with either pathway (using pharmacological treatments). For this, we looked for changes of volume and density affecting the functional neuropilar units of the olfactory pathway, respectively the antennal lobe glomeruli and mushroom body microglomeruli. Though the lack of reproducibility of data prevented us from concluding about a role for nitric oxide signaling, our results indicate the involvement of calcium signaling in the reorganization of the bee's olfactory system in the mushroom bodies, associated with l-LTM formation. Overall, these results have led to better characterize the mechanisms of long-term memory in the honeybee, showing similarities with other species. They open new perspectives for studying the mechanisms of synaptic plasticity associated with memory in this model

La dépression est une maladie chronique, récurrente et potentiellement mortelle qui affecte plus de 20 % de la population à travers le monde. Les mécanismes sous-jacents de la dépression demeurent incompris et la pharmacothérapie actuelle, largement basée sur l’hypothèse monoaminergique, fait preuve d’une efficacité sous optimale et d’une latence thérapeutique élevée. Par conséquent, la recherche est amenée à élaborer de nouveaux traitements pharmacologiques. Pour détecter leur action, il est avant tout nécessaire de développer des outils expérimentaux adéquats. Dans cette optique, notre but a été de mesurer l’anhédonie, un symptôme cardinal de la dépression, chez le rat de laboratoire. L’anhédonie a été définie comme une réduction de la récompense et a été mesurée avec le test de consommation de sucrose et la technique d’autostimulation intracérébrale. En vue d’induire l’anhédonie, nous avons effectué une bulbectomie olfactive, une procédure qui entraîne divers changements biochimiques, cellulaires et comportementaux similaires à ceux de l’état dépressif et qui peuvent être renversés par un traitement antidépresseur chronique. Nos résultats montrent que la bulbectomie olfactive produit également l’anhédonie, reflétée par une réduction durable de la consommation de sucrose et par une réduction de l’efficacité de l’amphétamine dans le test d’autostimulation intracérébrale. Ces effets ont été présents jusqu’à trois à quatre semaines suivant la chirurgie. La bulbectomie olfactive a aussi été associée à une augmentation de l’élément de réponse liant l’AMPc dans le striatum, un index moléculaire associé à l’anhédonie. Ces découvertes suggèrent que l’anhédonie peut être produite et étudiée de façon fiable dans le modèle de bulbectomie olfactive et que le circuit de récompense pourrait constituer une cible cohérente pour de nouvelles drogues en vue du traitement de la dépression.
Depression is a chronic, recurrent and potentially deadly disorder that affects over 20 % of the population worldwide. The mechanisms underlying depression are still not understood and current pharmacotherapy, based largely on monoaminergic hypotheses, is plagued by suboptimal efficacy and delayed therapeutic latency. This has lead to a search for novel pharmacological treatments. To achieve this, it is first necessary to develop adequate experimental tools. With this in mind, we aimed to measure anhedonia, a cardinal symptom of depression, in laboratory rats. We defined anhedonia as a reduction in reward, and measured it with the sucrose intake test and in the intracranial self-stimulation paradigm. In order to induce anhedonia, we surgically removed the olfactory bulbs, a procedure that results in a host of behavioral, cellular and biochemical changes that are qualitatively similar to those observed in clinical depression. These changes are long-lasting and reversed by chronic antidepressant treatment, validating olfactory bulbectomy as an animal model of depression. Our results show that olfactory bulbectomy also produces anhedonia, reflected by a stable and long-lasting reduction in sucrose intake as well as a reduction in the rewarding effectiveness of amphetamine in the self-stimulation paradigm. These effects were present even after three to four weeks post-surgery. Olfactory bulbectomy was also associated with increased striatal cAMP response element binding, a molecular index associated with depressive-like behaviour. These findings suggest that anhedonia can be reliably produced and studied within the olfactory bulbectomy model and that reward circuitry may comprise a logical target for novel drugs to treat depression.

Abstract This chapter provides a comprehensive overview linking the sense of smell to the field of emotion. The authors focus on the evolution of the olfactory system and its anatomical pathways, including overlapping structures with emotional processing circuits. The chapter proceeds with a detailed analysis of the functional relationship between olfaction and emotion based on the following topics: odors and leading emotional reactions; odors as chemosignals within social communication (focusing on a definition of chemosignals; chemosignals and emotional contagion; and chemosignals within intimate relationships); and odors as a basis for multisensory integration in the context of emotional processing. Throughout the chapter, the authors present a broad range of experimental studies including behavioral and neuroimaging investigations, as well as implications of research on patients with olfactory deficits.

Abstract The chemical senses—taste and smell—are the oldest animal senses. They are characterized by a multidimensional and diverse stimulus space, consisting of many molecules that cannot be classified along any narrow set of dimensions. In the case of the olfactory system, animals detect low molecular weight volatile chemicals (i.e. odorants) with the help of specialized olfactory sensory neurons that express one or a few ligand-binding odorant receptor proteins. Animals cope with the problem of recognizing an extremely large number of different odorants by programming a very large number of functionally different olfactory neurons. Odours activate these neurons and generate characteristic activity patterns across the neuron population, which are relayed to second-order olfactory neurons. The entire available raw information about the animal’s olfactory environment is present in these patterns; however, olfactory information is further processed before it is relayed to higher-order brain centres. Drosophila melanogaster provides an attractive model organism for studying olfaction, as it allows genetic, molecular, and physiological analyses. In recent years, immense progress has been achieved in understanding the olfactory neuronal circuits that underlie the coding and processing of odours in Drosophila. Here, the chapter reviews our present state of knowledge regarding the anatomical architecture of the fly’s olfactory system as well as giving recent insights into the coding strategies of the different neuronal populations involved.