Volume 28, Issue 3 - Special Issue: Olfactory Processing; Guest Editors: Marc Spehr and Ileana L. Hanganu-Opatz

The olfactory epithelium (OE) and its associated perireceptor space, i.e., the mucus layer (ML) covering the epithelium, are the most peripheral parts of the vertebrate olfactory system. The olfactory receptor neurons (ORNs), one of the cell types of the OE, are the odorant detectors of the olfactory system. These bipolar neurons extend their apical appendages, which express odorant receptors, into the ML. The binding of odorants to odorant receptors is the initial step of odor processing. The vast majority of research on the peripheral olfactory system has focused on the ORNs and the molecular components of the olfactory transduction cascades. Less attention has been directed to the other cell types of the OE and their physiological functions. For a long time, it was assumed that the olfactory signals detected in the OE are transmitted to the olfactory bulb without preprocessing, but this view turned out to be over-simplistic. It has been shown that the olfactory signals are critically modulated already in the OE. Despite compelling evidence, many descriptions of the olfactory system still ignore the existence of these peripheral modulatory mechanisms. The importance of peripheral modulation of the olfactory signals, the physiological functions of the other epithelial cell types, the extrinsic innervation of the olfactory mucosa, and the perireceptor space are only slowly coming into focus in the olfactory research. Furthermore, several intraepithelial signaling pathways that signal epithelial damage and initiate regenerative processes have recently been discovered. This review provides a concise overview of the current knowledge of peripheral events in the olfactory mucosa and the perireceptor space.

In this review, we aim to integrate our recent findings on the vasopressin system and its role in social discrimination with other known neuromodulatory mechanisms in the olfactory bulb that are involved in different experimental models of social memory. Behavioral paradigms commonly used to investigate odor-related social memory are individual social memory in rodents, lamb recognition in sheep, and the Bruce effect in female mice. All three cases involve neuromodulation in the main and/or the accessory olfactory bulb, the first centers for olfactory processing. As a large diversity of neuromodulators participate in social memory formation, here, we focus primarily on shared neuromodulatory systems and their physiological effects, in particular, the social neuropeptides, vasopressin and oxytocin, and the arousal-related modulators, acetylcholine and noradrenaline.

In order to survive, an organism has to adapt its behavioral actions to the current context by carefully balancing sensory input with physiological state and need. This challenge becomes particularly apparent for olfactory stimuli (volatile chemicals), which can signal not only food sources, mating partners, and offspring, but also pathogens, competitors, and predators. To achieve this difficult task, information processing in the olfactory system is strongly modulated by internal state (for example, metabolic or endocrine), environmental context, and previous experience. This article provides an overview how different internal states impact olfactory processing and discusses potential underlying mechanisms. It starts with a brief excurse on a leading model to study olfaction— Mus musculus —and concludes with implications for human health and disease.

Sensory input across modalities is highly dynamic, continuously confronting the brain with the task of making sense of the external world. Olfaction is a key sense that many species depend on for survival, for example to locate food sources and mating partners or to avoid encountering predators. In the absence of visual cues, olfactory cues are especially useful, as they provide information over a large range of distances. Natural odours form temporally complex plumes that show rapid fluctuations in odour concentration carrying information about the location of an odour source. This review focuses on how primarily mammals use this spatial information from olfactory cues to navigate their environment. I highlight progress made on the physical description of dynamically fluctuating odours, behavioural paradigms to investigate odour-guided navigation and review initial findings on the underlying neural mechanisms that allow mammals to extract spatial information from the dynamic odour landscape.

Olfaction, despite being evolutionarily one of the oldest senses, is complex in structure and function. It can distinguish between trillions of odorants, provides orientation, mediates social interactions, and serves as a warning system. Sensory signals from the periphery are first processed in the olfactory bulb (OB) and then distributed to several olfactory cortical structures. Unlike other sensory modalities, this primary sensory information is not relayed via the thalamus. One prominent olfactory cortical region is the anterior olfactory nucleus (AON), a two-layered structure located within the olfactory peduncle. The AON exerts strong reciprocal connections not only to the OB but also to higher brain areas, e.g., the piriform cortex (PCx), thereby serving as a hub for bottom-up and top-down information processing. However, the functional role of the AON is not well-understood. Here, we provide an overview of recent publications investigating the function of AON in olfactory processing and behavior and present a framework for future research on this fascinating archicortical structure.

Although mostly unaware, we constantly navigate a complex landscape of airborne molecules. The perception of these molecules helps us navigate, shapes our social life, and can trigger emotionally charged memories transporting us back to the past within a split second. While the processing of olfactory information in early sensory areas is well understood, how the sense of smell affects cognition only recently gained attention in the field of neuroscience. Here, we review links between olfaction and cognition and explore the idea that the activity in olfactory areas may be critical for coordinating cognitive networks. Further, we discuss how olfactory activity may shape the development of cognitive networks and associations between the decline of olfactory and cognitive abilities in aging. Olfaction provides a great tool to study large-scale networks underlying cognitive abilities and bears the potential for a better understanding of cognitive symptoms associated with many mental disorders.