Olfactory-driven beta band entrainment of limbic circuitry during neonatal development

ABSTRACTCognitive processing relies on the functional refinement of the limbic circuitry during the first two weeks of life. During this developmental period, when most sensory systems are still immature, the sense of olfaction acts as “door to the world”, providing the main source of environmental inputs. However, it is unknown whether early olfactory processing shapes the development of the limbic circuitry. Here, we address this question by combining simultaneous in vivo recordings from the olfactory bulb (OB), lateral entorhinal cortex (LEC), hippocampus (HP), and prefrontal cortex (PFC) with opto- and chemogenetic manipulations of mitral/tufted cells (M/TCs) in the OB of non-anesthetized neonatal mice. We show that the neonatal OB synchronizes the limbic circuity in beta frequency range. Moreover, it drives neuronal and network activity in LEC, as well as subsequently, HP and PFC via long-range projections from mitral cells (MCs) to HP-projecting LEC neurons. Thus, OB activity controls the communications within limbic circuits during neonatal development.

539 - The association between olfactory dysfunction and psychiatric disorders

It is known that olfactory dysfunction occurs early in neurodegenerative diseases, such as Alzheimer’s and Parkinson’s diseases and frontotemporal dementia (FTD). Dementia and psychiatric disorders share a number of clinical features, such as psychosis and depression. As such, misdiagnoses across these conditions are not uncommon. A variety of studies show smell dysfunction in schizophrenia, but little is known about other psychiatric disorders. In order to verify the link between olfaction and psychiatric disorders, a medical literature search was carried out in may 2021 using PubMed, and Cochrane Library, including the terms “olfaction” and “olfactory dysfunction” combined individually with “psychiatric disorder” and “depression”. Systematic reviews and meta-analyses written in English from 1991 to 2021 were included. Even thought one review suggested that patients with depression have reduced olfactory performance when compared with healthy, results show studies with different methodology and design which makes it difficult to reach definitive conclusions as how and if olfactory functioning is related to depression. Further studies with the same methodology that examines and separates central and peripheral olfactory processing are needed. Another review showed robust olfactory deficits in schizophrenia and at-risk youths, what indicates that olfactory measures may be a useful marker of schizophrenia risk status. Finally, a systematic review compared olfactory function in FTD, depression, schizophrenia and bipolar disorder. Results revealed that odor identification but not discrimination was severely impaired in FTD, both were impaired in schizophrenia, while no olfactory impairments were observed in depression. Findings in bipolar disorder were mixed. This review showed that testing odor identification and discrimination differentiates FTD from depression and schizophrenia, but not from bipolar disorder. It is possible to conclude that olfactory dysfunction occurs in schizophrenia and dementia but not in depression.

Chronic exposure to odors at naturally occurring concentrations triggers limited plasticity in early stages of Drosophila olfactory processing

ABSTRACTIn insects and mammals, chronic exposure to odors at high concentrations in early life alters olfactory function, but the role of odor experience-dependent plasticity in more naturalistic contexts is less clear. We investigated olfactory plasticity in the Drosophila antennal lobe by exposing flies to odors at concentrations that are typically encountered in natural odor sources. These stimuli also strongly and selectively activated only a single class of olfactory receptor neuron (ORN) input, facilitating the investigation of input-specific plasticity. Overall, chronic exposure to three such odors elicited limited plasticity in the odor responses of second-order projection neurons (PNs). Exposure to some odors elicited mild increases in PN responses to weak stimuli, extending the lower bound of the dynamic range of PN signaling. When present, plasticity was observed broadly in multiple PN types and thus was not selective for PNs receiving direct input from the chronically active ORNs. Chronic E2-hexenal exposure did not affect PN intrinsic properties, local inhibitory innervation, ORN responses, or ORN-PN synaptic strength, but modestly increased broad lateral excitation evoked by some odors. These results show that PN odor coding is only mildly affected by strong persistent activation of a single olfactory input and highlight the stability of early stages of insect olfactory processing to significant perturbations in the sensory environment.

Volumetry of Olfactory Structures in Mild CognitiveImpairment and Alzheimer’s Disease: A Systematic Review and a Meta-Analysis

Olfactory decline is an early symptom of Alzheimer’s disease (AD) and is a predictor of conversion from mild cognitive impairment (MCI) to AD. Olfactory decline could reflect AD-related atrophy of structures related to the sense of smell. The aim of this study was to verify whether the presence of a clinical diagnosis of AD or MCI is associated with a volumetric decrease in the olfactory bulbs (OB) and the primary olfactory cortex (POC). We conducted two systematic reviews, one for each region and a meta-analysis. We collected articles from PsychNet, PubMed, Ebsco, and ProQuest databases. Results showed large and heterogeneous effects indicating smaller OB volumes in patients with AD (k = 6, g = −1.21, 95% CI [−2.19, −0.44]) and in patients with MCI compared to controls. There is also a trend for smaller POC in patients with AD or MCI compared to controls. Neuroanatomical structures involved in olfactory processing are smaller in AD and these volumetric reductions could be measured as early as the MCI stage.

Human olfaction: A view from the top

Although our understanding of human olfactory perception has increased dramatically, it remains less well understood how olfaction interacts with cognitive processes. In this overview, we review the claim that olfaction evolved to predominantly embody “novelty detection”, a framework that leaves little room for cognitive representations and processes. We challenge this framework by pointing to several perceived limitations. Instead, we argue that human olfaction is best understood in terms of its reliance on top-down processes; visual or verbal contexts may generate predictions of odor qualities, and a fundamental role of olfaction is to evaluate such predictions. We hypothesize that olfaction is to a large extent dependent on “top-down” cognitive processes. We discuss how this framework differs from other contemporary approaches to human olfactory perception. We conclude that in contrast to other frameworks that characterize olfaction primarily in terms of stimulus-driven perception, human olfactory processing is best understood by invoking a “view from the top”.

Age and social experience induced plasticity across brain regions of the paper wasp Polistes fuscatus

Developmental studies of brain volumes can reveal which portions of neural circuits are sensitive to environmental inputs. In social insects, differences in relative investment across brain regions emerge as behavioural repertoires change during ontogeny or as a result of experience. Here, we test the effects of maturation and social experience on morphological brain development in Polistes fuscatus paper wasps , focusing on brain regions involved in visual and olfactory processing. We find that mature wasps regardless of social experience have relatively larger brains than newly emerged wasps and this difference is driven by changes to mushroom body calyx and visual regions but not olfactory processing neuropils. Notably, social wasps invest more in the anterior optic tubercle (AOT), a visual glomerulus involved in colour and object processing in other taxa, relative to other visual integration centres the mushroom body calyces compared with aged socially naive wasps. Differences in developmental plasticity between visual and olfactory neuropil volumes are discussed in light of behavioural maturation in paper wasps, especially as it relates to social recognition. Previous research has shown that P. fuscatus need social experience to develop specialized visual processing of faces, which is used to individually recognize conspecifics. The present study suggests that the AOT is a candidate brain region that could mediate facial processing in this species.

Clinical Implications of Psychophysical Olfactory Testing: Assessment, Diagnosis, and Treatment Outcome

Purpose of ReviewOlfactory dysfunction dramatically impairs quality of life with a prevalence of 20% in the general adult population. Psychophysical olfactory testing has been widely used to evaluate the ability to smell due to its validated utility and feasibility in clinic. This review summarizes the current literature regarding psychophysical olfactory testing and the clinical relevance of the olfactory testing with different components. Furthermore, the review highlights the diagnosis and treatment value of olfactory subtests in patients with olfactory dysfunction.Recent FindingsWith the accumulation of studies of psychophysical olfactory testing in olfactory disorders, the clinical relevance of olfactory testing with different components is expanding. Different olfactory domains present with distinct olfactory processing and cortical activity. Psychophysical assessment of olfaction with three domains reveals different levels of olfactory processing and might assist with analyzing the pathophysiologic mechanism of the various olfactory disorders. Furthermore, olfactory thresholds provided the largest amount of non-redundant information to the olfactory diagnosis. Sinonasal olfactory dysfunction and non-sinonasal-related olfactory dysfunction are emerging classifications of smell disorders with certain characteristics of olfactory impairment and different responses to the therapy including steroids, sinus surgery, and olfactory training.SummaryThese recent advancements should promote the understanding of psychophysical olfactory testing, the association between individual subcomponents and neurophysiological processes, and pave the way for precision assessment and treatment of the olfactory dysfunction.