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Journal of Neuroendocrinology Jun 2024Olfaction is the most ancient sense and is needed for food-seeking, danger protection, mating and survival. It is often the first sensory modality to perceive changes in... (Review)
Review
Olfaction is the most ancient sense and is needed for food-seeking, danger protection, mating and survival. It is often the first sensory modality to perceive changes in the external environment, before sight, taste or sound. Odour molecules activate olfactory sensory neurons that reside on the olfactory epithelium in the nasal cavity, which transmits this odour-specific information to the olfactory bulb (OB), where it is relayed to higher brain regions involved in olfactory perception and behaviour. Besides odour processing, recent studies suggest that the OB extends its function into the regulation of food intake and energy balance. Furthermore, numerous hormone receptors associated with appetite and metabolism are expressed within the OB, suggesting a neuroendocrine role outside the hypothalamus. Olfactory cues are important to promote food preparatory behaviours and consumption, such as enhancing appetite and salivation. In addition, altered metabolism or energy state (fasting, satiety and overnutrition) can change olfactory processing and perception. Similarly, various animal models and human pathologies indicate a strong link between olfactory impairment and metabolic dysfunction. Therefore, understanding the nature of this reciprocal relationship is critical to understand how olfactory or metabolic disorders arise. This present review elaborates on the connection between olfaction, feeding behaviour and metabolism and will shed light on the neuroendocrine role of the OB as an interface between the external and internal environments. Elucidating the specific mechanisms by which olfactory signals are integrated and translated into metabolic responses holds promise for the development of targeted therapeutic strategies and interventions aimed at modulating appetite and promoting metabolic health.
Topics: Olfactory Bulb; Animals; Humans; Neurosecretory Systems; Feeding Behavior; Smell; Energy Metabolism
PubMed: 38468186
DOI: 10.1111/jne.13382 -
Progress in Neurobiology Feb 2023Multisensory integration plays an important role in animal cognition. Although many studies have focused on visual-auditory integration, studies on olfactory-auditory...
Multisensory integration plays an important role in animal cognition. Although many studies have focused on visual-auditory integration, studies on olfactory-auditory integration are rare. Here, we investigated neural activity patterns and odor decoding in the lateral entorhinal cortex (LEC) under uni-sensory and multisensory stimuli in awake, head-fixed mice. Using specific retrograde tracing, we verified that the LEC receives direct inputs from the primary auditory cortex (AC) and the medial geniculate body (MGB). Strikingly, we found that mitral/tufted cells (M/Ts) in the olfactory bulb (OB) and neurons in the LEC respond to both olfactory and auditory stimuli. Sound decreased the neural responses evoked by odors in both the OB and LEC, for both excitatory and inhibitory responses. Interestingly, significant changes in odor decoding performance and modulation of odor-evoked local field potentials (LFPs) were observed only in the LEC. These data indicate that the LEC is a critical center for olfactory-auditory multisensory integration, with direct projections from both olfactory and auditory centers.
Topics: Mice; Animals; Entorhinal Cortex; Smell; Odorants; Olfactory Bulb; Evoked Potentials
PubMed: 36581184
DOI: 10.1016/j.pneurobio.2022.102399 -
Journal of the History of the... 2022This article presents a collection of previously overlooked, stereotyped, abstract, anatomical representations of the olfactory bulbs and tracts that were printed as...
This article presents a collection of previously overlooked, stereotyped, abstract, anatomical representations of the olfactory bulbs and tracts that were printed as part of schematic woodcuts of the medieval cell doctrine, generally in the early-sixteenth century but extending into the seventeenth century and, in at least one case, to the mid-nineteenth century. A representation of the olfactory bulbs is incorporated into many of these woodcuts, beginning with an illustration by German physician, philosopher, and theologian Magnus Hundt in 1501 in his , which showed central projections of the two olfactory bulbs joining in the meshwork of the . German physician and anatomist Johann Eichmann, known as Johannes Dryander, modified Hundt's figure for his own monograph in 1537 but retained the representation of the olfactory bulbs. In 1503, German Carthusian humanist writer Gregor Reisch published an influential and highly copied woodcut in his , showing connections from the olfactory bulbs overlying the bridge of the nose (as well as from other special sense organs) to the in the anterior cell or ventricle. In the following centuries, numerous authors derived similar figures from Reisch's original schematic illustration of the medieval cell doctrine, including Brunschwig (1512, 1525), Głogowczyk (1514), Romberch/Host (1520), Leporeus/Le Lièvre (1520, 1523), Dolce (1562), Lull/Bernardus de Lavinheta (1612), and Elliotson (1835). Similar representations were provided by Peyligk (1518) and Eck (1520). These stereotyped schematic images linked the olfactory bulbs to olfaction before the advent of more realistic images beginning in the mid-sixteenth century.
Topics: Anatomists; Humans; Olfactory Bulb
PubMed: 34788191
DOI: 10.1080/0964704X.2021.1976585 -
Journal of Alzheimer's Disease : JAD 2022Olfactory dysfunction is highly prevalent in dementia syndromes, including Alzheimer's disease (AD) and frontotemporal dementia (FTD). The structural integrity of the...
BACKGROUND
Olfactory dysfunction is highly prevalent in dementia syndromes, including Alzheimer's disease (AD) and frontotemporal dementia (FTD). The structural integrity of the olfactory bulb (OB) is thought to play a critical role in odor detection and identification, but no MRI study has measured OB volume in FTD, or measured OB volume longitudinally in AD.
OBJECTIVE
To measure OB volume in FTD and AD patients longitudinally using MRI.
METHODS
This study measured OB volumes using MRI in patients diagnosed with behavioral-variant FTD (n = 55), semantic dementia (n = 34), progressive non-fluent aphasia (n = 30), AD (n = 50), and healthy age-matched controls (n = 55) at their first visit to a dementia research clinic ('baseline'). Imaging data in patients 12-months later were analyzed where available (n = 84) for longitudinal assessment. Volumes of subcortical and cortical olfactory regions ('olfactory network') were obtained via surface-based morphometry.
RESULTS
Results revealed that in AD and FTD at baseline, OB volumes were similar to controls, whereas volumes of olfactory network regions were significantly reduced in all patient groups except in progressive non-fluent aphasia. Longitudinal data revealed that OB volume became significantly reduced (10-25% volume reduction) in all dementia groups with disease progression.
CONCLUSION
Olfactory dysfunction is common in patients diagnosed with AD or FTD, but our results indicate that there is no detectable volume loss to the OBs upon first presentation to the clinic. Our findings indicate that the OBs become detectably atrophied later in the disease process. OB atrophy indicates the potential usefulness for OBs to be targeted in interventions to improve olfactory function.
Topics: Alzheimer Disease; Aphasia; Atrophy; Frontotemporal Dementia; Humans; Magnetic Resonance Imaging; Olfaction Disorders; Olfactory Bulb
PubMed: 35848020
DOI: 10.3233/JAD-220080 -
Chemical Senses Oct 2020It is well established that early blindness results in enhancement of the remaining nonvisual sensory modalities accompanied by functional and anatomical brain...
It is well established that early blindness results in enhancement of the remaining nonvisual sensory modalities accompanied by functional and anatomical brain plasticity. While auditory and tactile functions have been largely investigated, the results regarding olfactory functions remained less explored and less consistent. In the present study, we investigated olfactory function in blind mice using 3 tests: the buried food test, the olfactory threshold test, and the olfactory performance test. The results indicated better performance of blind mice in the buried food test and odor performance test while there was no difference in the olfactory threshold test. Using histological measurements, we also investigated if there was anatomical plasticity in the olfactory bulbs (OB), the most salient site for olfactory processing. The results indicated a larger volume of the OB driven by larger glomerular and granular layers in blind mice compared with sighted mice. Structural plasticity in the OB may underlie the enhanced olfactory performance in blind mice.
Topics: Animals; Blindness; Disease Models, Animal; Female; Male; Mice; Olfactory Bulb; Olfactory Perception; Smell
PubMed: 32766717
DOI: 10.1093/chemse/bjaa052 -
Dynamic Impairment of Olfactory Behavior and Signaling Mediated by an Olfactory Corticofugal System.The Journal of Neuroscience : the... Sep 2020Processing of olfactory information is modulated by centrifugal projections from cortical areas, yet their behavioral relevance and underlying neural mechanisms remain...
Processing of olfactory information is modulated by centrifugal projections from cortical areas, yet their behavioral relevance and underlying neural mechanisms remain unclear in most cases. The anterior olfactory nucleus (AON) is part of the olfactory cortex, and its extensive connections to multiple upstream and downstream brain centers place it in a prime position to modulate early sensory information in the olfactory system. Here, we show that optogenetic activation of AON neurons in awake male and female mice was not perceived as an odorant equivalent cue. However, AON activation during odorant presentation reliably suppressed behavioral odor responses. This AON-mediated effect was fast and constant across odors and concentrations. Likewise, activation of glutamatergic AON projections to the olfactory bulb (OB) transiently inhibited the excitability of mitral/tufted cells (MTCs) that relay olfactory input to the cortex. Single-unit MTC recordings revealed that optogenetic activation of glutamatergic AON terminals in the OB transiently decreased sensory-evoked MTC spiking, regardless of the strength or polarity of the sensory response. The reduction in MTC firing during optogenetic stimulation was confirmed in recordings in awake mice. These findings suggest that glutamatergic AON projections to the OB impede early olfactory signaling by inhibiting OB output neurons, thereby dynamically gating sensory throughput to the cortex. The anterior olfactory nucleus (AON) as an olfactory information processing area sends extensive projections to multiple brain centers, but the behavioral consequences of its activation have been scarcely investigated. Using behavioral tests in combination with optogenetic manipulation, we show that, in contrast to what has been suggested previously, the AON does not seem to form odor percepts but instead suppresses behavioral odor responses across odorants and concentrations. Furthermore, this study shows that AON activation inhibits olfactory bulb output neurons in both anesthetized as well as awake mice, pointing to a potential mechanism by which the olfactory cortex can actively and dynamically gate sensory throughput to higher brain centers.
Topics: Animals; Female; Glutamic Acid; Male; Mice; Mice, Inbred C57BL; Neurons, Afferent; Olfactory Bulb; Olfactory Pathways; Olfactory Perception; Smell; Synaptic Transmission
PubMed: 32817250
DOI: 10.1523/JNEUROSCI.2667-19.2020 -
European Archives of... Mar 2022To evaluate the success of olfactory training in patients with olfactory loss and olfactory bulb (OB) atrophy detected on magnetic resonance imaging (MRI) and other...
PURPOSE
To evaluate the success of olfactory training in patients with olfactory loss and olfactory bulb (OB) atrophy detected on magnetic resonance imaging (MRI) and other characteristics.
METHODS
This study included 48 patients with olfactory loss who underwent a nasal endoscopic examination and MRI before olfactory training. The Korean Version of the Sniffin' Sticks Test was performed before and after training. The olfactory training success was defined as an improvement of more than 6 points in the Threshold-Discrimination-Identification (TDI) score. Patient characteristics and OB atrophy pre-training were compared between successful and unsuccessful groups.
RESULTS
The etiology of olfactory loss included respiratory viral infection in 30 (62.5%), trauma in 10 (20.8%), and idiopathic loss in 8 (16.7%) patients. Twenty-three (47.9%) of 48 patients exhibited successful olfactory training. Etiology, age, gender, and symptom duration were not different between unsuccessful and successful groups. Pre-training discrimination, identification, and TDI scores were significantly different between unsuccessful and successful groups (P < 0.05). Success rate of patients with bilateral OB atrophy was significantly lower than that of patients with unilateral OB atrophy and normal morphology (P = 0.006). OB height was significantly lower in the unsuccessful group than in the successful group (P < 0.05). Bilateral OB atrophy was an independent risk factor for failure of olfactory training according to the multivariate analysis.
CONCLUSION
Olfactory loss patients with bilateral OB atrophy may not be able to improve olfactory function after olfactory training.
Topics: Atrophy; Humans; Magnetic Resonance Imaging; Olfaction Disorders; Olfactory Bulb; Smell
PubMed: 34091728
DOI: 10.1007/s00405-021-06917-z -
Frontiers in Neural Circuits 2020Generation of neuronal diversity is a biological strategy widely used in the brain to process complex information. The olfactory bulb is the first relay station of... (Review)
Review
Generation of neuronal diversity is a biological strategy widely used in the brain to process complex information. The olfactory bulb is the first relay station of olfactory information in the vertebrate central nervous system. In the olfactory bulb, axons of the olfactory sensory neurons form synapses with dendrites of projection neurons that transmit the olfactory information to the olfactory cortex. Historically, the olfactory bulb projection neurons have been classified into two populations, mitral cells and tufted cells. The somata of these cells are distinctly segregated within the layers of the olfactory bulb; the mitral cells are located in the mitral cell layer while the tufted cells are found in the external plexiform layer. Although mitral and tufted cells share many morphological, biophysical, and molecular characteristics, they differ in soma size, projection patterns of their dendrites and axons, and odor responses. In addition, tufted cells are further subclassified based on the relative depth of their somata location in the external plexiform layer. Evidence suggests that different types of tufted cells have distinct cellular properties and play different roles in olfactory information processing. Therefore, mitral and different types of tufted cells are considered as starting points for parallel pathways of olfactory information processing in the brain. Moreover, recent studies suggest that mitral cells also consist of heterogeneous subpopulations with different cellular properties despite the fact that the mitral cell layer is a single-cell layer. In this review, we first compare the morphology of projection neurons in the olfactory bulb of different vertebrate species. Next, we explore the similarities and differences among subpopulations of projection neurons in the rodent olfactory bulb. We also discuss the timing of neurogenesis as a factor for the generation of projection neuron heterogeneity in the olfactory bulb. Knowledge about the subpopulations of olfactory bulb projection neurons will contribute to a better understanding of the complex olfactory information processing in higher brain regions.
Topics: Animals; Dendrites; Humans; Interneurons; Neurons; Olfactory Bulb; Olfactory Pathways; Olfactory Receptor Neurons; Synapses
PubMed: 32982699
DOI: 10.3389/fncir.2020.561822 -
Cell and Tissue Research Jan 2021Whether an odorant is perceived as pleasant or unpleasant (hedonic value) governs a range of crucial behaviors: foraging, escaping danger, and social interaction.... (Review)
Review
Whether an odorant is perceived as pleasant or unpleasant (hedonic value) governs a range of crucial behaviors: foraging, escaping danger, and social interaction. Despite its importance in olfactory perception, little is known regarding how odor hedonics is represented and encoded in the brain. Here, we review recent findings describing how odorant hedonic value is represented in the first olfaction processing center, the olfactory bulb. We discuss how olfactory bulb circuits might contribute to the coding of innate and learned odorant hedonics in addition to the odorant's physicochemical properties.
Topics: Animals; Odorants; Olfactory Bulb; Vertebrates
PubMed: 33515292
DOI: 10.1007/s00441-020-03372-w -
The Journal of Neuroscience : the... Jul 2022Metabolic state can alter olfactory sensitivity, but it is unknown whether the activity of the olfactory bulb (OB) may fine tune metabolic homeostasis. Our objective was...
Metabolic state can alter olfactory sensitivity, but it is unknown whether the activity of the olfactory bulb (OB) may fine tune metabolic homeostasis. Our objective was to use CRISPR gene editing in male and female mice to enhance the excitability of mitral/tufted projection neurons (M/TCs) of the OB to test for improved metabolic health. slice recordings of MCs in CRISPR mice confirmed increased excitability due the targeted loss of Kv1.3 channels, which resulted in a less negative resting membrane potential (RMP), enhanced action potential (AP) firing, and insensitivity to the selective channel blocker margatoxin (MgTx). CRISPR mice exhibited enhanced odor discrimination using a habituation/dishabituation paradigm. CRISPR mice were challenged for 25 weeks with a moderately high-fat (MHF) diet, and compared with littermate controls, male mice were resistance to diet-induced obesity (DIO). Female mice did not exhibit DIO. CRISPR male mice gained less body weight, accumulated less white adipose tissue, cleared a glucose challenge more quickly, and had less serum leptin and liver triglycerides. CRISPR male mice consumed equivalent calories as control littermates, and had unaltered energy expenditure (EE) and locomotor activity, but used more fats for metabolic substrate over that of carbohydrates. Counter to CRISPR-engineered mice, by using chemogenetics to decrease M/TC excitability in male mice, activation of inhibitory designer receptors exclusively activated by designer drugs (DREADDs) caused a decrease in odor discrimination, and resulted in a metabolic profile that was obesogenic, mice had reduced EE and oxygen consumption (VO). We conclude that the activity of M/TC projection neurons canonically carries olfactory information and simultaneously can regulate whole-body metabolism. The olfactory system drives food choice, and olfactory sensitivity is strongly correlated to hunger and fullness. Olfactory function thereby influences nutritional balance and obesity outcomes. Obesity has become a health and financial crisis in America, shortening life expectancy and increasing the severity of associated illnesses. It is expected that 51% of Americans will be obese by the year 2030. Using CRISPR gene editing and chemogenetic approaches, we discovered that changing the excitability of output neurons in the olfactory bulb (OB) affects metabolism and body weight stabilization in mice. Our results suggest that long-term therapeutic targeting of OB activity to higher processing centers may be a future clinical treatment of obesity or type II Diabetes.
Topics: Animals; Body Weight; Diabetes Mellitus, Type 2; Diet, High-Fat; Female; Humans; Male; Mice; Mice, Inbred C57BL; Neurons; Obesity; Olfactory Bulb
PubMed: 35710623
DOI: 10.1523/JNEUROSCI.0190-22.2022