Chronic stress leads to long-lasting deficits in olfactory-guided behaviors, and to neuroplastic changes in the nucleus of the lateral olfactory tract

Olfactory Impairment and Frailty: A Systematic Review and Meta-Analysis

Importance

Olfactory impairment (OI) and frailty are prevalent conditions associated with aging, but studies investigating their association with each other have been discordant.

Objective

To summarize current evidence surrounding the association between OI and frailty.

Data Sources

PubMed, Embase, Cochrane Library, SCOPUS, and CINAHL from inception to November 28, 2023.

Study Selection

This study included observational studies investigating the association between objectively or subjectively assessed OI and objectively evaluated frailty among adults.

Data Extraction and Synthesis

Two independent authors extracted data into a structured template. Maximally adjusted estimates were pooled using a random-effects model, and statistical heterogeneity was evaluated using I2 values. Additional prespecified subgroup and sensitivity analyses were performed. This study used the Newcastle-Ottawa Scale for bias assessment and the Grading of Recommendations Assessment, Development and Evaluation framework for overall evidence quality evaluation.

Main Outcomes and Measures

The primary outcome was the cross-sectional association between OI and frailty, for which the odds of frailty were compared between participants with and without OI. The secondary outcome was the cross-sectional association between frailty and OI, for which the odds of OI were compared between participants with and without frailty.

Results

This study included 10 studies with 10 624 patients (52.9% female; mean [SD] age, 62.9 [9.6] years). The Newcastle-Ottawa Scale score of studies ranged from low to moderate. Grading of Recommendations Assessment, Development and Evaluation scores ranged from low to moderate. OI was associated with a 2.32-fold (odds ratio [OR], 2.32; 95% CI, 1.63-3.31; I2 = 0%) greater odds of frailty compared with individuals with healthy olfactory function. The odds of OI was progressively greater with categorical frailty status, with a 1.55-fold (OR, 1.55; 95% CI, 1.32-1.82; I2 = 0%), 2.28-fold (OR, 2.28; 95% CI, 1.96-2.65; I2 = 0%), and 4.67-fold (OR, 4.67; 95% CI, 2.77-7.86; I2 = 0%) increase in odds for individuals with prefrailty, frailty, and the most frailty, respectively, compared with robust individuals. The results demonstrated stability in subgroup analyses (geographical continent of study, objective vs subjective olfactory assessment) and sensitivity tests.

Conclusions and Relevance

The results of this systematic review and meta-analysis suggest that there is an association between OI and frailty, with an increase in the odds of OI with worsening categorical frailty status among individuals with prefrailty, frailty, and the most frailty. OI may be a potential biomarker for frailty. Future studies could delve into whether OI may be a modifiable risk factor for frailty.

Odor awareness modulates the association between perceived stress and chemosensory anhedonia in women

Chronic stress alters reward sensitivity and contributes to anhedonia. Chemosensation is dominated by a hedonic dimension, but little is known about the association between chronic perceived stress and hedonic chemosensation in non-clinical populations. In the current study, 325 participants (201 females) completed a questionnaire-based survey measuring their chronic perceived stress (Perceived Stress Scale; PSS), chemosensory pleasure (Chemosensory Pleasure Scale; CPS), and olfactory metacognitive abilities (odor awareness, affective impact of odor, importance of olfaction). For females, higher PSS scores significantly predicted lower CPS scores, which is mediated by the positive odor awareness. Moreover, negative odor awareness was identified as a moderator underlying the relationship between PSS and CPS scores in females  but not in males. For females, higher PSS predicted lower CPS for those with lower, but not for those with higher levels of negative odor awareness. These results show that the link between chronic perceived stress and chemosensory anhedonia is pronounced in females, with olfactory perception playing a key role. The current study provides insights into the understanding of stress-related anhedonia and into the development of effective treatments.

Alcohol Dependence Modifies Brain Networks Activated During Withdrawal and Reaccess: A c-Fos-Based Analysis in Mice

BACKGROUND

High-level alcohol consumption causes neuroplastic changes in the brain that promote pathological drinking behavior. Some of these changes have been characterized in defined brain circuits and cell types, but unbiased approaches are needed to explore broader patterns of adaptations.

METHODS

We used whole-brain c-Fos mapping and network analysis to assess patterns of neuronal activity during alcohol withdrawal and following reaccess in a well-characterized model of alcohol dependence. Mice underwent 4 cycles of chronic intermittent ethanol to increase voluntary alcohol consumption, and a subset underwent forced swim stress to further escalate consumption. Brains were collected either 24 hours (withdrawal) or immediately following a 1-hour period of alcohol reaccess. c-fos counts were obtained for 110 brain regions using iDISCO and ClearMap. Then, we classified mice as high or low drinkers and used graph theory to identify changes in network properties associated with high-drinking behavior.

RESULTS

During withdrawal, chronic intermittent ethanol mice displayed widespread increased c-Fos expression relative to air-exposed mice, independent of forced swim stress. Reaccess drinking reversed this increase. Network modularity, a measure of segregation into communities, was increased in high-drinking mice after alcohol reaccess relative to withdrawal. The cortical amygdala showed increased cross-community coactivation during withdrawal in high-drinking mice, and cortical amygdala silencing in chronic intermittent ethanol mice reduced voluntary drinking.

CONCLUSIONS

Alcohol withdrawal in dependent mice causes changes in brain network organization that are attenuated by reaccess drinking. Olfactory brain regions, including the cortical amygdala, drive some of these changes and may play an important but underappreciated role in alcohol dependence.

Social Odour Perception and Stress Responses in Women’s Quality of Partner Relationship and Attachment Style

The perception of body and social odours (SOP) is crucial for interpersonal chemosensory signalling and mate choice, yet little is known about the role of the SOP on the quality of partnerships and the attachment style. The aim of this study was to investigate the role of the SOP in women’s stress responses by considering the role of biopsychosocial variables in the quality of interpersonal relationships (also considering intimate partner violence). In total, 253 women filled out an online survey that included a series of questionnaires to investigate self-perceived stress (PSS), emotional regulation (ERQ), olfactory social assessment (SOS), quality of partnership (RRQ), attachment style (RQ), and the Conflict Tactile Scale 2 (CTS-2). The main results highlight that a high awareness of social odours correlates with a good quality of relationship and with an emotional regulation capacity; the PSS correlates negatively with the ERQ (i.e., as the PSS increases, the ERQ decreases). The level of IPV predicts an interpersonal style characterized by a low desire to develop meaningful relationships but with a tendency to depend on and trust another. The idea of being hurt by the other is not central in women who experience this type of relationship. The study’s main conclusion is that social odour perception is important for emotional regulation and in partner relationships.

Chronic stress is associated with reward and emotion-related eating behaviors in college students

Introduction

Stress is related to altered olfactory perception and eating behaviors. The current study investigated the association between chronic stress, food reward and perception of food and non-food odors among college students.

Methods

Sixty-one participants completed the Perceived Stress Scale (PSS) and the Brief Daily Stressors Screening Tool (BDSST). The detective threshold and suprathreshold perception (pleasantness, intensity, and familiarity) of two food (chocolate, strawberry) odors and a non-food (rose) odor were measured. Food reward and macronutrient preference were measured using the computerized Leeds Food Preference Questionnaire and the Macronutrient and Taste Preference Ranking task, respectively. Reward-related eating, emotional eating and eating-related inhibitory control were measured by the Dutch Eating Behavior Questionnaire (DEBQ) and the Reward-Based Eating Drive Scale (RED) scales.

Results

Neither the perceived stress or the severity of daily life stressor exposure was related to odor sensitivity, however, the PSS score was significantly correlated with pleasantness for strawberry odor (r = 0.329, p = 0.013). Chronic stress (PSS and BDSST scores) was significantly correlated with the DEBQ emotional eating and reward-related eating measured by RED (all ps < 0.01). Moreover, the BDSST score was negatively correlated with subjective liking for low-calorie sweet foods (r = -0.46, p < 0.001).

Discussion

Together, our preliminary results suggest disassociated effect of chronic stress on odor perception and eating behaviors.

Chronic unpredictable stress shifts rat behavior from exploration to exploitation

Chronic stress evokes wide-ranging behavioral alterations, including risk avoidance, increased motoric output, and reduced consummatory behaviors. These are often interpreted as dysfunctions, but they may subserve adaptations for coping with existential threats. We tested this in a cohort of rats previously exposed to mild unpredictable stress for 5 weeks. Previously stressed rats exhibited the typically increased avoidance of open field and altered responses to predator odor, suggesting enhanced sensitivity to threatening contexts and cues. Interestingly, these animals collected rewards at a higher rate than controls, because they locomoted faster, spent less time in off-task (exploratory) behavior, and committed fewer licks at feeders. Further, they were not impaired in flexibly shifting choice as reward probabilities changed among feeders, suggesting that behavioral adaptations are not simply of transference to behavioral control to neural systems insensitive to reward (e.g. habits). These data add to a small but growing body of evidence indicating that stress shifts responses away from exploration and toward exploitation of resources, possibly to reduce threat exposure.HighlightsRats with a history of stress collected reward at a higher rate than controls on an operant task, owing to increase locomotion speed, reduced off-task behavior, and reduced time licking at feeders.Previously stressed rats exhibited increased win-stay responses than controls, suggesting the involvement of neural circuits related to goal-directed responding.Previously stressed rats performed equally to controls on a task requiring a shift of preferences based on reward probability, suggesting that they are not simply relying more on habit-based neural systems.

Highlighting Immune System and Stress in Major Depressive Disorder, Parkinson's, and Alzheimer's Diseases, with a Connection with Serotonin

There is recognition that both stress and immune responses are important factors in a variety of neurological disorders. Moreover, there is an important role of several neurotransmitters that connect these factors to several neurological diseases, with a special focus in this paper on serotonin. Accordingly, it is known that imbalances in stressors can promote a variety of neuropsychiatric or neurodegenerative pathologies. Here, we discuss some facts that link major depressive disorder, Alzheimer’s, and Parkinson’s to the stress and immune responses, as well as the connection between these responses and serotonergic signaling. These are important topics of investigation which may lead to new or better treatments, improving the life quality of patients that suffer from these conditions.

Odor Sensitivity Versus Odor Identification in Older US Adults: Associations With Cognition, Age, Gender, and Race

The ability to identify odors predicts morbidity, mortality, and quality of life. It varies by age, gender, and race and is used in the vast majority of survey and clinical literature. However, odor identification relies heavily on cognition. Other facets of olfaction, such as odor sensitivity, have a smaller cognitive component. Whether odor sensitivity also varies by these factors has not been definitively answered. We analyzed data from the National Social Life, Health, and Aging Project, a nationally representative study of older US adults (n = 2081). Odor identification was measured using 5 validated odors presented with Sniffin' Stick pens as was odor sensitivity in a 6-dilution n-butanol constant stimuli detection test. Multivariate ordinal logistic regression modeled relationships between olfaction and age, gender, race, cognition, education, socioeconomic status, social network characteristics, and physical and mental health. Odor sensitivity was worse in older adults (P < 0.01), without gender (P = 0.56) or race (P = 0.79) differences. Odor identification was also worse in older adults, particularly men (both P ≤ 0.01), without differences by race. Decreased cognitive function was associated with worse odor identification (P ≤ 0.01) but this relationship was weaker for odor sensitivity (P = 0.02) in analyses that adjusted for other covariates. Odor sensitivity was less strongly correlated with cognitive ability than odor identification, confirming that it may be a more specific measure of peripheral olfactory processing. Investigators interested in associations between olfaction and health should consider both odor sensitivity and identification when attempting to understand underlying neurosensory mechanisms.

Smell and Stress Response in the Brain: Review of the Connection between Chemistry and Neuropharmacology

The stress response in the brain is not fully understood, although stress is one of the risk factors for developing mental disorders. On the other hand, the stimulation of the olfactory system can influence stress levels, and a certain smell has been empirically known to have a stress-suppressing effect, indeed. In this review, we first outline what stress is and previous studies on stress-responsive biomarkers (stress markers) in the brain. Subsequently, we confirm the olfactory system and review previous studies on the relationship between smell and stress response by species, such as humans, rats, and mice. Numerous studies demonstrated the stress-suppressing effects of aroma. There are also investigations showing the effects of odor that induce stress in experimental animals. In addition, we introduce recent studies on the effects of aroma of coffee beans and essential oils, such as lavender, cypress, α-pinene, and thyme linalool on the behavior and the expression of stress marker candidates in the brain. The transfer of volatile components into the brain is also discussed while using the results of thyme linalool as an example. These studies may provide a good opportunity to connect chemical research at the molecular level with neuropharmacological approaches in the future.

The relationship of olfactory function and clinical traits in major depressive disorder

People who have developed a good sense of smell could experience much more happiness and pleasure, which would trigger a discussion that olfactory disorder might correlate with the pathogenesis of major depressive disorder (MDD). Similar experiments conducted on rats have confirmed that nerve damage of olfactory pathway can induce a series of depression-like changes, including behavior, neurobiochemistry, and neuroimmunity. These changes will recover progressively with anti-depression treatment. While in similar studies on human beings, olfactory dysfunction has been found in people suffering from depression. This review briefly discusses the correlation between olfactory deficits and clinical traits of depression in different dimensions, such as the severity, duration and cognitive impairment of depression. Improving olfactory function may be expected to be a potential antidepressant therapy.

The olfactory mucosa, first actor of olfactory detection, is sensitive to glucocorticoid hormone

The olfactory mucosa (OM) is the primary site of odorant detection, and its axonal projections relay information to brain structures for signal processing. We have previously observed that olfactory function can be affected during a prolonged stress challenge in Wistar rats. The stress response is a neuroendocrine retro-controlled loop allowing pleiotropic adaptive tissue alterations, which are partly mediated through the release of glucocorticoid hormones. We hypothesised that, as part of their wide-ranging pleiotropic effects, glucocorticoids might affect the first step of olfactory detection. To study this, we used a number of approaches ranging from the molecular detection and functional characterisation of glucocorticoid receptors (GRs) in OM cells, to the study of GR acute activation in vivo at the molecular, electrophysiological and behavioural levels. In contrast to previous reports, where GR was reported to be exclusive in olfactory sensory neurones, we located functional GR expression mostly in olfactory ensheathing cells. Dexamethasone (2 mg/kg) was injected intraperitoneally to activate GR in vivo, and this led to functional odorant electrophysiological response (electro-olfactogram) and OM gene expression changes. In a habituation/cross-habituation test of olfactory sensitivity, we observed that DEX-treated rats exhibited higher responsiveness to a complex odorant mixture. These findings support the idea that olfactory perception is altered in stressed animals, as glucocorticoids might enhance odour detection, starting at the first step of detection.

Comparing the Superficial Vasculature of the Central Nervous System in Six Laboratory Animals: A Hypothesis About the Role of the "Circle of Willis"

We provide images of the entire central nervous system vasculature, and compare the anatomical findings in six different laboratory animals. A detailed understanding of the specific anatomy for each is important in the design of experimental modeling and for understanding the specific function of each target organ. Six different types of animals, the Korean wild mouse, C57BL/6J mouse, F344 rat, mongolian gerbil, Syrian hamsters, and guinea pigs, were included. To stain the blood vessels in each of the animals, Alcian blue reagent was used to perfuse each species. The bifurcation and anastomotic patterns of the anterior cerebral arteries differed in each species. The vascular supply to the olfactory nerve was visualized as a single artery supplying both olfactory nerves, and arteries supplying the lateral portion of the olfactory nerves originating from the olfactory bulb area. The posterior communicating arteries of the six animals demonstrated unique morphologies. The shape of the hypophyseal portal system varied by species. Most animals used in this study had a hexagonal Circle of Willis, except for the Korean wild mouse. Using this approach, we successfully mapped the brain vascular system in six different species of animals. This information and the images created can guide other researchers as they design research studies and create experimental models for new surgical procedures and approaches. Anat Rec, 2019. © 2019 Wiley Periodicals, Inc. Anat Rec, 302:2049-2061, 2019. © 2019 American Association for Anatomy.