The relationship between olfaction and cognitive function in the elderly

Influence of age on nicotinic cholinergic regulation of blood flow in rat's olfactory bulb and neocortex

The olfactory bulb receives cholinergic basal forebrain inputs as does the neocortex. With a focus on nicotinic acetylcholine receptors (nAChRs), this review article provides an overview and discussion of the following findings: (1) the nAChRs-mediated regulation of regional blood flow in the neocortex and olfactory bulb, (2) the nAChR subtypes that mediate their responses, and (3) their activity in old rats. The activation of the α4β2-like subtype of nAChRs produces vasodilation in the neocortex, and potentiates olfactory bulb vasodilation induced by olfactory stimulation. The nAChR activity producing neocortical vasodilation was similarly maintained in 2-year-old rats as in adult rats, but was clearly reduced in 3-year-old rats. In contrast, nAChR activity in the olfactory bulb was reduced already in 2-year-old rats. Thus, age-related impairment of α4β2-like nAChR function may occur earlier in the olfactory bulb than in the neocortex. Given the findings, the vasodilation induced by α4β2-like nAChR activation may be beneficial for neuroprotection in the neocortex and the olfactory bulb.

Nicotinic cholinergic regulation of olfactory bulb blood flow response in aged rats

In our previous research, we had demonstrated the crucial role of neuronal nicotinic acetylcholine receptors (nAChRs) in potentiation of the olfactory bulb blood flow response to olfactory stimulation in adult rats. The present study examined the effects of nAChR activation on the olfactory bulb blood flow response in rats aged 24-27 months. We found that, under urethane anesthesia, unilateral olfactory nerve stimulation (300 μA, 20 Hz, 5 s) increased blood flow within the ipsilateral olfactory bulb, without changes in the systemic arterial pressure. The increase in blood flow was dependent upon the current and frequency of the stimulus. Intravenous administration of nicotine (30 μg/kg) had little effect on the olfactory bulb blood flow response to nerve stimulation at either 2 Hz or 20 Hz. These results suggest a reduction in nAChR-mediated potentiation of the olfactory bulb blood flow response in aged rats.

Associations of Olfaction With Longitudinal Trajectories of Brain Volumes and Neuropsychological Function in Older Adults

BACKGROUND AND OBJECTIVES

Olfactory function declines with aging, and olfactory deficits are one of the earliest features of neurodegenerative diseases, such as Parkinson disease and Alzheimer disease. Previous studies have shown that olfaction is associated with brain volumes and cognitive function, but data are exclusively cross-sectional. We aimed to examine longitudinal associations of olfaction with changes in brain volumes and neuropsychological function.

METHODS

In the Baltimore Longitudinal Study of Aging, we chose the first assessment of olfaction to examine the associations with retrospective and prospective changes in neuropsychological performance and brain volumes in participants aged 50 years or older using linear mixed-effects models, adjusted for demographic variables and cardiovascular disease. Olfaction was measured as odor identification scores through the 16-item Sniffin' Sticks.

RESULTS

We analyzed data from 567 (58% women, 42% men, 27% Black, 66% White, and 7% others) participants who had data on odor identification scores and brain volumetric MRI (n = 420 with retrospective repeats over a mean of 3.7 years, n = 280 with prospective repeats over a mean of 1.2 years). We also analyzed data from 754 participants (56% women, 44% men, 29% Black, 65% White, and 6% others) with neuropsychological assessments (n = 630 with retrospective repeats over a mean of 6.6 years, n = 280 with prospective repeats over a mean of 1.5 years). After adjustment, higher odor identification scores were associated with prior and subsequent slower brain atrophy in the entorhinal cortex (β ± SE = 0.0093 ± 0.0031, p = 0.0028 and β ± SE = 0.0176 ± 0.0073, p = 0.0169, respectively), hippocampus (β ± SE = 0.0070 ± 0.0030, p = 0.0192 and β ± SE = 0.0173 ± 0.0066, p = 0.0089, respectively), and additional frontal and temporal areas (all p < 0.05). Higher odor identification scores were also associated with prior slower decline in memory, attention, processing speed, and manual dexterity and subsequent slower decline in attention (all p < 0.05). Some associations were attenuated after exclusion of data points at and after symptom onset of cognitive impairment or dementia.

DISCUSSION

In older adults, olfaction is related to brain atrophy of specific brain regions and neuropsychological changes in specific domains over time. The observed associations are driven, in part, by those who developed cognitive impairment or dementia. Future longitudinal studies with longer follow-ups are needed to understand whether olfactory decline precedes cognitive decline and whether it is mediated through regionally specific brain atrophy.

How the sense of smell influences cognition throughout life

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.

Normative Data of the Trail Making Test Among Urban Community-Dwelling Older Adults in Japan

Introduction

Population aging is likely to increase the number of people with dementia living in urban areas. The Trail Making Test (TMT) is widely used as a cognitive task to measure attention and executive function among older adults. Normative data from a sample of community-dwelling older adults are required to evaluate the executive function of this population. The purpose of this study was to examine the Trail Making Test completion rate and completion time among urban community-dwelling older adults in Japan.

Methods

A survey was conducted at a local venue or during a home visit (n = 1,966). Cognitive tests were conducted as a part of the survey, and TMT Parts A (TMT-A) and B (TMT-B) were completed after the completion of the Japanese version of the Mini-Mental State Examination (MMSE-J). Testers recorded TMT completion status, completion time, and the number of errors observed.

Results

In the TMT-A, 1,913 (99.5%) participants understood the instructions, and 1,904 (99.1%) participants completed the task within the time limit of 240 s. In the TMT-B, 1,839 (95.9%) participants understood the instructions, and 1,584 (82.6%) participants completed the task within the time limit of 300 s. The completion rate of TMT-B was 90.2 and 41.8% for participants with an MMSE-J score of >23 points and ≦23 points, respectively. Results of multiple regression analyses showed that age, education, and the MMSE-J score were associated with completion time in both TMTs.

Conclusion

In both TMTs, completion time was associated with age, education, and general cognitive function. However, not all participants completed the TMT-B, and the completion rate was relatively low among participants with low MMSE-J scores. These findings may help interpret future TMT assessments.

Olfactory function and discrimination ability in the elderly: a pilot study

We recently reported that subjects with a higher olfactory identification threshold for rose odor declined more in attentional ability in the elderly. This study focuses on discrimination ability and olfactory identification threshold in twelve elderly subjects living in a community (age: 80.9 ± 1.6). Olfactory function was assessed by the rose odor identification threshold. We assessed the discrimination ability by distinguishing 5 similar odor pairs. Our results showed that the subjects with a higher olfactory identification threshold (≥ 5) declined more in discrimination ability (14% ± 14%, p = 0.03) compared to those with a lower threshold (≤ 4) (averaged value set at 100%). As discrimination ability is related to the basal forebrain cholinergic system, our results suggest that olfactory impairment links to the decline in cognitive function relating the cholinergic system.

The Role of Olfactory System in the Etiogenesis of Parkinson’s Diseases: An Overview

Parkinson’s disease (PD) mainly affects the dopaminergic neuronal networks of the substantia nigra, which leads to both motor and nonmotor symptoms of the disease. Based on the reports from the previous studies, 95% of the cases are presented along with olfactory dysfunction. The relevant publications from 2002 to 2021 were searched and shortlisted using PubMed and Google Scholar. In this review, we have discussed the correlation between olfactory dysfunction and PD. Olfactory damage presents earlier than the motor symptoms. Because there are no current methodologies for the early detection of PD, olfactory dysfunction can be used as a potential marker for the early detection of PD and hence paving the way for better therapeutic approaches.