Functional imaging studies of dopamine system and cognition in normal aging and Parkinson's disease

Aging leads to sex-dependent effects on pair bonding and increased number of oxytocin-producing neurons in monogamous prairie voles

Pair bonds powerfully modulate health, which becomes particularly important when facing the detrimental effects of aging. To examine the impact of aging on relationship formation and response to loss, we examined behavior in naive 6-, 12-, and 18-month male and female prairie voles, a monogamous species that forms mating-based pair bonds. We found that older males (18-months) bonded quicker than younger voles, while similarly aged female voles increased partner directed affiliative behaviors. Supporting sex differences in bonding behaviors, we found that males were more likely to sample both partner and stranger voles while females were more likely to display partner preference during the initial 20 min of the test. We also found that male voles of all ages show enduring bonding behavior despite four weeks of partner separation while females show an overall decrease in partner-directed affiliation, including an erosion of partner preference in 12-month females. Finally, we found that the number of oxytocin, but not vasopressin, cells in the paraventricular hypothalamus increased at 18 months of age. These results establish prairie voles as a novel model to study the effects of normal and abnormal aging on pair bonding.

Unraveling the complexity of human brain: Structure, function in healthy and disease states

The human brain stands as an intricate organ, embodying a nexus of structure, function, development, and diversity. This review delves into the multifaceted landscape of the brain, spanning its anatomical intricacies, diverse functional capacities, dynamic developmental trajectories, and inherent variability across individuals. The dynamic process of brain development, from early embryonic stages to adulthood, highlights the nuanced changes that occur throughout the lifespan. The brain, a remarkably complex organ, is composed of various anatomical regions, each contributing uniquely to its overall functionality. Through an exploration of neuroanatomy, neurophysiology, and electrophysiology, this review elucidates how different brain structures interact to support a wide array of cognitive processes, sensory perception, motor control, and emotional regulation. Moreover, it addresses the impact of age, sex, and ethnic background on brain structure and function, and gender differences profoundly influence the onset, progression, and manifestation of brain disorders shaped by genetic, hormonal, environmental, and social factors. Delving into the complexities of the human brain, it investigates how variations in anatomical configuration correspond to diverse functional capacities across individuals. Furthermore, it examines the impact of neurodegenerative diseases on the structural and functional integrity of the brain. Specifically, our article explores the pathological processes underlying neurodegenerative diseases, such as Alzheimer's, Parkinson's, and Huntington's diseases, shedding light on the structural alterations and functional impairments that accompany these conditions. We will also explore the current research trends in neurodegenerative diseases and identify the existing gaps in the literature. Overall, this article deepens our understanding of the fundamental principles governing brain structure and function and paves the way for a deeper understanding of individual differences and tailored approaches in neuroscience and clinical practice-additionally, a comprehensive understanding of structural and functional changes that manifest in neurodegenerative diseases.

Reduced neural distinctiveness of speech representations in the middle-aged brain

Speech perception declines independent of hearing thresholds in middle-age, and the neurobiological reasons are unclear. In line with the age-related neural dedifferentiation hypothesis, we predicted that middle-aged adults show less distinct cortical representations of phonemes and acoustic-phonetic features relative to younger adults. In addition to an extensive audiological, auditory electrophysiological, and speech perceptual test battery, we measured electroencephalographic responses time-locked to phoneme instances (phoneme-related potential; PRP) in naturalistic, continuous speech and trained neural network classifiers to predict phonemes from these responses. Consistent with age-related neural dedifferentiation, phoneme predictions were less accurate, more uncertain, and involved a broader network for middle-aged adults compared with younger adults. Representational similarity analysis revealed that the featural relationship between phonemes was less robust in middle-age. Electrophysiological and behavioral measures revealed signatures of cochlear neural degeneration (CND) and speech perceptual deficits in middle-aged adults relative to younger adults. Consistent with prior work in animal models, signatures of CND were associated with greater cortical dedifferentiation, explaining nearly a third of the variance in PRP prediction accuracy together with measures of acoustic neural processing. Notably, even after controlling for CND signatures and acoustic processing abilities, age-group differences in PRP prediction accuracy remained. Overall, our results reveal "fuzzier" phonemic representations, suggesting that age-related cortical neural dedifferentiation can occur even in middle-age and may underlie speech perceptual challenges, despite a normal audiogram.

Cortico-cortical connectivity is influenced by levodopa in tremor-dominant Parkinson's disease

Resting tremor is the most common presenting motor symptom in Parkinson's disease (PD). The supplementary motor area (SMA) is a main target of the basal-ganglia-thalamo-cortical circuit and has direct, facilitatory connections with the primary motor cortex (M1), which is important for the execution of voluntary movement. Dopamine potentially modulates SMA and M1 activity, and both regions have been implicated in resting tremor. This study investigated SMA-M1 connectivity in individuals with PD ON and OFF dopamine medication, and whether SMA-M1 connectivity is implicated in resting tremor. Dual-site transcranial magnetic stimulation was used to measure SMA-M1 connectivity in PD participants ON and OFF levodopa. Resting tremor was measured using electromyography and accelerometry. Stimulating SMA inhibited M1 excitability OFF levodopa, and facilitated M1 excitability ON levodopa. ON medication, SMA-M1 facilitation was significantly associated with smaller tremor than SMA-M1 inhibition. The current findings contribute to our understanding of the neural networks involved in PD which are altered by levodopa medication and provide a neurophysiological basis for the development of interventions to treat resting tremor.

Transcriptomic analysis to identify genes associated with hypothalamus vulnerability in aging mice with cognitive decline

The normal aging process is accompanied by cognitive decline, and previous studies have indicated the crucial role of the hypothalamus in regulating both aging and cognition. However, the precise molecular mechanism underlying this relationship remains unclear. Therefore, this present study aimed to identify potential predictors of cognitive decline associated with aging specifically within the hypothalamus. To achieve this, we employed Morris water maze (MWM) testing to assess learning and memory differences between young and aged mice. Additionally, transcriptome sequencing was conducted on the hypothalamus of young and aged mice to identify potential genes. Subsequently, GO and KEGG analyses were performed to investigate the functions of differentially expressed genes (DEGs) and their associated biological pathways. Finally, the results obtained from sequencing analysis were further validated using qRT-PCR. Notably, MWM testing revealed a significant decrease in spatial learning and memory ability among aged mice. According to KEGG analysis, the DEGs primarily encompassed various biochemical signaling pathways related to immune system (e.g., C3; C4b; Ccl2; Ccl7; Cebpb; Clec7a; Col3a1; Cxcl10; Cxcl2; Fosb; Fosl1; Gbp5; H2-Ab1; Hspa1a; Hspa1b; Icam1; Il1b; Itga5; Itgax; Lilrb4a; Plaur; Ptprc; Serpine1; Tnfrsf10b; Tnfsf10), neurodegenerative disease (e.g., Atp2a1; Creb5; Fzd10; Hspa1a; Hspa1b; Il1b; Kcnj10; Nxf3; Slc6a3; Tubb6; Uba1y; Wnt9b), nervous system function (e.g., Chrna4; Chrna6; Creb5; Slc6a3),and aging (e.g., Creb5; Hspa1a; Hspa1b) among others. These identified genes may serve as potential predictors for cognitive function in elderly individuals and will provide a crucial foundation for further exploration into the underlying molecular mechanisms.

Magnetic ionic liquid-based liquid-liquid microextraction followed by ultra-performance liquid chromatography coupled with triple-quadrupole tandem mass spectrometry for simultaneous determination of neurotransmitters in human cerebrospinal fluid and plasma

A green, efficient and easy sample pretreatment method of magnetic ionic liquid-based liquid-liquid microextraction (MIL-based LLME) combined with a sensitive, rapid and precise analytical method of ultra-performance liquid chromatography coupled with triple-quadrupole tandem mass spectrometry (UPLC-QqQ/MS²) was developed to simultaneously - determining of neurotransmitters (NTs) in biosamples. Two magnetic ionic liquids (MILs), [P_6,6,6,14]₃[GdCl₆] and [P_6,6,6,14]₂[CoCl₄] tested, and the latter was selected as the extraction solvent due to its advantages of visual recognition, paramagnetic behavior and higher extraction efficiency. Facile magnetic separation of MIL containing analytes from matrix was realized by applying external magnetic field without rather than centrifugation. Experimental parameters that would influence the extraction efficiency, including type and amount of MIL, extraction time, speed of the vortex process, salt concentration, and environmental pH, were optimized obtained. The proposed method was successfully applied to the simultaneous extraction and determination of 20 NTs in human cerebrospinal fluid and plasma samples. Excellent analytical performance indicates the broad potential of this method for clinical diagnosis and therapy of neurological diseases.

Investigation of the factors associated with hemichorea/hemiballismus in post-stroke patients

Classical knowledge highlights the role of lesions of the subthalamic nuclei (STN) in the pathophysiology of hemichorea/hemiballismus (HH). However, the published reports indicate various other lesion regions in the majority of post-stroke cases with HH. Ergo, we aimed to investigate the significance of the lesion site and clinical features for developing HH in post-stroke patients. Overall, we retrospectively scanned all the patients with stroke who were hospitalized between 01/06/2022 and 31/07/2022 in our neurology clinic. The data regarding the demographic features, comorbidities, stroke etiologies, and laboratory findings, including serum glucose and HBA1C were retrospectively recruited using the electronic-based medical record system. The cranial magnetic resonance imaging (MRI) and computed tomography images have been systematically evaluated for the presence of lesions in localizations that are previously associated with HH. We conducted comparative analyses between patients with and without HH to reveal the discrepancies between groups. The logistic regression analyses were also performed to reveal the predictive values of some features. Overall, the data of 124 post-stroke patients were analyzed. The mean age was 67.9 ± 12.4 years (F/M = 57/67). Six patients were determined to develop HH. The comparative analyses between patients with and without HH revealed that the mean age tended to be higher in the HH group (p = 0.08) and caudate nucleus involvement was more common in the HH group (p = 0.005). Besides cortical involvement was absent in all subjects developing HH. The logistic regression model revealed the presence of a caudate lesion and advanced age as factors associated with HH. We found that the caudate lesion was a crucial determinant of the occurrence of HH in post-stroke patients. With the significance of the other factors of increased age and cortical sparring, we observed differences in the HH group may be investigated also in future-related studies on larger groups.

Hippocampal and motor regions contribute to memory benefits after enacted encoding: cross-sectional and longitudinal evidence

The neurobiological underpinnings of action-related episodic memory and how enactment contributes to efficient memory encoding are not well understood. We examine whether individual differences in level (n = 338) and 5-year change (n = 248) in the ability to benefit from motor involvement during memory encoding are related to gray matter (GM) volume, white matter (WM) integrity, and dopamine-regulating genes in a population-based cohort (age range = 25-80 years). A latent profile analysis identified 2 groups with similar performance on verbal encoding but with marked differences in the ability to benefit from motor involvement during memory encoding. Impaired ability to benefit from enactment was paired with smaller HC, parahippocampal, and putamen volume along with lower WM microstructure in the fornix. Individuals with reduced ability to benefit from encoding enactment over 5 years were characterized by reduced HC and motor cortex GM volume along with reduced WM microstructure in several WM tracts. Moreover, the proportion of catechol-O-methyltransferase-Val-carriers differed significantly between classes identified from the latent-profile analysis. These results provide converging evidence that individuals with low or declining ability to benefit from motor involvement during memory encoding are characterized by low and reduced GM volume in regions critical for memory and motor functions along with altered WM microstructure.

Aging in nucleus accumbens and its impact on alcohol use disorders

Ethanol is one of the most widely consumed drugs in the world and prolonged excessive ethanol intake might lead to alcohol use disorders (AUDs), which are characterized by neuroadaptations in different brain regions, such as in the reward circuitry. In addition, the global population is aging, and it appears that they are increasing their ethanol consumption. Although research involving the effects of alcohol in aging subjects is limited, differential effects have been described. For example, studies in human subjects show that older adults perform worse in tests assessing working memory, attention, and cognition as compared to younger adults. Interestingly, in the field of the neurobiological basis of ethanol actions, there is a significant dichotomy between what we know about the effects of ethanol on neurochemical targets in young animals and how it might affect them in the aging brain. To be able to understand the distinct effects of ethanol in the aging brain, the following questions need to be answered: (1) How does physiological aging impact the function of an ethanol-relevant region (e.g., the nucleus accumbens)? and (2) How does ethanol affect these neurobiological systems in the aged brain? This review discusses the available data to try to understand how aging affects the nucleus accumbens (nAc) and its neurochemical response to alcohol. The data show that there is little information on the effects of ethanol in aged mice and rats, and that many studies had considered 2-3-month-old mice as adults, which needs to be reconsidered since more recent literature defines 6 months as young adults and >18 months as an older mouse. Considering the actual relevance of an aged worldwide population and that this segment is drinking more frequently, it appears at least reasonable to explore how ethanol affects the brain in adult and aged models.

Digitally embodied lifespan neurocognitive development and Tactile Internet: Transdisciplinary challenges and opportunities

Mechanisms underlying perceptual processing and inference undergo substantial changes across the lifespan. If utilized properly, technologies could support and buffer the relatively more limited neurocognitive functions in the still developing or aging brains. Over the past decade, a new type of digital communication infrastructure, known as the "Tactile Internet (TI)," is emerging in the fields of telecommunication, sensor and actuator technologies and machine learning. A key aim of the TI is to enable humans to experience and interact with remote and virtual environments through digitalized multimodal sensory signals that also include the haptic (tactile and kinesthetic) sense. Besides their applied focus, such technologies may offer new opportunities for the research tapping into mechanisms of digitally embodied perception and cognition as well as how they may differ across age cohorts. However, there are challenges in translating empirical findings and theories about neurocognitive mechanisms of perception and lifespan development into the day-to-day practices of engineering research and technological development. On the one hand, the capacity and efficiency of digital communication are affected by signal transmission noise according to Shannon's (1949) Information Theory. On the other hand, neurotransmitters, which have been postulated as means that regulate the signal-to-noise ratio of neural information processing (e.g., Servan-Schreiber et al., 1990), decline substantially during aging. Thus, here we highlight neuronal gain control of perceptual processing and perceptual inference to illustrate potential interfaces for developing age-adjusted technologies to enable plausible multisensory digital embodiments for perceptual and cognitive interactions in remote or virtual environments.

Biomolecular Markers of Brain Aging

Characterized by the gradual loss of physiological integrity, impaired function, and increased susceptibility to death, aging is considered the primary risk factor for major human diseases, such as cancer, diabetes, cardiovascular disorders, and neurodegenerative diseases. The time-dependent accumulation of cellular damage is widely considered the general cause of aging. While the mechanism of normal aging is still unresolved, researchers have identified different markers of aging, including genomic instability, telomere attrition, epigenetic alterations, loss of proteostasis, deregulated nutrient-sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and altered intercellular communication. Theories of aging can be divided into two categories: (1) aging is a genetically programmed process, and (2) aging is a random process caused by gradual damage to the organism over time as a result of its vital activities. Aging affects the entire human body, and aging of the brain is undoubtedly different from all other organs, as neurons are highly differentiated postmitotic cells, and the lifespan of most neurons in the postnatal period is equal to the lifespan of the brain. In this chapter, we discuss the conserved mechanisms of aging that may underlie the changes observed in the aging brain, with a focus on mitochondrial function and oxidative stress, autophagy and protein turnover, insulin/IGF signaling, target of rapamycin (TOR) signaling, and sirtuin function.

The structural changes of gray matter in Parkinson disease patients with mild cognitive impairments

Objectives

Parkinson disease (PD) is associated with cognitive impairments. However, the underlying neural mechanism of cognitive impairments in PD is still not clear. This study aimed to investigate the anatomic alternations of gray matter in PD patients with mild cognitive impairment (MCI) and their associations with neurocognitive measurements.

Methods

T1-weighted magnetic resonance imaging (MRI) data were acquired from 23 PD patients with MCI, 23 PD patients without MCI, and 23 matched healthy controls. The MRI data were analyzed using voxel-based morphometry (VBM) and surfaced-based morphometry (SBM) methods to assess the structural changes in gray matter volume and cortical thickness respectively. Receiver operating characteristic (ROC) analysis was used to examine the diagnostic accuracies of the indexes of interest. The correlations between the structural metrics and neurocognitive assessments (e.g., Montreal cognitive assessment, MOCA; Mini-mental state examination, MMSE) were further examined.

Results

PD patients with MCI showed reduced gray matter volume (GMV) in the frontal cortex (e.g., right inferior frontal gyrus and middle frontal gyrus) and extended to insula as well as cerebellum compared with the healthy controls and PD patients without MIC. Thinner of cortical thickens in the temporal lobe (e.g., left middle temporal gyrus and right superior temporal gyrus) extending to parietal cortex (e.g., precuneus) were found in the PD patients with MCI relative to the healthy controls and PD patients without MCI.ROC analysis indicated that the area under the ROC curve (AUC) values in the frontal, temporal, and subcortical structures (e.g., insula and cerebellum) could differentiate the PD patients with MCI and without MCI and healthy controls. Furthermore, GMV of the right middle frontal gyrus and cortical thickness of the right superior temporal gyrus were correlated with neurocognitive dysfunctions (e.g., MOCA and MMSE) in PD patients with MCI.

Conclusion

This study provided further evidence that PD with MCI was associated with structural alternations of brain. Morphometric analysis focusing on the cortical and subcortical regions could be biomarkers of cognitive impairments in PD patients.

Sex differences in neurobehavioral consequences of methamphetamine exposure in adult mice

RATIONALE

Recreational and medical use of stimulants is increasing, and their use may increase susceptibility to aging and promote neurobehavioral impairments. The long-term consequences of these psychostimulants and how they interact with age have not been fully studied.

OBJECTIVES

Our study investigated whether chronic exposure to the prototypical psychostimulant, methamphetamine (METH), at doses designed to emulate human therapeutic dosing, would confer a pro-oxidizing redox shift promoting long-lasting neurobehavioral impairments.

METHODS

Groups of 4-month-old male and female C57BL/6 J mice were administered non-contingent intraperitoneal injections of either saline or METH (1.4 mg/kg) twice a day for 4 weeks. Mice were randomly assigned to one experimental group: (i) short-term cognitive assessments (at 5 months), (ii) long-term cognitive assessments (at 9.5 months), and (ii) longitudinal motor assessments (at 5, 7, and 9 months). Brain regions were assessed for oxidative stress and markers of neurotoxicity after behavior testing.

RESULTS

Chronic METH exposure induced short-term effects on associative memory, gait speed, dopamine (DA) signaling, astrogliosis in females, and spatial learning and memory, balance, DA signaling, and excitotoxicity in males. There were no long-term effects of chronic METH on cognition; however, it decreased markers of excitotoxicity in the striatum and exacerbated age-associated motor impairments in males.

CONCLUSION

In conclusion, cognitive and motor functions were differentially and sex-dependently affected by METH exposure, and oxidative stress did not seem to play a role in the observed behavioral outcomes. Future studies are necessary to continue exploring the long-term neurobehavioral consequences of drug use in both sexes and the relationship between aging and drugs.

A Novel and Selective Dopamine Transporter Inhibitor, (S)-MK-26, Promotes Hippocampal Synaptic Plasticity and Restores Effort-Related Motivational Dysfunctions

Dopamine (DA), the most abundant human brain catecholaminergic neurotransmitter, modulates key behavioral and neurological processes in young and senescent brains, including motricity, sleep, attention, emotion, learning and memory, and social and reward-seeking behaviors. The DA transporter (DAT) regulates transsynaptic DA levels, influencing all these processes. Compounds targeting DAT (e.g., cocaine and amphetamines) were historically used to shape mood and cognition, but these substances typically lead to severe negative side effects (tolerance, abuse, addiction, and dependence). DA/DAT signaling dysfunctions are associated with neuropsychiatric and progressive brain disorders, including Parkinson’s and Alzheimer diseases, drug addiction and dementia, resulting in devastating personal and familial concerns and high socioeconomic costs worldwide. The development of low-side-effect, new/selective medicaments with reduced abuse-liability and which ameliorate DA/DAT-related dysfunctions is therefore crucial in the fields of medicine and healthcare. Using the rat as experimental animal model, the present work describes the synthesis and pharmacological profile of (S)-MK-26, a new modafinil analogue with markedly improved potency and selectivity for DAT over parent drug. Ex vivo electrophysiology revealed significantly augmented hippocampal long-term synaptic potentiation upon acute, intraperitoneally delivered (S)-MK-26 treatment, whereas in vivo experiments in the hole-board test showed only lesser effects on reference memory performance in aged rats. However, in effort-related FR5/chow and PROG/chow feeding choice experiments, (S)-MK-26 treatment reversed the depression-like behavior induced by the dopamine-depleting drug tetrabenazine (TBZ) and increased the selection of high-effort alternatives. Moreover, in in vivo microdialysis experiments, (S)-MK-26 significantly increased extracellular DA levels in the prefrontal cortex and in nucleus accumbens core and shell. These studies highlight (S)-MK-26 as a potent enhancer of transsynaptic DA and promoter of synaptic plasticity, with predominant beneficial effects on effort-related behaviors, thus proposing therapeutic potentials for (S)-MK-26 in the treatment of low-effort exertion and motivational dysfunctions characteristic of depression and aging-related disorders.

Dopaminergic medication increases motivation to exert cognitive control by reducing subjective effort costs in Parkinson's patients

Engaging in demanding mental activities requires the allocation of cognitive control, which can be effortful and aversive. Individuals thus tend to avoid exerting cognitive effort if less demanding behavioral options are available. Recent accounts propose a key role for dopamine in motivating behavior by increasing the sensitivity to rewards associated with effort exertion. Whether dopamine additionally plays a specific role in modulating the sensitivity to the costs of cognitive effort, even in the absence of any incentives, is much less clear. To address this question, we assessed cognitive effort avoidance in patients (n = 38) with Parkinson's disease, a condition characterized by loss of midbrain dopaminergic neurons, both ON and OFF dopaminergic medication and compared them to healthy controls (n = 24). Effort avoidance was assessed using the Demand Selection Task (DST), in which participants could freely choose between performing a high-demand or a low-demand version of a task-switching paradigm. Critically, participants were not offered any incentives to choose the more effortful option, nor for good performance. While healthy controls and patients OFF their dopaminergic medications consistently preferred the low-demand option, effort avoidance in patients ON dopaminergic medications was reduced compared to patients OFF, a difference which seems to lessen over trials. These differences in preference could not be explained by altered task-switching performance. Although patients ON were less accurate at detecting the different effort levels, as measured during instructed forced-choice blocks, their detection ability was not associated with effort avoidance, unlike in the healthy controls and the patients OFF. Our findings provide evidence that dopamine replacement in Parkinson's patients increases the willingness to engage in cognitively demanding behavior, and that this cannot be explained by possible effects of dopamine replacement on performance nor on the ability to detect effort demands. These results suggest that dopamine plays a role in reducing the sensitivity to effort costs that is independent of its role in enhancing the sensitivity to the benefits of effort exertion.

Is essential tremor a degenerative or an electrical disorder? Electrical disorder

Essential tremor (ET) is one of the most common movement disorders, yet we do not have a complete understanding of its pathophysiology. From a phenomenology standpoint, ET is an isolated tremor syndrome of bilateral upper limb action tremor with or without tremor in other body locations. ET is a pathological tremor that arises from excessive oscillation in the central motor network. The tremor network comprises of multiple brain regions including the inferior olive, cerebellum, thalamus, and motor cortex, and there is evidence that a dynamic oscillatory disturbance within this network leads to tremor. ET is a chronic disorder, and the natural history shows a slow progression of tremor intensity with age. There are reported data suggesting that ET follows the disease model of a neurodegenerative disorder, however whether ET is a degenerative or electrical disorder has been a subject of debate. In this chapter, we will review cumulative evidence that ET as a syndrome is a fundamentally electric disorder. The etiology is likely heterogenous and may not be primarily neurodegenerative.

Executive Functions in Aging: An Experimental and Computational Study of the Wisconsin Card Sorting and Brixton Spatial Anticipation Tests

In order to explore the effect of normal aging on executive function, we tested 25 younger adults and 25 neurologically healthy older adults on the Wisconsin Card Sorting Test (WCST) and the Brixton Spatial Anticipation Test (BRXT), two classic tests of executive function. We found that older participants were more likely than younger participants to err on both tasks, but the additional errors of older participants tended to be related to task set maintenance and rule inference rather than perseveration. We further found that the tendency to perseverate (across all participants) on the WCST was related to the tendency to produce stimulus or response perseverations on the BRXT, rather than any tendency to perseverate on BRXT rule application. Finally, on both tasks, older participants were also slower, particularly on trials following an error, than younger participants. To explore the neurocomputational basis for the observed behaviours we then extended an existing model of schema-modulated action selection on the WCST to the BRXT. We argue on the basis of the model that the performance of older participants on both tasks reflects a slower update of schema thresholds within the basal ganglia, coupled with a decrease in sensitivity to feedback.

Caffeine and attentional control: improved and impaired performance in healthy older adults and Parkinson's disease according to task demands

INTRODUCTION

Caffeine is frequently consumed to boost goal-directed attention. These procognitive effects may occur due to the adenosine-mediated enhancement of monoamines, such as dopamine, after caffeine administration. As such, caffeine's beneficial effects may be altered in conditions such as Parkinson's disease (PD). However, whether caffeine improves cognition, and at what cost, has not been experimentally established in patients with neurodegenerative disease.

METHODS

Single-dose trials to probe cognitive effects of caffeine are often confounded by short-term caffeine abstinence which conflates caffeine's effects with treatment of withdrawal. Using a placebo controlled, blinded, randomised trial design, we assessed the effect of 100 mg of caffeine across well-established tasks (Choice reaction time, Stroop Task and Rapid Serial Visual Presentation Task; RSVP) that probe different aspects of attention in PD patients (n = 24) and controls (n = 44). Critically, participants withdrew from caffeine for a week prior to testing to eliminate the possibility that withdrawal reversal explained any cognitive benefit.

RESULTS

Caffeine administration was found to reduce the overall number of errors in patients and controls on the Stroop (p = .018, η²_p = .086) and Choice reaction time (p < . 0001, η²_p = .588) tasks, but there was no specific effect of caffeine on ignoring irrelevant information in the Stroop task. On the RSVP task, caffeine improved dual item accuracy (p = .037) but impaired single item accuracy (p = .044). Across all tasks, there was little evidence that caffeine has different effects in PD participants and controls.

CONCLUSION

When removing withdrawal effects as a factor, we demonstrate caffeine has beneficial effects on selective attention but is a double-edge sword for visual temporal attention and would need careful targeting to be clinically useful.

The Effects of a Novel Non-catechol Dopamine Partial Agonist on Working Memory in the Aged Rhesus Monkey

Aged-related declines in cognition, especially working memory and executive function, begin in middle-age and these abilities are known to be mediated by the prefrontal cortex (PFC) and more specifically the dopamine (DA) system within the PFC. In both humans and monkeys, there is significant evidence that the PFC is the first cortical region to change with age and the PFC appears to be particularly vulnerable to age-related loss of dopamine (DA). Therefore, the DA system is a strong candidate for therapeutic intervention to slow or reverse age related declines in cognition. In the present study, we administered a novel selective, potent, non-catechol DA D1 R agonist PF-6294 (Pfizer, Inc.) to aged female rhesus monkeys and assessed their performance on two benchmark tasks of working memory - the Delayed Non-match to Sample Task (DNMS) and Delayed Recognition Span Task (DRST). The DNMS task was administered first with the standard 10 s delay and then with 5 min delays, with and without distractors. The DRST was administered each day with four trials with unique sequences and one trial of a repeated sequence to assess evidence learning and retention. Overall, there was no significant effect of drug on performance on any aspect of the DNMS task. In contrast, we demonstrated that a middle range dose of PF-6294 significantly increased memory span on the DRST on the first and last days of testing and by the last day of testing the increased memory span was driven by the performance on the repeated trials.

Reinstatement of synaptic plasticity in the aging brain through specific dopamine transporter inhibition

Aging-related neurological deficits negatively impact mental health, productivity, and social interactions leading to a pronounced socioeconomic burden. Since declining brain dopamine signaling during aging is associated with the onset of neurological impairments, we produced a selective dopamine transporter (DAT) inhibitor to restore endogenous dopamine levels and improve cognitive function. We describe the synthesis and pharmacological profile of (S,S)-CE-158, a highly specific DAT inhibitor, which increases dopamine levels in brain regions associated with cognition. We find both a potentiation of neurotransmission and coincident restoration of dendritic spines in the dorsal hippocampus, indicative of reinstatement of dopamine-induced synaptic plasticity in aging rodents. Treatment with (S,S)-CE-158 significantly improved behavioral flexibility in scopolamine-compromised animals and increased the number of spontaneously active prefrontal cortical neurons, both in young and aging rodents. In addition, (S,S)-CE-158 restored learning and memory recall in aging rats comparable to their young performance in a hippocampus-dependent hole board test. In sum, we present a well-tolerated, highly selective DAT inhibitor that normalizes the age-related decline in cognitive function at a synaptic level through increased dopamine signaling.

Differential Effects of a Nap on Motor Sequence Learning-Related Functional Connectivity Between Young and Older Adults

In older adults, motor sequence learning (MSL) is largely intact. However, consolidation of newly learned motor sequences is impaired compared to younger adults, and there is evidence that brain areas supporting enhanced consolidation via sleep degrade with age. It is known that brain activity in hippocampal-cortical-striatal areas is important for sleep-dependent, off-line consolidation of motor-sequences. Yet, the intricacies of how both age and sleep alter communication within this network of brain areas, which facilitate consolidation, are not known. In this study, 37 young (age 20-35) and 49 older individuals (age 55-75) underwent resting state functional magnetic resonance imaging (fMRI) before and after training on a MSL task as well as after either a nap or a period of awake rest. Young participants who napped showed strengthening of functional connectivity (FC) between motor, striatal, and hippocampal areas, compared to older subjects regardless of sleep condition. Follow-up analyses revealed this effect was driven by younger participants who showed an increase in FC between striatum and motor cortices, as well as older participants who showed decreased FC between the hippocampus, striatum, and precuneus. Therefore, different effects of sleep were observed in younger vs. older participants, where young participants primarily showed increased communication in the striatal-motor areas, while older participants showed decreases in key nodes of the default mode network and striatum. Performance gains correlated with FC changes in young adults, and this association was much greater in participants who napped compared to those who stayed awake. Performance gains also correlated with FC changes in older adults, but only in those who napped. This study reveals that, while there is no evidence of time-dependent forgetting/deterioration of performance, older adults exhibit a completely different pattern of FC changes during consolidation compared to younger adults, and lose the benefit that sleep affords to memory consolidation.

Exploring reward-related attention selectivity deficits in Parkinson's disease

An important aspect of managing a limited cognitive resource like attention is to use the reward value of stimuli to prioritize the allocation of attention to higher-value over lower-value stimuli. Recent evidence suggests this depends on dopaminergic signaling of reward. In Parkinson's disease, both reward sensitivity and attention are impaired, but whether these deficits are directly related to one another is unknown. We tested whether Parkinson's patients use reward information when automatically allocating their attention and whether this is modulated by dopamine replacement. We compared patients, tested both ON and OFF dopamine replacement medication, to older controls using a standard attention capture task. First, participants learned the different reward values of stimuli. Then, these reward-associated stimuli were used as distractors in a visual search task. We found that patients were generally distracted by the presence of the distractors but that the degree of distraction caused by the high-value and low-value distractors was similar. Furthermore, we found no evidence to support the possibility that dopamine replacement modulates the effect of reward on automatic attention allocation. Our results suggest a possible inability in Parkinson's patients to use the reward value of stimuli when automatically allocating their attention, and raise the possibility that reward-driven allocation of resources may affect the adaptive modulation of other cognitive processes.

Diverse midbrain dopaminergic neuron subtypes and implications for complex clinical symptoms of Parkinson's disease

Parkinson’s disease (PD), the most common degenerative movement disorder, is clinically manifested with various motor and non-motor symptoms. Degeneration of midbrain substantia nigra pas compacta (SNc) dopaminergic neurons (DANs) is generally attributed to the motor syndrome. The underlying neuronal mechanisms of non-motor syndrome are largely unexplored. Besides SNc, midbrain ventral tegmental area (VTA) DANs also produce and release dopamine and modulate movement, reward, motivation, and memory. Degeneration of VTA DANs also occurs in postmortem brains of PD patients, implying an involvement of VTA DANs in PD-associated non-motor symptoms. However, it remains to be established that there is a distinct segregation of different SNc and VTA DAN subtypes in regulating different motor and non-motor functions, and that different DAN subpopulations are differentially affected by normal ageing or PD. Traditionally, the distinction among different DAN subtypes was mainly based on the location of cell bodies and axon terminals. With the recent advance of single cell RNA sequencing technology, DANs can be readily classified based on unique gene expression profiles. A combination of specific anatomic and molecular markers shows great promise to facilitate the identification of DAN subpopulations corresponding to different behavior modules under normal and disease conditions. In this review, we first summarize the recent progress in characterizing genetically, anatomically, and functionally diverse midbrain DAN subtypes. Then, we provide perspectives on how the preclinical research on the connectivity and functionality of DAN subpopulations improves our current understanding of cell-type and circuit specific mechanisms of the disease, which could be critically informative for designing new mechanistic treatments.

Sarcopenia and Neuroscience: Learning to Communicate

In the 1990s and early 2000s, the common definition for sarcopenia was age-related loss of skeletal muscle, and low levels of muscle mass were central to sarcopenia diagnosis. In more recent consensus definitions, however, low muscle strength displaces low muscle mass as a defining feature of sarcopenia. The change stems from growing evidence that muscle weakness is a better predictor of adverse health outcomes (e.g., mobility limitations) than muscle mass. This evidence accompanies an emerging recognition that central neural mechanisms are critical determinants of age-related changes in strength and mobility that can occur independently of variations in muscle mass. However, strikingly little practical attention is typically given to the potential role of the central nervous system in the aetiology or remediation of sarcopenia (i.e., low muscle function). In this article, we provide an overview of some mechanisms that mediate neural regulation of muscle contraction and control, and highlight the specific contributions of neural hypoexcitability, dopaminergic dysfunction, and degradation of functional and structural brain connectivity in relation to sarcopenia. We aim to enhance the lines of communication between the domains of sarcopenia and neuroscience. We believe that appreciation of the neural regulation of muscle contraction and control is fundamental to understanding sarcopenia and to developing targeted therapeutic strategies for its treatment.

Distinct and Common Large-Scale Networks of the Hippocampal Long Axis in Older Age: Links to Episodic Memory and Dopamine D2 Receptor Availability

The hippocampal longitudinal axis has been linked to dissociated functional networks relevant to episodic memory. However, the organization of axis-dependent networks and their relation to episodic memory in aging remains less explored. Moreover, age-related deterioration of the dopamine (DA) system, affecting memory and functional network properties, might constitute a source of reduced specificity of hippocampal networks in aging. Here, we characterized axis-dependent large-scale hippocampal resting-state networks, their relevance to episodic memory, and links to DA in older individuals (n = 170, 64–68 years). Partial least squares identified 2 dissociated networks differentially connected to the anterior and posterior hippocampus. These overlapped with anterior–temporal/posterior–medial networks in young adults, indicating preserved organization of axis-dependent connectivity in old age. However, axis-specific networks were overall unrelated to memory and hippocampal DA D2 receptor availability (D2DR) measured with [¹¹C]-raclopride positron emission tomography. Further analyses identified a memory-related network modulated by hippocampal D2DR, equally connected to anterior–posterior regions. This network included medial frontal, posterior parietal, and striatal areas. The results add to the current understanding of large-scale hippocampal connectivity in aging, demonstrating axis-dependent connectivity with dissociated anterior and posterior networks, as well as a primary role in episodic memory of connectivity shared by regions along the hippocampalaxis.

Individual Variability in Reaction Time and Prediction of Clinical Response to Methylphenidate in Adult ADHD: A Prospective Open Label Study Using Conners' Continuous Performance Test II

Objective: The aim of this study was to examine whether reaction time parameters in adult patients with ADHD could predict their response to methylphenidate (MPH). Method: Previously unmedicated patients (N = 123) were administered the Conners' Continuous Performance Test II (CPT II) at baseline and after 6 weeks of treatment with immediate-release MPH. In addition to traditional CPT measures, we extracted intraindividual raw data and analyzed time series using linear and nonlinear mathematical models. Results: Clinical responders, assessed with the Clinical Global Impression-Improvement scale, showed significant normalization of target failures, reduced variability and skewness, and increased complexity of reaction time series after 6 weeks of treatment, while nonresponders showed no significant changes. Prior to treatment, responders had significantly higher variability and skewness, combined with lower complexity, compared with nonresponders. Conclusion: These results show that the CPT test is useful in the evaluation of treatment response to MPH.

Mild Motor Signs Matter in Typical Brain Aging: The Value of the UPDRS Score Within a Functionally Intact Cohort of Older Adults

Objectives: To characterize the clinical correlates of subclinical Parkinsonian signs, including longitudinal cognitive and neural (via functional connectivity) outcomes, among functionally normal older adults.

Methods: Participants included 737 functionally intact community-dwelling older adults who performed prospective comprehensive evaluations at ~15-months intervals for an average of 4.8 years (standard deviation 3.2 years). As part of these evaluations, participants completed the Unified Parkinson's Disease Rating Scale (UPDRS) longitudinally and measures of processing speed, executive functioning and verbal episodic memory. T1-weighted structural scans and task-free functional MRI scans were acquired on 330 participants. We conducted linear mixed-effects models to determine the relationship between changes in UPDRS with cognitive and neural changes, using age, sex, and education as covariates.

Results: Cognitive outcomes were processing speed, executive functioning, and episodic memory. Greater within-person increases in UPDRS were associated with more cognitive slowing over time. Although higher average UPDRS scores were significantly associated with overall poorer executive functions, there was no association between UPDRS and executive functioning longitudinally. UPDRS scores did not significantly relate to longitudinal memory performances. Regarding neural correlates, greater increases in UPDRS scores were associated with reduced intra-subcortical network connectivity over time. There were no relationships with intra-frontoparietal or inter-subcortical-frontoparietal connectivity.

Conclusions: Our findings add to the aging literature by indicating that mild motor changes are negatively associated with cognition and network connectivity in functionally intact adults.

Is impaired dopaminergic function associated with mobility capacity in older adults?

The capacity to move is essential for independence and declines with age. Slow movement speed, in particular, is strongly associated with negative health outcomes. Prior research on mobility (herein defined as movement slowness) and aging has largely focused on musculoskeletal mechanisms and processes. More recent work has provided growing evidence for a significant role of the nervous system in contributing to reduced mobility in older adults. In this article, we report four pieces of complementary evidence from behavioral, genetic, and neuroimaging experiments that, we believe, provide theoretical support for the assertion that the basal ganglia and its dopaminergic function are responsible, in part, for age-related reductions in mobility. We report four a posteriori findings from an existing dataset: (1) slower central activation of ballistic force development is associated with worse mobility among older adults; (2) older adults with the Val/Met intermediate catecholamine-O-methyl-transferase (COMT) genotype involved in dopamine degradation exhibit greater mobility than their homozygous counterparts; (3) there are moderate relationships between performance times from a series of lower and upper extremity tasks supporting the notion that movement speed in older adults is a trait-like attribute; and (4) there is a relationship of functional connectivity within the medial orbofrontal (mOFC) cortico-striatal network and measures of mobility, suggesting that a potential neural mechanism for impaired mobility with aging is the deterioration of the integrity of key regions within the mOFC cortico-striatal network. These findings align with recent basic and clinical science work suggesting that the basal ganglia and its dopaminergic function are mechanistically linked to age-related reductions in mobility capacity.

Phenylketonuria, co-morbidity, and ageing: A review

Phenylketonuria (PKU) is a metabolic condition which, left untreated, results in severe and irreversible brain damage. Newborn screening and the development of the low phenylalanine (Phe) diet have transformed the outcomes for people with PKU. Those who have benefited from early treatment are now approaching their fifth and sixth decade. It is therefore timely to consider multi-morbidity in PKU and the effects of ageing, in parallel with the wider benefits of emerging treatment options in addition to dietary relaxation. We have conducted the first literature review of co-morbidity and ageing in the context of PKU. Avenues explored have emerged from limited study of multi-morbidity to date and the knowledge and critical enquiry of the authors. Findings suggest PKU to have a wider impact than brain development, and result in several intriguing questions that require investigation to attain the best outcomes for people with PKU in adulthood moving through to older age. We recognise the difficulty in studying longitudinal outcomes in rare diseases and emphasise the necessity to develop PKU registries and cohorts that facilitate well-designed studies to answer some of the questions raised in this review. Whilst awaiting new information in these areas we propose that clinicians engage with patients to make personalised and well-informed decisions around Phe control and assessment for co-morbidity.

Age-differential relationships among dopamine D1 binding potential, fusiform BOLD signal, and face-recognition performance

Facial recognition ability declines in adult aging, but the neural basis for this decline remains unknown. Cortical areas involved in face recognition exhibit lower dopamine (DA) receptor availability and lower blood-oxygen-level-dependent (BOLD) signal during task performance with advancing adult age. We hypothesized that changes in the relationship between these two neural systems are related to age differences in face-recognition ability. To test this hypothesis, we leveraged positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) to measure D1 receptor binding potential (BP_ND) and BOLD signal during face-recognition performance. Twenty younger and 20 older participants performed a face-recognition task during fMRI scanning. Face recognition accuracy was lower in older than in younger adults, as were D1 BP_ND and BOLD signal across the brain. Using linear regression, significant relationships between DA and BOLD were found in both age-groups in face-processing regions. Interestingly, although the relationship was positive in younger adults, it was negative in older adults (i.e., as D1 BP_ND decreased, BOLD signal increased). Ratios of BOLD:D1 BP_ND were calculated and relationships to face-recognition performance were tested. Multiple linear regression revealed a significant Group × BOLD:D1 BP_ND Ratio interaction. These results suggest that, in the healthy system, synchrony between neurotransmitter (DA) and hemodynamic (BOLD) systems optimizes the level of BOLD activation evoked for a given DA input (i.e., the gain parameter of the DA input-neural activation function), facilitating task performance. In the aged system, however, desynchronization between these brain systems would reduce the gain parameter of this function, adversely impacting task performance and contributing to reduced face recognition in older adults.

Context-Dependent Behavior in Parkinson’s Disease With Freezing of Gait

Background. Context-dependent behavior is a phenomenon in which people demonstrate superior performance in the context where a motor task was originally learned, but show poorer performance in an unfamiliar context. Previous studies found that people with Parkinson’s disease (PD) demonstrated greater context-dependency than nondisabled adults. Moreover, the frontostriatal circuit appeared to play a role in mediating context-dependent behavior. Neuroimaging studies showed that people with PD and freezing of gait (FoG) had difficulty recruiting the frontostriatal circuit when performing a set-shifting task, known to be mediated by this neural network. Objective. This study aimed to investigate whether individuals with PD and FoG (PD + FoG) would be more context-dependent than those without FoG (PD − FoG). Furthermore, the association between context-dependent behavior and set-shifting ability would be determined. Methods. Sixteen individuals with PD + FoG, 15 participants with PD − FoG, and 15 nondisabled adults (Control) were recruited. The participants practiced 3 numerical sequences, each associated with a specific context. One day following practice, the participants were tested under 2 conditions: the sequence-context associations remained the same as practice or were changed. Set-shifting ability was measured by the Trail Making Test (TMT). Results. Compared to the PD − FoG group, the PD + FoG group showed a greater decrement in normalized motor performance when the sequence-context associations were changed. Context-dependency correlated with the TMT in the PD − FoG group but not in the PD + FoG or Control groups. Conclusion. While people with PD + FoG appeared to be more context-dependent than individuals without FoG, a relationship between context-dependent behavior and set-shifting existed only in those without FoG.

Associations of Motor Symptom Severity and Quality of Life to Motor Task Performance in Upper and Lower Extremities Across Task Complexity in Parkinson's Disease

The authors examined the associations between the performance of upper- and lower-extremity motor tasks across task complexity and motor symptom severity, overall disease severity, and the physical aspects of quality of life in persons with Parkinson's disease. The performance was assessed for three lower-extremity tasks and two upper-extremity tasks of different levels of complexity. The motor symptoms and overall disease severity correlated significantly with all motor tasks with higher correlation coefficients in the complex tasks. Thus, the strength of the association between disease severity or severity of motor symptoms and motor performance is task-specific, with higher values in complex motor tasks than in simpler motor tasks. Mobility-related and activity-of-daily-living-related quality of life correlated with lower-extremity tasks of low and medium complexity and with the complex upper-extremity task, respectively; this suggests that Parkinson's Disease Questionnaire-39 is capable of differentiating between the impact of gross and fine motor function on quality of life.

Dopamine D2 receptor stimulation modulates the balance between ignoring and updating according to baseline working memory ability

BACKGROUND

Working memory (WM) deficits in neuropsychiatric disorders have often been attributed to altered dopaminergic signalling. Specifically, D₂ receptor stimulation is thought to affect the ease with which items can be gated into and out of WM. In addition, this effect has been hypothesised to vary according to baseline WM ability, a putative index of dopamine synthesis levels. Moreover, whether D₂ stimulation affects WM vicariously through modulating relatively WM-free cognitive control processes has not been explored.

AIMS

We examined the effect of administering a dopamine agonist on the ability to ignore or update information in WM.

METHOD

A single dose of cabergoline (1 mg) was administered to healthy older adult humans in a within-subject, double-blind, placebo-controlled study. In addition, we obtained measures of baseline WM ability and relatively WM-free cognitive control (overcoming response conflict).

RESULTS

Consistent with predictions, baseline WM ability significantly modulated the effect that drug administration had on the proficiency of ignoring and updating. High-WM individuals were relatively better at ignoring compared to updating after drug administration. Whereas the opposite occurred in low-WM individuals. Although the ability to overcome response conflict was not affected by cabergoline, a negative relationship between the effect the drug had on response conflict performance and ignoring was observed. Thus, both response conflict and ignoring are coupled to dopaminergic stimulation levels.

CONCLUSIONS

Cumulatively, these results provide evidence that dopamine affects subcomponents of cognitive control in a diverse, antagonistic fashion and that the direction of these effects is dependent upon baseline WM.

The Neuropsychiatry of Parkinson Disease: A Perfect Storm

Affective disorders, cognitive decline, and psychosis have long been recognized as common in Parkinson disease (PD), and other psychiatric disorders include impulse control disorders, anxiety symptoms, disorders of sleep and wakefulness, and apathy. Psychiatric aspects of PD are associated with numerous adverse outcomes, yet in spite of this and their frequent occurrence, there is incomplete understanding of epidemiology, presentation, risk factors, neural substrate, and management strategies. Psychiatric features are typically multimorbid, and there is great intra- and interindividual variability in presentation. The hallmark neuropathophysiological changes that occur in PD, plus the association between exposure to dopaminergic medications and certain psychiatric disorders, suggest a neurobiological basis for many psychiatric symptoms, although psychological factors are involved as well. There is evidence that psychiatric disorders in PD are still under-recognized and undertreated and although psychotropic medication use is common, controlled studies demonstrating efficacy and tolerability are largely lacking. Future research on neuropsychiatric complications in PD should be oriented toward determining modifiable correlates or risk factors and establishing efficacious and well-tolerated treatment strategies.

The Aging Brain & the Dorsal Basal Ganglia: Implications for Age-Related Limitations of Mobility

The capacity to move is essential for independence and declines with age. Limitations in mobility impact ~35% of adults over 70 and the majority of adults over 85. These limitations are highly associated with disability, dependency, and survival. More than 25-years ago the term "sarcopenia" was coined to highlight the age-related loss of muscle mass and strength with the assumption being that sarcopenia led to limitations in mobility. However, contrary to expectations, recent findings clearly indicate these variables only modestly explain limitations in mobility. One likely reason the current sarcopenia variables of muscle mass and strength do not discriminate, or predict, mobility limitations well is because they are heavily influenced by musculoskeletal mechanisms and do not incorporate measures reflective of the central neural control of mobility. Unfortunately, the precise central neural changes associated with aging that lead to decreased mobility are poorly understood. This knowledge gap has hampered the development of effective interventions for mobility limitations and the subsequent reduction of major functional disability for older adults. Here, we discuss the potential role of the motor control circuit of the dorsal basal ganglia as well as dopaminergic function in age-related reductions in mobility.

Dopamine D2/D3 receptor abnormalities after traumatic brain injury and their relationship to post-traumatic depression

Objective

To investigate dopamine D2/D3 receptor availability following traumatic brain injury (TBI) and their relationship to the presence of DSM-IV Major Depressive Disorder (MDD) and patterns of axonal injury.

Methods

Twelve moderate-severe TBI patients and 26 controls were imaged using [¹¹C]PHNO positron emission tomography (PET) and structural magnetic resonance imaging (MRI). TBI patients and a second group of 32 controls also underwent diffusion tensor imaging (DTI) and neuropsychological assessment. Patients included six with post-injury MDD (TBI-MDD) and six without (TBI-NON). Non-displaceable binding potential (BP_ND) [¹¹C]PHNO values were used to index D2/D3 receptor availability, and were calculated using a reference region procedure. Differences in BP_ND were examined using voxelwise and region-of-interest analyses. White matter microstructure integrity, quantified by fractional anisotropy (FA), was assessed and correlated with BP_ND.

Results

Lower [¹¹C]PHNO BP_ND was found in the caudate across all TBI patients when compared to controls. Lower [¹¹C]PHNO BP_ND was observed in the caudate of TBI-MDD patients and increased [¹¹C]PHNO BP_ND in the Amygdala of TBI-NON patients compared to controls. There were no significant differences in [¹¹C]PHNO BP_ND between TBI-MDD and TBI-NON patients. Furthermore, DTI provided evidence of axonal injury following TBI. The uncinate fasciculus and cingulum had abnormally low FA, with the uncinate particularly affected in TBI-MDD patients. Caudate [¹¹C]PHNO BP_ND correlated with FA within the nigro-caudate tract.

Conclusions

[¹¹C]PHNO BP_ND is abnormal following TBI, which indicates post-traumatic changes in D2/D3 receptors. Patterns of [¹¹C]PHNO BP_ND seen in patients with and without MDD suggest that further research would be beneficial to determine whether the use of dopaminergic treatment might be effective in the treatment of post-traumatic depression.

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[¹¹C]PHNO PET is used for the first time in traumatic brain injury (TBI) patients.

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Post-traumatic changes in dopamine D2/D3 receptors were observed.

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Patients with major depression showed more prominent reductions in [¹¹C]PHNO BP_ND.

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Non-depressed TBI patients had greater [¹¹C]PHNO BP_ND in the Amygdala.

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These findings suggest a potential role of D2/D3 changes in post-TBI depression.

Age-Related Differences in Motivational Integration and Cognitive Control

Motivational incentives play an influential role in value-based decision-making and cognitive control. A compelling hypothesis in the literature suggests that the motivational value of diverse incentives are integrated in the brain into a common currency value signal that influences decision-making and behavior. To investigate whether motivational integration processes change during healthy aging, we tested older (N = 44) and younger (N = 54) adults in an innovative incentive integration task paradigm that establishes dissociable and additive effects of liquid (e.g., juice, neutral, saltwater) and monetary incentives on cognitive task performance. The results reveal that motivational incentives improve cognitive task performance in both older and younger adults, providing novel evidence demonstrating that age-related cognitive control deficits can be ameliorated with sufficient incentive motivation. Additional analyses revealed clear age-related differences in motivational integration. Younger adult task performance was modulated by both monetary and liquid incentives, whereas monetary reward effects were more gradual in older adults and more strongly impacted by trial-by-trial performance feedback. A surprising discovery was that older adults shifted attention from liquid valence toward monetary reward throughout task performance, but younger adults shifted attention from monetary reward toward integrating both monetary reward and liquid valence by the end of the task, suggesting differential strategic utilization of incentives. These data suggest that older adults may have impairments in incentive integration and employ different motivational strategies to improve cognitive task performance. The findings suggest potential candidate neural mechanisms that may serve as the locus of age-related change, providing targets for future cognitive neuroscience investigations.

White matter integrity in brain structures supporting semantic processing is associated with value-directed remembering in older adults

White matter microstructure changes substantially in aging. To better understand how the integrity of white matter structures supports the selective learning of rewarding material, 23 healthy older adults were tested on a value-directed remembering task. This task involved successive free recall word lists where items differed in importance, as denoted by value cues preceding each word. White matter structure was measured using diffusion tensor imaging (DTI). We found that greater structural integrity (as measured by lower mean diffusivity) in left inferior fronto-occipital fasciculus was associated with greater recall for high-value items, but not low-value items. Older adults with greater structural integrity in a tract involved in semantic processing are thus able to more successfully encode high-value items for subsequent recall. However, unlike prior findings in younger adults, older adults' memory for high value-items was not significantly correlated with the structural integrity of the uncinate fasciculus, nor with the strength of anatomical connectedness between the bilateral nucleus accumbens to ventral tegmental area reward pathway. These structural imaging findings add support to recent functional neuroimaging demonstrations that value-related modulation of memory in older adults depends heavily on brain circuits implicated in controlled processing of semantic knowledge.

Neurophysiology and genetics of burning mouth syndrome

BACKGROUND AND AIMS

Neuropathic mechanisms are involved in burning mouth syndrome (BMS), and variation of the dopamine D2 receptor (DRD2) gene contributes to experimental pain perception. We investigated whether neurophysiologic findings differ in BMS patients compared to healthy controls, and whether 957C>T polymorphism of the DRD2 gene influences thermal sensitivity or pain experience in BMS.

METHODS

Forty-five BMS patients (43 women), mean age 62.5 years, and 32 healthy controls (30 women), mean age 64.8 years, participated. Patients estimated pain intensity, interference, suffering and sleep with Numeric Rating Scale. Blink reflex tests of the supraorbital (SON), mental (MN) and lingual (LN) nerves, and thermal quantitative sensory testing were done. The results were analysed with ANOVA. DRD2 gene 957C>T polymorphism was determined in 31 patients, and its effects on neurophysiologic and clinical variables were analysed.

RESULTS

Cool (p = 0.0090) and warm detection thresholds (p = 0.0229) of the tongue were higher in BMS patients than controls. The stimulation threshold for SON BR was higher in patients than in controls (p = 0.0056). The latencies of R2 component were longer in BMS patients than in controls (p = 0.0005) at the SON distribution. Habituation of SON BR did not differ between the groups. The heat pain thresholds were highest (p = 0.0312) in homozygous patients with 957TT, who also reported most interference (p = 0.0352) and greatest suffering (p = 0.0341). Genotype 957CC associated with sleep disturbances (p = 0.0254).

CONCLUSIONS

Burning mouth syndrome patients showed thermal hypoesthesia within LN distribution compatible with small fibre neuropathy. The DRD2 957C>T genotype influences perception and experience of BMS pain.

SIGNIFICANCE

The results confirm earlier findings of neuropathic pain in BMS. The DRD2 957 C>T genotype influences perception and experience of clinical pain in BMS.

Label-free imaging of neurotransmitters in live brain tissue by multi-photon ultraviolet microscopy

Visualizing small biomolecules in living cells remains a difficult challenge. Neurotransmitters provide one of the most frustrating examples of this difficulty, as our understanding of signaling in the brain critically depends on our ability to follow the neurotransmitter traffic. Last two decades have seen considerable progress in probing some of the neurotransmitters, e.g. by using false neurotransmitter mimics, chemical labeling techniques, or direct fluorescence imaging. Direct imaging harnesses the weak UV fluorescence of monoamines, which are some of the most important neurotransmitters controlling mood, memory, appetite, and learning. Here we describe the progress in imaging of these molecules using the least toxic direct excitation route found so far, namely multi-photon (MP) imaging. MP imaging of serotonin, and more recently that of dopamine, has allowed researchers to determine the location of the vesicles, follow their intracellular dynamics, probe their content, and monitor their release. Recent developments have even allowed ratiometric quantitation of the vesicular content. This review shows that MP ultraviolet (MP-UV) microscopy is an effective but underutilized method for imaging monoamine neurotransmitters in neurones and brain tissue.

Comparing Effects of Reward Anticipation on Working Memory in Younger and Older Adults

Goal-directed behavior requires sufficient resource allocation of cognitive control processes, such as the ability to prioritize relevant over less relevant information in working memory. Findings from neural recordings in animals and human multimodal imaging studies suggest that reward incentive mechanisms could facilitate the encoding and updating of context representations, which can have beneficial effects on working memory performance in young adults. In order to investigate whether these performance enhancing effects of reward on working memory processes are still preserved in old age, the current study aimed to investigate whether aging alters the effects of reward anticipation on the encoding and updating mechanisms in working memory processing. Therefore, a reward modulated verbal n-back task with age-adjusted memory load manipulation was developed to investigate reward modulation of working memory in younger (age 20-27) and older (age 65-78) adults. Our results suggest that the mechanism of reward anticipation in enhancing the encoding and updating of stimulus representations in working memory is still preserved in old age. EZ-diffusion modeling showed age distinct patterns of reward modulation of model parameters that correspond to different processes of memory-dependent decision making. Whereas processes of memory evidence accumulation and sensorimotor speed benefited from reward modulation, responses did not become more cautious with incentive motivation for older adults as it was observed in younger adults. Furthermore, individual differences in reward-related enhancement of decision speed correlated with cognitive processing fluctuation and memory storage capacity in younger adults, but no such relations were observed in older adults. These findings indicate that although beneficial effects of reward modulation on working memory can still be observed in old age, not all performance aspects are facilitated. Whereas reward facilitation of content representations in working memory seems to be relatively preserved, aging seems to affect the updating of reward contexts. Future research is needed to elucidate potential mechanisms for motivational regulation of the plasticity of working memory in old age.

Neuro-Cognitive Effects of Acute Tyrosine Administration on Reactive and Proactive Response Inhibition in Healthy Older Adults

The aging brain is characterized by altered dopamine signaling. The amino acid tyrosine, a catecholamine precursor, is known to improve cognitive performance in young adults, especially during high environmental demands. Tyrosine administration might also affect catecholamine transmission in the aging brain, thereby improving cognitive functioning. In healthy older adults, impairments have been demonstrated in two forms of response inhibition: reactive inhibition (outright stopping) and proactive inhibition (anticipatory response slowing) under high information load. However, no study has directly compared the effects of a catecholamine precursor on reactive and load-dependent proactive inhibition. In this study we explored the effects of tyrosine on reactive and proactive response inhibition and signal in dopaminergically innervated fronto-striatal regions. Depending on age, tyrosine might lead to beneficial or detrimental neurocognitive effects. We aimed to address these hypotheses in 24 healthy older human adults (aged 61-72 years) using fMRI in a double blind, counterbalanced, placebo-controlled, within-subject design. Across the group, tyrosine did not alter reactive or proactive inhibition behaviorally but did increase fronto-parietal proactive inhibition-related activation. When taking age into account, tyrosine affected proactive inhibition both behaviorally and neurally. Specifically, increasing age was associated with a greater detrimental effect of tyrosine compared with placebo on proactive slowing. Moreover, with increasing age, tyrosine decreased fronto-striatal and parietal proactive signal, which correlated positively with tyrosine's effects on proactive slowing. Concluding, tyrosine negatively affected proactive response slowing and associated fronto-striatal activation in an age-dependent manner, highlighting the importance of catecholamines, perhaps particularly dopamine, for proactive response inhibition in older adults.

Curiosity in old age: A possible key to achieving adaptive aging

Curiosity is a fundamental part of human motivation that supports a variety of human intellectual behaviors ranging from early learning in children to scientific discovery. However, there has been little attention paid to the role of curiosity in aging populations. By bringing together broad but sparse neuroscientific and psychological literature on curiosity and related concepts (e.g., novelty seeking in older adults), we propose that curiosity, although it declines with age, plays an important role in maintaining cognitive function, mental health, and physical health in older adults. We identify the dopaminergic reward system and the noradrenergic system as the key brain systems implicated in curiosity processing and discuss how these brain systems contribute to the relationship between curiosity and adaptive aging.

Human aging reduces the neurobehavioral influence of motivation on episodic memory

The neural circuitry mediating the influence of motivation on long-term declarative or episodic memory formation is delineated in young adults, but its status is unknown in healthy aging. We examined the effect of reward and punishment anticipation on intentional declarative memory formation for words using an event-related functional magnetic resonance imaging (fMRI) monetary incentive encoding task in twenty-one younger and nineteen older adults. At 24-hour memory retrieval testing, younger adults were significantly more likely to remember words associated with motivational cues than neutral cues. Motivational enhancement of memory in younger adults occurred only for recollection ("remember" responses) and not for familiarity ("familiar" responses). Older adults had overall diminished memory and did not show memory gains in association with motivational cues. Memory encoding associated with monetary rewards or punishments activated motivational (substantia nigra/ventral tegmental area) and memory-related (hippocampus) brain regions in younger, but not older, adults during the target word periods. In contrast, older and younger adults showed similar activation of these brain regions during the anticipatory motivational cue interval. In a separate monetary incentive delay task that did not require learning, we found evidence for relatively preserved striatal reward anticipation in older adults. This supports a potential dissociation between incidental and intentional motivational processes in healthy aging. The finding that motivation to obtain rewards and avoid punishments had reduced behavioral and neural influence on intentional episodic memory formation in older compared to younger adults is relevant to life-span theories of cognitive aging including the dopaminergic vulnerability hypothesis.

The effect of proprioceptive acuity variability on motor adaptation in older adults

Motor adaptation requires efficient integration of sensory information with predicted sensory consequences of one's own action. However, the effect of reduced sensory acuity on motor adaptation in humans remains to be further investigated. Here, we examined the variability of proprioceptive acuity during an arm-position matching task and the pattern of visuomotor adaptation in older and young adults, and determined the relationship between the two variables. The older adults, a known example of impaired proprioceptive acuity, exhibited greater trial-to-trial variability during the arm-position matching task as compared with the young adults. Furthermore, the older adults showed a slower rate of adaptation to a 30° visuomotor rotation during targeted reaching movements, as well as larger movement errors in the later phase of adaptation, than the young adults. Our correlation analyses indicated a negative association between the variability in proprioceptive acuity and the rate of visuomotor adaptation in the older adults; and no association was observed in the young adults. These findings point to a possibility that an increase in the variability of proprioceptive acuity due to aging may weaken the integration of predicted and actual sensory feedback, which in turn may result in poor visuomotor adaptation in older adults.