Reproducibility of the correlative triad among aging, dopamine receptor availability, and cognition

Aging-related losses in dopamine D2/3 receptor availability are linked to working-memory decline across five years

Although age differences in the dopamine system have been suggested to contribute to age-related cognitive decline based on cross-sectional data, recent large-scale cross-sectional studies reported only weak evidence for a correlation among aging, dopamine receptor availability, and cognition. Regardless, longitudinal data remain essential to make robust statements about dopamine losses as a basis for cognitive aging. We present correlations between changes in D2/3 dopamine receptor availability and changes in working memory measured over 5 yr in healthy, older adults (n = 128, ages 64 to 68 yr at baseline). Greater decline in D2/3 dopamine receptor availability in working memory-relevant regions (caudate, middle frontal cortex, hippocampus) was related to greater decline in working memory performance in individuals who exhibited working memory reductions across time (n = 43; caudate: rs = 0.494; middle frontal cortex: rs = 0.506; hippocampus; rs = 0.423), but not in individuals who maintained performance (n = 41; caudate: rs = 0.052; middle frontal cortex: rs = 0.198; hippocampus; rs = 0.076). The dopamine-working memory link in decliners was not observed in the orbitofrontal cortex, which does not belong to the core working memory network. Our longitudinal analyses support the notion that aging-related changes in the dopamine system contribute to working memory decline in aging.

Longitudinal support for the correlative triad among aging, dopamine D2-like receptor loss, and memory decline

Dopamine decline is suggested to underlie aging-related cognitive decline, but longitudinal examinations of this link are currently missing. We analyzed 5-year longitudinal data for a sample of healthy, older adults (baseline: n = 181, age: 64-68 years; 5-year follow-up: n = 129) who underwent positron emission tomography with 11C-raclopride to assess dopamine D2-like receptor (DRD2) availability, magnetic resonance imaging to evaluate structural brain measures, and cognitive tests. Health, lifestyle, and genetic data were also collected. A data-driven approach (k-means cluster analysis) identified groups that differed maximally in DRD2 decline rates in age-sensitive brain regions. One group (n = 47) had DRD2 decline exclusively in the caudate and no cognitive decline. A second group (n = 72) had more wide-ranged DRD2 decline in putamen and nucleus accumbens and also in extrastriatal regions. The latter group showed significant 5-year working memory decline that correlated with putamen DRD2 decline, along with higher dementia and cardiovascular risk and a faster biological pace of aging. Taken together, for individuals with more extensive DRD2 decline, dopamine decline is associated with memory decline in aging.

Substantial decline of phasic dopamine signaling in senescent male rats does not impact dopamine-dependent Pavlovian conditioning

Normal aging is associated with cognitive decline which impacts financial decision making. One of the underlying features of decision making is probability estimation, in which nucleus accumbens dopamine signaling has been implicated. Here we used fast-scan cyclic voltammetry to probe for age differences in dopamine signaling, and pharmacological manipulation to test for age differences in the dopamine dependence of Pavlovian conditioning. We found differences in phasic dopamine signaling to reward delivery, and unconditioned and conditioned stimuli, but no difference in conditioned approach between adult and senescent groups. In addition, we found that dopamine receptor antagonism with flupenthixol (225 μg/kg, i.p.) partially inhibited conditioned approach in the adult group, whereas it completely blocked conditioned approach in the senescent group. Further increase in concentration to 300 μg/kg, i.p. resulted in complete inhibition of conditioned approach behavior in both age groups. Therefore, while phasic dopamine signaling in the nucleus accumbens of senescent animals is greatly diminished in concentration, these animals maintain dopamine dependent Pavlovian conditioning.

Aging, uncertainty, and decision making-A review

There is a great deal of uncertainty in the world. One common source of uncertainty results from incomplete or missing information about probabilistic outcomes (i.e., outcomes that may occur), which influences how people make decisions. The impact of this type of uncertainty may particularly pronounced for older adults, who, as the primary leaders around the world, make highly impactful decisions with lasting outcomes. This review examines the ways in which uncertainty about probabilistic outcomes is perceived, handled, and represented in the aging brain, with an emphasis on how uncertainty may specifically affect decision making in later life. We describe the role of uncertainty in decision making and aging from four perspectives, including 1) theoretical, 2) self-report, 3) behavioral, and 4) neuroscientific. We report evidence of any age-related differences in uncertainty among these contexts and describe how these changes may affect decision making. We then integrate the findings across the distinct perspectives, followed by a discussion of important future directions for research on aging and uncertainty, including prospection, domain-specificity in risk-taking behaviors, and choice overload.

Multivariate associations between dopamine receptor availability and risky investment decision-making across adulthood

Enhancing dopamine increases financial risk taking across adulthood but it is unclear whether baseline individual differences in dopamine function are related to risky financial decisions. Here, thirty-five healthy adults completed an incentive-compatible risky investment decision task and a PET scan at rest using [11C]FLB457 to assess dopamine D2-like receptor availability. Participants made choices between a safe asset (bond) and a risky asset (stock) with either an expected value less than the bond ("bad stock") or expected value greater than the bond ("good stock"). Five measures of behavior (choice inflexibility, risk seeking, suboptimal investment) and beliefs (absolute error, optimism) were computed and D2-like binding potential was extracted from four brain regions of interest (midbrain, amygdala, anterior cingulate, insula). We used canonical correlation analysis to evaluate multivariate associations between decision-making and dopamine function controlling for age. Decomposition of the first dimension (r = 0.76) revealed that the strongest associations were between measures of choice inflexibility, incorrect choice, optimism, amygdala binding potential, and age. Follow-up univariate analyses revealed that amygdala binding potential and age were both independently associated with choice inflexibility. The findings suggest that individual differences in dopamine function may be associated with financial risk taking in healthy adults.

Children’s ADHD and Dysregulation Problems, DAT1 Genotype and Methylation, and their Interplay with Family Environment

Background

International literature has underlined the complex interplay between genetic and environmental variables in shaping children’s emotional-behavioral functioning.

Objective

This study aimed to explore the dynamic relationship between children’s Dopamine Transporter (DAT1) genotype and methylation, and maternal and paternal affective environment, on children’s Attention Deficit Hyperactivity Disorder (ADHD) problems and dysregulation problems.

Method

In a community sample of 76 families with school-aged children, we assessed children’s DAT1 genotype and methylation, their own ADHD problems and dysregulation profile (CBCL 6–18 DP), and maternal and paternal psychopathological risk, parenting stress, and marital adjustment. Hierarchical regressions were carried out to verify the possible moderation of children’s genotype on the relationship between children’s methylation and psychopathological risk, parental environment and children’s methylation, and parental environment and children’s psychopathological risk.

Results

The levels of methylation at M1 CpG significantly predicted ADHD problems among children with 10/10 genotype, whereas high levels of methylation at M6 CpG predicted low ADHD problems for children with 9/x genotype. High levels of methylation at M3 CpG were associated with high scores of CBCL DP. DAT1 genotype moderated the relationship between maternal and paternal variables with children’s methylation and psychopathological risk. The scores of maternal and paternal Dyadic Adjustment Scale showed indirect effects on children’s methylation and psychopathological risk in relation to those exerted by risk factors.

Conclusion

Our study has supported the emerging evidence on the complex nature of children’s emotional-behavioral functioning and the associated risk and protective factors, with important implications for the planning of preventive programs.

The Aged Striatum: Evidence of Molecular and Structural Changes Using a Longitudinal Multimodal Approach in Mice

To study the aging human brain requires significant resources and time. Thus, mice models of aging can provide insight into changes in brain biological functions at a fraction of the time when compared to humans. This study aims to explore changes in dopamine D₁ and D₂ receptor availability and of gray matter density in striatum during aging in mice and to evaluate whether longitudinal imaging in mice may serve as a model for normal brain aging to complement cross-sectional research in humans. Mice underwent repeated structural magnetic resonance imaging (sMRI), and [¹¹C]Raclopride and [¹¹C]SCH23390 positron emission tomography (PET) was performed on a subset of aging mice. PET and sMRI data were analyzed by binding potential (BP_ND), voxel- and tensor-based morphometry (VBM and TBM, respectively). Longitudinal PET revealed a significant reduction in striatal BP_ND for D₂ receptors over time, whereas no significant change was found for D₁ receptors. sMRI indicated a significant increase in modulated gray matter density (mGMD) over time in striatum, with limited clusters showing decreased mGMD. Mouse [¹¹C]Raclopride data is compatible with previous reports in human cross-sectional studies, suggesting that a natural loss of dopaminergic D₂ receptors in striatum can be assessed in mice, reflecting estimates from humans. No changes in D₁ were found, which may be attributed to altered [¹¹C]SCH23390 kinetics in anesthetized mice, suggesting that this tracer is not yet able to replicate human findings. sMRI revealed a significant increase in mGMD. Although contrary to expectations, this increase in modulated GM density may be attributed to an age-related increase in non-neuronal cells.

Influences of dopaminergic system dysfunction on late-life depression

Deficits in cognition, reward processing, and motor function are clinical features relevant to both aging and depression. Individuals with late-life depression often show impairment across these domains, all of which are moderated by the functioning of dopaminergic circuits. As dopaminergic function declines with normal aging and increased inflammatory burden, the role of dopamine may be particularly salient for late-life depression. We review the literature examining the role of dopamine in the pathogenesis of depression, as well as how dopamine function changes with aging and is influenced by inflammation. Applying a Research Domain Criteria (RDoC) Initiative perspective, we then review work examining how dopaminergic signaling affects these domains, specifically focusing on Cognitive, Positive Valence, and Sensorimotor Systems. We propose a unified model incorporating the effects of aging and low-grade inflammation on dopaminergic functioning, with a resulting negative effect on cognition, reward processing, and motor function. Interplay between these systems may influence development of a depressive phenotype, with an initial deficit in one domain reinforcing decline in others. This model extends RDoC concepts into late-life depression while also providing opportunities for novel and personalized interventions.

Diabetes and Cognitive Impairment: A Role for Glucotoxicity and Dopaminergic Dysfunction

Diabetes mellitus (DM) is a chronic metabolic disorder characterized by hyperglycemia, responsible for the onset of several long-term complications. Recent evidence suggests that cognitive dysfunction represents an emerging complication of DM, but the underlying molecular mechanisms are still obscure. Dopamine (DA), a neurotransmitter essentially known for its relevance in the regulation of behavior and movement, modulates cognitive function, too. Interestingly, alterations of the dopaminergic system have been observed in DM. This review aims to offer a comprehensive overview of the most relevant experimental results assessing DA’s role in cognitive function, highlighting the presence of dopaminergic dysfunction in DM and supporting a role for glucotoxicity in DM-associated dopaminergic dysfunction and cognitive impairment. Several studies confirm a role for DA in cognition both in animal models and in humans. Similarly, significant alterations of the dopaminergic system have been observed in animal models of experimental diabetes and in diabetic patients, too. Evidence is accumulating that advanced glycation end products (AGEs) and their precursor methylglyoxal (MGO) are associated with cognitive impairment and alterations of the dopaminergic system. Further research is needed to clarify the molecular mechanisms linking DM-associated dopaminergic dysfunction and cognitive impairment and to assess the deleterious impact of glucotoxicity.

Elevated Dopamine Synthesis as a Mechanism of Cognitive Resilience in Aging

Aging is associated with declines in multiple components of the dopamine system including loss of dopamine-producing neurons, atrophy of the dopamine system's cortical targets, and reductions in the density of dopamine receptors. Countering these patterns, dopamine synthesis appears to be stable or elevated in older age. We tested the hypothesis that elevation in dopamine synthesis in aging reflects a compensatory response to neuronal loss rather than a nonspecific monotonic shift in older age. We measured individual differences in striatal dopamine synthesis capacity in cognitively normal older adults using [18F]Fluoro-l-m-tyrosine positron emission tomography cross-sectionally and tested relationships with longitudinal reductions in cortical thickness and working memory decline beginning up to 13 years earlier. Consistent with a compensation account, older adults with the highest dopamine synthesis capacity were those with greatest atrophy in posterior parietal cortex. Elevated dopamine synthesis capacity was not associated with successful maintenance of working memory performance overall, but had a moderating effect such that higher levels of dopamine synthesis capacity reduced the impact of atrophy on cognitive decline. Together, these findings support a model by which upregulation of dopamine synthesis represents a mechanism of cognitive resilience in aging.

Differential prioritization of intramaze cue and boundary information during spatial navigation across the human lifespan

Spatial learning can be based on intramaze cues and environmental boundaries. These processes are predominantly subserved by striatal- and hippocampal-dependent circuitries, respectively. Maturation and aging processes in these brain regions may affect lifespan differences in their contributions to spatial learning. We independently manipulated an intramaze cue or the environment's boundary in a navigation task in 27 younger children (6-8 years), 30 older children (10-13 years), 29 adolescents (15-17 years), 29 younger adults (20-35 years) and 26 older adults (65-80 years) to investigate lifespan age differences in the relative prioritization of either information. Whereas learning based on an intramaze cue showed earlier maturation during the progression from younger to later childhood and remained relatively stable across adulthood, maturation of boundary-based learning was more protracted towards peri-adolescence and showed strong aging-related decline. Furthermore, individual differences in prioritizing intramaze cue- over computationally more demanding boundary-based learning was positively associated with cognitive processing fluctuations and this association was partially mediated by spatial working memory capacity during adult, but not during child development. This evidence reveals different age gradients of two modes of spatial learning across the lifespan, which seem further influenced by individual differences in cognitive processing fluctuations and working memory, particularly during aging.

Exercise, Dopamine, and Cognition in Older Age

Jonasson et al. investigated whether individual differences in human dopamine receptors (D2R) were related to cognitive performance before and after a 6-month aerobic exercise intervention (compared with active control). While D2R decreased (perhaps counterintuitively) with exercise, there was no relationship between D2R and working memory at baseline or following exercise.