Effects of aging on executive functioning and mesocorticolimbic dopamine markers in male Fischer 344 × brown Norway rats

Neurosteroids and the mesocorticolimbic system

The mesocorticolimbic system coordinates executive functions, such as working memory and behavioral flexibility. This circuit includes dopaminergic projections from the ventral tegmental area to the nucleus accumbens and medial prefrontal cortex. In this review, we summarize evidence that cells in multiple nodes of the mesocorticolimbic system produce neurosteroids (steroids synthesized in the nervous system) and express steroid receptors. Here, we focus on neuroandrogens (androgens synthesized in the nervous system), neuroestrogens (estrogens synthesized in the nervous system), and androgen and estrogen receptors. We also summarize how (neuro)androgens and (neuro)estrogens affect dopamine signaling in the mesocorticolimbic system and regulate executive functions. Taken together, the data suggest that steroids produced in the gonads and locally in the brain modulate higher-order cognition and executive functions.

Polyoxidovanadates as a pharmacological option against brain aging

The world population is aging rapidly, and chronic diseases associated are cardiometabolic syndrome, cancer, and neurodegenerative diseases. Oxidative stress and inflammation are typical hallmarks in them. Polyoxidovanadates (POVs) have shown interesting pharmacological actions against chronic diseases. This work aimed to evaluate the POV effect on hippocampal neuroinflammation, redox balance, and recognition memory in the aging of rats. Rats 18 months old were administered a daily dose of sodium metavanadate (MV), decavanadate (DV), Metformin (Metf), or MetfDeca for two months. Results showed that short-term and long-term recognition memory improved by 28 % and 16 % (DV), 19 % and 20 % (Metf), and 21 % and 27 % (MetfDeca). In hippocampi, reactive oxygen species, IL-1β, and TNF-α, after DV, Metf, and MetfDeca decreased at similar concentrations to young adult control, while lipid peroxidation substantially ameliorated. Additionally, superoxide dismutase and catalase activity increased by 41 % and 42 % (DV), 39 % and 41 % (Metf), and 75 % and 73 % (MetfDeca). POV treatments reduced Nrf2 and GFAP immunoreactivity in CA1 (70-87.5 %), CA3 (60-80 %), and DG (57-89 %). Metformin treatment showed a minor effect, while MV treatment did not improve any parameters. Although DV, Metf, and MetfDeca treatments showed similar results, POVs doses were 16-fold fewer than Metformin. In conclusion, DV and MetfDeca could be pharmacological options to reduce age-related neuronal damage.

Development of cross-species translational paradigms for psychiatric research in the Research Domain Criteria era

The past 30 years of IBNS has included research attempting to treat the cognitive and behavioral deficits observed in people with psychiatric conditions. Early work utilized drugs identified from tests thought to be cognition-relevant, however the high failure rate crossing the translational-species barrier led to focus on developing valid cross-species translational tests. The face, predictive, and neurobiological validities used to assess animal models of psychiatry can be used to validate these tests. Clinical sensitivity is another important aspect however, for if the clinical population targeted for treatment does not exhibit task deficits, then why develop treatments? This review covers some work validating cross-species translational tests and suggests future directions. Also covered is the contribution IBNS made to fostering such research and my role in IBNS, making it more available to all including fostering mentor/mentee programs plus spearheading diversity and inclusivity initiatives. All science needs support and IBNS has supported research recreating the behavioral abnormalities that define psychiatric conditions with the aim to improve the lives of people with such conditions.

Age-Related Changes in Risky Decision Making and Associated Neural Circuitry in a Rat Model

Altered decision making at advanced ages can have a significant impact on an individual's quality of life and the ability to maintain personal independence. Relative to young adults, older adults make less impulsive and less risky choices; although these changes in decision making could be considered beneficial, they can also lead to choices with potentially negative consequences (e.g., avoidance of medical procedures). Rodent models of decision making have been invaluable for dissecting cognitive and neurobiological mechanisms that contribute to age-related changes in decision making, but they have predominantly used costs related to timing or probability of reward delivery and have not considered other equally important costs, such as the risk of adverse consequences. The current study therefore used a rat model of decision making involving risk of explicit punishment to examine age-related changes in this form of choice behavior in male rats, and to identify potential cognitive and neurobiological mechanisms that contribute to these changes. Relative to young rats, aged rats displayed greater risk aversion, which was not attributable to reduced motivation for food, changes in shock sensitivity, or impaired cognitive flexibility. Functional MRI analyses revealed that, overall, functional connectivity was greater in aged rats compared with young rats, particularly among brain regions implicated in risky decision making such as basolateral amygdala, orbitofrontal cortex, and ventral tegmental area. Collectively, these findings are consistent with greater risk aversion found in older humans, and reveal age-related changes in brain connectivity.

Androgen synthesis inhibition increases behavioural flexibility and mPFC tyrosine hydroxylase in gonadectomized male rats

Behavioural flexibility is essential to adapt to a changing environment and depends on the medial prefrontal cortex (mPFC). Testosterone administration decreases behavioural flexibility. It is well known that testosterone is produced in the gonads, but testosterone is also produced in the brain, including the mPFC and other nodes of the mesocorticolimbic system. It is unclear how testosterone produced in the brain versus the gonads influences behavioural flexibility. Here, in adult male rats, we assessed the effects of the androgen synthesis inhibitor abiraterone acetate (ABI) and long-term gonadectomy (GDX) on behavioural flexibility in two paradigms. In Experiment 1, ABI but not GDX reduced the number of errors to criterion and perseverative errors in a strategy set-shifting task. In Experiment 2, with a separate cohort of rats, ABI but not GDX reduced perseverative errors in a reversal learning task. In Experiment 1, we also examined tyrosine hydroxylase immunoreactivity (TH-ir), and ABI but not GDX increased TH-ir in the mPFC. Our findings suggest that neurally-produced androgens modulate behavioural flexibility via modification of dopamine signalling in the mesocorticolimbic system. These results indicate that neurosteroids regulate executive functions and that ABI treatment for prostate cancer might affect cognition.

Effect of exergaming versus combined exercise on cognitive function and brain activation in frail older adults: A randomised controlled trial

BACKGROUND

Cognitive impairment is prevalent among frail older adults. Traditional exercise and exergaming positively affect cognition in healthy older people. However, few studies have investigated the effects of exergaming on cognition and brain activation in frail older adults.

OBJECTIVE

This study compared the effect of Kinect based exergaming (EXER) and combined physical exercise (CPE) training on cognitive function and brain activation in frail older adults in Taiwan. We hypothesised that EXER would be superior to CPE in this population.

METHODS

We randomised 46 community-dwelling frail older adults to the EXER or CPE group for 36 sessions (three 60-min training sessions per week) over 12 weeks. Outcome measures for cognitive function included global cognition measured by the Montreal Cognitive Assessment, executive function measured by the Executive Interview 25, verbal memory measured by the Chinese version of the California Verbal Learning Test, attention measured by the Stroop Colour and Word Test and Trail Making Test (part B), and working memory measured by spatial n-back tests. Prefrontal cortex activation during the global cognition test was documented with functional near-infrared spectroscopy (fNIRS).

RESULTS

Both groups improved significantly in global cognition (P<0.05), executive function (P<0.05), and attention (P<0.05) after the 12-week intervention. The group×time interaction indicated that EXER training significantly enhanced global cognition more than CPE training (F(1,44)=5.277, P=0.026). Moreover, only the EXER group showed significant improvements in verbal (P<0.05) and working (P<0.05) memory after the intervention. The fNIRS hemodynamics data revealed decreased activation in prefrontal cortices of both groups (P<0.05) during the post-training cognitive assessment, thereby suggesting greater neural efficiency; however, we found no significant group difference.

CONCLUSION

In frail older adults, exergaming and CPE could improve cognitive function, most likely by increasing neural efficiency. Moreover, exergaming may be superior to CPE, particularly in improving global cognition.

Attenuated NMDAR signaling on fast-spiking interneurons in prefrontal cortex contributes to age-related decline of cognitive flexibility

Ionotropic glutamate receptors of the NMDA and AMPA subtypes transduce excitatory signaling on neurons in the prefrontal cortex (PFC) in support of cognitive flexibility. Cognitive flexibility is reliably observed to decline at advanced ages, coinciding with changes in PFC glutamate receptor expression and neuronal physiology. However, the relationship between age-related impairment of cognitive flexibility and changes to excitatory signaling on distinct classes of PFC neurons is not known. In this study, one cohort of young adult (4 months) and aged (20 months) male F344 rats were characterized for cognitive flexibility on an operant set-shifting task. Expression of the essential NMDAR subunit, NR1, was correlated with individual differences in set-shifting abilities such that lower NR1 in the aged PFC was associated with worse set-shifting. In contrast, lower expression of two AMPAR subunits, GluR1 and GluR2, was not associated with set-shift abilities in aging. As NMDARs are expressed by both pyramidal cells and fast-spiking interneurons (FSI) in PFC, whole-cell patch clamp recordings were performed in a second cohort of age-matched rats to compare age-associated changes on these neuronal subtypes. Evoked excitatory postsynaptic currents were generated using a bipolar stimulator while AMPAR vs. NMDAR-mediated components were isolated using pharmacological tools. The results revealed a clear increase in AMPA/NMDA ratio in FSIs that was not present in pyramidal neurons. Together, these data indicate that loss of NMDARs on interneurons in PFC contributes to age-related impairment of cognitive flexibility.

Medial orbitofrontal cortex dopamine D1/D2 receptors differentially modulate distinct forms of probabilistic decision-making

Efficient decision-making involves weighing the costs and benefits associated with different actions and outcomes to maximize long-term utility. The medial orbitofrontal cortex (mOFC) has been implicated in guiding choice in situations involving reward uncertainty, as inactivation in rats alters choice involving probabilistic rewards. The mOFC receives considerable dopaminergic input, yet how dopamine (DA) modulates mOFC function has been virtually unexplored. Here, we assessed how mOFC D₁ and D₂ receptors modulate two forms of reward seeking mediated by this region, probabilistic reversal learning and probabilistic discounting. Separate groups of well-trained rats received intra-mOFC microinfusions of selective D₁ or D₂ antagonists or agonists prior to task performance. mOFC D₁ and D₂ blockade had opposing effects on performance during probabilistic reversal learning and probabilistic discounting. D₁ blockade impaired, while D₂ blockade increased the number of reversals completed, both mediated by changes in errors and negative feedback sensitivity apparent during the initial discrimination of the task, which suggests changes in probabilistic reinforcement learning rather than flexibility. Similarly, D₁ blockade reduced, while D₂ blockade increased preference for larger/risky rewards. Excess D₁ stimulation had no effect on either task, while excessive D₂ stimulation impaired probabilistic reversal performance, and reduced both profitable risky choice and overall task engagement. These findings highlight a previously uncharacterized role for mOFC DA, showing that D₁ and D₂ receptors play dissociable and opposing roles in different forms of reward-related action selection. Elucidating how DA biases behavior in these situations will expand our understanding of the mechanisms regulating optimal and aberrant decision-making.

Alterations in dopamine system and in its connectivity with serotonin in a rat model of Alzheimer's disease

Dopamine pathways alterations are reported in Alzheimer’s disease. However, it is difficult in humans to establish when these deficits appear and their impact in the course of Alzheimer’s disease. In the TgF344-Alzheimer’s disease rat model at the age of 6 months, we showed a reduction in in vivo release of striatal dopamine due to serotonin 5HT_2A-receptor blockade, in the absence of alterations in 5HT_2A-receptor binding, suggesting a reduction in 5HT_2A-receptor-dopamine system connectivity. In addition, a functional hypersensitivity of postsynaptic dopamine D₂-receptors and D₂-autoreceptors was also reported without any change in D₂-receptor density and in the absence of amyloid plaques or overexpression of the 18 kDa translocator protein (an inflammatory marker) in areas of the dopamine system. Citalopram, a selective serotonin reuptake inhibitor, induced functional 5HT_2A-receptor−D₂-receptor connectivity changes but had no effect on D₂-autoreceptor hypersensitivity. In older rats, dopamine cell bodies overexpressed translocator protein and dopamine projection sites accumulated amyloid. Interestingly, the 5HT_2A-receptor density is decreased in the accumbens subdivisions and the substantia nigra pars compacta. This reduction in the striatum is related to the astrocytic expression of 5HT_2A-receptor. Our results indicate that both serotonin/dopamine connectivity and dopamine signalling pathways are dysregulated and potentially represent novel early diagnostic and therapeutic avenues.

Spatial reference learning deficits in absence of dysfunctional working memory in the TgF344-AD rat model of Alzheimer's disease

Alzheimer's disease (AD) is characterized by cognitive disorders and alterations of behavioral traits such as anhedonia and anxiety. Contribution of nonphysiological forms of amyloid and tau peptides to the onset of neurological dysfunctions remains unclear because most preclinical models only present one of those pathological AD-related biomarkers. A more recently developed model, the TgF344-AD rat has the advantage of overexpressing amyloid and naturally developing tauopathy, thus making it close to human familial forms of AD. We showed the presence of a learning dysfunction in a reference memory test, without spatial working memory impairment but with an increase in anxiety levels and a decrease in motivation to participate in the test. In the sucrose preference test, TgF344-AD rats did not show signs of anhedonia but did not increase the volume of liquid consumed when the water was replaced by sucrose solution. These behavioral phenomena were observed at an age when tau accumulation are absent, and where amyloid deposits are predominant in the hippocampus and the entorhinal cortex. Within the hippocampus itself, amyloid accumulation is heterogenous between the subiculum, the dorsal hippocampus and the ventral hippocampus. Thus, our data demonstrated heterogeneity in the appearance of various behavioral and neurochemical markers in the TgF344-AD rat. This multivariate analysis will therefore make it possible to define the stage of the pathology, to measure its evolution and the effects of future therapeutic treatments.

Memory and dendritic spines loss, and dynamic dendritic spines changes are age-dependent in the rat

Brain aging is a widely studied process, but due to its complexity, much of its progress is unknown. There are many studies linking memory loss and reduced interneuronal communication with brain aging. However, only a few studies compare young and old animals. In the present study, in male rats aged 3, 6, and 18 months, we analyzed the locomotor activity and also short and long-term memory using the novel object recognition test (NORT), in addition to evaluating the dendritic length and the number of dendritic spines in the prefrontal cortex (PFC) and in the CA1, CA3 and DG regions of the dorsal hippocampus using Golgi-Cox staining. We also analyzed the types of dendritic spines in the aforementioned regions. 6- and 18-month old animals showed a reduction in locomotor activity, while long-term memory deficit was observed in 18-month old rats. At 18 months old, the dendritic length was reduced in all the studied regions. The dendritic spine number was also reduced in layer 5 of the PFC, and the CA1 and CA3 of the hippocampus. The dynamics of dendritic spines changed with age, with a reduction of the mushroom spines in all the studied regions, with an increase of the stubby spines in all the studied regions except from the CA3 region, that showed a reduction. Our data suggest that age causes changes in behavior, which may be the result of morphological changes at the dendrite level, both in their length and in the dynamics of their spines.

Age-related variability in decision-making: Insights from neurochemistry

Despite dopamine's significant role in models of value-based decision-making and findings demonstrating loss of dopamine function in aging, evidence of systematic changes in decision-making over the life span remains elusive. Previous studies attempting to resolve the neural basis of age-related alteration in decision-making have typically focused on physical age, which can be a poor proxy for age-related effects on neural systems. There is growing appreciation that aging has heterogeneous effects on distinct components of the dopamine system within subject in addition to substantial variability between subjects. We propose that some of the conflicting findings in age-related effects on decision-making may be reconciled if we can observe the underlying dopamine components within individuals. This can be achieved by incorporating in vivo imaging techniques including positron emission tomography (PET) and neuromelanin-sensitive MR. Further, we discuss how affective factors may contribute to individual differences in decision-making performance among older adults. Specifically, we propose that age-related shifts in affective attention ("positivity effect") can, in some cases, counteract the impact of altered dopamine function on specific decision-making processes, contributing to variability in findings. In an effort to provide clarity to the field and advance productive hypothesis testing, we propose ways in which in vivo dopamine imaging can be leveraged to disambiguate dopaminergic influences on decision-making, and suggest strategies for assessing individual differences in the contribution of affective attentional focus.

Effects of aging on testosterone and androgen receptors in the mesocorticolimbic system of male rats

As males age, systemic testosterone (T) levels decline. T regulates executive function, a collection of cognitive processes that are mediated by the mesocorticolimbic system. Here, we examined young adult (5 months) and aged (22 months) male Fischer 344 × Brown Norway rats, and measured systemic T levels in serum and local T levels in microdissected nodes of the mesocorticolimbic system (ventral tegmental area (VTA), nucleus accumbens (NAc), medial prefrontal cortex (mPFC), and orbitofrontal cortex (OFC)). We also measured androgen receptor (AR) immunoreactivity (-ir) in the mesocorticolimbic system. As expected, systemic T levels decreased with age. Local T levels in mesocorticolimbic regions - except the VTA - also decreased with age. Mesocorticolimbic T levels were higher than serum T levels at both ages. AR-ir was present in the VTA, NAc, mPFC, and OFC and decreased with age in the mPFC. Taken together with previous results, the data suggest that changes in androgen signaling may contribute to changes in executive function during aging.

Palatability of Goat's versus Cow's Milk: Insights from the Analysis of Eating Behavior and Gene Expression in the Appetite-Relevant Brain Circuit in Laboratory Animal Models

Goat's (GM) and cow's milk (CM) are dietary alternatives with select health benefits shown in human and animal studies. Surprisingly, no systematic analysis of palatability or preference for GM vs. CM has been performed to date. Here, we present a comprehensive investigation of short-term intake and palatability profiles of GM and CM in laboratory mice and rats. We studied consumption in no-choice and choice scenarios, including meal microstructure, and by using isocaloric milks and milk-enriched solid diets. Feeding results are accompanied by qPCR data of relevant genes in the energy balance-related hypothalamus and brain stem, and in the nucleus accumbens, which regulates eating for palatability. We found that GM and CM are palatable to juvenile, adult, and aged rodents. Given a choice, animals prefer GM- to CM-based diets. Analysis of meal microstructure using licking patterns points to enhanced palatability of and, possibly, greater motivation toward GM over CM. Most profound changes in gene expression after GM vs. CM were associated with the brain systems driving consumption for reward. We conclude that, while both GM and CM are palatable, GM is preferred over CM by laboratory animals, and this preference is driven by central mechanisms controlling eating for pleasure.