Cardiorespiratory fitness and mnemonic discrimination across the adult lifespan.

With a rising aging population, it is important to develop behavioral tasks that assess and track cognitive decline, and to identify protective factors that promote healthy brain aging. Mnemonic discrimination tasks that rely on pattern separation mechanisms are a promising metric to detect subtle age-related memory impairments. Behavioral performance on these tasks rely on the integrity of the hippocampus and surrounding circuitry, which are brain regions known to be adversely affected in aging and neurodegenerative disorders. Aerobic exercise, which improves cardiorespiratory fitness (CRF), has been shown to counteract aging-related decreases in structural and functional brain integrity and attenuate decline of cognitive performance. Here, we tested the hypothesis that higher CRF attenuates age-related deficits in mnemonic discrimination in both a nonspatial mnemonic discrimination (Mnemonic Similarity Task) and a virtual navigation task (Route Disambiguation Task). Importantly, we included individuals across the lifespan (aged 18-83 yr), including the middle-age range, to determine mnemonic discrimination performance across adulthood. Participants completed two mnemonic discrimination tasks and a treadmill test to assess CRF. Our results demonstrate robust negative age-related effects on mnemonic discrimination performance across both the nonspatial and spatial domains. Critically, higher CRF mitigated age-related attenuation in spatial contextual discrimination task performance, but did not show an attenuation effect on performance for object-based mnemonic discrimination. These results suggest that performance on spatial mnemonic discrimination may be a useful tool to track vulnerability in older individuals at risk for cognitive decline, and that higher CRF may lead to cognitive preservation across the adult lifespan, particularly for spatial disambiguation of similar contexts.

Improving fitness increases dentate gyrus/CA3 volume in the hippocampal head and enhances memory in young adults.

Converging evidence suggests a relationship between aerobic exercise and hippocampal neuroplasticity that interactively impacts hippocampally dependent memory. The majority of human studies have focused on the potential for exercise to reduce brain atrophy and attenuate cognitive decline in older adults, whereas animal studies often center on exercise-induced neurogenesis and hippocampal plasticity in the dentate gyrus (DG) of young adult animals. In the present study, initially sedentary young adults (18-35 years) participated in a moderate-intensity randomized controlled exercise intervention trial (ClinicalTrials.gov; NCT02057354) for a duration of 12 weeks. The aims of the study were to investigate the relationship between change in cardiorespiratory fitness (CRF) as determined by estimated V˙O2MAX , hippocampally dependent mnemonic discrimination, and change in hippocampal subfield volume. Results show that improving CRF after exercise training is associated with an increased volume in the left DG/CA3 subregion in young adults. Consistent with previous studies that found exercise-induced increases in anterior hippocampus in older adults, this result was specific to the hippocampal head, or most anterior portion, of the subregion. Our results also demonstrate a positive relationship between change in CRF and change in corrected accuracy for trials requiring the highest level of discrimination on a putative behavioral pattern separation task. This relationship was observed in individuals who were initially lower-fit, suggesting that individuals who show greater improvement in their CRF may receive greater cognitive benefit. This work extends animal models by providing evidence for exercise-induced neuroplasticity specific to the neurogenic zone of the human hippocampus.

Cardiorespiratory fitness predicts effective connectivity between the hippocampus and default mode network nodes in young adults.

Rodent and human studies examining the relationship between aerobic exercise, brain structure, and brain function indicate that the hippocampus (HC), a brain region critical for episodic memory, demonstrates striking plasticity in response to exercise. Beyond the hippocampal memory system, human studies also indicate that aerobic exercise and cardiorespiratory fitness (CRF) are associated with individual differences in large-scale brain networks responsible for broad cognitive domains. Examining network activity in large-scale resting-state brain networks may provide a link connecting the observed relationships between aerobic exercise, hippocampal plasticity, and cognitive enhancement within broad cognitive domains. Previously, CRF has been associated with increased functional connectivity of the default mode network (DMN), specifically in older adults. However, how CRF relates to the magnitude and directionality of connectivity, or effective connectivity, between the HC and other DMN nodes remains unknown. We used resting-state fMRI and conditional Granger causality analysis (CGCA) to test the hypothesis that CRF positively predicts effective connectivity between the HC and other DMN nodes in healthy young adults. Twenty-six participants (ages 18-35 years) underwent a treadmill test to determine CRF by estimating its primary determinant, maximal oxygen uptake (V. O2max ), and a 10-min resting-state fMRI scan to examine DMN effective connectivity. We identified the DMN using group independent component analysis and examined effective connectivity between nodes using CGCA. Linear regression analyses demonstrated that CRF significantly predicts causal influence from the HC to the ventromedial prefrontal cortex, posterior cingulate cortex, and lateral temporal cortex and to the HC from the dorsomedial prefrontal cortex. The observed relationship between CRF and hippocampal effective connectivity provides a link between the rodent literature, which demonstrates a relationship between aerobic exercise and hippocampal plasticity, and the human literature, which demonstrates a relationship between aerobic exercise and CRF and the enhancement of broad cognitive domains including, but not limited to, memory.

Converging meta-analytic and connectomic evidence for functional subregions within the human retrosplenial region.

Interest in the retrosplenial cortex (RSC) has surged in recent years, as this region has been implicated in a range of cognitive processes. Previously reported anatomical and functional definitions of the human RSC encompass a larger area than expected from underlying cytoarchitectonic profiles. Here, we used a large-scale, unbiased, and data-driven approach combining functional MRI meta-analysis and resting-state functional connectivity (rsFC) methods to test the nature of this heterogeneity. The automated toolset Neurosynth was used to conduct meta-analyses in order to (a) identify heterogeneous areas in the retrosplenial region (RS region) associated with one or more cognitive domains, and (b) contrast the activation profiles related to these domains. These analyses yielded several functional subregions across the RS region, highlighting differences between anterior RS regions associated with episodic memory and posterior RS regions in the parietal-occipital sulcus associated with scenes and navigation. These regions were subsequently used as seeds to conduct whole brain rsFC analyses using data from the Human Connectome Project. In support of the meta-analysis findings, rsFC revealed divergent connectivity profiles, with anterior regions demonstrating connectivity to the default mode network (DMN) and posterior regions demonstrating connectivity to visual regions. Anterior RS regions and the parietal-occipital sulcus connected to different subnetworks of the DMN. This convergent evidence supports the conclusion that the broad cortical RS region incorporating both anatomical and functional RSC consists of functionally heterogeneous subregions. This study combines two large databases to provide a novel methodological blueprint for understanding brain function in the RS region and beyond. (PsycINFO Database Record (c) 2018 APA, all rights reserved).

Hippocampal subfield and medial temporal cortical persistent activity during working memory reflects ongoing encoding.

Previous neuroimaging studies support a role for the medial temporal lobes in maintaining novel stimuli over brief working memory (WM) delays, and suggest delay period activity predicts subsequent memory. Additionally, slice recording studies have demonstrated neuronal persistent spiking in entorhinal cortex, perirhinal cortex (PrC), and hippocampus (CA1, CA3, subiculum). These data have led to computational models that suggest persistent spiking in parahippocampal regions could sustain neuronal representations of sensory information over many seconds. This mechanism may support both WM maintenance and encoding of information into long term episodic memory. The goal of the current study was to use high-resolution fMRI to elucidate the contributions of the MTL cortices and hippocampal subfields to WM maintenance as it relates to later episodic recognition memory. We scanned participants while they performed a delayed match to sample task with novel scene stimuli, and assessed their memory for these scenes post-scan. We hypothesized stimulus-driven activation that persists into the delay period-a putative correlate of persistent spiking-would predict later recognition memory. Our results suggest sample and delay period activation in the parahippocampal cortex (PHC), PrC, and subiculum (extending into DG/CA3 and CA1) was linearly related to increases in subsequent memory strength. These data extend previous neuroimaging studies that have constrained their analysis to either the sample or delay period by modeling these together as one continuous ongoing encoding process, and support computational frameworks that predict persistent activity underlies both WM and episodic encoding.

Exogenous activation of Wnt/ß-catenin signaling attenuates binge alcohol-induced deficient bone fracture healing.

AIMS: Excessive alcohol consumption is associated with fracture non-union. Canonical Wnt pathway signaling activity regulates normal fracture healing. We previously demonstrated that binge alcohol exposure modulates ß-catenin levels in the fracture callus of mice. Here, we sought to determine whether exogenous enhancement ß-catenin signaling activity could restore normal fracture healing to binge-exposed mice. METHODS: C57BL/6 male mice were exposed to episodic alcohol or saline for 6 total days of alcohol exposure over a 2-week period. Following alcohol exposure, mice were subjected to a stabilized mid-shaft tibia fracture. Beginning 4 days post-injury, mice received daily injections of either lithium chloride or saline subcutaneously. Protein levels of activated, inactivated, and total ß-catenin and GSK-3ß in fracture calluses were measured at post-injury day 9. Biomechanical strength testing and histology of callus tissue was assessed at post fracture day 14. RESULTS: Binge alcohol was associated with decreased callus biomechanical strength, and reduced cartilaginous callus formation. Alcohol decreased levels of callus-associated activated ß-catenin while concomitantly increasing the levels of inactive ß-catenin at post-injury day 9. Alcohol also increased callus associated activated GSK-3ß at post-injury day 9. Lithium chloride (an inhibitor of GSK-3ß) treatment increased activated ß-catenin protein levels, significantly decreased activated GSK-3ß and restored cartilaginous callus formation and endochondral ossification. CONCLUSION: These data link alcohol-impaired fracture healing with deregulation of Canonical Wnt signaling activity in the fracture callus. Exogenous activation of the Wnt pathway using LiCl attenuated the damaging effects of binge alcohol exposure on the fracture healing process by modulating canonical Wnt signaling activity.

Ketorolac administration does not delay early fracture healing in a juvenile rat model: a pilot study.

BACKGROUND: Nonsteroidal anti-inflammatory drugs (NSAIDs) are effective at controlling pain in children, especially in the treatment of fractures. Adult animal and adult clinical studies demonstrate conflicting evidence for the inhibitory relationship between NSAIDs and fracture healing. Published pediatric orthopaedic clinical studies do not demonstrate an inhibitory effect of ketorolac on bone healing. Little is known about the effects of any NSAID on bone formation in juvenile animals. This study investigates the effects of the NSAID ketorolac on fracture healing in a juvenile rat model. METHODS: Unilateral surgically induced and stabilized tibial shaft fractures were created in 45 juvenile (3 to 4 wk old) male Sprague-Dawley rats. Either ketorolac (5 mg/kg; n=24) or saline (0.9% normal saline; n=21) was then administered to the rats 6 d/wk by intraperitoneal injections. Animals were then randomly assigned into time groups and euthanized at 7 days (n=8 ketorolac, n=7 saline), 14 days (n=8 ketorolac, n=7 saline), or 21 days (n=8 ketorolac, n=7 saline) postfracture. Biomechanical analysis was performed using a custom-designed 4-point bending loading apparatus. Statistics for tibial stiffness and strength data were performed using software package Systat 11. Specimens were also evaluated histologically using hematoxylin and eosin staining. RESULTS: Strength and stiffness of all fractured tibiae increased over time from day 7 to day 21 regardless of treatment type. No statistical difference was found between the fractured tibiae strength or stiffness in the ketorolac or control-treated specimens at the same time point. In addition, the quality of the fracture callus was similar in both groups at each of the time points. CONCLUSIONS: In this study of a juvenile rat model with a stabilized tibia fracture, fracture callus strength, stiffness, and histologic characteristics were not affected by the administration of ketorolac during the first 21 days of fracture healing. CLINICAL RELEVANCE: The absence of inhibitory effects of ketorolac on early juvenile rat fracture healing supports the clinical practice of utilizing NSAIDs for analgesia in children with long bone fractures.

Mesenchymal stem cells facilitate fracture repair in an alcohol-induced impaired healing model.

OBJECTIVES: Clinical studies have shown alcohol to be a risk factor for traumatic orthopaedic injuries and for nonunion. Data from animal studies suggest that alcohol exposure inhibits fracture healing. This report presents a novel rodent model of impaired fracture healing caused by repeated alcohol exposure. Using this model, we examined the regenerative effects of an intravenously administered population of isolated and expanded mesenchymal stem cells (MSCs) on fracture healing. METHODS: Bone marrow-derived MSC were isolated from transgenic green fluorescent protein C57BL/6 mice, and culture expanded using a lineage depletion protocol. Adult wild-type C57BL/6 mice were subjected to a 2-week binge alcohol exposure paradigm (3 days during which they received daily intraperitoneal injections of a 20% alcohol/saline solution followed by a 4-day rest period and another binge cycle for 3 consecutive days). At completion of the second binge cycle, mice were subjected to a mid-shaft tibia fracture while intoxicated. Twenty-four hours after the fracture, animals were administered an intravenous transplant of green fluorescent protein-labeled MSC. Two weeks after the fracture, animals were euthanized and injured tibiae were collected and subjected to biomechanical, histologic, and microcomputed tomography analysis. RESULTS: Pre-injury binge alcohol exposure resulted in a significant impairment in biomechanical strength and decrease in callus volume. MSC transplants restored both fracture callus volume (P < 0.05) and biomechanical strength (P < 0.05) in animals with alcohol-impaired healing. In vivo imaging demonstrated a time-dependent MSC migration to the fracture site. CONCLUSIONS: These data suggest that a 2-week binge alcohol exposure significantly impairs fracture healing in a murine tibia fracture model. Intravenously administered MSC were capable of specifically homing to the fracture site and of normalizing biomechanical, histologic, and microcomputed tomography parameters of healing in animals exposed to alcohol. Understanding MSC recruitment patterns and functional contributions to fracture repair may lead to their use in patients with impaired fracture healing and nonunion.

Acute alcohol exposure impairs fracture healing and deregulates ß-catenin signaling in the fracture callus.

BACKGROUND: Alcohol abuse is a risk factor for bone damage and fracture-related complications. Through precise ß-catenin signaling, canonical Wnt signaling plays a key role in fracture repair by promoting the differentiation of new bone and cartilage cells. In this study, we examined the effects of alcohol on the Wnt pathway in injured bone using a murine model of alcohol-induced impaired fracture healing. METHODS: Male C57Bl/6 or T cell factor (TCF)-transgenic mice were administered 3 daily intraperitoneal doses of alcohol or saline. One hour following the final injection, mice were subjected to a stabilized, mid-shaft tibial fracture. Injured and contralateral tibias were harvested at 6, 9, or 14 days post-fracture for the analysis of biomechanical strength, callus tissue composition, and Wnt/ß-catenin signaling. RESULTS: Acute alcohol treatment was associated with a significant decrease in fracture callus volume, diameter, and biomechanical strength at day 14 post-fracture. Histology revealed an alcohol-related reduction in cartilage and bone formation at the fracture site, and that alcohol inhibited normal cartilage maturation. Acute alcohol exposure caused a significant 2.3-fold increase in total ß-catenin protein at day 6 and a significant decrease of 53 and 56% at days 9 and 14, respectively. lacZ staining in ß-galactosidase-expressing TCF-transgenic mice revealed spatial and quantitative differences in Wnt-specific transcriptional activation at day 6 in the alcohol group. Days 9 and 14 post-fracture showed that acute alcohol exposure decreased Wnt transcriptional activation, which correlates with the modulation of total ß-catenin protein levels observed at these time points. CONCLUSIONS: Acute alcohol exposure resulted in significant impairment of fracture callus tissue formation, perturbation of the key Wnt pathway protein ß-catenin, and disruption of normal Wnt-mediated transcription. These data suggest that the canonical Wnt pathway is a target for alcohol in bone and may partially explain why impaired fracture healing is observed in alcohol-abusing individuals.

Antioxidant therapy attenuates deficient bone fracture repair associated with binge alcohol exposure.

OBJECTIVES: Alcohol consumption is a known risk factor for traumatic injuries of all types and has been shown to produce detrimental effects on bone metabolism. Although the mechanisms responsible for these detrimental effects are not well characterized, oxidative stress from alcohol exposure appears to play a central role. This study was designed to examine the effect of a short-term binge alcohol consumption pattern on fracture repair and the effect of an antioxidant, N-acetylcysteine, on fracture healing after binge alcohol consumption. METHODS: One hundred forty-four adult male Sprague-Dawley rats underwent unilateral closed femur fracture after injection of either saline or alcohol to simulate a binge alcohol cycle. Animals in the antioxidant treatment group received daily N-acetylcysteine after fracture. Femurs were harvested at 1, 2, 4, and 6 weeks after injury and underwent biomechanical testing and histologic analysis. RESULTS: Binge alcohol administration was associated with significant decreases in biomechanical strength at 1- and 2-week time points with a trend toward decreased strength at 4- and 6-week time points as well. Alcohol-treated animals had less cartilage component within the fracture callus and healed primarily by intramembranous ossification. Administration of N-acetylcysteine in alcohol-treated animals improved biomechanical strength to levels comparable to the control animals and was associated with increased endochondral ossification. CONCLUSIONS: Our results indicate that binge alcohol alters the quality of fracture healing after a traumatic injury and that concurrent administration of an antioxidant is able to reverse these effects.