Hippocampal functional hyperemia mediated by NMDA receptor/NO signaling in rats during mild exercise

Light-exercise-induced dopaminergic and noradrenergic stimulation in the dorsal hippocampus: Using a rat physiological exercise model

Exercise activates the dorsal hippocampus that triggers synaptic and cellar plasticity and ultimately promotes memory formation. For decades, these benefits have been explored using demanding and stress-response-inducing exercise at moderate-to-vigorous intensities. In contrast, our translational research with animals and humans has focused on light-intensity exercise (light exercise) below the lactate threshold (LT), which almost anyone can safely perform with minimal stress. We found that even light exercise can stimulate hippocampal activity and enhance memory performance. Although the circuit mechanism of this boost remains unclear, arousal promotion even with light exercise implies the involvement of the ascending monoaminergic system that is essential to modulate hippocampal activity and impact memory. To test this hypothesis, we employed our physiological exercise model based on the LT of rats and immunohistochemically assessed the neuronal activation of the dorsal hippocampal sub-regions and brainstem monoaminergic neurons. Also, we monitored the extracellular concentration of monoamines in the dorsal hippocampus using in vivo microdialysis. We found that even light exercise increased neuronal activity in the dorsal hippocampal sub-regions and elevated the extracellular concentrations of noradrenaline and dopamine. Furthermore, we found that tyrosine hydroxylase-positive neurons in the locus coeruleus (LC) and the ventral tegmental area (VTA) were activated even by light exercise and were both positively correlated with the dorsal hippocampal activation. In conclusion, our findings demonstrate that light exercise stimulates dorsal hippocampal neurons, which are associated with LC-noradrenergic and VTA-dopaminergic activation. This shed light on the circuit mechanisms responsible for hippocampal neural activation during exercise, consequently enhancing memory function.

Muscle-brain crosstalk mediated by exercise-induced myokines - insights from experimental studies

Over the past couple of decades, it has become apparent that skeletal muscles might be engaged in endocrine signaling, mostly as a result of exercise or physical activity in general. The importance of this phenomenon is currently studied in terms of the impact that exercise- or physical activity -induced signaling factors have, in the interaction of the "muscle-brain crosstalk." So far, skeletal muscle-derived myokines were demonstrated to intercede in the connection between muscles and a plethora of various organs such as adipose tissue, liver, or pancreas. However, the exact mechanism of muscle-brain communication is yet to be determined. It is speculated that, in particular, brain-derived neurotrophic factor (BDNF), irisin, cathepsin B (CTSB), interleukin 6 (IL-6), and insulin-like growth factor-1 (IGF-1) partake in this crosstalk by promoting neuronal proliferation and synaptic plasticity, also resulting in improved cognition and ameliorated behavioral alterations. Researchers suggest that myokines might act directly on the brain parenchyma via crossing the blood-brain barrier (BBB). The following article reviews the information available regarding rodent studies on main myokines determined to cross the BBB, specifically addressing the association between exercise-induced myokine release and central nervous system (CNS) impairments. Although the hypothesis of skeletal muscles being critical sources of myokines seems promising, it should not be forgotten that the origin of these factors might vary, depending on the cell types engaged in their synthesis. Limited amount of research providing information on alterations in myokines expression in various organs at the same time, results in taking them only as circumstantial evidence on the way to determine the actual involvement of skeletal muscles in the overall state of homeostasis. The following article reviews the information available regarding rodent studies on main myokines determined to cross the BBB, specifically addressing the association between exercise-induced myokine release and CNS impairments.

An acute bout of resistance exercise increases BDNF in hippocampus and restores the long-term memory of insulin-resistant rats

A sedentary lifestyle, inadequate diet, and obesity are substantial risk factors for Type 2 diabetes mellitus (T2DM) development. A major picture of T2DM is insulin resistance (IR), which causes many impairments in brain physiology, such as increased proinflammatory state and decreased brain-derived neurotrophic factor (BDNF) concentration, hence reducing cognitive function. Physical exercise is a non-pharmacological tool for managing T2DM/IR and its complications. Thus, this study investigated the effects of IR induction and the acute effects of resistance exercise (RE) on memory, neurotrophic, and inflammatory responses in the hippocampus and prefrontal cortex of insulin-resistant rats. IR was induced by a high-fat diet and fructose-rich beverage. Insulin-resistant rats performed acute resistance exercise (IR.RE; vertical ladder climb at 50-100% of the maximum load) or rest (IR.REST; 20 min). Cognitive parameters were assessed by novel object recognition (NOR) tasks, and biochemical analyses were performed to assess BDNF concentrations and inflammatory profile in the hippocampus and prefrontal cortex. Insulin-resistant rats had 20% worse long-term memory (LTM) (p < 0.01) and lower BDNF concentration in the hippocampus (-14.6%; p < 0.05) when compared to non-insulin-resistant rats (CON). An acute bout of RE restored LTM (-9.7% pre vs. post; p > 0.05) and increased BDNF concentration in the hippocampus (9.1%; p < 0.05) of insulin-resistant rats compared to REST. Thus, an acute bout of RE can attenuate the adverse effects of IR on memory and neurotrophic factors in rats, representing a therapeutic tool to alleviate the IR impact on the brain.

Optimal inhaled oxygen and carbon dioxide concentrations for post-cardiac arrest cerebral reoxygenation and neurological recovery

Prolonged cerebral hypoperfusion after the return of spontaneous circulation (ROSC) from cardiac arrest (CA) may lead to poor neurological recovery. In a 7-min asphyxia-induced CA rat model, four combinations of inhaled oxygen (iO2) and carbon dioxide (iCO2) were administered for 150 min post-ROSC and compared in a randomized animal trial. At the end of administration, the partial pressure of brain tissue oxygenation (PbtO2) monitored in the hippocampal CA1 region returned to the baseline for the 88% iO2 [ΔPbtO2, median: -0.39 (interquartile range: 5.6) mmHg] and 50% iO2 [ΔpbtO2, -2.25 (10.9) mmHg] groups; in contrast, PbtO2 increased substantially in the 88% iO2+12% iCO2 [ΔpbtO2, 35.05 (16.0) mmHg] and 50% iO2+12% iCO2 [ΔpbtO2, 42.03 (31.7) mmHg] groups. Pairwise comparisons (post hoc Dunn's test) indicated the significant role of 12% iCO2 in augmenting PbtO2 during the intervention and improving neurological recovery at 24 h post-ROSC. Facilitating brain reoxygenation may improve post-CA neurological outcomes.

Molecular mechanisms underlying physical exercise-induced brain BDNF overproduction

Accumulating evidence supports that physical exercise (EX) is the most effective non-pharmacological strategy to improve brain health. EX prevents cognitive decline associated with age and decreases the risk of developing neurodegenerative diseases and psychiatric disorders. These positive effects of EX can be attributed to an increase in neurogenesis and neuroplastic processes, leading to learning and memory improvement. At the molecular level, there is a solid consensus to involve the neurotrophin brain-derived neurotrophic factor (BDNF) as the crucial molecule for positive EX effects on the brain. However, even though EX incontestably leads to beneficial processes through BDNF expression, cellular sources and molecular mechanisms underlying EX-induced cerebral BDNF overproduction are still being elucidated. In this context, the present review offers a summary of the different molecular mechanisms involved in brain's response to EX, with a specific focus on BDNF. It aims to provide a cohesive overview of the three main mechanisms leading to EX-induced brain BDNF production: the neuronal-dependent overexpression, the elevation of cerebral blood flow (hemodynamic hypothesis), and the exerkine signaling emanating from peripheral tissues (humoral response). By shedding light on these intricate pathways, this review seeks to contribute to the ongoing elucidation of the relationship between EX and cerebral BDNF expression, offering valuable insights into the potential therapeutic implications for brain health enhancement.

Mechanisms underlying the effect of voluntary running on adult hippocampal neurogenesis

Adult hippocampal neurogenesis is important for preserving learning and memory-related cognitive functions. Physical exercise, especially voluntary running, is one of the strongest stimuli to promote neurogenesis and has beneficial effects on cognitive functions. Voluntary running promotes exit of neural stem cells (NSCs) from the quiescent stage, proliferation of NSCs and progenitors, survival of newborn cells, morphological development of immature neuron, and integration of new neurons into the hippocampal circuitry. However, the detailed mechanisms driving these changes remain unclear. In this review, we will summarize current knowledge with respect to molecular mechanisms underlying voluntary running-induced neurogenesis, highlighting recent genome-wide gene expression analyses. In addition, we will discuss new approaches and future directions for dissecting the complex cellular mechanisms driving change in adult-born new neurons in response to physical exercise.

Inhaled Carbon Dioxide Improves Neurological Outcomes by Downregulating Hippocampal Autophagy and Apoptosis in an Asphyxia‐Induced Cardiac Arrest and Resuscitation Rat Model

Background

Protracted cerebral hypoperfusion following cardiac arrest (CA) may cause poor neurological recovery. We hypothesized that inhaled carbon dioxide (CO₂) could augment cerebral blood flow (CBF) and improve post‐CA neurological outcomes.

Methods and Results

After 6‐minute asphyxia‐induced CA and resuscitation, Wistar rats were randomly allocated to 4 groups (n=25/group) and administered with different inhaled CO₂ concentrations, including control (0% CO₂), 4% CO₂, 8% CO₂, and 12% CO₂. Invasive monitoring was maintained for 120 minutes, and neurological outcomes were evaluated with neurological function score at 24 hours post‐CA. After the 120‐minute experiment, CBF was 242.3% (median; interquartile range, 221.1%–267.4%) of baseline in the 12% CO₂ group while CBF fell to 45.8% (interquartile range, 41.2%–58.1%) of baseline in the control group (P<0.001). CBF increased along with increasing inhaled CO₂ concentrations with significant linear trends (P<0.001). At 24 hours post‐CA, compared with the control group (neurological function score, 9 [interquartile range, 8–9]), neurological recovery was significantly better in the 12% CO₂ group (neurological function score, 10 [interquartile range, 9.8–10]) (P<0.001) while no survival difference was observed. Brain tissue malondialdehyde (P=0.02) and serum neuron‐specific enolase (P=0.002) and S100β levels (P=0.002) were significantly lower in the 12% CO₂ group. TUNEL (terminal deoxynucleotidyl transferase–mediated biotin–deoxyuridine triphosphate nick‐end labeling)‐positive cell densities in hippocampal CA1 (P<0.001) and CA3 (P<0.001) regions were also significantly reduced in the 12% CO₂ group. Western blotting showed that beclin‐1 (P=0.02), p62 (P=0.02), and LAMP2 (lysosome‐associated membrane protein 2) (P=0.01) expression levels, and the LC3B‐II:LC3B‐I ratio (P=0.02) were significantly lower in the 12% CO₂ group.

Conclusions

Administering inhaled CO₂ augmented post‐CA CBF, mitigated oxidative brain injuries, ameliorated neuronal injury, and downregulated apoptosis and autophagy, thereby improving neurological outcomes.

Exercise type influences the effect of an acute bout of exercise on hippocampal neuronal activation in mice

The effects of exercise on the hippocampus depend on exercise conditions. Exercise intensity is thought to be a dominant factor that influences the effects of exercise on the hippocampus; however, it is uncertain whether the type of exercise influences its effectiveness. This study investigated whether the effect of an acute bout of exercise on hippocampal neuronal activation differs between two different types of exercise: treadmill and rotarod exercise. The intensities of both exercises were matched at just below the lactate threshold (LT), based on blood lactate concentration. Immunohistochemical examination of c-Fos, a marker of neuronal activation, revealed that treadmill exercise at 15 m/min (T15) significantly increased c-Fos expression in all subfields of the hippocampus (dentate gyrus DG, CA1, CA3), but rotarod exercise at 30 rpm (R30) did not, as compared with the respective control groups. These results demonstrate that moderate treadmill exercise more efficiently evokes hippocampal neuronal activation than does intensity-matched rotarod exercise. This suggests that exercise type is another important factor affecting the effects of exercise on the hippocampus.

Region-Dependent Increase of Cerebral Blood Flow During Electrically Induced Contraction of the Hindlimbs in Rats

Elevation of cerebral blood flow (CBF) may contribute to the cerebral benefits of the regular practice of physical exercise. Surprisingly, while electrically induced contraction of a large muscular mass is a potential substitute for physical exercise to improve cognition, its effect on CBF remains to be investigated. Therefore, the present study investigated CBF in the cortical area representing the hindlimb, the hippocampus and the prefrontal cortex in the same anesthetized rats subjected to either acute (30 min) or chronic (30 min for 7 days) electrically induced bilateral hindlimb contraction. While CBF in the cortical area representing the hindlimb was assessed from both laser doppler flowmetry (LDF_CBF) and changes in p-eNOS^Ser1177 levels (p-eNOS_CBF), CBF was evaluated only from changes in p-eNOS^Ser1177 levels in the hippocampus and the prefrontal cortex. The contribution of increased cardiac output and increased neuronal activity to CBF changes were examined. Stimulation was associated with tachycardia and no change in arterial blood pressure. It increased LDF_CBF with a time- and intensity-dependent manner as well as p-eNOS_CBF in the area representing the hindlimb. By contrast, p-eNOS_CBF was unchanged in the two other regions. The augmentation of LDF_CBF was partially reduced by atenolol (a ß1 receptor antagonist) and not reproduced by the administration of dobutamine (a ß1 receptor agonist). Levels of c-fos as a marker of neuronal activation selectively increased in the area representing the hindlimb. In conclusion, electrically induced bilateral hindlimb contraction selectively increased CBF in the cortical area representing the stimulated muscles as a result of neuronal hyperactivity and increased cardiac output. The absence of CBF changes in cognition-related brain regions does not support flow-dependent neuroplasticity in the pro-cognitive effect of electrically induced contraction of a large muscular mass.

Lactate Is Answerable for Brain Function and Treating Brain Diseases: Energy Substrates and Signal Molecule

Research to date has provided novel insights into lactate's positive role in multiple brain functions and several brain diseases. Although notable controversies and discrepancies remain, the neurobiological role and the metabolic mechanisms of brain lactate have now been described. A theoretical framework on the relevance between lactate and brain function and brain diseases is presented. This review begins with the source and route of lactate formation in the brain and food; goes on to uncover the regulatory effect of lactate on brain function; and progresses to gathering the application and concentration variation of lactate in several brain diseases (diabetic encephalopathy, Alzheimer's disease, stroke, traumatic brain injury, and epilepsy) treatment. Finally, the dual role of lactate in the brain is discussed. This review highlights the biological effect of lactate, especially L-lactate, in brain function and disease studies and amplifies our understanding of past research.

High-intensity Intermittent Training Enhances Spatial Memory and Hippocampal Neurogenesis Associated with BDNF Signaling in Rats

High-intensity intermittent (or interval) training (HIIT) has started to gain popularity as a time-effective approach to providing beneficial effects to the brain and to peripheral organs. However, it still remains uncertain whether HIIT enhances hippocampal functions in terms of neurogenesis and spatial memory due to unconsidered HIIT protocol for rodents. Here, we established the HIIT regimen for rats with reference to human study. Adult male Wistar rats were assigned randomly to Control, moderate-intensity continuous training (MICT; 20 m/min, 30 min/day, 5 times/week), and HIIT (60 m/min, 10 30-s bouts of exercise, interspaced with 2.5 min of recovery, 5 times/week) groups. The ratios of exercise time and volume between MICT and HIIT were set as 6:1 and 2:1-4:1, respectively. After 4 weeks of training, all-out time in the incremental exercise test was prolonged for exercise training. In skeletal muscle, the plantaris citrate synthase activity significantly increased only in the HIIT group. Simultaneously, both HIIT and MICT led to enhanced spatial memory and adult hippocampal neurogenesis (AHN) as well as enhanced protein levels of hippocampal brain-derived neurotrophic factor (BDNF) signaling. Collectively, we suggest that HIIT could be a time-efficient exercise protocol that enhances hippocampal memory and neurogenesis in rats and is associated with hippocampal BDNF signaling.

Cerebral Blood Flow-Guided Manipulation of Arterial Blood Pressure Attenuates Hippocampal Apoptosis After Asphyxia-Induced Cardiac Arrest in Rats

Background In most post-cardiac arrest patients, the autoregulation mechanism of cerebral blood flow (CBF) is dysregulated. We examined whether recovery of CBF by adjusting mean arterial pressure mitigates post-cardiac arrest neuronal damage. Methods and Results Wistar rats that underwent 8-minute asphyxia-induced cardiac arrest and resuscitation were computer-randomized to norepinephrine or control groups. The CBF was measured at the dorsal hippocampal CA1 region of the left hemisphere. In the norepinephrine group, the mean arterial pressure was adjusted to recover CBF to 80% to 100% of baseline. Twenty-four hours following resuscitation, neurological outcomes were assessed, and brain tissues and blood samples were harvested for neuronal apoptosis and injury assessment. Thirty resuscitated rats were randomized into 2 groups, each containing 12 rats that completed the experiments. Norepinephrine infusion effectively prevented posthyperemia hypoperfusion and recovered CBF to pre-arrest baseline levels; a moderate positive linear correlation between mean arterial pressure and CBF during this period was also observed (P<0.001). There were no significant between-group differences in neurological recovery. In the norepinephrine group compared with the control group, upregulated cleaved caspase-3 protein expression in brain tissue determined by Western blot was reduced (P=0.02) and the densities of apoptotic cells in hippocampal CA1 and CA3 regions determined by terminal deoxynucleotidyl transferase-mediated dUTP biotin nick-end labeling were decreased (P<0.001). No significant differences in serum neuron-specific enolase or S100β levels were detected between the 2 groups. Conclusions CBF recovery demonstrated neuroprotective effects by reducing activation of cerebral apoptosis and number of apoptotic neurons. However, these effects did not significantly improve clinical neurological function, necessitating further investigation.

Neurovascular Coupling in the Dentate Gyrus Regulates Adult Hippocampal Neurogenesis

Newborn dentate granule cells (DGCs) are continuously generated in the adult brain. The mechanism underlying how the adult brain governs hippocampal neurogenesis remains poorly understood. In this study, we investigated how coupling of pre-existing neurons to the cerebrovascular system regulates hippocampal neurogenesis. Using a new in vivo imaging method in freely moving mice, we found that hippocampus-engaged behaviors, such as exploration in a novel environment, rapidly increased microvascular blood-flow velocity in the dentate gyrus. Importantly, blocking this exploration-elevated blood flow dampened experience-induced hippocampal neurogenesis. By imaging the neurovascular niche in combination with chemogenetic manipulation, we revealed that pre-existing DGCs actively regulated microvascular blood flow. This neurovascular coupling was linked by parvalbumin-expressing interneurons, primarily through nitric-oxide signaling. Further, we showed that insulin growth factor 1 signaling participated in functional hyperemia-induced neurogenesis. Together, our findings revealed a neurovascular coupling network that regulates experience-induced neurogenesis in the adult brain.

Tyrosine as a Mechanistic-Based Biomarker for Brain Glycogen Decrease and Supercompensation With Endurance Exercise in Rats: A Metabolomics Study of Plasma

Brain glycogen, localized in astrocytes, produces lactate as an energy source and/or a signal factor to serve neuronal functions involved in memory formation and exercise endurance. In rodents, 4 weeks of chronic moderate exercise-enhancing endurance and cognition increases brain glycogen in the hippocampus and cortex, which is an adaption of brain metabolism achieved through exercise. Although this brain adaptation is likely induced due to the accumulation of acute endurance exercise–induced brain glycogen supercompensation, its molecular mechanisms and biomarkers are unidentified. Since noradrenaline synthesized from blood-borne tyrosine activates not only glycogenolysis but also glycogenesis in astrocytes, we hypothesized that blood tyrosine is a mechanistic-based biomarker of acute exercise–induced brain glycogen supercompensation. To test this hypothesis, we used a rat model of endurance exercise, a microwave irradiation for accurate detection of glycogen in the brain (the cortex, hippocampus, and hypothalamus), and capillary electrophoresis mass spectrometry–based metabolomics to observe the comprehensive metabolic profile of the blood. Endurance exercise induced fatigue factors such as a decrease in blood glucose, an increase in blood lactate, and the depletion of muscle glycogen, but those parameters recovered to basal levels within 6 h after exercise. Brain glycogen decreased during endurance exercise and showed supercompensation within 6 h after exercise. Metabolomics detected 186 metabolites in the plasma, and 110 metabolites changed significantly during and following exhaustive exercise. Brain glycogen levels correlated negatively with plasma glycogenic amino acids (serine, proline, threonine, glutamate, methionine, tyrosine, and tryptophan) (r < −0.9). This is the first study to produce a broad picture of plasma metabolite changes due to endurance exercise–induced brain glycogen supercompensation. Our findings suggest that plasma glycogenic amino acids are sensitive indicators of brain glycogen levels in endurance exercise. In particular, plasma tyrosine as a precursor of brain noradrenaline might be a valuable mechanistic-based biomarker to predict brain glycogen dynamics in endurance exercise.

A new perspective of the hippocampus in the origin of exercise-brain interactions

Exercising regularly is a highly effective strategy for maintaining cognitive health throughout the lifespan. Over the last 20 years, many molecular, physiological and structural changes have been documented in response to aerobic exercise training in humans and animals, particularly in the hippocampus. However, how exercise produces such neurological changes remains elusive. A recent line of investigation has suggested that muscle-derived circulating factors cross into the brain and may be the key agents driving enhancement in synaptic plasticity and hippocampal neurogenesis from aerobic exercise. Alternatively, or concurrently, the signals might originate from within the brain itself. Physical activity also produces instantaneous and robust neuronal activation of the hippocampal formation and the generation of theta oscillations which are closely correlated with the force of movements. The repeated acute activation of the hippocampus during physical movement is likely critical for inducing the long-term neuroadaptations from exercise. Here we review the evidence which establishes the association between physical movement and hippocampal neuronal activation and discuss implications for long-term benefits of physical activity on brain function.

On the Run for Hippocampal Plasticity

Accumulating research in rodents and humans indicates that exercise benefits brain function and may prevent or delay onset of neurodegenerative conditions. In particular, exercise modifies the structure and function of the hippocampus, a brain area important for learning and memory. This review addresses the central and peripheral mechanisms underlying the beneficial effects of exercise on the hippocampus. We focus on running-induced changes in adult hippocampal neurogenesis, neural circuitry, neurotrophins, synaptic plasticity, neurotransmitters, and vasculature. The role of peripheral factors in hippocampal plasticity is also highlighted. We discuss recent evidence that systemic factors released from peripheral organs such as muscle (myokines), liver (hepatokines), and adipose tissue (adipokines) during exercise contribute to hippocampal neurotrophin and neurogenesis levels, and memory function. A comprehensive understanding of the body-brain axis is needed to elucidate how exercise improves hippocampal plasticity and cognition.

Moderate treadmill exercise ameliorates amyloid-β-induced learning and memory impairment, possibly via increasing AMPK activity and up-regulation of the PGC-1α/FNDC5/BDNF pathway

Alzheimer's disease (AD) is a neurodegenerative disorder associated with loss of memory and cognitive abilities. Previous evidence suggested that exercise ameliorates learning and memory deficits by increasing brain derived neurotrophic factor (BDNF) and activating downstream pathways in AD animal models. However, upstream pathways related to increase BDNF induced by exercise in AD animal models are not well known. We investigated the effects of moderate treadmill exercise on Aβ-induced learning and memory impairment as well as the upstream pathway responsible for increasing hippocampal BDNF in an animal model of AD. Animals were divided into five groups: Intact, Sham, Aβ_1-42, Sham-exercise (Sham-exe) and Aβ_1-42-exercise (Aβ-exe). Aβ was microinjected into the CA1 area of the hippocampus and then animals in the exercise groups were subjected to moderate treadmill exercise (for 4 weeks with 5 sessions per week) 7 days after microinjection. In the present study the Morris water maze (MWM) test was used to assess spatial learning and memory. Hippocampal mRNA levels of BDNF, peroxisome proliferator-activated receptor gamma co-activator 1 alpha (PGC-1α), fibronectin type III domain-containing 5 (FNDC5) as well as protein levels of AMPK-activated protein kinase (AMPK), PGC-1α, BDNF, phosphorylation of AMPK were measured. Our results showed that intra-hippocampal injection of Aβ_1-42 impaired spatial learning and memory which was accompanied by reduced AMPK activity (p-AMPK/total-AMPK ratio) and suppression of the PGC-1α/FNDC5/BDNF pathway in the hippocampus of rats. In contrast, moderate treadmill exercise ameliorated the Aβ_1-42-induced spatial learning and memory deficit, which was accompanied by restored AMPK activity and PGC-1α/FNDC5/BDNF levels. Our results suggest that the increased AMPK activity and up-regulation of the PGC-1α/FNDC5/BDNF pathway by exercise are likely involved in mediating the beneficial effects of exercise on Aβ-induced learning and memory impairment.

Exercise and cerebrovascular plasticity

Aging impairs cerebrovascular plasticity and subsequently leads cerebral hypoperfusion, which synergistically accelerates aging-associated cognitive dysfunction and neurodegenerative diseases associated with impaired neuronal plasticity. On the other hand, over two decades of researches have successfully demonstrated that exercise, or higher level of physical activity, is a powerful and nonpharmacological approach to improve brain function. Most of the studies have focused on the neuronal aspects and found that exercise triggers improvements in neuronal plasticity, such as neurogenesis; however, exercise can improve cerebrovascular plasticity as well. In this chapter, to understand these beneficial effects of exercise on the cerebral vasculature, we first discuss the issue of changes in cerebral blood flow and its regulation during acute bouts of exercise. Then, how regular exercise improves cerebrovascular plasticity will be discussed. In addition, to shed light on the importance of understanding interactions between the neuron and cerebral vasculature, we describe neuronal activity-driven uptake of circulating IGF-I into the brain.

A bout of treadmill exercise increases matrix metalloproteinase-9 activity in the rat hippocampus

Regular exercise induces a variety of structural changes in the hippocampus of rodents, although the underlying mechanisms remain obscure. Particularly, the possible involvement of molecules regulating the remodeling of the extracellular matrix (ECM) is under-studied. Matrix metalloproteinase-9 (MMP-9), an extracellular protease, plays a critical role in regulating neuronal plasticity by remodeling the ECM in the brain. The current study used gel zymography to examine for changes in the proteolytic activity of MMP-9 in the rat hippocampus following a bout of treadmill exercise at mild (10m/min) or moderate (25m/min) intensity. We found that MMP-9 activity was significantly increased at 12h after mild treadmill exercise. However, the activity of MMP-2 and the expression level of the tissue inhibitor of metalloproteinase-1 (TIMP-1) were unchanged following exercise. These findings suggest that exercise triggers MMP-9 activation in the hippocampus, which might be a new molecular mechanism of exercise-induced hippocampal plasticity.

Voluntary wheel running ameliorates symptoms of MK-801-induced schizophrenia in mice

Schizophrenia is a chronic and severe mental disorder characterized by the disintegration of cognitive thought processes and emotional responses. Despite the precise cause of schizophrenia remains unclear, it is hypothesized that a dysregulation of the N‑methyl‑D‑aspartate (NMDA) receptor in the brain is a major contributing factor to its development. Brain‑derived neurotrophic factor (BDNF) is a member of the neurotrophin family and is implicated in learning and memory processes. In the present study, we investigated in vivo the effects of voluntary wheel running on behavioral symptoms associated with NMDA receptor expression, using MK‑801‑induced schizophrenic mice. Abilify (aripiprazole), a drug used to treat human schizophrenia patients, was used as the positive control. For the assessment of behavioral symptoms affecting locomotion, social interaction and spatial working memory, the open‑field, social interaction and Morris water maze tests were conducted. For investigating the biochemical parameters, NMDA receptor expression in the hippocampal CA2‑3 regions and prefrontal cortex was detected by NMDA immunofluorescence and BDNF expression in the hippocampus was measured using western blot analysis. MK‑801 injection for 14 days induced schizophrenia‑like behavioral abnormalities with decreased expression of the NMDA receptor and BDNF in the brains of mice. The results indicated that free access to voluntary wheel running for 2 weeks alleviated schizophrenia‑like behavioral abnormalities and increased the expression of NMDA receptor and BDNF, comparable to the effects of aripiprazole treatment. In the present study, the results suggest that NMDA receptor hypofunctioning induced schizophrenia‑like behaviors, and that voluntary wheel running was effective in reducing these symptoms by increasing NMDA receptor and BDNF expression, resulting in an improvement of disease related behavioral deficits.

Long-term exercise is a potent trigger for ΔFosB induction in the hippocampus along the dorso-ventral axis

Physical exercise improves multiple aspects of hippocampal function. In line with the notion that neuronal activity is key to promoting neuronal functions, previous literature has consistently demonstrated that acute bouts of exercise evoke neuronal activation in the hippocampus. Repeated activating stimuli lead to an accumulation of the transcription factor ΔFosB, which mediates long-term neural plasticity. In this study, we tested the hypothesis that long-term voluntary wheel running induces ΔFosB expression in the hippocampus, and examined any potential region-specific effects within the hippocampal subfields along the dorso–ventral axis. Male C57BL/6 mice were housed with or without a running wheel for 4 weeks. Long-term wheel running significantly increased FosB/ΔFosB immunoreactivity in all hippocampal regions measured (i.e., in the DG, CA1, and CA3 subfields of both the dorsal and ventral hippocampus). Results confirmed that wheel running induced region-specific expression of FosB/ΔFosB immunoreactivity in the cortex, suggesting that the uniform increase in FosB/ΔFosB within the hippocampus is not a non-specific consequence of running. Western blot data indicated that the increased hippocampal FosB/ΔFosB immunoreactivity was primarily due to increased ΔFosB. These results suggest that long-term physical exercise is a potent trigger for ΔFosB induction throughout the entire hippocampus, which would explain why exercise can improve both dorsal and ventral hippocampus-dependent functions. Interestingly, we found that FosB/ΔFosB expression in the DG was positively correlated with the number of doublecortin-immunoreactive (i.e., immature) neurons. Although the mechanisms by which ΔFosB mediates exercise-induced neurogenesis are still uncertain, these data imply that exercise-induced neurogenesis is at least activity dependent. Taken together, our current results suggest that ΔFosB is a new molecular target involved in regulating exercise-induced hippocampal plasticity.

Long-term aerobic exercise increases redox-active iron through nitric oxide in rat hippocampus

Adult hippocampus is highly vulnerable to iron-induced oxidative stress. Aerobic exercise has been proposed to reduce oxidative stress but the findings in the hippocampus are conflicting. This study aimed to observe the changes of redox-active iron and concomitant regulation of cellular iron homeostasis in the hippocampus by aerobic exercise, and possible regulatory effect of nitric oxide (NO). A randomized controlled study was designed in the rats with swimming exercise treatment (for 3 months) and/or an unselective inhibitor of NO synthase (NOS) (L-NAME) treatment. The results from the bleomycin-detectable iron assay showed additional redox-active iron in the hippocampus by exercise treatment. The results from nonheme iron content assay, combined with the redox-active iron content, showed increased storage iron content by exercise treatment. NOx (nitrate plus nitrite) assay showed increased NOx content by exercise treatment. The results from the Western blot assay showed decreased ferroportin expression, no changes of TfR1 and DMT1 expressions, increased IRP1 and IRP2 expression, increased expressions of eNOS and nNOS rather than iNOS. In these effects of exercise treatment, the increased redox-active iron content, storage iron content, IRP1 and IRP2 expressions were completely reversed by L-NAME treatment, and decreased ferroportin expression was in part reversed by L-NAME. L-NAME treatment completely inhibited increased NOx and both eNOS and nNOS expression in the hippocampus. Our findings suggest that aerobic exercise could increase the redox-active iron in the hippocampus, indicating an increase in the capacity to generate hydroxyl radicals through the Fenton reactions, and aerobic exercise-induced iron accumulation in the hippocampus might mainly result from the role of the endogenous NO.

Variable alteration of regional tissue oxygen pressure in rat hippocampus by acute swimming exercise

One of the events in the brain is an increasing cerebral blood flow during exercise. The tissue oxygen level may be increased because blood flow correlates with tissue oxygen level. However, it is little known whether the tissue oxygen pressure in hippocampal region (Hip-pO2) will be affected by exercise.

AIMS

The aim of this study is to examine Hip-pO2 levels in the hippocampus and its changes during exercise.

MAIN METHODS

We applied improved Clark-type electrodes to measure Hip-pO2 level in the hippocampus of rats that were subjected to three groups, 2h swimming without weights (low intensity, n=6), 2h swimming with a 5 g weight (moderate intensity, n=6), and 2h swimming with a 10 g weight (high intensity, n=6).

KEY FINDINGS

Exercise affected the Hip-pO2 level, the responses varied with the exercise intensity and duration. Interestingly during and after the Low intensity swimming the Hip-pO2 level showed long lasting enhancement (10-20% above resting level). But the moderate and high intensity swimming increased Hip-pO2 level at the start of the swimming (50%, P<0.05 and slightly above resting level, respectively, at 10 min of 2h swimming) and then began to decrease (at 120 min and 10 min of 2h swimming, respectively), and suppressed the Hip-pO2 levels during post exercise resting period (2h) (85-95% of resting level, NS and 60-70% of resting level P<0.05, respectively).

SIGNIFICANCE

We propose that exercise-induced hippocampal hyper/hypo oxygen condition may participate in beneficial exercise effects on brain function.

Long-term exercise treatment reduces oxidative stress in the hippocampus of aging rats

Exercise can exert beneficial effects on cognitive functions of older subjects and it can also play an important role in the prevention of neurodegenerative diseases. At the same time it is perceivable that limited information is available on the nature of molecular pathways supporting the antioxidant effects of exercise in the brain. In this study 12-month old, middle-aged female Wistar rats were subjected to daily moderate intensity exercise on a rodent treadmill for a period of 15weeks which covered the early aging period unmasking already some aging-related molecular disturbances. The levels of reactive oxygen species (ROS), the amount of protein carbonyls, the levels of antioxidant intracellular enzymes superoxide dismutases (SOD-1, SOD-2) and glutathione peroxidase (GPx) were determined in the hippocampus. In addition, to identify the molecular pathways that may be involved in ROS metabolism and mitochondrial biogenesis, the activation of 5'-AMP-activated protein kinase (AMPK), the protein level of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), nuclear respiratory factor 1 (NRF-1) and mitochondrial transcription factor A (mtTFA) were measured. Our results revealed a lower level of ROS associated with a reduced amount of protein carbonyls in the hippocampus of physically trained rats compared to sedentary controls. Furthermore, exercise induced an up-regulation of SOD-1 and GPx enzymes, p-AMPK and PGC-1α, that can be related to an improved redox balance in the hippocampus. These results suggest that long-term physical exercise can comprises antioxidant properties and by this way protect neurons against oxidative stress at the early stage of aging.

Mild exercise increases dihydrotestosterone in hippocampus providing evidence for androgenic mediation of neurogenesis

Mild exercise activates hippocampal neurons through the glutamatergic pathway and also promotes adult hippocampal neurogenesis (AHN). We hypothesized that such exercise could enhance local androgen synthesis and cause AHN because hippocampal steroid synthesis is facilitated by activated neurons via N-methyl-D-aspartate receptors. Here we addressed this question using a mild-intense treadmill running model that has been shown to be a potent AHN stimulator. A mass-spectrometric analysis demonstrated that hippocampal dihydrotestosterone increased significantly, whereas testosterone levels did not increase significantly after 2 wk of treadmill running in both orchidectomized (ORX) and sham castrated (Sham) male rats. Furthermore, analysis of mRNA expression for the two isoforms of 5α-reductases (srd5a1, srd5a2) and for androgen receptor (AR) revealed that both increased in the hippocampus after exercise, even in ORX rats. All rats were injected twice with 5'-bromo-2'deoxyuridine (50 mg/kg body weight, i.p.) on the day before training. Mild exercise significantly increased AHN in both ORX and Sham rats. Moreover, the increase of doublecortin or 5'-bromo-2'deoxyuridine/NeuN-positive cells in ORX rats was blocked by s.c. flutamide, an AR antagonist. It was also found that application of an estrogen receptor antagonist, tamoxifen, did not suppress exercise-induced AHN. These results support the hypothesis that, in male animals, mild exercise enhances hippocampal synthesis of dihydrotestosterone and increases AHN via androgenenic mediation.

Brain glycogen supercompensation following exhaustive exercise

Brain glycogen localized in astrocytes, a critical energy source for neurons, decreases during prolonged exhaustive exercise with hypoglycaemia. However, it is uncertain whether exhaustive exercise induces glycogen supercompensation in the brain as in skeletal muscle. To explore this question, we exercised adult male rats to exhaustion at moderate intensity (20 m min(-1)) by treadmill, and quantified glycogen levels in several brain loci and skeletal muscles using a high-power (10 kW) microwave irradiation method as a gold standard. Skeletal muscle glycogen was depleted by 82-90% with exhaustive exercise, and supercompensated by 43-46% at 24 h after exercise. Brain glycogen levels decreased by 50-64% with exhaustive exercise, and supercompensated by 29-63% (whole brain 46%, cortex 60%, hippocampus 33%, hypothalamus 29%, cerebellum 63% and brainstem 49%) at 6 h after exercise. The brain glycogen supercompensation rates after exercise positively correlated with their decrease rates during exercise. We also observed that cortical and hippocampal glycogen supercompensation were sustained until 24 h after exercise (long-lasting supercompensation), and their basal glycogen levels increased with 4 weeks of exercise training (60 min day(-1) at 20 m min(-1)). These results support the hypothesis that, like the effect in skeletal muscles, glycogen supercompensation also occurs in the brain following exhaustive exercise, and the extent of supercompensation is dependent on that of glycogen decrease during exercise across brain regions. However, supercompensation in the brain preceded that of skeletal muscles. Further, the long-lasting supercompensation of the cortex and hippocampus is probably a prerequisite for their training adaptation (increased basal levels), probably to meet the increased energy demands of the brain in exercising animals.