Brian R. Christie

Effects of voluntary exercise on synaptic plasticity and gene expression in the dentate gyrus of adult male sprague–dawley rats in vivo

We have previously shown that voluntary exercise produces enhanced neurogenesis and long-term potentiation (LTP) in the dentate gyrus (DG) of mice in vitro. In the present experiments we show that rats given access to a running wheel (Runners) exhibit significantly more short-term potentiation and LTP with theta-patterned conditioning stimulation in vivo than do age-matched litter mates (Controls). This increase in LTP appears to reflect an alteration in the induction threshold for synaptic plasticity that accompanies voluntary exercise. Weak theta-patterned stimulation, which did not produce LTP in control subjects, produced a robust and long-lasting LTP in Runners. LTP induction in both groups was dependent upon the activation of N-methyl-D-aspartate (NMDA) receptors, and could be blocked by the competitive antagonist [+/-]-3-[2-carboxypiperazin-4-yl] propanephosphonic acid. Consistent with these findings, we found that mRNA levels for NR2B subtype of NMDA receptor were increased specifically in the DG of Runners. In addition to changes in NR2B mRNA levels, quantitative polymerase chain reaction analysis revealed that brain-derived neurotrophic factor (BDNF) and glutamate receptor 5 mRNA levels were also significantly elevated in the DG of Runners, but not in other areas of the hippocampus. Thus, alterations in the expression of BDNF, and specific glutamate receptor subtypes, may underlie the ability of exercise to enhance neurogenesis and reduce the threshold for LTP in the DG.

Mice lacking methyl-CpG binding protein 1 have deficits in adult neurogenesis and hippocampal function

DNA methylation-mediated epigenetic regulation plays critical roles in regulating mammalian gene expression, but its role in normal brain function is not clear. Methyl-CpG binding protein 1 (MBD1), a member of the methylated DNA-binding protein family, has been shown to bind methylated gene promoters and facilitate transcriptional repression in vitro. Here we report the generation and analysis of MBD1-/- mice. MBD1-/- mice had no detectable developmental defects and appeared healthy throughout life. However, we found that MBD1-/- neural stem cells exhibited reduced neuronal differentiation and increased genomic instability. Furthermore, adult MBD1-/- mice had decreased neurogenesis, impaired spatial learning, and a significant reduction in long-term potentiation in the dentate gyrus of the hippocampus. Our findings indicate that DNA methylation is important in maintaining cellular genomic stability and is crucial for normal neural stem cell and brain functions.

Voluntary exercise alters the cytoarchitecture of the adult dentate gyrus by increasing cellular proliferation, dendritic complexity, and spine density.

Voluntary exercise produces a dramatic increase in the number of bromodeoxyuridine (BrdU)‐positive cells in the adult dentate gyrus (DG); however, it has never been determined whether this increase reflects neurogenic activity or some exercise‐induced change in the metabolic processing of systemically injected BrdU. In these experiments, we show that 1) 200 mg/kg is a saturating dose for single injections of BrdU in both control and voluntary exercise animals; 2) there is significantly more cell labeling in animals that exercise when saturating doses of BrdU are employed; 3) high doses of BrdU do not affect the number, appearance, or distribution of labeled cells; 4) voluntary exercise leads to similar increases in the number of cells expressing Ki67, an intrinsic marker of cellular proliferation; 5) both dendritic length and complexity are significantly increased in the DG of animals that exercise; and 6) spine density is significantly greater on dendrites in the DG following voluntary exercise. This study demonstrates that exercise up‐regulates neurogenic activity in the DG of adult rats, independently of any putative changes in altered BrdU metabolism, and that it also substantially alters the morphology of dentate granule cell dendrites. The dramatic changes in the cytoarchitecture of the DG induced by voluntary exercise might underlie the enhancement of hippocampal long‐term potentiation and hippocampal‐dependent memory that our group has previously described. These results suggest that exercise may be an effective component of therapeutic regimes aimed at improving the functioning of individuals with neuropathologies that involve the degradation of cells in the hippocampus. J. Comp. Neurol. 486:39–47, 2005.

Exercise-induced changes in dendritic structure and complexity in the adult hippocampal dentate gyrus.

Neurogenesis is a constitutive activity in the adult dentate gyrus whereby new cells are created in the subgranular zone, before becoming neurons in the dentate gyrus granule cell layer. New granule cells are thought to migrate from the subgranular zone outwards to the edge of the cell layer as they mature. In these experiments we examined the dendritic morphology of granule cells in the subgranular zone, and the inner and outer regions of the granule cell zone in Sprague-Dawley rats with low and high rates of neurogenesis. In animals with lower rates of neurogenesis, the number of primary dendrites, degree of dendritic complexity and total dendritic length was lowest in cells located in the subgranular zone, higher in inner granule cell zone neurons, and highest in outer granule cell zone granule cells. Subgranular zone granule cells typically extended one primary dendrite and had a simple, immature dendritic tree, while granule cells in the outer granule cell zone had an increased number of primary dendrites, greater dendritic complexity, and greater total dendritic length. Animals that engaged in voluntary exercise showed increased neurogenesis, and the proportion of cells with one or two primary dendrites was increased in all of the granule cell zones. Despite having fewer primary processes, these cells showed enhanced dendritic complexity and an overall increase in their total dendritic length. These results indicate that granule cell dendritic morphology may be indicative of the age and position of a cell in the granule cell layer, but that in animals with increased rates of neurogenesis, the proportion of cells exhibiting what is considered an immature phenotype is increased throughout the all regions of the dentate gyrus cell layer.

Hippocampal long-term depression mediates acute stress-induced spatial memory retrieval impairment

Acute stress impairs memory retrieval and facilitates the induction of long-term depression (LTD) in the hippocampal CA1 region of the adult rodent brain. However, whether such alterations in synaptic plasticity cause the behavioral effects of stress is not known. Here, we report that two selective inhibitors of the induction or expression of stress-enabled, N-methyl-d-aspartate receptor-dependent hippocampal LTD also block spatial memory retrieval impairments caused by acute stress. Additionally, we demonstrate that facilitating the induction of hippocampal LTD in vivo by blockade of glutamate transport mimics the behavioral effects of acute stress by impairing spatial memory retrieval. Thus, the present study demonstrates that hippocampal LTD is both necessary and sufficient to cause acute stress-induced impairment of spatial memory retrieval and provides a new perspective from which to consider the nature of cognitive deficits in disorders whose symptoms are aggravated by stress.

Hippocampal cell loss and neurogenesis after fetal alcohol exposure: insights from different rodent models.

Prenatal ethanol exposure is invariably detrimental to the developing central nervous system and the hippocampus is particularly sensitive to the teratogenic effects of ethanol. Prenatal ethanol exposure has been shown to result in hippocampal cell loss, altered neuronal morphology and impaired performance on hippocampal-dependent learning and memory tasks in rodents. The dentate gyrus (DG) of the hippocampus is one of the few brain regions where neurogenesis continues into adulthood. This process appears to have functional significance and these newly generated neurons are believed to play important functions in learning and memory. Recently, several groups have shown that adult hippocampal neurogenesis is compromised in animal models of fetal alcohol spectrum disorders (FASD). The direction and magnitude of any changes in neurogenesis, however, appear to depend on a variety of factors that include: the rodent model used; the blood alcohol concentration achieved; the developmental time point when alcohol was administered; and the frequency of ethanol exposure. In this review we will provide an overview of the different rodent models of FASD that are commonly used in this research, emphasizing each of their strengths and limitations. We will also present an up-to-date summary on the effects of prenatal/neonatal ethanol exposure on adult hippocampal neurogenesis and cell loss, highlighting some of the possible molecular mechanisms that might be involved.

Voluntary exercise rescues deficits in spatial memory and long-term potentiation in prenatal ethanol-exposed male rats

Prenatal ethanol exposure can lead to long‐lasting impairments in the ability to process spatial information in rats, as well as produce long‐lasting deficits in the ability of animals to exhibit long‐term potentiation, a biological model of learning and memory processing. Conversely, we have recently shown that both spatial memory and long‐term potentiation can be enhanced in animals that are given access to a running wheel in their home cage. In the present study, Sprague–Dawley rat dams were given one of three diets throughout gestation: (i) a liquid diet containing ethanol (35.5% ethanol‐derived calories); (ii) a liquid diet, isocaloric to the ethanol diet, but with maltose‐dextrin substituting for the ethanol derived calories and (iii) an ad libitum diet of standard rat chow. At weaning (28 days) animals were housed individually in either a standard rat cage, or a cage that contained a running wheel. Adult offspring were tested on a two trial version of the Morris water maze beginning at postnatal day 60, for five consecutive days. Following this, the capacity of the perforant path to dentate gyrus pathway to sustain long‐term potentiation was examined in these animals using theta‐patterned conditioning stimuli. Our results demonstrate that prenatal ethanol exposure can produce pronounced deficits in both spatial memory and long‐term potentiation, but that allowing animals access to voluntary exercise can attenuate these deficits to the point that those exposed to ethanol prenatally can no longer be differentiated from control animals. These findings indicate that voluntary exercise may have therapeutic benefits for individuals that have undergone prenatal ethanol exposure.

NGF Is Essential for Hippocampal Plasticity and Learning

Nerve growth factor (NGF) is produced in the hippocampus throughout life and is retrogradely trafficked to septal cholinergic neurons, providing a potential mechanism for modulating cholinergic inputs and, thereby, hippocampal plasticity. To explore NGF modulation of hippocampal plasticity and function, NGF levels were augmented or blocked in intact adult rats, and subsequent in vivo effects on cholinergic neurons, hippocampal long-term potentiation (LTP), and learning were examined. NGF augmentation significantly enhanced cholinergic neuronal markers and facilitated induction of hippocampal LTP. Blockade of endogenous NGF significantly reduced hippocampal LTP and impaired retention of spatial memory. These findings reveal an essential role for NGF in regulating biological mechanisms related to plasticity and memory in the intact adult brain.

Physical exercise-induced hippocampal neurogenesis and antidepressant effects are mediated by the adipocyte hormone adiponectin.

Significance This study unmasks a previously unidentified functional role of adiponectin (a hormone secreted by adipocytes) in modulating hippocampal neurogenesis and alleviating depression-like behaviors. To our knowledge, this is the first report showing that adiponectin may be an essential factor that mediates the antidepressant effects of physical exercise on the brain by adiponectin receptor 1-mediated activation of AMP-activated protein kinase. Our results reveal a possible mechanism by which exercise increases hippocampal neurogenesis and also suggest a promising therapeutic treatment for depression. Adiponectin (ADN) is an adipocyte-secreted protein with insulin-sensitizing, antidiabetic, antiinflammatory, and antiatherogenic properties. Evidence is also accumulating that ADN has neuroprotective activities, yet the underlying mechanism remains elusive. Here we show that ADN could pass through the blood–brain barrier, and elevating its levels in the brain increased cell proliferation and decreased depression-like behaviors. ADN deficiency did not reduce the basal hippocampal neurogenesis or neuronal differentiation but diminished the effectiveness of exercise in increasing hippocampal neurogenesis. Furthermore, exercise-induced reduction in depression-like behaviors was abrogated in ADN-deficient mice, and this impairment in ADN-deficient mice was accompanied by defective running-induced phosphorylation of AMP-activated protein kinase (AMPK) in the hippocampal tissue. In vitro analyses indicated that ADN itself could increase cell proliferation of both hippocampal progenitor cells and Neuro2a neuroblastoma cells. The neurogenic effects of ADN were mediated by the ADN receptor 1 (ADNR1), because siRNA targeting ADNR1, but not ADNR2, inhibited the capacity of ADN to enhance cell proliferation. These data suggest that adiponectin may play a significant role in mediating the effects of exercise on hippocampal neurogenesis and depression, possibly by activation of the ADNR1/AMPK signaling pathways, and also raise the possibility that adiponectin and its agonists may represent a promising therapeutic treatment for depression.

The role of oxidative stress in fetal alcohol spectrum disorders

The ingestion of alcohol/ethanol during pregnancy can result in abnormal fetal development in both humans and a variety of experimental animal models. Depending on the pattern of consumption, the dose, and the period of exposure to ethanol, a myriad of structural and functional deficits can be observed. These teratogenic effects are thought to result from the ethanol-induced dysregulation of a variety of intracellular pathways ultimately culminating in toxicity and cell death. For instance, ethanol exposure can lead to the generation of reactive oxygen species (ROS) and produce an imbalance in the intracellular redox state, leading to an overall increase in oxidative stress. In the present review we will provide an up-to-date summary on the effects of prenatal/neonatal ethanol exposure on the levels of oxidative stress in the central nervous system (CNS) of experimental models of fetal alcohol spectrum disorders (FASD). We will also review the evidence for the use of antioxidants as potential therapeutic strategies for the treatment of some of the neuropathological deficits characteristic of both rodent models of FASD and children afflicted with these disorders. We conclude that an imbalance in the intracellular redox state contributes to the deficits seen in FASD and suggest that antioxidants are potential candidates for the development of novel therapeutic strategies for the treatment of these developmental disorders.