Network activity changes in the pathophysiology of Alzheimer's disease: the role of aging and early entorhinal cortex dysfunction.

The greatest risk factor for development of the deadly neurodegenerative disorder known as Alzheimer's disease (AD) is advancing age. Currently unknown is what mediates the impact of advanced age on development of AD. Also unknown is what impact activity alterations in the entorhinal cortex (EC) has on the spread of AD pathology such as pathological tau through the brain as AD progresses. This review focuses on evidence in the literature that describes how one potential age-related change, that of glutamate-mediated increases in neuronal activity, may ultimately increase the risk of developing AD and promote the spread of tau pathology in AD-affected brains from the EC to later regions such as the hippocampus and prefrontal cortex. A better understanding of these detrimental alterations may allow for earlier detection of AD, offering a better prognosis for affected individuals.

ß-hydroxybutyric acid attenuates oxidative stress and improves markers of mitochondrial function in the HT-22 hippocampal cell line.

Ketone bodies have been the topic of research for their possible therapeutic neurotropic effects in various neurological diseases such as Parkinson's disease, dementia, and seizures. However, continuing research on ketone bodies as a prophylactic agent for decreasing the risk for various neurodegenerative diseases is currently required. In this paper, hippocampal HT-22 cells were treated with ß-hydroxybutyric acid at different doses to elucidate the neurotropic effects. In addition, markers of oxidative stress, mitochondrial function, and apoptosis were investigated. As a result, the ketone body (ß-hydroxybutyric acid) showed a significant increase in hippocampal neuronal viability at a moderate dose. Results show that ß-hydroxybutyric acid exhibited antioxidant effect by decreasing prooxidant oxidative stress markers such as reactive oxygen species, nitrite content, and increasing glutathione content leading to decreased lipid peroxidation. Results show that ß-hydroxybutyric acid improved mitochondrial functions by increasing Complex-I and Complex-IV activities and showing that ß-hydroxybutyric acid significantly reduces caspase-1 and caspase-3 activities. Finally, using computational pharmacokinetics and molecular modeling software, we validated the pharmacokinetic effects and pharmacodynamic (N-Methyl-D-aspartic acid and acetylcholinesterase) interactions of ß-hydroxybutyric acid. The computational studies demonstrate that ß-hydroxybutyric acid can interact with N-Methyl-D-aspartic acid receptor and cholinesterase enzyme (the prime pharmacodynamic targets for cognitive impairment) and further validates its oral absorption, distribution into the central nervous system. Therefore, this work highlights the neuroprotective potential of ketone bodies in cognitive-related neurodegenerative diseases.

Differential Effects of Human P301L Tau Expression in Young versus Aged Mice.

The greatest risk factor for developing Alzheimer's disease (AD) is increasing age. Understanding the changes that occur in aging that make an aged brain more susceptible to developing AD could result in novel therapeutic targets. In order to better understand these changes, the current study utilized mice harboring a regulatable mutant P301L human tau transgene (rTg(TauP301L)4510), in which P301L tau expression can be turned off or on by the addition or removal of doxycycline in the drinking water. This regulatable expression allowed for assessment of aging independent of prolonged mutant tau expression. Our results suggest that P301L expression in aged mice enhances memory deficits in the Morris water maze task. These behavioral changes may be due to enhanced late-stage tau pathology, as evidenced by immunoblotting and exacerbated hippocampal dysregulation of glutamate release and uptake measured by the microelectrode array technique. We additionally observed changes in proteins important for the regulation of glutamate and tau phosphorylation that may mediate these age-related changes. Thus, age and P301L tau interact to exacerbate tau-induced detrimental alterations in aged animals.

Western diet-induced obesity disrupts the diurnal rhythmicity of hippocampal core clock gene expression in a mouse model.

Western diet (WD) feeding disrupts core clock gene expression in peripheral tissues and contributes to WD-induced metabolic disease. The hippocampus, the mammalian center for memory, is also sensitive to WD feeding, but whether the WD disrupts its core clock is unknown. To this end, male mice were maintained on a WD for 16 weeks and diurnal metabolism, gene expression and memory were assessed. WD-induced obesity disrupted the diurnal rhythms of whole-body metabolism, markers of inflammation and hepatic gene expression, but did not disrupt diurnal expression of hypothalamic Bmal1, Npas2 and Per2. However, all measured core clock genes were disrupted in the hippocampus after WD feeding and the expression pattern of genes implicated in Alzheimer's disease and synaptic function were altered. Finally, WD feeding disrupted hippocampal memory in a task- and time-dependent fashion. Our results implicate WD-induced alterations in the rhythmicity of hippocampal gene expression in the etiology of diet-induced memory deficits.

Hippocampal Genetic Knockdown of PPARδ Causes Depression-Like Behaviors and Neurogenesis Suppression.

BACKGROUND: Although depression is the leading cause of disability worldwide, its pathophysiology is poorly understood. Our previous study showed that hippocampal peroxisome proliferator-activated receptor δ (PPARδ) overexpression displays antidepressive effect and enhances hippocampal neurogenesis during chronic stress. Herein, we further extended our curiosity to investigate whether downregulating PPARδ could cause depressive-like behaviors through downregulation of neurogenesis. METHODS: Stereotaxic injection of lentiviral vector, expressing short hairpin RNA complementary to the coding exon of PPARδ, was done into the bilateral dentate gyri of the hippocampus, and the depression-like behaviors were observed in mice. Additionally, hippocampal neurogenesis, brain-derived neurotrophic factor and cAMP response element-binding protein were measured both in vivo and in vitro. RESULTS: Hippocampal PPARδ knockdown caused depressive-like behaviors and significantly decreased neurogenesis, neuronal differentiation, levels of mature brain-derived neurotrophic factor and phosphorylated cAMP response element-binding protein in the hippocampus. In vitro study further confirmed that PPARδ knockdown could inhibit proliferation and differentiation of neural stem cells. Furthermore, these effects were mimicked by repeated systemic administration of a PPARδ antagonist, GSK0660 (1 or 3 mg/kg i.p. for 21 d). CONCLUSIONS: These findings suggest that downregulation of hippocampal PPARδ is associated with depressive behaviors in mice through an inhibitory effect on cAMP response element-binding protein/brain-derived neurotrophic factor-mediated adult neurogenesis in the hippocampus, providing new insights into the pathogenesis of depression.

Neuroprotective effects of INT-777 against Aß1-42-induced cognitive impairment, neuroinflammation, apoptosis, and synaptic dysfunction in mice.

Increasing evidence demonstrates that the neurotoxicity of amyloid-beta (Aß) deposition plays a causative role in Alzheimer's disease (AD). Herein, we evaluated the neuroprotective effects of 6α-ethyl-23(S)-methylcholic acid (S-EMCA, INT-777), a specific G-protein coupled bile acid receptor 1 (TGR5) agonist, in the Aß1-42-treated mouse model of acute neurotoxicity. Single intracerebroventricular (i.c.v.) injection of aggregated Aß1-42 (410 pmol/mouse; 5 µl) into the mouse brain induced cognitive impairment, neuroinflammation, apoptosis, and synaptic dysfunction. In contrast, INT-777 (1.5 or 3.0 µg/mouse, i.c.v.) significantly improved Aß1-42-induced cognitive impairment, as reflected by better performance in memory tests. Importantly, INT-777 treatment reversed Aß1-42-induced TGR5 down-regulation, suppressed the increase of nuclear NF-κB p65, and mitigated neuroinflammation, as evidenced by lower proinflammatory cytokines and less Iba1-positive cells in the hippocampus and frontal cortex. INT-777 treatment also pronouncedly suppressed apoptosis through the reduction of TUNEL-positive cells, decreased caspase-3 activation, increased the ratio of Bcl-2/Bax, and ameliorated synaptic dysfunction by promoting dendritic spine generation with the upregulation of postsynaptic and presynaptic proteins (PSD95 and synaptophysin) in Aß1-42-treated mice. Our results indicate that INT-777 has potent neuroprotective effects against Aß1-42-induced neurotoxicity. Taken together, these findings suggest that the activation of TGR5 could be a novel and promising strategy for the treatment of AD.

Role of Adiponectin in Central Nervous System Disorders.

Adiponectin, the most abundant plasma adipokine, plays an important role in the regulation of glucose and lipid metabolism. Adiponectin also possesses insulin-sensitizing, anti-inflammatory, angiogenic, and vasodilatory properties which may influence central nervous system (CNS) disorders. Although initially not thought to cross the blood-brain barrier, adiponectin enters the brain through peripheral circulation. In the brain, adiponectin signaling through its receptors, AdipoR1 and AdipoR2, directly influences important brain functions such as energy homeostasis, hippocampal neurogenesis, and synaptic plasticity. Overall, based on its central and peripheral actions, recent evidence indicates that adiponectin has neuroprotective, antiatherogenic, and antidepressant effects. However, these findings are not without controversy as human observational studies report differing correlations between plasma adiponectin levels and incidence of CNS disorders. Despite these controversies, adiponectin is gaining attention as a potential therapeutic target for diverse CNS disorders, such as stroke, Alzheimer's disease, anxiety, and depression. Evidence regarding the emerging role for adiponectin in these disorders is discussed in the current review.

Peripheral viral challenge elevates extracellular glutamate in the hippocampus leading to seizure hypersusceptibility.

Peripheral viral infections increase seizure propensity and intensity in susceptible individuals. We have modeled this comorbidity by demonstrating that the acute phase response instigated by an intraperitoneal (i.p.) injection of a viral mimetic, polyinosinic-polycytidylic acid (PIC), induces protracted hypersusceptibility to kainic acid-induced seizures. We have further demonstrated that PIC challenge robustly increases the level of tonic extracellular glutamate and neuronal excitability in the hippocampus. This study was undertaken to determine a relationship between tonic glutamate and seizure susceptibility following PIC challenge. Briefly, glutamate-sensing microelectrodes were permanently implanted into the CA1 of 8-week-old female C57BL/6 mice. Following a 3-day recovery, acute phase response was induced by i.p. injection of 12 mg/kg of PIC, while saline-injected mice served as controls. Tonic glutamate was measured at 1, 2, 3 and 4 days after PIC challenge. PIC challenge induced an approximately fourfold increase in tonic glutamate levels measured after 24 h. The levels gradually declined to the baseline values within 4 days. Twenty-four hours after PIC challenge, the mice featured an approximately threefold increase in cumulative seizure scores and twofold increase in the duration of status epilepticus induced by subcutaneous injection of 12 mg/kg of kainic acid. Seizure scores positively correlated with pre-seizure tonic glutamate. Moreover, seizures resulted in a profound (76%) elevation of extracellular glutamate in the CA1 of PIC-challenged but not saline-injected mice. Our results implicate the increase in extracellular glutamate as a mediator of seizure hypersusceptibility induced by peripheral viral challenge.

Altered AMPA receptor expression plays an important role in inducing bidirectional synaptic plasticity during contextual fear memory reconsolidation.

Retrieval of a memory appears to render it unstable until the memory is once again re-stabilized or reconsolidated. Although the occurrence and consequences of reconsolidation have received much attention in recent years, the specific mechanisms that underlie the process of reconsolidation have not been fully described. Here, we present the first electrophysiological model of the synaptic plasticity changes underlying the different stages of reconsolidation of a conditioned fear memory. In this model, retrieval of a fear memory results in immediate but transient alterations in synaptic plasticity, mediated by modified expression of the glutamate receptor subunits GluA1 and GluA2 in the hippocampus of rodents. Retrieval of a memory results in an immediate impairment in LTP, which is enhanced 6h following memory retrieval. Conversely, memory retrieval results in an immediate enhancement of LTD, which decreases with time. These changes in plasticity are accompanied by decreased expression of GluA2 receptor subunits. Recovery of LTP and LTD correlates with progressive overexpression of GluA2 receptor subunits. The contribution of the GluA2 receptor was confirmed by interfering with receptor expression at the postsynaptic sites. Blocking GluA2 endocytosis restored LTP and attenuated LTD during the initial portion of the reconsolidation period. These findings suggest that altered GluA2 receptor expression is one of the mechanisms that controls different forms of synaptic plasticity during reconsolidation.

Peripherally restricted viral challenge elevates extracellular glutamate and enhances synaptic transmission in the hippocampus.

Peripheral infections increase the propensity and severity of seizures in susceptible populations. We have previously shown that intraperitoneal injection of a viral mimic, polyinosinic-polycytidylic acid (PIC), elicits hypersusceptibility of mice to kainic acid (KA)-induced seizures. This study was undertaken to determine whether this seizure hypersusceptibility entails alterations in glutamate signaling. Female C57BL/6 mice were intraperitoneally injected with PIC, and after 24 h, glutamate homeostasis in the hippocampus was monitored using the enzyme-based microelectrode arrays. PIC challenge robustly increased the level of resting extracellular glutamate. While pre-synaptic potassium-evoked glutamate release was not affected, glutamate uptake was profoundly impaired and non-vesicular glutamate release was augmented, indicating functional alterations of astrocytes. Electrophysiological examination of hippocampal slices from PIC-challenged mice revealed a several fold increase in the basal synaptic transmission as compared to control slices. PIC challenge also increased the probability of pre-synaptic glutamate release as seen from a reduction of paired-pulse facilitation and synaptic plasticity as seen from an enhancement of long-term potentiation. Altogether, our results implicate a dysregulation of astrocytic glutamate metabolism and an alteration of excitatory synaptic transmission as the underlying mechanism for the development of hippocampal hyperexcitability, and consequently seizure hypersusceptibility following peripheral PIC challenge. Peripheral infections/inflammations enhance seizure susceptibility. Here, we explored the effect of peritoneal inflammation induced by a viral mimic on glutamate homeostasis and glutamatergic neurotransmission in the mouse hippocampus. We found that peritoneal inflammation elevated extracellular glutamate concentration and enhanced the probability of pre-synaptic glutamate release resulting in hyperexcitability of neuronal networks. These mechanisms are likely to underlie the enhanced seizure propensity.

Riluzole rescues alterations in rapid glutamate transients in the hippocampus of rTg4510 mice.

Those at risk for Alzheimer's disease (AD) often exhibit hippocampal hyperexcitability in the years preceding diagnosis. Our previous work with the rTg(TauP301L)4510 tau mouse model of AD suggests that this increase in hyperexcitability is likely mediated by an increase in depolarization-evoked glutamate release and a decrease in glutamate uptake, alterations of which correlate with learning and memory deficits. Treatment with riluzole restored glutamate regulation and rescued memory deficits in the TauP301L model. Here, we used enzyme-based ceramic microelectrode array technology to measure real-time phasic glutamate release and uptake events in the hippocampal subregions of TauP301L mice. For the first time, we demonstrate that perturbations in glutamate transients (rapid, spontaneous bursts of glutamate) exist in a tau mouse model of AD mouse model and that riluzole mitigates these alterations. These results help to inform our understanding of how glutamate signaling is altered in the disease process and also suggest that riluzole may serve as a clinically applicable therapeutic approach in AD.

P301L tau expression affects glutamate release and clearance in the hippocampal trisynaptic pathway.

Individuals at risk of developing Alzheimer's disease (AD) often exhibit hippocampal hyperexcitability. A growing body of evidence suggests that perturbations in the glutamatergic tripartite synapse may underlie this hyperexcitability. Here, we used a tau mouse model of AD (rTg(TauP301L)4510) to examine the effects of tau pathology on hippocampal glutamate regulation. We found a 40% increase in hippocampal vesicular glutamate transporter, which packages glutamate into vesicles, and has previously been shown to influence glutamate release, and a 40% decrease in hippocampal glutamate transporter 1, the major glutamate transporter responsible for removing glutamate from the extracellular space. To determine whether these alterations affected glutamate regulation in vivo, we measured tonic glutamate levels, potassium-evoked glutamate release, and glutamate uptake/clearance in the dentate gyrus, cornu ammonis 3(CA3), and cornu ammonis 1(CA1) regions of the hippocampus. P301L tau expression resulted in a 4- and 7-fold increase in potassium-evoked glutamate release in the dentate gyrus and CA3, respectively, and significantly decreased glutamate clearance in all three regions. Both release and clearance correlated with memory performance in the hippocampal-dependent Barnes maze task. Alterations in mice expressing P301L were observed at a time when tau pathology was subtle and before readily detectable neuron loss. These data suggest novel mechanisms by which tau may mediate hyperexcitability. Pre-synaptic vesicular glutamate transporters (vGLUTs) package glutamate into vesicles before exocytosis into the synaptic cleft. Once in the extracellular space, glutamate acts on glutamate receptors. Glutamate is removed from the extracellular space by excitatory amino acid transporters, including GLT-1, predominantly localized to glia. P301L tau expression increases vGLUT expression and glutamate release, while also decreasing GLT-1 expression and glutamate clearance.

Riluzole rescues glutamate alterations, cognitive deficits, and tau pathology associated with P301L tau expression.

Hyperexcitability of the hippocampus is a commonly observed phenomenon in the years preceding a diagnosis of Alzheimer's disease (AD). Our previous work suggests a dysregulation in glutamate neurotransmission may mediate this hyperexcitability, and glutamate dysregulation correlates with cognitive deficits in the rTg(TauP301L)4510 mouse model of AD. To determine whether improving glutamate regulation would attenuate cognitive deficits and AD-related pathology, TauP301L mice were treated with riluzole (~ 12.5 mg/kg/day p.o.), an FDA-approved drug for amyotrophic lateral sclerosis that lowers extracellular glutamate levels. Riluzole-treated TauP301L mice exhibited improved performance in the water radial arm maze and the Morris water maze, associated with a decrease in glutamate release and an increase in glutamate uptake in the dentate gyrus, cornu ammonis 3 (CA3), and cornu ammonis 1 (CA1) regions of the hippocampus. Riluzole also attenuated the TauP301L-mediated increase in hippocampal vesicular glutamate transporter 1, which packages glutamate into vesicles and influences glutamate release; and the TauP301L-mediated decrease in hippocampal glutamate transporter 1, the major transporter responsible for removing glutamate from the extracellular space. The TauP301L-mediated reduction in PSD-95 expression, a marker of excitatory synapses in the hippocampus, was also rescued by riluzole. Riluzole treatment reduced total levels of tau, as well as the pathological phosphorylation and conformational changes in tau associated with the P301L mutation. These findings open new opportunities for the development of clinically applicable therapeutic approaches to regulate glutamate in vulnerable circuits for those at risk for the development of AD.

Synergistic antidepressant-like effect of ferulic acid in combination with piperine: involvement of monoaminergic system.

The lifetime prevalence rate for major depressive disorder (MDD) is approximately 17 % for most developed countries around the world. Dietary polyphenols are currently used as an adjuvant therapy to accelerate the therapeutic efficacy on depression. Ferulic acid (FA) or 4-hydroxy-3-methoxy-cinnamic acid (Fig. 1a) is a main polyphenolic component of Chinese herb Radix Angelicae Sinensis, which is found to have antidepressant-like effects through regulating serotonergic and noradrenergic function. The present study examined the synergistic effect of low doses of FA combined with subthreshold dose of piperine, a bioavailability enhancer, on depression-like behaviors in mice, and investigated the possible mechanism. The administration of FA, even in the highest dose tested, reduced immobility time by 60 % in the tail suspension and forced swimming tests (TST and FST) in mice when compared to control. The maximal antidepressant-like effect of FA was obtained with 200 mg/kg. In addition, piperine only produced a weak antidepressant-like effect in the TST and FST. However, the evidence from the interaction analysis suggested a synergistic effect when low doses of FA were combined with a subthreshold dose of piperine. Further neurochemical evidence such as monoamine levels in the frontal cortex, hippocampus, and hypothalamus and measurements of monoamine oxidase activity also supported a synergistic effect of FA and piperine in the enhancement of monoaminergic function. This finding supports the concept that the combination strategy might be an alternative therapy in the treatment of psychiatric disorders with high efficacy and low side effects.

Effects of an α5GABAA inverse agonist on MK-801-induced learning deficits in an incremental repeated acquisition task.

N-methyl-D-aspartate receptors (NMDARs) are essential for several kinds of synaptic plasticity and play a critical role in learning and memory. Deficits in NMDAR functioning may be partially responsible for the learning and memory deficits associated with aging and numerous diseases. Administration of MK-801, a noncompetitive NMDAR antagonist, is commonly used as a preclinical model of NMDAR dysfunction. The objective of this study was to assess the effects of α5GABAA receptor inhibition on learning deficits in the incremental repeated acquisition (IRA) task induced by acute MK-801 administration. The IRA task, commonly used to examine factors that affect learning, begins with a single response and increments to progressively longer chains throughout a single session as behavior meets preset criteria. MK-801 (0.03-0.5 mg/kg, intraperitoneally), administered 10 min pretesting, produced a significant dose-dependent decrease in measures of IRA performance at doses greater than or equal to 0.25 mg/kg. The MK-801-induced deficit was attenuated after treatment with an α5GABAA receptor inverse agonist, L-655,708 (1 mg/kg, intraperitoneally). The present study provides the focus for, and supports the feasibility of, further in-depth definitive studies examining α5GABAA receptor inhibition as a suitable candidate for the attenuation of NMDAR-related deficits.

Recent Insights on Glutamatergic Dysfunction in Alzheimer's Disease and Therapeutic Implications.

Alzheimer's disease (AD) poses a critical public health challenge, and there is an urgent need for novel treatment options. Glutamate, the principal excitatory neurotransmitter in the human brain, plays a critical role in mediating cognitive and behavioral functions; and clinical symptoms in AD patients are highly correlated with the loss of glutamatergic synapses. In this review, we highlight how dysregulated glutamatergic mechanisms can underpin cognitive and behavioral impairments and contribute to the progression of AD via complex interactions with neuronal and neural network hyperactivity, Aß, tau, glial dysfunction, and other disease-associated factors. We focus on the tripartite synapse, where glutamatergic neurotransmission occurs, and evidence elucidating how the tripartite synapse can be pathologically altered in AD. We also discuss promising therapeutic approaches that have the potential to rescue these deficits. These emerging data support the development of novel glutamatergic drug candidates as compelling approaches for treating AD.

Prenatal Cannabinoid Exposure Elicits Memory Deficits Associated with Reduced PSA-NCAM Expression, Altered Glutamatergic Signaling, and Adaptations in Hippocampal Synaptic Plasticity.

Cannabis is now one of the most commonly used illicit substances among pregnant women. This is particularly concerning since developmental exposure to cannabinoids can elicit enduring neurofunctional and cognitive alterations. This study investigates the mechanisms of learning and memory deficits resulting from prenatal cannabinoid exposure (PCE) in adolescent offspring. The synthetic cannabinoid agonist WIN55,212-2 was administered to pregnant rats, and a series of behavioral, electrophysiological, and immunochemical studies were performed to identify potential mechanisms of memory deficits in the adolescent offspring. Hippocampal-dependent memory deficits in adolescent PCE animals were associated with decreased long-term potentiation (LTP) and enhanced long-term depression (LTD) at hippocampal Schaffer collateral-CA1 synapses, as well as an imbalance between GluN2A- and GluN2B-mediated signaling. Moreover, PCE reduced gene and protein expression of neural cell adhesion molecule (NCAM) and polysialylated-NCAM (PSA-NCAM), which are critical for GluN2A and GluN2B signaling balance. Administration of exogenous PSA abrogated the LTP deficits observed in PCE animals, suggesting PSA mediated alterations in GluN2A- and GluN2B- signaling pathways may be responsible for the impaired hippocampal synaptic plasticity resulting from PCE. These findings enhance our current understanding of how PCE affects memory and how this process can be manipulated for future therapeutic purposes.

Phenyl-2-aminoethyl selenide ameliorates hippocampal long-term potentiation and cognitive deficits following doxorubicin treatment.

Chemotherapy-induced memory loss ("chemobrain") can occur following treatment with the widely used chemotherapeutic agent doxorubicin (DOX). However, the mechanisms through which DOX induces cognitive dysfunction are not clear, and there are no commercially available therapies for its treatment or prevention. Therefore, the aim of this study was to determine the therapeutic potential of phenyl-2-aminoethyl selenide (PAESe), an antioxidant drug previously demonstrated to reduce cardiotoxicity associated with DOX treatment, against DOX-induced chemobrain. Four groups of male athymic NCr nude (nu/nu) mice received five weekly tail-vein injections of saline (Control group), 5 mg/kg of DOX (DOX group), 10 mg/kg PAESe (PAESe group), or 5 mg/kg DOX and 10 mg/kg PAESe (DOX+PAESe group). Spatial memory was evaluated using Y-maze and novel object location tasks, while synaptic plasticity was assessed through the measurement of field excitatory postsynaptic potentials from the Schaffer collateral circuit. Western blot analyses were performed to assess hippocampal protein and phosphorylation levels. In this model, DOX impaired synaptic plasticity and memory, and increased phosphorylation of protein kinase B (Akt) and extracellular-regulated kinase (ERK). Co-administration of PAESe reduced Akt and ERK phosphorylation and ameliorated the synaptic and memory deficits associated with DOX treatment.

Impacts of neonatal methylmercury on behavioral flexibility and learning in spatial discrimination reversal and visual signal detection tasks.

Early postnatal development in rodents is sensitive to neurotoxic effects of the environmental contaminant, methylmercury. While juvenile and adolescent exposure also produce long-term impairments in behavior, the outcome of neonatal exposure is less understood. Neural development during the neonatal period in rodents is akin to that seen in humans during the third trimester of pregnancy but methylmercury exposure occurring during the neonatal period has not been modeled, partly because breast milk is a poor source of bioavailable methylmercury. To examine this developmental period, male Long-Evans rats were exposed to 0, 80, or 350 µg/kg/day methylmercuric chloride from postnatal days 1-10, the rodent neonatal period. As adults, behavioral flexibility, attention, memory, and expression of the dopamine transporter in these rats was assessed. Rats exhibited changes in behavioral flexibility assessed in a spatial discrimination reversal procedure. Those rats exposed to the highest dose of methylmercury displayed subtly altered patterns of perseveration compared to control animals. During acquisition of the attention/memory procedure, rats exposed to this dose also had slower acquisition, and achieved lower overall accuracy during training, compared to controls despite neither attention nor memory being affected once the task was acquired. Finally, dopamine transporter expression in the striatum, prefrontal cortex, and hippocampus was unchanged in these adult rats. The results of this study replicate the trend of findings seen with exposure during gestation or during adolescence.

rTg(TauP301L)4510 mice exhibit increased VGlut1 in hippocampal presynaptic glutamatergic vesicles and increased extracellular glutamate release.

The molecular pathways that contribute to the onset of symptoms in tauopathy models, including Alzheimer's disease (AD), are difficult to distinguish because multiple changes can happen simultaneously at different stages of disease progression. Understanding early synaptic alterations and their supporting molecular pathways is essential to develop better pharmacological targets to treat AD. Here, we focus on an early onset rTg(TauP301L )4510 tauopathy mouse model that exhibits hyperexcitability in hippocampal neurons of adult mice that is correlated with presynaptic changes and increased extracellular glutamate levels. However, it is not clear if increased extracellular glutamate is caused by presynaptic changes alone, or if presynaptic changes are a contributing factor among other factors. To determine whether pathogenic tau alters presynaptic function and glutamate release, we studied cultured hippocampal neurons at 14-18 days in vitro (DIV) from animals of both sexes to measure presynaptic changes in tauP301L positive mice. We observed that presynaptic vesicles exhibit increased vesicular glutamate transporter 1 (VGlut1) using immunohistochemistry of fixed cells and an established pH-sensitive green fluorescent protein approach. We show that tauP301L positive neurons exhibit a 40% increase in VGlut1 per vesicle compared to tauP301L negative littermates. Further, we use the extracellular glutamate reporter iGluSnFR to show that increased VGlut1 per vesicle directly translates into a 40% increase in extracellular glutamate. Together, these results show that increased extracellular glutamate levels observed in tauP301L mice are not caused by increased vesicle exocytosis probability but rather are directly related to increased VGlut1 transporters per synaptic vesicle.