SIRT2 Inhibition Rescues Neurodegenerative Pathology but Increases Systemic Inflammation in a Transgenic Mouse Model of Alzheimer's Disease

Sirtuin 2 (SIRT2) has been proposed to have a central role on aging, inflammation, cancer and neurodegenerative diseases; however, its specific function remains controversial. Recent studies propose SIRT2 pharmacological inhibition as a therapeutic strategy for several neurodegenerative diseases including Alzheimer's disease (AD). Surprisingly, none of these published studies regarding the potential interest of SIRT2 inhibition has assessed the peripheral adverse side consequences of this treatment. In this study, we demonstrate that the specific SIRT2 inhibitor, the compound 33i, does not exhibit genotoxic or mutagenic properties. Moreover, pharmacological treatment with 33i, improved cognitive dysfunction and long-term potentiation, reducing amyloid pathology and neuroinflammation in the APP/PS1 AD mouse model. However, this treatment increased peripheral levels of the inflammatory cytokines IL-1β, TNF, IL-6 and MCP-1. Accordingly, peripheral SIRT2 inhibition with the blood brain barrier impermeable compound AGK-2, worsened the cognitive capacities and increased systemic inflammation. The analysis of human samples revealed that SIRT2 is increased in the brain but not in the serum of AD patients. These results suggest that, although SIRT2 pharmacological inhibition may have beneficial consequences in neurodegenerative diseases, its pharmacological inhibition at the periphery would not be recommended and the systemic adverse side effects should be considered. This information is essential to maximize the therapeutic potential of SIRT2 inhibition not only for AD but also for other neurodegenerative diseases.

Prolactin Reduces Hippocampal Parvalbumin and GABAA Receptor Expression in Female Mice

INTRODUCTION

Parvalbumin (PV)-positive cells are strategic elements of neuronal networks capable of influencing memory and learning processes. However, it is not known whether pituitary hormones may be related to PV expression in the hippocampus - a part of the limbic system with important functions in learning and memory.

OBJECTIVE

Since previous studies indicate that prolactin (PRL) plays a significant role in hippocampal-dependent learning and synaptic plasticity, we hypothesized that a rise in PRL levels can modify PV expression in the hippocampus.

METHODS

We employed biochemical, immunohistochemistry, and densitometry techniques - as well as a behavioural assay - in a hyperprolactinemia model using subcutaneous osmotic pumps in female mice.

RESULTS

PRL treatment via osmotic pump induced an increase in PRL receptor (PRLR) expression in most regions of the hippocampus analysed by Western blotting and immunohistochemistry methods. Fluorescent densitometry analysis revealed that PV expression decreases in the same layers in the hippocampus following PRL treatment, while double labelling immunostaining indicated close localization of PV and PRLR in PV-positive interneurons. In addition, we found that PRL induced a reduction in the β2/3 subunit of GABAA receptor (GABAAR) expression that was linearly correlated with the reduction in PV expression. This reduction in the β2/3 subunit of GABAAR expression was maintained in trained animals in which PRL treatment improved the learning of a spatial memory task.

CONCLUSIONS

These data show, for the first time, that an increase in PRL level is associated with changes in key constituent elements of inhibitory circuits in the hippocampus and may be of relevance for the alterations in cognitive function reported in hyperprolactinemia.

Increase in serum prolactin levels in females improves the performance of spatial learning by promoting changes in the circuital dynamics of the hippocampus

Beyond the direct physiological functions associated with motherhood in mammals, previous studies have suggested the potential role of prolactin (Prl) in distinct brain processes such as neuroprotection, neurogenesis, and stress responses. However, the cognitive influence of Prl remains unclear, particularly regarding the mechanisms of acquisition, consolidation and retrieval of information in the brain. Using chronic implanted electrodes in freely moving female mice combined with behavioral tests, we investigated the rhythmic activity changes induced by Prl in a model of hippocampus-dependent learning and memory. Our results show that Prl improves the learning of a spatial memory task in the acquisition stage. The main variations at the circuitry level were in the theta frequency band (4-8 Hz and 8-12 Hz), marked by a faster change in oscillatory activity with no modifications to higher frequencies. These results show that Prl plays a significant role in the acquisition of information during learning of a spatial memory task, suggesting that an increase in Prl levels may induce changes in circuital network plasticity.

Prolactin enhances hippocampal synaptic plasticity in female mice of reproductive age

Dynamic signaling between the endocrine system (ES) and the nervous system (NS) is essential for brain and body homeostasis. In particular, reciprocal interaction occurs during pregnancy and motherhood that may involve changes in some brain plasticity processes. Prolactin (PRL), a hormone with pleiotropic effects on the NS, promotes maternal behavior and has been linked to modifications in brain circuits during motherhood; however, it is unclear whether PRL may regulate synaptic plasticity. Therefore, the main aim of the present work was to determine the cellular and molecular mechanisms triggered by PRL that regulate synaptic plasticity in the hippocampus. By analyzing extracellular recordings in CA3‐CA1 synapses of hippocampal slices, we report that PRL modifies short and long‐term synaptic plasticity in female mice of reproductive age, but not in sexually immature females or adult males. This effect is carried out through mechanisms that include participation of GABA_A receptors and activation of the JAK2‐mediated signaling pathway. These findings show for the first time how PRL enhances the synaptic strength in hippocampal circuits and that this effect is sexually dimorphic, which would influence complex brain processes in physiological conditions like pregnancy and lactation.

Diabetes Causes Dysfunctional Dopamine Neurotransmission Favoring Nigrostriatal Degeneration in Mice

BACKGROUND

Numerous studies indicate an association between neurodegenerative and metabolic diseases. Although still a matter of debate, growing evidence from epidemiological and animal studies indicate that preexisting diabetes increases the risk to develop Parkinson's disease. However, the mechanisms of such an association are unknown.

OBJECTIVES

We investigated whether diabetes alters striatal dopamine neurotransmission and assessed the vulnerability of nigrostriatal neurons to neurodegeneration.

METHODS

We used streptozotocin-treated and genetically diabetic db/db mice. Expression of oxidative stress and nigrostriatal neuronal markers and levels of dopamine and its metabolites were monitored. Dopamine release and uptake were assessed using fast-scan cyclic voltammetry. 6-Hydroxydopamine was unilaterally injected into the striatum using stereotaxic surgery. Motor performance was scored using specific tests.

RESULTS

Diabetes resulted in oxidative stress and decreased levels of dopamine and its metabolites in the striatum. Levels of proteins regulating dopamine release and uptake, including the dopamine transporter, the Girk2 potassium channel, the vesicular monoamine transporter 2, and the presynaptic vesicle protein synaptobrevin-2, were decreased in diabetic mice. Electrically evoked levels of extracellular dopamine in the striatum were enhanced, and altered dopamine uptake was observed. Striatal microinjections of a subthreshold dose of the neurotoxin 6-hydroxydopamine in diabetic mice, insufficient to cause motor alterations in nondiabetic animals, resulted in motor impairment, higher loss of striatal dopaminergic axons, and decreased neuronal cell bodies in the substantia nigra.

CONCLUSIONS

Our results indicate that diabetes promotes striatal oxidative stress, alters dopamine neurotransmission, and increases vulnerability to neurodegenerative damage leading to motor impairment. © 2020 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Proteolytic Degradation of Hippocampal STEP61 in LTP and Learning

Striatal-enriched protein tyrosine phosphatase (STEP) modulates key signaling molecules involved in synaptic plasticity and neuronal function. It is postulated that STEP opposes the development of long-term potentiation (LTP) and that it exerts a restraint on long-term memory (LTM). Here, we examined whether STEP₆₁ levels are regulated during hippocampal LTP and after training in hippocampal-dependent tasks. We found that after inducing LTP by high frequency stimulation or theta-burst stimulation STEP₆₁ levels were significantly reduced, with a concomitant increase of STEP₃₃ levels, a product of calpain cleavage. Importantly, inhibition of STEP with TC-2153 improved LTP in hippocampal slices. Moreover, we observed that after training in the passive avoidance and the T-maze spontaneous alternation task, hippocampal STEP₆₁ levels were significantly reduced, but STEP₃₃ levels were unchanged. Yet, hippocampal BDNF content and TrkB levels were increased in trained mice, and it is known that BDNF promotes STEP degradation through the proteasome. Accordingly, hippocampal pTrkB^Tyr816, pPLCγ^Tyr783, and protein ubiquitination levels were increased in T-SAT trained mice. Remarkably, injection of the TrkB antagonist ANA-12 (2 mg/Kg, but not 0.5 mg/Kg) elicited LTM deficits and promoted STEP₆₁ accumulation in the hippocampus. Also, STEP knockout mice outperformed wild-type animals in an age- and test-dependent manner. Summarizing, STEP₆₁ undergoes proteolytic degradation in conditions leading to synaptic strengthening and memory formation, thus highlighting its role as a molecular constrain, which is removed to enable the activation of pathways important for plasticity processes.

Caffeine-mediated BDNF release regulates long-term synaptic plasticity through activation of IRS2 signaling

Caffeine has cognitive‐enhancing properties with effects on learning and memory, concentration, arousal and mood. These effects imply changes at circuital and synaptic level, but the mechanism by which caffeine modifies synaptic plasticity remains elusive. Here we report that caffeine, at concentrations representing moderate to high levels of consumption in humans, induces an NMDA receptor‐independent form of LTP (_CAFLTP) in the CA1 region of the hippocampus by promoting calcium‐dependent secretion of BDNF, which subsequently activates TrkB‐mediated signaling required for the expression of _CAFLTP. Our data include the novel observation that insulin receptor substrate 2 (IRS2) is phosphorylated during induction of _CAFLTP, a process that requires cytosolic free Ca²⁺. Consistent with the involvement of IRS2 signals in caffeine‐mediated synaptic plasticity, phosphorylation of Akt (Ser473) in response to LTP induction is defective in Irs2^−/− mice, demonstrating that these plasticity changes are associated with downstream targets of the phosphoinositide 3‐kinase (PI3K) pathway. These findings indicate that TrkB‐IRS2 signals are essential for activation of PI3K during the induction of LTP by caffeine.

Activity-dependent switch of GABAergic inhibition into glutamatergic excitation in astrocyte-neuron networks

Interneurons are critical for proper neural network function and can activate Ca²⁺ signaling in astrocytes. However, the impact of the interneuron-astrocyte signaling into neuronal network operation remains unknown. Using the simplest hippocampal Astrocyte-Neuron network, i.e., GABAergic interneuron, pyramidal neuron, single CA3-CA1 glutamatergic synapse, and astrocytes, we found that interneuron-astrocyte signaling dynamically affected excitatory neurotransmission in an activity- and time-dependent manner, and determined the sign (inhibition vs potentiation) of the GABA-mediated effects. While synaptic inhibition was mediated by GABA_A receptors, potentiation involved astrocyte GABA_B receptors, astrocytic glutamate release, and presynaptic metabotropic glutamate receptors. Using conditional astrocyte-specific GABA_B receptor (Gabbr1) knockout mice, we confirmed the glial source of the interneuron-induced potentiation, and demonstrated the involvement of astrocytes in hippocampal theta and gamma oscillations in vivo. Therefore, astrocytes decode interneuron activity and transform inhibitory into excitatory signals, contributing to the emergence of novel network properties resulting from the interneuron-astrocyte interplay.

DOI:http://dx.doi.org/10.7554/eLife.20362.001

A role for Kalirin-7 in corticostriatal synaptic dysfunction in Huntington's disease

Cognitive dysfunction is an early clinical hallmark of Huntington's disease (HD) preceding the appearance of motor symptoms by several years. Neuronal dysfunction and altered corticostriatal connectivity have been postulated to be fundamental to explain these early disturbances. However, no treatments to attenuate cognitive changes have been successful: the reason may rely on the idea that the temporal sequence of pathological changes is as critical as the changes per se when new therapies are in development. To this aim, it becomes critical to use HD mouse models in which cognitive impairments appear prior to motor symptoms. In this study, we demonstrate procedural memory and motor learning deficits in two different HD mice and at ages preceding motor disturbances. These impairments are associated with altered corticostriatal long-term potentiation (LTP) and specific reduction of dendritic spine density and postsynaptic density (PSD)-95 and spinophilin-positive clusters in the cortex of HD mice. As a potential mechanism, we described an early decrease of Kalirin-7 (Kal7), a guanine-nucleotide exchange factor for Rho-like small GTPases critical to maintain excitatory synapse, in the cortex of HD mice. Supporting a role for Kal7 in HD synaptic deficits, exogenous expression of Kal7 restores the reduction of excitatory synapses in HD cortical cultures. Altogether, our results suggest that cortical dysfunction precedes striatal disturbances in HD and underlie early corticostriatal LTP and cognitive defects. Moreover, we identified diminished Kal7 as a key contributor to HD cortical alterations, placing Kal7 as a molecular target for future therapies aimed to restore corticostriatal function in HD.

Neurotrophin receptor p75(NTR) mediates Huntington's disease-associated synaptic and memory dysfunction

Learning and memory deficits are early clinical manifestations of Huntington's disease (HD). These cognitive impairments have been mainly associated with frontostriatal HD pathology; however, compelling evidence provided by several HD murine models suggests that the hippocampus may contribute to synaptic deficits and memory dysfunction in HD. The neurotrophin receptor p75(NTR) negatively regulates spine density, which is associated with learning and memory; therefore, we explored whether disturbed p75(NTR) function in the hippocampus could contribute to synaptic dysfunction and memory deficits in HD. Here, we determined that levels of p75(NTR) are markedly increased in the hippocampus of 2 distinct mouse models of HD and in HD patients. Normalization of p75(NTR) levels in HD mutant mice heterozygous for p75(NTR) prevented memory and synaptic plasticity deficits and ameliorated dendritic spine abnormalities, likely through normalization of the activity of the GTPase RhoA. Moreover, viral-mediated overexpression of p75(NTR) in the hippocampus of WT mice reproduced HD learning and memory deficits, while knockdown of p75(NTR) in the hippocampus of HD mice prevented cognitive decline. Together, these findings provide evidence of hippocampus-associated memory deficits in HD and demonstrate that p75(NTR) mediates synaptic, learning, and memory dysfunction in HD.

Prostaglandin E2 EP1 receptor antagonist improves motor deficits and rescues memory decline in R6/1 mouse model of Huntington's disease

In this study, we evaluated the potential beneficial effects of antagonizing prostaglandin E2 (PGE2) EP1 receptor on motor and memory deficits in Huntington's disease (HD). To this aim, we implanted an osmotic mini-pump system to chronically administrate an EP1 receptor antagonist (SC-51089) in the R6/1 mouse model of HD, from 13 to 18 weeks of age, and used different paradigms to assess motor and memory function. SC-51089 administration ameliorated motor coordination and balance dysfunction in R6/1 mice as analyzed by rotarod, balance beam, and vertical pole tasks. Long-term memory deficit was also rescued after EP1 receptor antagonism as assessed by the T-maze spontaneous alternation and the novel object recognition tests. Additionally, treatment with SC-51089 improved the expression of specific synaptic markers and reduced the number of huntingtin nuclear inclusions in the striatum and hippocampus of 18-week-old R6/1 mice. Moreover, electrophysiological studies showed that hippocampal long-term potentiation was significantly recovered in R6/1 mice after EP1 receptor antagonism. Altogether, these results show that the antagonism of PGE2 EP1 receptor has a strong therapeutic effect on R6/1 mice and point out a new therapeutic candidate to treat motor and memory deficits in HD.

Dopamine release regulation by astrocytes during cerebral ischemia

Brain ischemia triggers excessive release of neurotransmitters that mediate neuronal damage following ischemic injury. The striatum is one of the areas most sensitive to ischemia. Release of dopamine (DA) from ischemic neurons is neurotoxic and directly contributes to the cell death in affected areas. Astrocytes are known to be critically involved in the physiopathology of cerebrovascular disease. However, their response to ischemia and their role in neuroprotection in striatum are not completely understood. In this study, we used an in vitro model to evaluate the mechanisms of ischemia-induced DA release, and to study whether astrocytes modulate the release of DA in response to short-term ischemic conditions. Using slices of adult mouse brain exposed to oxygen and glucose deprivation (OGD), we measured the OGD-evoked DA efflux using fast cyclic voltammetry and also assessed metabolic impairment by 2,3,5-triphenyltetrazolium chloride (TTC) and tissue viability by propidium iodide (PI) staining. Our data indicate that ischemia induces massive release of DA by dual mechanisms: one which operates via vesicular exocytosis and is action potential dependent and another involving reverse transport by the dopamine transporter (DAT). Simultaneous blockade of astrocyte glutamate transporters and DAT prevented the massive release of dopamine and reduced the brain tissue damage. The present results provide the first experimental evidence that astrocytes function as a key cellular element of ischemia-induced DA release in striatum, constituting a novel and promising therapeutic target in ischemia.

Confocal microscopy for astrocyte in vivo imaging: Recycle and reuse in microscopy

In vivo imaging is one of the ultimate and fundamental approaches for the study of the brain. Two-photon laser scanning microscopy (2PLSM) constitutes the state-of-the-art technique in current neuroscience to address questions regarding brain cell structure, development and function, blood flow regulation and metabolism. This technique evolved from laser scanning confocal microscopy (LSCM), which impacted the field with a major improvement in image resolution of live tissues in the 1980s compared to widefield microscopy. While nowadays some of the unparalleled features of 2PLSM make it the tool of choice for brain studies in vivo, such as the possibility to image deep within a tissue, LSCM can still be useful in this matter. Here we discuss the validity and limitations of LSCM and provide a guide to perform high-resolution in vivo imaging of the brain of live rodents with minimal mechanical disruption employing LSCM. We describe the surgical procedure and experimental setup that allowed us to record intracellular calcium variations in astrocytes evoked by sensory stimulation, and to monitor intact neuronal dendritic spines and astrocytic processes as well as blood vessel dynamics. Therefore, in spite of certain limitations that need to be carefully considered, LSCM constitutes a useful, convenient, and affordable tool for brain studies in vivo.

Normalization of Dyrk1A expression by AAV2/1-shDyrk1A attenuates hippocampal-dependent defects in the Ts65Dn mouse model of Down syndrome

The cognitive dysfunctions of Down Syndrome (DS) individuals are the most disabling alterations caused by the trisomy of human chromosome 21 (HSA21). In trisomic Ts65Dn mice, a genetic model for DS, the overexpression of HSA21 homologous genes has been associated with strong visuo-spatial cognitive alterations, ascribed to hippocampal dysfunction. In the present study, we evaluated whether the normalization of the expression levels of Dyrk1A (Dual specificity tyrosine-phosphorylation-regulated kinase 1A), a candidate gene for DS, might correct hippocampal defects in Ts65Dn mice. In the hippocampus of 2 month-old Ts65Dn mice, such normalization was achieved through the stereotaxical injection of adeno-associated viruses containing a short hairpin RNA against Dyrk1A (AAV2/1-shDyrk1A) and a luciferase reporter gene. The injected hippocampi were efficiently transduced, as shown by bioluminescence in vivo imaging, luciferase activity quantification and immunohistochemical analysis. At the molecular level, viral infusion allowed the normalization of the targeted Dyrk1A expression, as well as of the key players of the MAPK/CREB pathway. The electrophysiological recordings of hippocampal slices from Ts65Dn mice injected with AAV2/1-shDyrk1A displayed attenuation of the synaptic plasticity defects of trisomic mice. In contrast, contralateral hippocampal injection with an AAV2/1 control virus containing a scrambled sequence, showed neither the normalization of Dyrk1A levels nor changes of synaptic plasticity. In the Morris water maze task, although long-term consolidation of the task was not achieved, treated Ts65Dn mice displayed initially a normalized thigmotactic behavior, similar to euploid littermates, indicating the partial improvement in their hippocampal-dependent search strategy. Taken together, these results show Dyrk1A as a critical player in the pathophysiology of DS and define Dyrk1A as a therapeutic target in adult trisomic mice.

L-DOPA-induced increase in TH-immunoreactive striatal neurons in parkinsonian mice: insights into regulation and function

Tyrosine hydroxylase (TH)-immunoreactive (ir) neurons have been found in the striatum after dopamine depletion; however, little is known about the mechanism underlying their appearance or their functional significance. We previously showed an increase in striatal TH-ir neurons after L-DOPA treatment in mice with unilateral 6-OHDA lesions in the striatum. In the present study, we further examined the time-course and persistence of the effects of chronic L-DOPA treatment on the appearance and regulation of TH-ir neurons as well as their possible function. We found that the L-DOPA-induced increase in striatal TH-ir neurons is dose-dependent and persists for days after L-DOPA withdrawal, decreasing significantly 10 days after L-DOPA treatment ends. Using hemiparkinsonian D1 receptor knock-out (D1R-/-) and D2 receptor knock-out (D2R-/-) mice, we found that the D1R, but not the D2R, is required for the L-DOPA-induced appearance of TH-ir neurons in the dopamine-depleted striatum. Interestingly, our experiments in aphakia mice, which lack Pitx3 expression in the brain, indicate that the L-DOPA-dependent increase in the number of TH-ir neurons is independent of Pitx3, a transcription factor necessary for the development of mesencephalic dopaminergic neurons. To explore the possible function of L-DOPA-induced TH-ir neurons in the striatum, we examined dopamine overflow and forelimb use in L-DOPA-treated parkinsonian mice. These studies revealed a tight spatio-temporal correlation between the presence of striatal TH-ir neurons, the recovery of electrically stimulated dopamine overflow in the lesioned striatum, and the recovery of contralateral forelimb use with chronic L-DOPA treatment. Our results suggest that the presence of TH-ir neurons in the striatum may underlie the long-duration response to L-DOPA following withdrawal. Promotion of these neurons in the early stages of Parkinson's disease, when dopamine denervation is incomplete, may be beneficial for maintaining motor function.

Facilitation of AMPA receptor synaptic delivery as a molecular mechanism for cognitive enhancement

Cell adhesion molecules and downstream growth factor-dependent signaling are critical for brain development and synaptic plasticity, and they have been linked to cognitive function in adult animals. We have previously developed a mimetic peptide (FGL) from the neural cell adhesion molecule (NCAM) that enhances spatial learning and memory in rats. We have now investigated the cellular and molecular basis of this cognitive enhancement, using biochemical, morphological, electrophysiological, and behavioral analyses. We have found that FGL triggers a long-lasting enhancement of synaptic transmission in hippocampal CA1 neurons. This effect is mediated by a facilitated synaptic delivery of AMPA receptors, which is accompanied by enhanced NMDA receptor-dependent long-term potentiation (LTP). Both LTP and cognitive enhancement are mediated by an initial PKC activation, which is followed by persistent CaMKII activation. These results provide a mechanistic link between facilitation of AMPA receptor synaptic delivery and improved hippocampal-dependent learning, induced by a pharmacological cognitive enhancer.

CCL2-expressing astrocytes mediate the extravasation of T lymphocytes in the brain. Evidence from patients with glioma and experimental models in vivo

CCL2 is a chemokine involved in brain inflammation, but the way in which it contributes to the entrance of lymphocytes in the parenchyma is unclear. Imaging of the cell type responsible for this task and details on how the process takes place in vivo remain elusive. Herein, we analyze the cell type that overexpresses CCL2 in multiple scenarios of T-cell infiltration in the brain and in three different species. We observe that CCL2⁺ astrocytes play a part in the infiltration of T-cells in the brain and our analysis shows that the contact of T-cells with perivascular astrocytes occurs, suggesting that may be an important event for lymphocyte extravasation.

Astrocytes mediate in vivo cholinergic-induced synaptic plasticity

Long-term potentiation (LTP) of synaptic transmission represents the cellular basis of learning and memory. Astrocytes have been shown to regulate synaptic transmission and plasticity. However, their involvement in specific physiological processes that induce LTP in vivo remains unknown. Here we show that in vivo cholinergic activity evoked by sensory stimulation or electrical stimulation of the septal nucleus increases Ca²⁺ in hippocampal astrocytes and induces LTP of CA3-CA1 synapses, which requires cholinergic muscarinic (mAChR) and metabotropic glutamate receptor (mGluR) activation. Stimulation of cholinergic pathways in hippocampal slices evokes astrocyte Ca²⁺ elevations, postsynaptic depolarizations of CA1 pyramidal neurons, and LTP of transmitter release at single CA3-CA1 synapses. Like in vivo, these effects are mediated by mAChRs, and this cholinergic-induced LTP (c-LTP) also involves mGluR activation. Astrocyte Ca²⁺ elevations and LTP are absent in IP₃R2 knock-out mice. Downregulating astrocyte Ca²⁺ signal by loading astrocytes with BAPTA or GDPβS also prevents LTP, which is restored by simultaneous astrocyte Ca²⁺ uncaging and postsynaptic depolarization. Therefore, cholinergic-induced LTP requires astrocyte Ca²⁺ elevations, which stimulate astrocyte glutamate release that activates mGluRs. The cholinergic-induced LTP results from the temporal coincidence of the postsynaptic activity and the astrocyte Ca²⁺ signal simultaneously evoked by cholinergic activity. Therefore, the astrocyte Ca²⁺ signal is necessary for cholinergic-induced synaptic plasticity, indicating that astrocytes are directly involved in brain storage information.

IRS-2 Deficiency impairs NMDA receptor-dependent long-term potentiation

The beneficial effects of insulin and insulin-like growth factor I on cognition have been documented in humans and animal models. Conversely, obesity, hyperinsulinemia, and diabetes increase the risk for neurodegenerative disorders including Alzheimer's disease (AD). However, the mechanisms by which insulin regulates synaptic plasticity are not well understood. Here, we report that complete disruption of insulin receptor substrate 2 (Irs2) in mice impairs long-term potentiation (LTP) of synaptic transmission in the hippocampus. Basal synaptic transmission and paired-pulse facilitation were similar between the 2 groups of mice. Induction of LTP by high-frequency conditioning tetanus did not activate postsynaptic N-methyl-D-aspartate (NMDA) receptors in hippocampus slices from Irs2(-/-) mice, although the expression of NR2A, NR2B, and PSD95 was equivalent to wild-type controls. Activation of Fyn, AKT, and MAPK in response to tetanus stimulation was defective in Irs2(-/-) mice. Interestingly, IRS2 was phosphorylated during induction of LTP in control mice, revealing a potential new component of the signaling machinery which modulates synaptic plasticity. Given that IRS2 expression is diminished in Type 2 diabetics as well as in AD patients, these data may reveal an explanation for the prevalence of cognitive decline in humans with metabolic disorders by providing a mechanistic link between insulin resistance and impaired synaptic transmission.

Flufenamic acid suppresses epileptiform activity in hippocampus by reducing excitatory synaptic transmission and neuronal excitability

PURPOSE

In this study, we explore the antiepileptic effects of flufenamic acid (FFA) in order to identify the cellular mechanisms that underlie the potential anticonvulsant properties of this nonsteroidal antiinflammatory compound.

METHODS

The mechanisms of FFA action were analyzed using an in vitro model in which epileptiform activity was induced in hippocampal slices by perfusion with 100 microm 4-aminopyridine (4-AP) added to a modified Mg(2+)-free solution. The activity of CA1 pyramidal neurons as well as the synaptic connection between CA3 and CA1 was monitored using extracellular and patch-clamp recordings.

RESULTS

Epileptiform activity was suppressed in hippocampal neurons by FFA at concentrations between 50 and 200 microm. Glutamatergic excitatory synaptic transmission was diminished by FFA without modifying recurrent gamma-aminobutyric acid (GABA)ergic synaptic inhibition. Several lines of evidence indicated that FFA did not decrease neurotransmitter release probability, implicating a postsynaptic mechanism of action. FFA also potently reduced neuronal excitability, but did not alter the amplitude, duration, or undershoot of action potentials.

CONCLUSIONS

Our results suggest that FFA exerts an anticonvulsive effect on hippocampal pyramidal neurons by simultaneously decreasing glutamatergic excitatory synaptic activity and reducing neuronal excitability. Therefore, our study provides experimental evidence that FFA may represent an effective pharmacologic agent in the treatment of epilepsy in the mammalian central nervous system.

Brain-derived neurotrophic factor modulates the severity of cognitive alterations induced by mutant huntingtin: involvement of phospholipaseCgamma activity and glutamate receptor expression

The involvement of brain-derived neurotrophic factor (BDNF) in cognitive processes and the decrease in its expression in Huntington's disease suggest that this neurotrophin may play a role in learning impairment during the disease progression. We therefore analyzed the onset and severity of cognitive deficits in two different mouse models with the same mutant huntingtin but with different levels of BDNF (R6/1 and R6/1:BDNF+/- mice). We observed that BDNF modulates cognitive function in different learning tasks, even before the onset of motor symptoms. R6/1:BDNF+/- mice showed earlier and more accentuated cognitive impairment than R6/1 mice at 5 weeks of age in discrimination learning; at 5 weeks of age in procedural learning; and at 9 weeks of age in alternation learning. At the earliest age at which cognitive impairment was detected, electrophysiological analysis was performed in the hippocampus. All mutant genotypes showed reduced hippocampal long term potentiation (LTP) with respect to wild type but did not show differences between them. Thus, we evaluated the involvement of BDNF-trkB signaling and glutamate receptor expression in the hippocampus of these mice. We observed a decrease in phospholipaseCgamma activity, but not ERK, in R61, BDNF+/- and R6/1:BDNF+/- hippocampus at the age when LTP was altered. However, a specific decrease in the expression of glutamate receptors NR1, NR2A and GluR1 was detected only in R6/1:BDNF+/- hippocampus. Therefore, these results show that BDNF modulates the learning and memory alterations induced by mutant huntingtin. This interaction leads to intracellular changes, such as specific changes in glutamate receptors and in BDNF-trkB signaling through phospholipaseCgamma.

Mechanical lesion activates newly identified NFATc1 in primary astrocytes: implication of ATP and purinergic receptors

Ca2+-dependent calcineurin is upregulated in reactive astrocytes in neuroinflammatory models. Therefore, the fact that the nuclear factor of activated T cells (NFAT) is activated in response to calcineurin qualifies this family of transcription factors with immune functions as candidates to mediate astrogliosis. Brain trauma induces a neuroinflammatory state in which ATP is released from astrocytes, stimulating calcium signalling. Our goal here is to characterize NFATc1 and NFATc2 in mouse primary astrocyte cultures, also exploring the implication of NFAT in astrocyte activation by mechanical lesion. Quantitative reverse transcriptase-polymerase chain reaction, Western blot analysis and immunofluorescence microscopy identified NFATc1 in astrocytes, but not NFATc2. Moreover, NFATc1 was expressed in the cytosol of resting astrocytes, whereas activation of the Ca2+-calcineurin pathway by ionomycin translocated NFATc1 to the nucleus, which is a requirement for activation. The implication of astrocytic NFAT in brain trauma was analysed using an in vitro scratch lesion model. Mechanical lesion caused a rapid NFATc1 translocation that progressed throughout the culture as a gradient and was maintained for at least 4 h. We also demonstrate that ATP, released by lesion, is a potent inducer of NFATc1 translocation and activation. Moreover, the use of P2Y receptor modulators showed that such ATP action is mediated by stimulation of several G(q)-protein-coupled P2Y purinergic receptors, among which P2Y(1) and P2Y(6) are included. In conclusion, this work provides evidence that newly identified NFATc1 is translocated in astrocytes in response to lesion following a pathway that involves ATP release and activation of metabotropic purinergic receptors.

D1 but not D5 Dopamine Receptors Are Critical for LTP, Spatial Learning, and LTP-Induced arc and zif268 Expression in the Hippocampus

Recent evidence suggests that glutamatergic and dopaminergic afferents must be activated to induce persistent long-term potentiation (LTP) in the hippocampus. Whereas extensive evidence supports the role of glutamate receptors in long-lasting synaptic plasticity and spatial learning and memory, there is less evidence regarding the role of dopamine receptors in these processes. Here, we used dopamine D(1) receptor knockout (D(1)R(-/-)) mice to explore the role of D(1)R in hippocampal LTP and its associated gene expression. We show that the magnitude of early and late phases of LTP (E-LTP and L-LTP) was markedly reduced in hippocampal slices from D(1)R(-/-) mice compared with wild-type mice. SCH23390, a D(1)/D(5)R antagonist, did not further reduce L-LTP in D(1)R(-/-) mice, suggesting that D(5)Rs are not involved. D(1)R(-/-) mice also showed a significant reduction of D(1)R-induced potentiation of N-Methyl-D-aspartic acid-mediated currents, via protein kinase activated by cyclic adenosine 3',5'-monophosphate activation. Finally, LTP-induced expression of the immediate early genes zif268 and arc in the hippocampal CA1 area was abolished in D(1)R(-/-) mice, and these mice showed impaired learning. These results indicate that D(1)R but not D(5)R are critical for hippocampal LTP and for the induction of Zif268 and Arc, proteins required for the transition from E-LTP to L-LTP and for memory consolidation in mammals.

Adenosine released by astrocytes contributes to hypoxia-induced modulation of synaptic transmission

Astrocytes play a critical role in brain homeostasis controlling the local environment in normal as well as in pathological conditions, such as during hypoxic/ischemic insult. Since astrocytes have recently been identified as a source for a wide variety of gliotransmitters that modulate synaptic activity, we investigated whether the hypoxia-induced excitatory synaptic depression might be mediated by adenosine release from astrocytes. We used electrophysiological and Ca2+ imaging techniques in hippocampal slices and transgenic mice, in which ATP released from astrocytes is specifically impaired, as well as chemiluminescent and fluorescence photometric Ca2+ techniques in purified cultured astrocytes. In hippocampal slices, hypoxia induced a transient depression of excitatory synaptic transmission mediated by activation of presynaptic A1 adenosine receptors. The glia-specific metabolic inhibitor fluorocitrate (FC) was as effective as the A1 adenosine receptor antagonist CPT in preventing the hypoxia-induced excitatory synaptic transmission reduction. Furthermore, FC abolished the extracellular adenosine concentration increase during hypoxia in astrocyte cultures. Several lines of evidence suggest that the increase of extracellular adenosine levels during hypoxia does not result from extracellular ATP or cAMP catabolism, and that astrocytes directly release adenosine in response to hypoxia. Adenosine release is negatively modulated by external or internal Ca2+ concentrations. Moreover, adenosine transport inhibitors did not modify the hypoxia-induced effects, suggesting that adenosine was not released by facilitated transport. We conclude that during hypoxia, astrocytes contribute to regulate the excitatory synaptic transmission through the release of adenosine, which acting on A1 adenosine receptors reduces presynaptic transmitter release. Therefore, adenosine release from astrocytes serves as a protective mechanism by down regulating the synaptic activity level during demanding conditions such as transient hypoxia.

Cholinergic modulation of status epilepticus in the rat barrel field region of primary somatosensory cortex

Status epilepticus (SE) represents a serious medical emergency that can produce long-lasting brain damage as well as cognitive and memory deficits. However, the mechanisms that determine the emergence of SE from a single seizure and the prolonged duration of SE are unknown. Therefore, we used pharmacological tools to investigate the cellular mechanisms that underlie this prolonged epileptic activity in the rat barrel field region of somatosensory cortex (S1BF). Electrocortical and unitary extracellular field recording in the rat S1BF region was used to assess abnormal epileptiform activity induced by intracerebral application of 4-aminopyridine (4-AP). Simultaneously, electromyographic (EMG) activity was recorded from mystacial pad musculature. Intracerebral injection of 4-AP induced an SE that was paralleled by an increase of whisker activity that was not synchronized with the electrocortical recording. The seizures were originated ipsilaterally in the cortex of the injected hemisphere and propagated to the contralateral cortex with lower amplitude. The application of the glutamatergic NMDA receptor antagonist D (-)-2-amino-5-phosphonopentanoic acid (AP5) strongly increased the seizure-onset latency. The muscarinic receptor antagonist atropine changed the continuous rapid spiking pattern of SE to periodic discharges, while glutamatergic or GABAergic antagonist did not modify the electrographic features of SE. Our data suggest that the muscarinic cholinergic system plays an important role in the seizure modulation during SE in the somatosensory cortex, while their emergence is controlled, in part, by glutamatergic NMDA receptors.

Stabilizing effects of extracellular ATP on synaptic efficacy and plasticity in hippocampal pyramidal neurons

The role of adenosine triphosphate (ATP) as a neurotransmitter and extracellular diffusible messenger has recently received considerable attention because of its possible participation in the regulation of synaptic plasticity. However, the possible contribution of extracellular ATP in maintaining and regulating synaptic efficacy during intracellular ATP depletion is understudied. We tested the effects of extracellular ATP on excitatory postsynaptic currents (EPSCs) evoked in CA1 pyramidal neurons by Schaffer collateral stimulation. In the absence of intracellular ATP, EPSC rundown was neutralized when a low concentration of ATP (1 microm) was added to the extracellular solution. Adenosine and ATP analogues did not prevent the EPSC rundown. The P(2) antagonists piridoxal-5'-phosphate-azophenyl 2',4'-disulphonate (PPADS) and reactive blue-2, and the P(1) adenosine receptor antagonist 8-cyclopentyltheophylline (CPT) had no detectable effects in cells depleted of ATP. However, the protective action of extracellular ATP on synaptic efficacy was blocked by extracellular application of the protein kinase inhibitors K252b and staurosporine. In contrast, K252b and staurosporine per se did not interfere with synaptic transmission in ATP loaded cells. Without intracellular ATP, bath-applied caffeine induced a transient (< 35 min) EPSC potentiation that was transformed into a persistent long-term potentiation (> 80 min) when 1 microm ATP was added extracellularly. An increased probability of transmitter release paralleled the long-term potentiation induced by caffeine, suggesting that it originated presynaptically. Therefore, we conclude that extracellular ATP may operate to maintain and regulate synaptic efficacy and plasticity in conditions of abnormal intracellular ATP depletion by phosphorylation of a surface protein substrate via activation of ecto-protein kinases.

Stressor-related impairment of synaptic transmission in hippocampal slices from α-synuclein knockout mice

The role of alpha-synuclein (alpha-Syn) has recently received considerable attention because it seems to play a role in Parkinson's disease (PD). Missense mutations in the alpha-Syn gene were found in autosomal dominant PD and alpha-Syn was shown to be a major constituent of protein aggregates in sporadic PD and other synucleinopathies. Under normal conditions, alpha-Syn protein is found exclusively in synaptic terminals. However, the potential participation of alpha-synuclein in maintaining and regulating synaptic efficacy is unknown. We have investigated the excitatory synaptic modulation of alpha-synuclein in CA1 pyramidal neurons, using the in vitro hippocampal slice technique. The 4-aminopyridine-induced increase of both spontaneous excitatory postsynaptic current (EPSC) frequency and amplitude was significantly higher in alpha-Syn wild-type than knockout mice, whereas basal spontaneous EPSC frequency and amplitude was similar in both animals. As the spontaneous synaptic activity was abolished by tetrodotoxin, which indicates that it was a result of action potential-mediated transmitter release from presynaptic terminals, spontaneous EPSC changes observed in alpha-Syn knockout mice suggest that these animals present a modification of synaptic transmission with a presynaptic origin. Presynaptic depression of evoked EPSCs by hypoxia or adenosine was significantly larger in alpha-Syn knockout than in wild-type mice, further supporting the hypothesis of regulation of synaptic transmission by alpha-Syn. Together, these observations indicate that the loss of alpha-Syn reduces synaptic efficacy when the probability of transmitter release is modified. We conclude that alpha-Syn might have important actions on the maintenance of the functional integrity of synaptic transmission and its regulation in hippocampus.

Valproate reduced synaptic activity increase induced by 4-aminopyridine at the hippocampal CA3-CA1 synapse

PURPOSE

We investigated the effects of valproate (VPA) on excitatory synaptic transmission changes induced by 4-aminopyridine (4-AP) to determine whether the antiepileptic effects shown by VPA can be ascribed to a modulation of spontaneous excitatory postsynaptic currents (sEPSCs) in the CA3-CA1 synapse.

METHODS

Rat hippocampal slices were prepared and maintained in vitro with standard methods. Whole-cell current and voltage-clamp recordings were obtained from CA1 pyramidal neurons by using the "blind" patch-clamp technique in an immersion recording chamber. Increase in the spontaneous excitatory synaptic activity was induced by addition of 4-AP to the medium.

RESULTS

Perfusion with VPA significantly counteracted the increase of frequency and amplitude of the sEPSCs induced by application of 4-AP and suppressed the epileptiform activity.

CONCLUSIONS

We conclude that VPA decreases the 4-AP-induced enhancement of excitatory synaptic activity at the CA3-CA1 synapse, and that this reduction of excitation input to CA1 contributes to the anticonvulsant effects of VPA.

Valproate reduced excitatory postsynaptic currents in hippocampal CA1 pyramidal neurons

Valproate (VPA) is one of the most widely used antiepileptic drugs, and it is also increasingly used for the treatment of neuropsychological disorders and neuropathic pain, as well as migraine prophylaxis. However, the underlying cellular mechanisms of VPA on the synaptic physiology remain unclear. We investigated the effects of VPA on synaptic transmission using the in vitro rat hippocampal slice technique and whole-cell patch clamp recordings from CA1 pyramidal neurons. Perfusion with VPA, at therapeutically attainable concentrations, decreased the amplitude of excitatory postsynaptic currents (EPSCs) evoked by Schaffer collateral stimulation, without modifying inhibitory postsynaptic currents (IPSCs). Furthermore, VPA induced a significant reduction of the non-NMDA EPSC (non-NMDA(EPSC)) component, without modifying the NMDA EPSC (NMDA(EPSC)) component. Paired pulse facilitation and EPSC variance were not significantly affected by VPA, indicating that VPA did not decrease transmitter release probability, which suggests a postsynaptic mechanism of action. We therefore conclude that VPA decreases excitatory synaptic activity through the modulation of postsynaptic non-NMDA receptors, without modifying synaptic inhibition, and that this reduction of excitation is, at least in part, responsible for the effects of VPA.

Valproate suppresses status epilepticus induced by 4-aminopyridine in CA1 hippocampus region

PURPOSE

We investigated the effects of valproate (VPA) on an in vivo model of status epilepticus (SE) induced by intrahippocampal application of 4-aminopyridine (4-AP).

METHODS

To induce continuous epileptiform activity without a clinical component, 4-AP (100 mM) was slowly injected in the hippocampus of adult rats. Extracellular field potential from the CA1 region of the rat hippocampus was recorded to assess abnormal epileptiform activity. Once the SE seizures were induced by 4-AP, the test drug was injected. In some experiments to test the ability of a drug to prevent the induction of SE, the drug was administered before 4-AP injection.

RESULTS

Intrahippocampal injection of 4-AP induced continuous epileptic activity without a clinical component that lasted >60 min. The intravenous injection of 400-600 mg/kg VPA rapidly (approximately 100 s) abolished the SE, and this effect persisted for >/=4 h in our experimental model. The intravenous injection of 100-300 mg/kg VPA did not abolish previously induced SE, but prevented the appearance of SE when applied before the induction of SE. The intravenous injection of 80 mg/kg phenytoin or carbamazepine did not abolish or prevent SE.

CONCLUSIONS

We conclude that 4-AP-induced SE was suppressed by VPA at 400-600 mg/kg, whereas minor doses (100-300 mg/kg) only prevent the 4-AP-induced SE. Present results suggest the revisiting of VPA as a useful drug for the treatment of SE.

Epinephrine reduces the nerve conduction blockage induced by high-potassium in mammalian sciatic nerve

We investigated the effects of epinephrine on the reversible nerve conduction block induced by high [K(+)](o) using electrophysiological extracellular recordings in the isolated rat sciatic nerve in vitro. Bath application of 400 micro M epinephrine (EN) or norepinephrine (NE) reduced the high-potassium-induced compound action potentials (CAPs) blockage in both sensory and motor fibers. The beta-adrenoreceptor agonist isoproterenol mimicked the EN effect while the alpha-adrenoreceptor agonists phenylephrine and guanfacine did not affect the CAPs reduction. Addition of EN to normal ACSF partially reversed the nerve conduction blockage induced by high frequency stimulation. These results suggest that EN and NE modulate the electrophysiological properties of both sensory and motor axons, and they improve the nerve conduction under high [K(+)](o) by modulating nerve excitability.

Caffeine-mediated presynaptic long-term potentiation in hippocampal CA1 pyramidal neurons

We report a new form of long-term potentiation (LTP) in Schaffer collateral (SC)-CA1 pyramidal neuron synapses that originates presynaptically and does not require N-methyl-d-aspartate (NMDA) receptor activation nor increases in postsynaptic-free Ca2+. Using rat hippocampal slices, application of a brief "pulse" of caffeine in the bath evoked a nondecremental LTP (CAFLTP) of SC excitatory postsynaptic currents. An increased probability of transmitter release paralleled the CAFLTP, suggesting that it originated presynaptically. The P1 adenosine receptor antagonist 8-cyclopentyltheophylline and the P2 purinoreceptor antagonists suramin and piridoxal-5'-phosphate-azophenyl 2',4'-disulphonate blocked the CAFLTP. Inhibition of Ca2+ release from caffeine/ryanodine stores by bath-applied ryanodine inhibited the CAFLTP, but ryanodine in the pipette solution was ineffective, suggesting a presynaptic effect of ryanodine. Previous induction of the "classical" LTP did not prevent the CAFLTP, suggesting that the LTP and the CAFLTP have different underlying cellular mechanisms. The CAFLTP is insensitive to the block of NMDA receptors by 2-amino-5-phosphonopentanoic acid and to Ca2+ chelation with intracellular 1,2-bis (2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid, indicating that neither postsynaptic NMDA receptors nor increases in cytosolic-free Ca2+ participate in the CAFLTP. We conclude that the CAFLTP requires the interaction of caffeine with presynaptic P1, P2 purinoreceptors, and ryanodine receptors and is caused by an increased probability of glutamate release at SC terminals.

Synaptic regulation of the slow Ca2+-activated K+ current in hippocampal CA1 pyramidal neurons: implication in epileptogenesis

The slow Ca2+-activated K+ current (sI(AHP)) plays a critical role in regulating neuronal excitability, but its modulation during abnormal bursting activity, as in epilepsy, is unknown. Because synaptic transmission is enhanced during epilepsy, we investigated the synaptically mediated regulation of the sI(AHP) and its control of neuronal excitability during epileptiform activity induced by 4-aminopyridine (4AP) or 4AP+Mg2+-free treatment in rat hippocampal slices. We used electrophysiological and photometric Ca2+ techniques to analyze the sI(AHP) modifications that parallel epileptiform activity. Epileptiform activity was characterized by slow, repetitive, spontaneous depolarizations and action potential bursts and was associated with increased frequency and amplitude of spontaneous excitatory postsynaptic currents and a reduced sI(AHP.) The metabotropic glutamate receptor (mGluR) antagonist (S)-alpha-methyl-4-carboxyphenylglycine did not modify synaptic activity enhancement but did prevent sI(AHP) inhibition and epileptiform discharges. The mGluR-dependent regulation of the sI(AHP) was not caused by modulated intracellular Ca2+ signaling. Histamine, isoproterenol, and (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid reduced the sI(AHP) but did not increase synaptic activity and failed to evoke epileptiform activity. We conclude that 4AP or 4AP+Mg-free-induced enhancement of synaptic activity reduced the sI(AHP) via activation of postsynaptic group I/II mGluRs. The increased excitability caused by the lack of negative feedback provided by the sI(AHP) contributes to epileptiform activity, which requires the cooperative action of increased synaptic activity.