NMDA Receptor Blocker Research Articles

Novel conjugates of aminoadamantanes with carbazole derivatives as potential multitarget agents for AD treatment

A new group of compounds, promising for the design of original multitarget therapeutic agents for treating neurodegenerative diseases, based on conjugates of aminoadamantane and carbazole derivatives was synthesized and investigated. Compounds of these series were found to interact with a group of targets that play an important role in the development of this type of diseases. First of all, these compounds selectively inhibit butyrylcholinesterase, block NMDA receptors containing NR2B subunits while maintaining the properties of MK-801 binding site blockers, exert microtubules stabilizing properties, and possess the ability to protect nerve cells from death at the calcium overload conditions. The leading compound C-2h has been shown the most promising effects on all analyzed parameters. Thus, these compounds can be regarded as promising candidates for the design of multi-target disease-modifying drugs for treatment of AD and/or similar neuropathologies.

Alterations in expression of Cat-315 epitope of perineuronal nets during normal ageing, and its modulation by an open-channel NMDA receptor blocker, memantine.

The perineuronal net (PNN), a specialized aggregate of the extracellular matrix, is involved in neuroprotection against oxidative stress, which is now recognized as a major contributor to age-related decline in brain functions. In this study, we investigated the age-related molecular changes of PNNs using monoclonal antibody Cat-315, which recognizes human natural killer-1 (HNK-1) glycan on aggrecan-based PNNs. Western blot analysis showed that the expression levels of Cat-315 epitope in the hippocampus were higher in middle-aged (MA, 12-month-old) mice than in young adult (YA, 2-month-old) mice. Although there were no differences in the expression levels of Cat-315 epitope between old age (OA, 20-month-old) and MA mice, Cat-315 immunoreactivity was also detected in astrocytes of OA mice. To focus on Cat-315 epitope in PNNs, we used YA and MA mice in the following experiments. Optical disector analysis showed that there were no differences in the numbers of Cat-315-positive (Cat-315+ ) PNNs between YA and MA mice. Fluorescence intensity analysis indicated that Cat-315 immunoreactivity in PNNs increased with age in the dorsal hippocampus, which is mainly involved in cognitive functions. Administration of an open-channel blocker of NMDA receptor, memantine, reduced the expression levels of Cat-315 epitope in the hippocampus. Furthermore, the numbers of glutamatergic and GABAergic terminals colocalized with Cat-315 epitope around parvalbumin-positive neurons were decreased by memantine. These findings provide novel insight into the involvement of PNNs in normal brain ageing, and suggest that memantine may counteract the age-related alterations in expression levels of Cat-315 epitope via regulation of its subcellular localization.

T021 Synaptic plasticity mechanism explains the specificity of direct current stimulation

Abstract Long term effects of transcranial electric stimulation with direct currents (tDCS) have been often attributed to synaptic plasticity. A number of human and animal studies provide support for this hypothesis. However, the exact mechanisms by which direct current stimulation affects synaptic plasticity remain unclear. Here we will present data on long term potentiation (LTP) and long term depression (LTD) of synaptic efficacy – the presumed cellular mechanism of learning. The experiments leverage classic plasticity induction protocols in hippocampal rodent brain slices (high frequency pulses, low frequency pulses, and theta burst stimulation). The main finding is that direct current stimulation in isolation does not affect synaptic efficacy. Only when stimulation is paired with one of these induction protocols do we observe effects on synaptic efficacy. The effects vanish in the presence of NMDA receptor blockers. The polarity of the effect depends on the induction mechanism and the site of plasticity, suggesting that the local polarization of the cellular membrane is the mediating factor. Based on these data we propose that tDCS in human experiments is task specific, not because of the exact placement of electrodes, but because stimulation only affects those synapses that are already undergoing plasticity. We predict that the most specific and effective tDCS interventions will be those that pair stimulation with a concurrent adaptation or learning protocol.

Вплив блокаторів НМДА-рецепторів на динаміку внутрішньоочного тиску у кролів

Experiments on rabbits with a model of acute temporary ocular hypertension created by intragastric administration of water (100 ml/kg) have been performed. It was found that intravenous administration or instillation into the eye of blocker of NMDA-receptor 1-adamantylethyloxy-3-morpholino-2- propanol hydrochloride (Ademol) unlike amantadine sulfate results in a significant decrease in intraocular pressure (IOP). It was also discovered that such ocular hypotensive effect takes place in animals with unchanged ophthalmotonus. Taking into account neuroretinoprotective and hypotensive ocular hypotensive properties of Ademol this drug appears to be perspective in the treatment of ischemic disorders of the retina and optic nerve, especially under the conditions of increased IOP.

Maintained LTP and Memory Are Lost by Zn2+ Influx into Dentate Granule Cells, but Not Ca2+ Influx.

The idea that maintained LTP and memory are lost by either increase in intracellular Zn2+ in dentate granule cells or increase in intracellular Ca2+ was examined to clarify significance of the increases induced by excess synapse excitation. Both maintained LTP and space memory were impaired by injection of high K+ into the dentate gyrus, but rescued by co-injection of CaEDTA, which blocked high K+-induced increase in intracellular Zn2+ but not high K+-induced increase in intracellular Ca2+. High K+-induced disturbances of LTP and intracellular Zn2+ are rescued by co-injection of 6-cyano-7-nitroquinoxakine-2,3-dione, an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonist, but not by co-injection of blockers of NMDA receptors, metabotropic glutamate receptors, and voltage-dependent calcium channels. Furthermore, AMPA impaired maintained LTP and the impairment was also rescued by co-injection of CaEDTA, which blocked increase in intracellular Zn2+, but not increase in intracellular Ca2+. NMDA and glucocorticoid, which induced Zn2+ release from the internal stores, did not impair maintained LTP. The present study indicates that increase in Zn2+ influx into dentate granule cells through AMPA receptors loses maintained LTP and memory. Regulation of Zn2+ influx into dentate granule cells is more critical for not only memory acquisition but also memory retention than that of Ca2+ influx.

Differential Neural Responses Underlying the Inhibition of the Startle Response by Pre-Pulses or Gaps in Mice.

Gap pre-pulse inhibition of the acoustic startle (GPIAS) is a behavioral paradigm used for inferring the presence of tinnitus in animal models as well as humans. In contrast to pre-pulse inhibition (PPI), the neural circuitry controlling GPIAS is poorly understood. To increase our knowledge on GPIAS, a comparative study with PPI was performed in mice combining these behavioral tests and c-Fos activity mapping in brain areas involved in the inhibition of the acoustic startle reflex (ASR). Both pre-pulses and gaps efficiently inhibited the ASR and abolished the induction of c-Fos in the pontine reticular nucleus. Differential c-Fos activation was found between PPI and GPIAS in the forebrain whereby PPI activated the lateral globus pallidus and GPIAS activated the primary auditory cortex. Thus, different neural maps are regulating the inhibition of the startle response by pre-pulses or gaps. To further investigate this differential response to PPI and GPIAS, we pharmacologically disrupted PPI and GPIAS with D-amphetamine or Dizocilpine (MK-801) to target dopamine efflux and to block NMDA receptors, respectively. Both D-amp and MK-801 efficiently decreased PPI and GPIAS. We administered Baclofen, an agonist GABAB receptor, but failed to detect any robust rescue of the effects of D-amp and MK-801 suggesting that PPI and GPIAS are GABAB-independent. These novel findings demonstrate that the inhibition of the ASR by pre-pulses or gaps is orchestrated by different neural pathways.

The Role of GluN2A in Cerebral Ischemia: Promoting Neuron Death and Survival in the Early Stage and Thereafter

Over-activation of NMDA receptors is a crucial step required for brain damage following a stroke. Although clinical trials for NMDA receptor blockers have failed, the role of GluN2A subunit in cerebral ischemia has been extensively evaluated in recent years. However, the effect of GluN2A on neuron damage induced by cerebral ischemia remains a matter of controversy. The underlying reason may be that GluN2A mediates both pro-death and pro-survival effects. These two effects result from two mutually excluding pathways, Ca2+ overload-dependent pro-death signaling and C-terminal-dependent pro-survival signaling, respectively. During the early stage of cerebral ischemia, over-activation of GluN2A plays an important role in Ca2+ overload. Under this condition, pro-death signaling might overcome pro-survival signaling. When GluN2A activity is restored almost to the normal level over time, pro-survival signaling of GluN2A will be dominant. The hypothesis that GluN2A promotes neuron death and survival in the early stage of cerebral ischemia and thereafter will be introduced in detail in this review.

Different functional networks observed in multiple sclerosis during rest and motor task fMRI

Event Abstract Back to Event Computational Neurosciences of Cerebellar Circuit Disorders Shyam Diwakar1 1 Amrita School of Biotechnology, Amrita Vishwa Vidyapeetham (Amrita University), India Spinocerebellar ataxias, Parkinson’s disease, Alzheimer’s dementia and other neurological disorders are associated with dysfunction attributed to various brain regions, including the cerebellum. Neuronal dynamics of the cerebellum correlate with learning and memory processes, via interactions at the synaptic levels forming neuronal microcircuits. Brain regions, including the cerebellum, have been known to include representations of internal models, transferring relevant information via their inputs and outputs [1]. The cerebellum, also known as the little brain, previously known for its role in motor coordination and timing [2], is now being implicated in autism [3], ataxias [4], dyskinesia [5], Alzheimer’s disease [6] and Parkinson’s disease [7]. As part of this study, we will discuss perspectives of cerebellum function as well as its interaction with basal ganglia and thalamo-cortical-thalamic circuitry. Computational models of individual circuits within the cerebellum allow validating and predicting behavioral functions and disease conditions observed during neurological disorders. The study will also look into multiple levels of analysis as this is important for determining physiological function and upstream and downstream roles during disease states and dysfunction. A modeling study of cerebellar function [8] during impaired motor control involved interlinking ion channel mutations to function at cellular and circuit level computations [9]. A study had looked into sodium channel excitability disruptions caused by fibroblast growth factor homologous factor mutations where an ataxia-like condition were observed in adult Wistar rats [10]. Sodium excitability has also been noted and mathematical modeling show spike suppression roles in juvenile prion protein knock-out mice with impaired motor control [9]. Epileptic seizure-like symptoms observed in mutant animals’ granule neurons suggest that sparse and asynchronous neuronal activity evolves into a single hyper-synchronous cluster with elevated spiking rates at seizure initiation [10]. In another study, blocking NMDA receptors in granule neurons showed reduced excitation. A selective blocking of NMDA receptors is seen during NR2A/NR2B mutations. Such simulations implicate a decreased number of spikes as seen via a change in N2A amplitude compared to controls in the generated local field response. A similar selective loss of neural activity in thalamo-cortical circuitry had resulted in glaucoma in human subjects [11]. Computational neuroscience of cerebellar [12] and cortical circuits [13] allowed reconstructing local field potentials[14] and fMRI Blood Oxygen-Level Dependent (BOLD) [15] signals via approximations of intercellular [16] and extracellular spiking activity [17]. We were also able to model local field potentials, cortical EEG and activity-dependent fMRI BOLD signals to correlate neural activity and dysfunction to population behavior. Acknowledgements This work derives direction and ideas from the Chancellor of Amrita University, Sri Mata Amritanandamayi Devi. This work was partially funded by Grants SR/CSI/49/2010, SR/CSI/60/2011, SR/CSRI/60/2013, SR/CSRI/61/2014 and Indo-Italy POC 2012-2013 from DST and BT/PR5142/MED/30/764/2012 from DBT, Government of India and by Embracing The World.

Potentiation of NMDA receptor-mediated synaptic transmission at the parabrachial-central amygdala synapses by CGRP in mice

The capsular part of the central amygdala (CeC) is called the “nociceptive amygdala,” as it receives nociceptive information from various pathways, including monosynaptic input from the lateral part of the parabrachial nucleus (LPB), a major target of ascending neurons in the spinal and medullary dorsal horn. LPB-CeC synaptic transmission is mediated by glutamate but the fibers from the LPB also contain calcitonin gene-related peptide (CGRP) and the CeC is rich in CGRP-binding sites. CGRP might be released in response to strong nociception and activate these CGRP receptors. Though it has been shown that CGRP affects the excitatory postsynaptic current (EPSC) amplitude at this synapse in a manner sensitive to NMDA receptor (NMDA-R) blockers, the effect of CGRP on postsynaptic NMDA-R-mediated current recorded in isolation has never been directly examined. Thus, we evaluated the effects of CGRP on NMDA-R-mediated EPSCs that were pharmacologically isolated in brain slices from naïve mice. CGRP significantly increased the amplitude of EPSCs mediated by NMDA-Rs in a manner dependent on protein kinase A activation, but not that mediated by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, in concentration-dependent and antagonist-sensitive manners. This CGRP-induced potentiation of synaptic NMDA-R function would have a potent impact on the strengthening of the nociception-emotion link in persistent pain.

Additive and antagonistic antinociceptive interactions between magnesium sulfate and ketamine in the rat formalin test.

Because ketamine and magnesium block NMDA receptor activation by distinct mechanisms of action, we hypothesized that in a model of inflammatory pain in rats the combination of ketamine and magnesium might be more effective than ketamine alone. Antinociceptive activity was assessed by the formalin test in male Wistar rats (200-250 g). Animals were injected with 100 μL of 2.5% formalin to the plantar surface of the right hind paw. Data were recorded as the total time spent in pain-related behavior after the injection of formalin or vehicle (0.9% NaCl). Ketamine and magnesium sulfate given separately reduced nocifensive behavior in the second phase of the formalin test in rats. When ketamine was applied after magnesium sulfate, the log dose-response curves for the effects of ketamine and the magnesium sulfate-ketamine combination revealed antagonistic interaction, and about 1.6 (CL 1.2-2.4) fold increment in ketamine dosage. A low dose of magnesium sulfate (5 mg/kg, subcutaneously) administered after ketamine increased the antinociceptive effect of ketamine by a factor of only 1.2 (CL 0.95-1.38), indicating an additive interaction. There was a 1.8-fold reduction in dosage of ketamine when ketamine was administered before rather than after the magnesium sulfate. The present study revealed that both ketamine and magnesium reduced pain-related behavior in the second phase of the formalin test in rats. Ketamine, when administered before or after the magnesium, provided additive or antagonistic antinociceptive interactions, respectively. Whether there will be an additive or antagonistic antinociceptive interaction between ketamine and magnesium depends on the order of drug administration.

Amphetamine and Methamphetamine Increase NMDAR-GluN2B Synaptic Currents in Midbrain Dopamine Neurons.

The psychostimulants amphetamine (AMPH) and methamphetamine (MA) are widely abused illicit drugs. Here we show that both psychostimulants acutely increase NMDA receptor (NMDAR)-mediated synaptic currents and decrease AMPA receptor (AMPAR)/NMDAR ratios in midbrain dopamine neurons. The potentiation depends on the transport of AMPH into the cell by the dopamine transporter. NMDAR-GluN2B receptor inhibitors, ifenprodil, RO 25-6981, and RO 04-5595, inhibit the potentiation without affecting basal-evoked NMDA currents, indicating that NMDAR-GluN2B receptors are activated by AMPH. A selective peptide inhibitor of AMPH-dependent trafficking of the neuronal excitatory amino acid transporter 3 (EAAT3) blocks potentiation, suggesting that EAAT3 internalization increases extracellular glutamate concentrations and activates GluN2B-containing NMDARs. Experiments with the use-dependent NMDAR blocker, MK-801, indicate that potentiated NMDARs reside on the plasma membrane and are not inserted de novo. In behavioral studies, GluN2B inhibitors reduce MA-mediated locomotor activity, without affecting basal activity. These results reveal an important interaction between dopamine and glutamatergic signaling in midbrain dopamine neurons in response to acute administration of psychostimulants.

The NMDA receptor GluN2C subunit controls cortical excitatory-inhibitory balance, neuronal oscillations and cognitive function.

Despite strong evidence for NMDA receptor (NMDAR) hypofunction as an underlying factor for cognitive disorders, the precise roles of various NMDAR subtypes remains unknown. The GluN2C-containing NMDARs exhibit unique biophysical properties and expression pattern, and lower expression of GluN2C subunit has been reported in postmortem brains from schizophrenia patients. We found that loss of GluN2C subunit leads to a shift in cortical excitatory-inhibitory balance towards greater inhibition. Specifically, pyramidal neurons in the medial prefrontal cortex (mPFC) of GluN2C knockout mice have reduced mEPSC frequency and dendritic spine density and a contrasting higher frequency of mIPSCs. In addition a greater number of perisomatic GAD67 puncta was observed suggesting a potential increase in parvalbumin interneuron inputs. At a network level the GluN2C knockout mice were found to have a more robust increase in power of oscillations in response to NMDAR blocker MK-801. Furthermore, GluN2C heterozygous and knockout mice exhibited abnormalities in cognition and sensorimotor gating. Our results demonstrate that loss of GluN2C subunit leads to cortical excitatory-inhibitory imbalance and abnormal neuronal oscillations associated with neurodevelopmental disorders.

An NMDA receptor-dependent mechanism for subcellular segregation of sensory inputs in the tadpole optic tectum.

In the vertebrate CNS, afferent sensory inputs are targeted to specific depths or layers of their target neuropil. This patterning exists ab initio, from the very beginning, and therefore has been considered an activity-independent process. However, here we report that, during circuit development, the subcellular segregation of the visual and mechanosensory inputs to specific regions of tectal neuron dendrites in the tadpole optic tectum requires NMDA receptor activity. Blocking NMDARs during the formation of these sensory circuits, or removing the visual set of inputs, leads to less defined segregation, and suggests a correlation-based mechanism in which correlated inputs wire to common regions of dendrites. This can account for how two sets of inputs form synapses onto different regions of the same dendrite. Blocking NMDA receptors during later stages of circuit development did not disrupt segregation, indicating a critical period for activity-dependent shaping of patterns of innervation.

Amide of Lambertian Acid Suppresses Hyperactivation of Inotropic Glutamate Receptors, but not Synaptic Potentiation in Hippocampal Sections.

Amide of lambertian acid suppresses hyperactivation of inotropic glutamate receptors in hippocampal sections induced by a decrease in the level of magnesium ions (a selective blocker of glutamate NMDA receptors). Treatment of the sections with amide of lambertian acid in standard physiological saline does not prevent development of NMDA-dependent synaptic potentiation. Lambertian acid isolated from needles and turpentine of Siberian pine (Pinus sibirica R. Mayr), and its derivatives may become a source of substances with glutamatergic mechanism of action for treatment of cognitive and neurodegenerative disorders.

Characterization of brevetoxin (PbTx-3) exposure in neurons of the anoxia-tolerant freshwater turtle (Trachemys scripta)

Harmful algal blooms are increasing in frequency and extent worldwide and occur nearly annually off the west coast of Florida where they affect both humans and wildlife. The dinoflagellate Karenia brevis is a key organism in Florida red tides that produces a suite of potent neurotoxins collectively referred to as the brevetoxins (PbTx). Brevetoxins bind to and open voltage gated sodium channels (VGSC), increasing cell permeability in excitable cells and depolarizing nerve and muscle tissue. Exposed animals may thus show muscular and neurological symptoms including head bobbing, muscle twitching, paralysis, and coma; large HABs can result in significant morbidity and mortality of marine life, including fish, birds, marine mammals, and sea turtles. Brevetoxicosis however is difficult to treat in endangered sea turtles as the physiological impacts have not been investigated and the magnitude and duration of brevetoxin exposure are generally unknown. In this study we used the freshwater turtle Trachemys scripta as a model organism to investigate the effects of the specific brevetoxin PbTx-3 in the turtle brain. Primary turtle neuronal cell cultures were exposed to a range of PbTx-3 concentrations to determine excitotoxicity. Agonists and antagonists of voltage-gated sodium channels and downstream targets were utilized to confirm the toxin’s mode of action. We found that turtle neurons are highly resistant to PbTx-3; while cell viability decreased in a dose dependent manner across PbTx-3 concentrations of 100–2000nM, the EC50 was significantly higher than has been reported in mammalian neurons. PbTx-3 exposure resulted in significant Ca2+ influx, which could be fully abrogated by the VGSC antagonist tetrodotoxin, NMDA receptor blocker MK-801, and tetanus toxin, indicating that the mode of action in turtle neurons is the same as in mammalian cells. As both turtle and mammalian VGSCs have a high affinity for PbTx-3, we suggest that the high resistance of the turtle neuron to PbTx-3 may be related to its ability to withstand anoxic depolarization. The ultimate goal of this work is to design treatment protocols for sea turtles exposed to red tides worldwide.

Dysregulated NMDA-Receptor Signaling Inhibits Long-Term Depression in a Mouse Model of Fragile X Syndrome.

Fragile X syndrome (FXS) is a neurodevelopmental disease. It is one of the leading monogenic causes of intellectual disability among boys with most also displaying autism spectrum disorder traits. Here we investigated the role of NMDA receptors on mGluR-dependent long-term depression (mGluR-LTD), a key biomarker in the disease, at four different developmental stages. First, we applied the mGluR agonist 3,5-dihydroxyphenylglycine in the absence or presence of the NMDAR blocker, APV, hereby unmasking the NMDAR component in this process. As expected, in the presence of APV, we found more LTD in the mouse KO than in WT. This, however, was only observed in the p30-60 age group. At all other age groups tested, mGluR-LTD was almost identical between KO and WT. Interestingly, at p60, in the absence of APV, no or very little LTD was found in KO that was completely restored by application of APV. This suggests that the underlying cause of the enhanced mGluR-LTD in KO (at p30) is caused by dysregulated NMDAR signaling. To investigate this further, we next used NMDAR-subunit-specific antagonists. Inhibition of GluN2B, but not GluN2A, blocked mGluR-LTD only in WT. This was in contrast in the KO where blocking GluN2B rescued mGluR-LTD, suggesting GluN2B-containing NMDARs in the KO are hyperactive. Thus, these findings suggest strong involvement of GluN2B-containing-NMDARs in the pathophysiology of FXS and highlight a potential path for treatment for the disease. There is currently no cure for fragile X, although medications targeting specific FXS symptoms do exist. The FXS animal model, the Fmr1 knock-out mouse, has demonstrated an increased mGluR5-mediated long-term depression (LTD) leading to several clinical trials of mGluR5 inhibitors/modulators, yet all have failed. In addition, surprisingly little information exists about the possible role of other ion channels/receptors, including NMDA receptors (NMDAR), in mGluR-LTD. Here we focus on NMDARs and their regulation of mGluR-mediated LTD at different developmental stages using several different NMDAR blockers/antagonists. Our findings suggest dysregulated NMDARs in the pathophysiology of FXS leading to altered mGluR-mediated LTD. Together, these data will help to develop new drug candidates that could lead to reversal of the FXS phenotype.

Galvanic Vestibular Stimulation: Cellular Substrates and Response Patterns of Neurons in the Vestibulo-Ocular Network.

Galvanic vestibular stimulation (GVS) uses modulated currents to evoke neuronal activity in vestibular endorgans in the absence of head motion. GVS is typically used for a characterization of vestibular pathologies; for studies on the vestibular influence of gaze, posture, and locomotion; and for deciphering the sensory-motor transformation underlying these behaviors. At variance with the widespread use of this method, basic aspects such as the activated cellular substrate at the sensory periphery or the comparability to motion-induced neuronal activity patterns are still disputed. Using semi-intact preparations of Xenopus laevis tadpoles, we determined the cellular substrate and the spatiotemporal specificity of GVS-evoked responses and compared sinusoidal GVS-induced activity patterns with motion-induced responses in all neuronal elements along the vestibulo-ocular pathway. As main result, we found that, despite the pharmacological block of glutamatergic hair cell transmission by combined bath-application of NMDA (7-chloro-kynurenic acid) and AMPA (CNQX) receptor blockers, GVS-induced afferent spike activity persisted. However, the amplitude modulation was reduced by ∼30%, suggesting that both hair cells and vestibular afferent fibers are normally recruited by GVS. Systematic alterations of electrode placement with respect to bilateral semicircular canal pairs or alterations of the bipolar stimulus phase timing yielded unique activity patterns in extraocular motor nerves, compatible with a spatially and temporally specific activation of vestibulo-ocular reflexes in distinct planes. Despite the different GVS electrode placement in semi-intact X. laevis preparations and humans and the more global activation of vestibular endorgans by the latter approach, this method is suitable to imitate head/body motion in both circumstances. Galvanic vestibular stimulation is used frequently in clinical practice to test the functionality of the sense of balance. The outcome of the test that relies on the activation of eye movements by electrical stimulation of vestibular organs in the inner ear helps to dissociate vestibular impairments that cause vertigo and imbalance in patients. This study uses an amphibian model to investigate at the cellular level the underlying mechanism on which this method depends. The outcome of this translational research unequivocally revealed the cellular substrate at the vestibular sensory periphery that is activated by electrical currents, as well as the spatiotemporal specificity of the evoked eye movements, thus facilitating the interpretation of clinical test results.

Effects of memantine on the excitation-inhibition balance in prefrontal cortex

Memantine is one of the few drugs currently approved for treatment of Alzheimer's disease (AD). The clinical effects of memantine are thought to be associated with inhibition of NMDA receptors (NMDARs). Surprisingly, other open-channel NMDAR blockers have unacceptable side effects that prevent their consideration for AD treatment. One of the mechanisms proposed to explain the therapeutic benefits of memantine involves preferential decrease of excitatory drive to inhibitory neurons in the cortical circuitry and consequent changes in balance between excitation and inhibition (E/I). In this study we addressed effects of memantine on E/I balance in the prefrontal cortex (PFC). We found that a moderate concentration of memantine shifted E/I balance away from inhibition in the PFC circuitry. Indeed, memantine decreased the frequency and amplitude of spontaneous inhibitory postsynaptic currents in pyramidal neurons while leaving spontaneous excitatory postsynaptic currents unaffected. These circuitry effects of memantine were occluded by the competitive NMDAR inhibitor AP-5, and thus are associated with NMDAR inhibition. We also found that memantine decreased feed-forward disynaptic inhibitory input to pyramidal neurons, which is thought to be mediated by parvalbumin (PV)-positive interneurons. Accordingly, memantine caused a greater decrease of the amplitude of NMDAR-mediated synaptic responses in PV-positive interneurons than in pyramidal neurons. Finally, memantine reduced firing activity in PV-positive interneurons while increasing firing in pyramidal neurons. This study elucidates a novel mechanism of action of memantine associated with shifting of the E/I balance away from inhibition in neocortical circuitry, and provides important insights for AD drug development.

Differential Regulation of Human Paired Associative Stimulation-Induced and Theta-Burst Stimulation-Induced Plasticity by L-type and T-type Ca2+ Channels.

Activity-dependent changes of postsynaptic Ca2+-concentration are influenced by a variety of different Ca2+-channels and play an important role in synaptic plasticity. Paired associative stimulation (PAS) and theta-burst stimulation (TBS) are noninvasive magnetic stimulation protocols used in human subjects to induce lasting corticospinal excitability changes that have been likened to synaptic long-term potentiation and long-term depression. To better characterize the Ca2+-related physiological mechanisms underlying PAS- and TBS-induced plasticity, we examined the impact of different Ca2+-sources. PAS-induced facilitation of corticospinal excitability was blocked by NMDA-receptor blocker dextromethorphan (DXM) and L-type voltage gated Ca2+ channels (VGCC) blocker nimodipine (NDP), but turned into depression by T-type VGCC blocker ethosuximide (ESM). Although, surprisingly, static corticospinal excitability was increased by the combination of DXM and NDP, PAS-induced facilitation was blocked. TBS-induced facilitation of corticospinal excitability, which has previously been shown to be turned into depression by L-type VGCC blocker NDP (Wankerl K, Weise D, Gentner R, Rumpf J, Classen J. 2010. L-type voltage-gated Ca2+ channels: a single molecular switch for long-term potentiation/long-term depression-like plasticity and activity-dependent metaplasticity in humans. J Neurosci. 30(18):6197-6204.), was blocked, but not reverted, by T-type VGCC blocker ESM. The different patterns of Ca2+-channel modulation of PAS- and TBS-induced plasticity may point to an important role of backpropagating action potentials in PAS-induced plasticity, similar as in spike-timing dependent synaptic plasticity, and to a requirement of dendritic Ca2+-dependent spikes in TBS-induced plasticity.

Activation of sodium channels by α-scorpion toxin, BmK NT1, produced neurotoxicity in cerebellar granule cells: an association with intracellular Ca2+ overloading.

Voltage-gated sodium channels (VGSCs) are responsible for the action potential generation in excitable cells including neurons and involved in many physiological and pathological processes. Scorpion toxins are invaluable tools to explore the structure and function of ion channels. BmK NT1, a scorpion toxin from Buthus martensii Karsch, stimulates sodium influx in cerebellar granule cells (CGCs). In this study, we characterized the mode of action of BmK NT1 on the VGSCs and explored the cellular response in CGC cultures. BmK NT1 delayed the fast inactivation of VGSCs, increased the Na+ currents, and shifted the steady-state activation and inactivation to more hyperpolarized membrane potential, which was similar to the mode of action of α-scorpion toxins. BmK NT1 stimulated neuron death (EC50=0.68µM) and produced massive intracellular Ca2+ overloading (EC50=0.98µM). TTX abrogated these responses, suggesting that both responses were subsequent to the activation of VGSCs. The Ca2+ response of BmK NT1 was primary through extracellular Ca2+ influx since reducing the extracellular Ca2+ concentration suppressed the Ca2+ response. Further pharmacological evaluation demonstrated that BmK NT1-induced Ca2+ influx and neurotoxicity were partially blocked either by MK-801, an NMDA receptor blocker, or by KB-R7943, an inhibitor of Na+/Ca2+ exchangers. Nifedipine, an L-type Ca2+ channel inhibitor, slightly suppressed both Ca2+ response and neurotoxicity. A combination of these three inhibitors abrogated both responses. Considered together, these data ambiguously demonstrated that activation of VGSCs by an α-scorpion toxin was sufficient to produce neurotoxicity which was associated with intracellular Ca2+ overloading through both NMDA receptor- and Na+/Ca2+ exchanger-mediated Ca2+ influx.