Differential responses to NMDA receptor activation in rat hippocampal interneurons and pyramidal cells may underlie enhanced pyramidal cell vulnerability

Multiscale neuro-inspired models for interpretation of EEG signals in patients with epilepsy

OBJECTIVE

The aim is to gain insight into the pathophysiological mechanisms underlying interictal epileptiform discharges observed in electroencephalographic (EEG) and stereo-EEG (SEEG, depth electrodes) recordings performed during pre-surgical evaluation of patients with drug-resistant epilepsy.

METHODS

We developed novel neuro-inspired computational models of the human cerebral cortex at three different levels of description: i) microscale (detailed neuron models), ii) mesoscale (neuronal mass models) and iii) macroscale (whole brain models). Although conceptually different, micro- and mesoscale models share some similar features, such as the typology of neurons (pyramidal cells and three types of interneurons), their spatial arrangement in cortical layers, and their synaptic connectivity (excitatory and inhibitory). The whole brain model consists of a large-scale network of interconnected neuronal masses, with connectivity based on the human connectome.

RESULTS

For these three levels of description, the fine-tuning of free parameters and the quantitative comparison with real data allowed us to reproduce interictal epileptiform discharges with a high degree of fidelity and to formulate hypotheses about the cell- and network-related mechanisms underlying the generation of fast ripples and SEEG-recorded epileptic spikes and spike-waves.

CONCLUSIONS

The proposed models provide valuable insights into the pathophysiological mechanisms underlying the generation of epileptic events. The knowledge gained from these models effectively complements the clinical analysis of SEEG data collected during the evaluation of patients with epilepsy.

SIGNIFICANCE

These models are likely to play a key role in the mechanistic interpretation of epileptiform activity.

Hippocampal metabolic recovery as a manifestation of the protective effect of ischemic preconditioning in rats

The ever-present risk of brain ischemic events in humans and its full prevention make the detailed studies of an organism's response to ischemia at different levels essential to understanding the mechanism of the injury as well as protection. We used the four-vessel occlusion as an animal model of forebrain ischemia to investigate its impact on the metabolic alterations in both the hippocampus and the blood plasma to see changes on the systemic level. By inducing sublethal ischemic stimuli, we focused on the endogenous phenomena known as ischemic tolerance. NMR spectroscopy was used to analyze relative metabolite levels in tissue extracts from rats' hippocampus and blood plasma in three various ischemic/reperfusion times: 3 h, 24 h, and 72 h. Hippocampal tissues were characterized by postischemically decreased glutamate and GABA (4-aminobutyrate) tissue content balanced with increased glutamine level, with most pronounced changes at 3 h reperfusion time. Glutamate (as well as glutamine) levels recovered towards the control levels on the third day, as if the glutamate re-synthesis would be firstly preferred before GABA. These results are indicating the higher feasibility of re-establishing of glutamatergic transmission three days after an ischemic event, in contrast to GABA-ergic. Tissue levels of N-acetylaspartate (NAA), as well as choline, were decreased without the tendency to recover three days after the ischemic event. Metabolomic analysis of blood plasma revealed that ischemically preconditioned rats, contrary to the non-preconditioned animals, did not show hyperglycemic conditions. Ischemically induced semi-ketotic state, manifested in increased plasma ketone bodies 3-hydroxybutyrate and acetoacetate, seems to be programmed to support the brain tissue revitalization after the ischemic event. These and other metabolites changes found in blood plasma as well as in the hippocampus were observed to a lower extent or recovered faster in preconditioned animals. Some metabolomic changes in hippocampal tissue extract were so strong that even single metabolites were able to differentiate between ischemic, ischemically preconditioned, and control brain tissues.

HIV-1 Tat causes cognitive deficits and selective loss of parvalbumin, somatostatin, and neuronal nitric oxide synthase expressing hippocampal CA1 interneuron subpopulations

Memory deficits are characteristic of HIV-associated neurocognitive disorders (HAND) and co-occur with hippocampal pathology. The HIV-1 transactivator of transcription (Tat), a regulatory protein, plays a significant role in these events, but the cellular mechanisms involved are poorly understood. Within the hippocampus, diverse populations of interneurons form complex networks; even subtle disruptions can drastically alter synaptic output, resulting in behavioral dysfunction. We hypothesized that HIV-1 Tat would impair cognitive behavior and injure specific hippocampal interneuron subtypes. Male transgenic mice that inducibly expressed HIV-1 Tat (or non-expressing controls) were assessed for cognitive behavior or had hippocampal CA1 subregions evaluated via interneuron subpopulation markers. Tat exposure decreased spatial memory in a Barnes maze and mnemonic performance in a novel object recognition test. Tat reduced the percentage of neurons expressing neuronal nitric oxide synthase (nNOS) without neuropeptide Y immunoreactivity in the stratum pyramidale and the stratum radiatum, parvalbumin in the stratum pyramidale, and somatostatin in the stratum oriens, which are consistent with reductions in interneuron-specific interneuron type 3 (IS3), bistratified, and oriens-lacunosum-moleculare interneurons, respectively. The findings reveal that an interconnected ensemble of CA1 nNOS-expressing interneurons, the IS3 cells, as well as subpopulations of parvalbumin- and somatostatin-expressing interneurons are preferentially vulnerable to HIV-1 Tat. Importantly, the susceptible interneurons form a microcircuit thought to be involved in feedback inhibition of CA1 pyramidal cells and gating of CA1 pyramidal cell inputs. The identification of vulnerable CA1 hippocampal interneurons may provide novel insight into the basic mechanisms underlying key functional and neurobehavioral deficits associated with HAND.

Rat intra-hippocampal NMDA infusion induces cell-specific damage and changes in expression of NMDA and GABAA receptor subunits

Excessive stimulation of NMDA receptors with glutamate or other potent agonists such as NMDA leads to excitotoxicity and neural injury. In this study, we aimed to provide insight into an animal model of brain excitotoxic damage; single unilateral infusion of NMDA at mild dose into the hippocampal formation. NMDA infusion induced chronic, focal neurodegeneration in the proximity of the injection site. The lesion was accompanied by severe and progressive neuroinflammation and affected preferentially principal neurons while sparing GABAergic interneurons. Furthermore, the unilateral lesion did not cause significant impairment of spatial learning abilities. Finally, GluN1 and GluN2B subunits of NMDA receptor were significantly upregulated up to 3 days after the NMDA infusion, while GABAA α5 subunit was downregulated at 30 days after the lesion. Taken together, a single infusion of NMDA into the hippocampal formation represents an animal model of excitotoxicity-induced chronic neurodegeneration of principal neurons accompanied by severe neuroinflammation and subunit specific changes in NMDA and GABAA receptors.

Selective neuronal vulnerability of human hippocampal CA1 neurons: lesion evolution, temporal course, and pattern of hippocampal damage in diffusion-weighted MR imaging

The CA1 (cornu ammonis) region of hippocampus is selectively vulnerable to a variety of metabolic and cytotoxic insults, which is mirrored in a delayed neuronal death of CA1 neurons. The basis and mechanisms of this regional susceptibility of CA1 neurons are poorly understood, and the correlates in human diseases affecting the hippocampus are not clear. Adopting a translational approach, the lesion evolution, temporal course, pattern of diffusion changes, and damage in hippocampal CA1 in acute neurologic disorders were studied using high-resolution magnetic resonance imaging. In patients with hippocampal ischemia (n=50), limbic encephalitis (n=30), after status epilepticus (n=17), and transient global amnesia (n=53), the CA1 region was selectively affected compared with other CA regions of the hippocampus. CA1 neurons exhibited a maximum decrease of apparent diffusion coefficient (ADC) 48 to 72 hours after the insult, irrespective of the nature of the insult. Hypoxic-ischemic insults led to a significant lower ADC suggesting that the ischemic insult results in a stronger impairment of cellular metabolism. The evolution of diffusion changes show that CA1 diffusion lesions mirror the delayed time course of the pathophysiologic cascade typically observed in animal models. Studying the imaging correlates of hippocampal damage in humans provides valuable insight into the pathophysiology and neurobiology of the hippocampus.

The hippocampus in aging and disease: From plasticity to vulnerability

The hippocampus has a pivotal role in learning and in the formation and consolidation of memory and is critically involved in the regulation of emotion, fear, anxiety, and stress. Studies of the hippocampus have been central to the study of memory in humans and in recent years, the regional specialization and organization of hippocampal functions have been elucidated in experimental models and in human neurological and psychiatric diseases. The hippocampus has long been considered a classic model for the study of neuroplasticity as many examples of synaptic plasticity such as long-term potentiation and -depression have been identified and demonstrated in hippocampal circuits. Neuroplasticity is the ability to adapt and reorganize the structure or function to internal or external stimuli and occurs at the cellular, population, network or behavioral level and is reflected in the cytological and network architecture as well as in intrinsic properties of hippocampal neurons and circuits. The high degree of hippocampal neuroplasticity might, however, be also negatively reflected in the pronounced vulnerability of the hippocampus to deleterious conditions such as ischemia, epilepsy, chronic stress, neurodegeneration and aging targeting hippocampal structure and function and leading to cognitive deficits. Considering this framework of plasticity and vulnerability, we here review basic principles of hippocampal anatomy and neuroplasticity on various levels as well as recent findings regarding the functional organization of the hippocampus in light of the regional vulnerability in Alzheimer's disease, ischemia, epilepsy, neuroinflammation and aging.

Separation of electrophysiologically distinct neuronal populations in the rat hippocampus for neuropharmacological testing under in vivo conditions

Microiontophoresis combined with extracellular spike recording is an excellent method for investigating local neuropharmacological effects under in vivo conditions. However, its application has recently become relatively rare in neuroscience research. Now, we aimed to revisit microiontophoresis and demonstrate that it provides valuable data about the pharmacophysiology of neurons and local neuronal networks, in vivo. Extracellular recordings were performed through the central recording channel of multibarrel carbon-fiber microelectrodes in the CA1 pyramidal layer of the hippocampus of anesthetized rats, while N-methyl-D-aspartate (NMDA) was locally administrated by means of microiontophoresis through the surrounding micropipettes of the microelectrode. Various separation procedures were used to distinguish putative pyramidal cells and interneurons. Quality of separation was verified by electrophysiological parameters. After the delivery of NMDA in the vicinity of the examined neurons, firing rate of putative pyramidal cells was increased with a significantly higher grade then that of putative interneurons. The present results in line with previous data indicate that pyramidal cells are more responsive to pharmacological manipulation through NMDA receptors, also confirming the reliability of the separation of different types of neurons in in vivo microiontophoretic experiments.

A positive allosteric modulator of α7 nAChRs augments neuroprotective effects of endogenous nicotinic agonists in cerebral ischaemia

BACKGROUND AND PURPOSE

Activation of α7 nicotinic acetylcholine receptors (nAChRs) can be neuroprotective. However, endogenous choline and ACh have not been regarded as potent neuroprotective agents because physiological levels of choline/ACh do not produce neuroprotective levels of α7 activation. This limitation may be overcome by the use of type-II positive allosteric modulators (PAMs-II) of α7 nAChRs, such as 1-(5-chloro-2,4-dimethoxyphenyl)-3-(5-methylisoxazol-3-yl)-urea (PNU-120596). This proof-of-concept study presents a novel neuroprotective paradigm that converts endogenous choline/ACh into potent neuroprotective agents in cerebral ischaemia by inhibiting α7 nAChR desensitization using PNU-120596.

EXPERIMENTAL APPROACH

An electrophysiological ex vivo cell injury assay (to quantify the susceptibility of hippocampal neurons to acute injury by complete oxygen and glucose deprivation; COGD) and an in vivo middle cerebral artery occlusion model of ischaemia were used in rats.

KEY RESULTS

Choline (20-200 μM) in the presence, but not absence of 1 μM PNU-120596 significantly delayed anoxic depolarization/injury of hippocampal CA1 pyramidal neurons, but not CA1 stratum radiatum interneurons, subjected to COGD in acute hippocampal slices and these effects were blocked by 20 nM methyllycaconitine, a selective α7 antagonist, thus, activation of α7 nAChRs was required. PNU-120596 alone was ineffective ex vivo. In in vivo experiments, both pre- and post-ischaemia treatments with PNU-120596 (30 mg·kg(-1) , s.c. and 1 mg·kg(-1) , i.v., respectively) significantly reduced the cortical/subcortical infarct volume caused by transient focal cerebral ischaemia. PNU-120596 (1 mg·kg(-1) , i.v., 30 min post-ischaemia) remained neuroprotective in rats subjected to a choline-deficient diet for 14 days prior to experiments.

CONCLUSIONS AND IMPLICATIONS

PNU-120596 and possibly other PAMs-II significantly improved neuronal survival in cerebral ischaemia by augmenting neuroprotective effects of endogenous choline/ACh.

Blockers of adenosine A1, but not muscarinic acetylcholine, receptors improve excessive extracellular glutamate-induced synaptic depression

We investigated adenosinergic and cholinergic effects on excessive glutamate-induced depressions of central excitatory synaptic transmissions in vitro. From the CA1 region in rat hippocampal slices, orthodromically elicited population spikes (PSs) and field excitatory postsynaptic potentials (fEPSPs) at 0.1Hz were simultaneously recorded. ANOVA was used for statistics, and p<0.05 was accepted as significant. Glutamate (10mM for 10min) completely depressed PSs and fEPSPs, which were partially recovered by the following washout for 40min (67.5±15.7% and 65.4±13.9% of the control, respectively, p<0.01, n=12). The recoveries in PSs and fEPSPs were exacerbated by edrophonium and carbamoylcholine but improved by non- and A1-selective adenosine receptor antagonists (p<0.01, n=6). The recovery in PSs, not that in fEPSPs, was exacerbated by adenosine, adenosine A1-receptor agonist and A2a-receptor antagonist (p<0.01, n=6). The effects of edrophonium were blocked by non-, M2- and M4-selective muscarinic acetylcholine receptor antagonists (p<0.01, n=6). Excessive glutamate depresses glutamatergic excitatory synaptic transmissions, which are exacerbated by muscarinic acetylcholine receptor stimulation but improved by adenosine A1 receptor block. Somatic excitability is impaired by excessive glutamate with adenosine A1 receptor stimulation.

Mild hyperthermia worsens the neuropathological damage associated with mild traumatic brain injury in rats

The effects of slight variations in brain temperature on the pathophysiological consequences of acute brain injury have been extensively described in models of moderate and severe traumatic brain injury (TBI). In contrast, limited information is available regarding the potential consequences of temperature elevations on outcome following mild TBI (mTBI) or concussions. One potential confounding variable with mTBI is the presence of elevated body temperature that occurs in the civilian or military populations due to hot environments combined with exercise or other forms of physical exertion. We therefore determined the histopathological effects of pre- and post-traumatic hyperthermia (39°C) on mTBI. Adult male Sprague-Dawley rats were divided into 3 groups: pre/post-traumatic hyperthermia, post-traumatic hyperthermia alone for 2 h, and normothermia (37°C). The pre/post-hyperthermia group was treated with hyperthermia starting 15 min before mild parasagittal fluid-percussion brain injury (1.4-1.6 atm), with the temperature elevation extending for 2 h after trauma. At 72 h after mTBI, the rats were perfusion-fixed for quantitative histopathological evaluation. Contusion areas and volumes were significantly larger in the pre/post-hyperthermia treatment group compared to the post-hyperthermia and normothermic groups. In addition, pre/post-traumatic hyperthermia caused the most severe loss of NeuN-positive cells in the dentate hilus compared to normothermia. These neuropathological results demonstrate that relatively mild elevations in temperature associated with peri-traumatic events may affect the long-term functional consequences of mTBI. Because individuals exhibiting mildly elevated core temperatures may be predisposed to aggravated brain damage after mTBI or concussion, precautions should be introduced to target this important physiological variable.

Tonic NMDA receptor-mediated current in prefrontal cortical pyramidal cells and fast-spiking interneurons

Tonically activated neuronal currents mediated by N-methyl-d-aspartate receptors (NMDARs) have been hypothesized to contribute to normal neuronal function as well as to neuronal pathology resulting from excessive activation of glutamate receptors (e.g., excitotoxicity). Whereas cortical excitatory cells are very vulnerable to excitotoxic insult, the data regarding resistance of inhibitory cells (or interneurons) are inconsistent. Types of neurons with more pronounced tonic NMDAR current potentially associated with the activation of extrasynaptic NMDARs could be expected to be more vulnerable to excessive activation by glutamate. In this study, we compared tonic activation of NMDARs in excitatory pyramidal cells and inhibitory fast-spiking interneurons in prefrontal cortical slices. We assessed tonic NMDAR current by measuring holding current shift as well as noise reduction following NMDAR blockade after removal of spontaneous glutamate release. In addition, we compared NMDAR miniature excitatory postsynaptic currents (EPSCs) in both cell types. We have demonstrated for the first time that tonic NMDAR currents are present in inhibitory fast-spiking interneurons. We found that the magnitude of tonic NMDAR current is similar in pyramidal cells and fast-spiking interneurons, and that quantal release of glutamate does not significantly impact tonic NMDAR current.

Energy deprivation transiently enhances rhythmic inhibitory events in the CA3 hippocampal network in vitro

Oxygen glucose deprivation (OGD) leads to rapid suppression of synaptic transmission. Here we describe an emergence of rhythmic activity at 8 to 20 Hz in the CA3 subfield of hippocampal slice cultures occurring for a few minutes prior to the OGD-induced cessation of evoked responses. These oscillations, dominated by inhibitory events, represent network activity, as they were abolished by tetrodotoxin. They were also completely blocked by the GABAergic antagonist picrotoxin, and strongly reduced by the glutamatergic antagonist NBQX. Applying CPP to block NMDA receptors had no effect and neither did UBP302, an antagonist of GluK1-containing kainate receptors. The gap junction blocker mefloquine disrupted rhythmicity. Simultaneous whole-cell voltage-clamp recordings from neighboring or distant CA3 pyramidal cells revealed strong cross-correlation of the incoming rhythmic activity. Interneurons in the CA3 area received similar correlated activity. Interestingly, oscillations were much less frequently observed in the CA1 area. These data, together with the observation that the recorded activity consists primarily of inhibitory events, suggest that CA3 interneurons are important for generating these oscillations. This transient increase in inhibitory network activity during OGD may represent a mechanism contributing to the lower vulnerability to ischemic insults of the CA3 area as compared to the CA1 area.

Brain plasticity after ischemic episode

Brain plasticity describes the potential of the organ for adaptive changes involved in various phenomena in health and disease. A substantial amount of experimental evidence, received in animal and cell models, shows that a cascade of plastic changes at the molecular, cellular, and tissue levels, is initiated in different regions of the postischemic brain. Underlying mechanisms include neurochemical alterations, functional changes in excitatory and inhibitory synapses, axonal and dendritic sprouting, and reorganization of sensory and motor central maps. Multiple lines of evidence indicate numerous points in which the process of postischemic recovery may be influenced with the aim to restore the full capacity of the brain tissue injured by an ischemic episode.

The role of lysosomal rupture in neuronal death

Apoptosis research in the past two decades has provided an enormous insight into its role in regulating cell death. However, apoptosis is only part of the story, and inhibition of neuronal necrosis may have greater impact than apoptosis, on the treatment of stroke, traumatic brain injury, and neurodegenerative diseases. Since the "calpain-cathepsin hypothesis" was first formulated, the calpain- and cathepsin-mediated regulation of necrotic cascades observed in monkeys, has been demonstrated to be a common neuronal death mechanism occurring from simpler organisms to humans. However, the detailed mechanism inducing lysosomal destabilization still remains poorly understood. Heat-shock protein-70 (Hsp70) is known to stabilize lysosomal membrane and protect cells from oxidative stress and apoptotic stimuli in many cell death pathways. Recent proteomics approach comparing pre- and post-ischemic hippocampal CA1 neurons as well as normal and glaucoma-suffered retina of primates, suggested that the substrate protein upon which activated calpain acts at the lysosomal membrane of neurons might be Hsp70. Understanding the interaction between activated calpains and Hsp70 will help to unravel the mechanism that destabilizes the lysosomal membrane, and will provide new insights into clarifying the whole cascade of neuronal necrosis. Although available evidence is circumferential, it is hypothesized that activated calpain cleaves oxidative stress-induced carbonylated Hsp70.1 (a major human Hsp70) at the lysosomal membrane, which result in lysosomal rupture/permeabilization. This review aims at highlighting the possible mechanism of lysosomal rupture in neuronal death by a modified "calpain-cathepsin hypothesis". As the autophagy-lysosomal degradation pathway is a target of oxidative stress, the implication of autophagy is also discussed.

Intracellular Zn2+ increases contribute to the progression of excitotoxic Ca2+ increases in apical dendrites of CA1 pyramidal neurons

Sustained intracellular Ca(2+) elevation is a well-established contributor to neuronal injury following excessive activation of N-methyl-d-aspartic acid (NMDA)-type glutamate receptors. Zn(2+) can also be involved in excitotoxic degeneration, but the relative contributions of these two cations to the initiation and progression of excitotoxic injury is not yet known. We previously concluded that extended NMDA exposure led to sustained Ca(2+) increases that originated in apical dendrites of CA1 neurons and then propagated slowly throughout neurons and caused rapid necrotic injury. However the fluorescent indicator used in those studies (Fura-6F) may also respond to Zn(2+), and in the present work we examine possible contributions of Zn(2+) to indicator signals and to the progression of degenerative signaling along murine CA1 dendrites. Selective chelation of Zn(2+) with N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) significantly delayed, but did not prevent the development and progression of sustained high-level Fura-6F signals from dendrites to somata. Rapid indicator loss during the Ca(2+) overload response, which corresponds to rapid neuronal injury, was also not prevented by TPEN. The relationship between cytosolic Zn(2+) and Ca(2+) levels was assessed in single CA1 neurons co-loaded with Fura-6F and the Zn(2+)-selective indicator FluoZin-3. NMDA exposure resulted in significant initial increases in FluoZin-3 increases that were prevented by TPEN, but not by extracellular Zn(2+) chelation with Ca-EDTA. Consistent with this result, Ca-EDTA did not delay the progression of Fura-6F signals during NMDA. Removal of extracellular Ca(2+) reduced, but did not prevent FluoZin-3 increases. These results suggest that sustained Ca(2+) increases indeed underlie Fura-6F signals that slowly propagate throughout neurons, and that Ca(2+) (rather than Zn(2+)) increases are ultimately responsible for neuronal injury during NMDA. However, mobilization of Zn(2+) from endogenous sources leads to significant neuronal Zn(2+) increases, that in turn contribute to mechanisms of initiation and progression of progressive Ca(2+) deregulation.

Localized loss of Ca2+ homeostasis in neuronal dendrites is a downstream consequence of metabolic compromise during extended NMDA exposures

Excessive Ca(2+) loading is central to most hypotheses of excitotoxic neuronal damage. We examined dendritic Ca(2+) signals in single CA1 neurons, injected with fluorescent indicators, after extended exposures to a low concentration of NMDA (5 microM). As shown previously, NMDA produces an initial transient Ca(2+) elevation of several micromolar, followed by recovery to submicromolar levels. Then after a delay of approximately 20-40 min, a large Ca(2+) elevation appears in apical dendrites and propagates to the soma. We show here that this large delayed Ca(2+) increase is required for ultimate loss of membrane integrity. However, transient removal of extracellular Ca(2+) for varying epochs before and after NMDA exposure does not delay the propagation of these events. In contrast to compound Ca(2+) elevations, intracellular Na(+) elevations are monophasic and were promptly reversed by the NMDA receptor antagonist MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate]. MK-801 applied after the transient Ca(2+) elevations blocked the delayed propagating Ca(2+) increase. Even if applied after the propagating response was visualized, MK-801 restored resting Ca(2+) levels. Propagating Ca(2+) increases in dendrites were delayed or prevented by (1) reducing extracellular Na(+), (2) injecting ATP together with the Ca(2+) indicator, or (3) provision of exogenous pyruvate. These results show that extended NMDA exposure initiates degenerative signaling generally in apical dendrites. Although very high Ca(2+) levels can report the progression of these responses, Ca(2+) itself may not be required for the propagation of degenerative signaling along dendrites. In contrast, metabolic consequences of sustained Na(+) elevations may lead to failure of ionic homeostasis in dendrites and precede Ca(2+)-dependent cellular compromise.

Potentiation of N-methyl-D-aspartate receptor-mediated neuronal injury during methamphetamine withdrawal in vitro requires co-activation of IP3 receptors

Recent findings suggest that methamphetamine (METH) functions acutely to inhibit N-methyl-d-aspartate (NMDA) receptor function. Protracted withdrawal from METH exposure may increase the sensitivity of NMDA receptors to agonist exposure, promoting neuronal excitability. However, the relevance of METH effects on NMDA receptor activity with regard to neuronal viability has not been fully studied. The present studies examined the effects of protracted METH exposure (6 or 7 days; 1.0-100 microM) and withdrawal (1 or 7 days) on NMDA receptor-dependent neurotoxicity, determined with use of the non-vital fluorescent marker propidium iodide, in organotypic slice cultures of male and female rats. Prolonged exposure to METH (100 microM) produced only modest toxicity in the granule cell layer of the dentate gyrus. Withdrawal from METH exposure (1 or 7 days) did not produce overt neuronal injury in any region of slice cultures. Exposure to NMDA (5 microM) produced marked neurotoxicity in the CA1 pyramidal cell layer. Neither co-exposure to METH nor 1 day of METH withdrawal in combination with NMDA exposure altered NMDA-induced neurotoxicity. In contrast, protracted withdrawal from METH exposure (7 days) was associated with a marked (approximately 400%) increase in NMDA-induced neurotoxicity in CA1 region pyramidal cells. This potentiation of neurotoxicity was prevented by co-exposure to the selective NMDA receptor antagonist 5-2-amino-5-phosphonovaleric acid (20 microM) and was markedly attenuated by co-exposure of slices to xestospongin C (1 microM), an antagonist of IP(3) receptors. The results of the present studies suggest that long-term METH withdrawal functionally sensitizes the NMDA receptor to agonist exposure and requires the co-activation of NMDA and IP(3) receptors.

Impaired firing and sodium channel function in CA1 hippocampal interneurons after transient cerebral ischemia

Although interneurons in area CA1 of the hippocampus are less vulnerable to cerebral ischemia than CA1 pyramidal cells, it is not clear whether their relatively intact cellular morphology implies preservation of normal function. As maintenance of cellular excitability and firing properties is essential for interneurons to regulate neural networks, we investigated these aspects of interneuronal function after transient cerebral ischemia in rats. Cerebral ischemia in rats was induced for 8 mins by a combination of bilateral common carotid artery occlusion and hypovolemic hypotension, and whole cell patch clamp recordings were made in hippocampal slices prepared 24 h after reperfusion. Interneurons located within stratum pyramidale of area CA1 exhibited normal membrane properties and action potentials under these conditions. However, their excitability had declined, as evidenced by an increased action potential threshold and a rightward shift in the relationship between injected depolarizing current and firing rate. Voltage-clamp experiments revealed that transient cerebral ischemia reduced the peak Na(+) current and shifted Na(+) channel activation to more depolarized values, but did not alter steady-state inactivation of the channel. Double immunofluorescence cytochemistry showed that transient cerebral ischemia also reduced Na(v)1.1 subunit immunoreactivity in interneurons that coexpressed parvalbumin. We conclude that transient cerebral ischemia renders CA1 interneurons less excitable, that depressed excitability involves impaired Na(+) channel activation and that Na(+) channel dysfunction is explained, at least in part, by reduced expression of the Na(v)1.1 subunit. These changes may promote interneuron survival, but might also contribute to pyramidal cell death.

Methamphetamine exposure antagonizes N-methyl-D-aspartate receptor-mediated neurotoxicity in organotypic hippocampal slice cultures

Glutamatergic systems have been increasingly recognized as mediators of methamphetamine's (METH) pharmacological effects though little is known about the means by which METH interacts with glutamate receptors. The present studies examined effects of METH (0.1-100 microM) on [3H]MK-801 binding to membranes prepared from adult rat cortex, hippocampus and cerebellum, as well as the neurotoxicity produced by 24-h exposure to N-methyl-D-aspartate (5-10 microM; NMDA) employing organotypic hippocampal slice cultures of neonatal rat. Co-incubation of [3H]MK-801 with METH (0.1-100 microM) did not reduce dextromethorphan (1 mM)-displaceable ligand binding. Exposure of slice cultures to NMDA for 24-h produced increases in uptake of the non-vital fluorescent marker propidium iodide (PI) of 150-500% above control levels, most notably, in the CA1 region pyramidal cell layer. Co-exposure to METH (>1.0 microM) with NMDA (5 microM) reduced PI uptake by approximately 50% in each subregion, though the CA1 pyramidal cell layer was markedly more sensitive to the protective effects of METH exposure. In contrast, METH exposure did not reduce PI uptake stimulated by 24-h exposure to 10 microM NMDA. Co-exposure to the NMDA receptor antagonist D-2-amino-5-phosphonovaleric acid (20 microM) prevented toxicity produced by exposure to 5 or 10 microM NMDA. These findings indicate that the pharmacological effects of short-term METH exposure involve inhibition of NMDA receptor-mediated neuronal signaling, not reflective of direct channel inhibition at an MK-801-sensitive site.

Protective effect of ginsenoside Rg1 on glutamate-induced lung injury

AIM

To examine the possible protective effect of ginsenoside Rg1, an active component of ginseng, on lung injury caused by glutamate in vivo.

METHODS

The lungs of mice receiving glutamate (0.5 g/kg) and/or ginsenoside Rg1 (0.03 g/kg) via intraperitoneal administration were collected. The indexes of lung wet weight/ body weight ratios (LW/BW), lung wet/dry weight ratios (W/D), heart rate (HR), and breathing rate (BR) were determined. The activity of nitric oxide synthase (NOS), xanthine oxidase (XOD), superoxide dismutase (SOD), catalase (CAT), the content of NO, and malondialdehyde in the lung homogenate were measured.

RESULTS

Treatment with glutamate for 2 h increased LW/BW, W/D, HR, and BR. These changes were nearly abolished by pretreatment with ginsenoside Rg1 for 30 min before glutamate injection. An analysis of the lung homogenate demonstrated the protective effect as evidenced by the inhibition of NOS (12%) and XOD (50%) inactivity, the enhanced activity of SOD (20%) and CAT (25%).

CONCLUSION

Ginsenoside Rg1 has a potential protective role in lung diseases associated with glutamate toxicity.

Calcium-dependent NMDA-induced dendritic injury and MAP2 loss in acute hippocampal slices

Excessive glutamate receptor stimulation can produce rapid disruption of dendritic morphology, including dendritic beading. We recently showed that transient N-methyl-d-aspartic acid (NMDA) exposure resulted in irreversible loss of synaptic function and loss of microtubule associated protein 2 (MAP2) from apical dendrites. The present study examined the initiation and progression of dendritic injury in mouse hippocampal slices following this excitotoxic stimulus. NMDA exposure (30 microM, 10 min) produced irregularly shaped dendritic swellings, evident first in distal apical dendrite branches, and later (20-90 min) involving most proximal dendrites. Over the same time course, immunoreactivity for the microtubule-associated protein MAP2 was progressively lost from apical dendrites, and increased in CA1 somata. This damage and MAP2 loss was Ca2+-dependent, and was not reversible within the time course of these experiments (90 min post-NMDA washout). Formation of regularly-spaced, spherical dendritic varicosities (dendritic beading) was rarely observed, except when NMDA was applied in Ca2+-free ACSF. Under these conditions, beading appeared predominant in interneurons, as assessed from experiments with GAD67-GFP (Deltaneo) mice. Ca2+-removal was associated with significantly better preservation of dendritic structure (MAP2) following NMDA exposure, and other ionic fluxes (sensitive to Gd3+ and spermine) may contribute to residual damage occurring in Ca2+-free conditions. These results suggest that irregularly shaped dendritic swelling is a Ca2+-dependent degenerative event that may be quite different from Ca2+-independent dendritic beading, and can be a predominant type of injury in CA1 pyramidal neurons in slices.

Presynaptic kainate receptor activation is a novel mechanism for target cell-specific short-term facilitation at Schaffer collateral synapses

Target cell-specific differences in short-term plasticity have been attributed to differences in the initial release probability of synapses. Using GIN (GFP-expressing inhibitory neurons) transgenic mice that express enhanced green fluorescent protein (EGFP) in a subset of interneurons containing somatostatin, we show that Schaffer collateral synapses onto the EGFP-expressing somatostatin interneurons in CA1 have very large short-term facilitation, even larger facilitation than onto pyramidal cells, in contrast to the majority of interneurons that have little or no facilitation. Using a combination of electrophysiological recordings and mathematical modeling, we show that the large short-term facilitation is caused both by a very low initial release probability and by synaptic activation of presynaptic kainate receptors that increase release probability on subsequent stimuli. Thus, we have discovered a novel mechanism for target cell-specific short-term plasticity at Schaffer collateral synapses in which the activation of presynaptic kainate receptors by synaptically released glutamate contributes to large short-term facilitation, enabling selective enhancement of the inputs to a subset of interneurons.