Strain-dependent differences in calcium signaling predict excitotoxicity in murine hippocampal neurons

Novel mechanism of hypoxic neuronal injury mediated by non-excitatory amino acids and astroglial swelling

In ischemic stroke and post-traumatic brain injury (TBI), blood-brain barrier disruption leads to leaking plasma amino acids (AA) into cerebral parenchyma. Bleeding in hemorrhagic stroke and TBI also release plasma AA. Although excitotoxic AA were extensively studied, little is known about non-excitatory AA during hypoxic injury. Hypoxia-induced synaptic depression in hippocampal slices becomes irreversible with non-excitatory AA, alongside their intracellular accumulation and increased tissue electrical resistance. Four non-excitatory AA (l-alanine, glycine, l-glutamine, l-serine: AGQS) at plasmatic concentrations were applied to slices from mice expressing EGFP in pyramidal neurons or astrocytes during normoxia or hypoxia. Two-photon imaging, light transmittance (LT) changes, and electrophysiological field recordings followed by electron microscopy in hippocampal CA1 st. radiatum were used to monitor synaptic function concurrently with cellular swelling and injury. During normoxia, AGQS-induced increase in LT was due to astroglial but not neuronal swelling. LT raise during hypoxia and AGQS manifested astroglial and neuronal swelling accompanied by a permanent loss of synaptic transmission and irreversible dendritic beading, signifying acute damage. Neuronal injury was not triggered by spreading depolarization which did not occur in our experiments. Hypoxia without AGQS did not cause cell swelling, leaving dendrites intact. Inhibition of NMDA receptors prevented neuronal damage and irreversible loss of synaptic function. Deleterious effects of AGQS during hypoxia were prevented by alanine-serine-cysteine transporters (ASCT2) and volume-regulated anion channels (VRAC) blockers. Our findings suggest that astroglial swelling induced by accumulation of non-excitatory AA and release of excitotoxins through antiporters and VRAC may exacerbate the hypoxia-induced neuronal injury.

Activation and function of murine primary microglia in the absence of the prion protein

The prion protein (PrP(C)) is predominantly expressed in the nervous and immune systems and is involved in relevant cell signaling. Microglia participate in neuroimmune interactions, and their regulatory mechanisms are critical for both health and disease. Despite recent reports with a microglial cell line, little is known about the relevance of PrP(C) in brain microglia. We investigated the role of PrP(C) in mouse primary microglia, and found no differences between wild type and Prnp-null cells in cell morphology or the expression of a microglial marker. Translocation of NF-κB to the nucleus also did not differ, nor did cytokine production. The levels of iNOS were also similar and, finally, microglia of either genotype showed no differences in either rates of phagocytosis or migration, even following activation. Thus, functional roles of PrP(C) in primary microglial cells are - if present - much more subtle than in transformed microglial cell lines.

Ciliary neurotrophic factor (CNTF) activation of astrocytes decreases spreading depolarization susceptibility and increases potassium clearance

Waves of spreading depolarization (SD) have been implicated in the progressive expansion of acute brain injuries. SD can persist over several days, coincident with the time course of astrocyte activation, but little is known about how astrocyte activation may influence SD susceptibility. We examined whether activation of astrocytes modified SD threshold in hippocampal slices. Injection of a lentiviral vector encoding Ciliary neurotrophic factor (CNTF) into the hippocampus in vivo, led to sustained astrocyte activation, verified by up-regulation of glial fibrillary acidic protein (GFAP) at the mRNA and protein levels, as compared to controls injected with vector encoding LacZ. In acute brain slices from LacZ controls, localized 1M KCl microinjections invariably generated SD in CA1 hippocampus, but SD was never induced with this stimulus in CNTF tissues. No significant change in intrinsic excitability was observed in CA1 neurons, but excitatory synaptic transmission was significantly reduced in CNTF samples. mRNA levels of the predominantly astrocytic Na(+) /K(+) -ATPase pump α2 subunit were higher in CNTF samples, and the kinetics of extracellular K(+) transients during matched synaptic activation were consistent with increased K(+) uptake in CNTF tissues. Supporting a role for the Na(+) /K(+) -ATPase pump in increased SD threshold, ouabain, an inhibitor of the pump, was able to generate SD in CNTF tissues. These data support the hypothesis that activated astrocytes can limit SD onset via increased K(+) clearance and suggest that therapeutic strategies targeting these glial cells could improve the outcome following acute brain injuries associated with SD.

Evolution of neuronal and astroglial disruption in the peri-contusional cortex of mice revealed by in vivo two-photon imaging

In traumatic brain injury mechanical forces applied to the cranium and brain cause irreversible primary neuronal and astroglial damage associated with terminal dendritic beading and spine loss representing acute damage to synaptic circuitry. Oedema develops quickly after trauma, raising intracranial pressure that results in a decrease of blood flow and consequently in cerebral ischaemia, which can cause secondary injury in the peri-contusional cortex. Spreading depolarizations have also been shown to occur after traumatic brain injury in humans and in animal models and are thought to accelerate and exacerbate secondary tissue injury in at-risk cortical territory. Yet, the mechanisms of acute secondary injury to fine synaptic circuitry within the peri-contusional cortex after mild traumatic brain injury remain unknown. A mild focal cortical contusion model in adult mouse sensory-motor cortex was implemented by the controlled cortical impact injury device. In vivo two-photon microscopy in the peri-contusional cortex was used to monitor via optical window yellow fluorescent protein expressing neurons, enhanced green fluorescent protein expressing astrocytes and capillary blood flow. Dendritic beading in the peri-contusional cortex developed slowly and the loss of capillary blood flow preceded terminal dendritic injury. Astrocytes were swollen indicating oedema and remained swollen during the next 24 h throughout the imaging session. There were no recurrent spontaneous spreading depolarizations in this mild traumatic brain injury model; however, when spreading depolarizations were repeatedly induced outside the peri-contusional cortex by pressure-injecting KCl, dendrites undergo rapid beading and recovery coinciding with passage of spreading depolarizations, as was confirmed with electrophysiological recordings in the vicinity of imaged dendrites. Yet, accumulating metabolic stress resulting from as few as four rounds of spreading depolarization significantly added to the fraction of beaded dendrites that were incapable to recover during repolarization, thus facilitating terminal injury. In contrast, similarly induced four rounds of spreading depolarization in another set of control healthy mice caused no accumulating dendritic injury as dendrites fully recovered from beading during repolarization. Taken together, our data suggest that in the mild traumatic brain injury the acute dendritic injury in the peri-contusional cortex is gated by the decline in the local blood flow, most probably as a result of developing oedema. Furthermore, spreading depolarization is a specific mechanism that could accelerate injury to synaptic circuitry in the metabolically compromised peri-contusional cortex, worsening secondary damage following traumatic brain injury.

Sustained NMDA receptor activation by spreading depolarizations can initiate excitotoxic injury in metabolically compromised neurons

Spreading depolarizations (SDs) are slowly propagating waves of near-complete neuronal and glial depolarization. SDs have been recorded in patients with brain injury, and the incidence of SD significantly correlates with outcome severity. Although it is well accepted that the ionic dyshomeostasis of SD presents a severe metabolic burden, there is currently limited understanding of SD-induced injury processes at a cellular level. In the current study we characterized events accompanying SD in the hippocampal CA1 region of murine brain slices, using whole-cell recordings and single-cell Ca(2+) imaging. We identified an excitatory phase that persisted for approximately 2 min following SD onset, and accompanied with delayed dendritic ionic dyshomeostasis. The excitatory phase coincided with a significant increase in presynaptic glutamate release, evidenced by a transient increase in spontaneous EPSC frequency and paired-pulse depression of evoked EPSCs. Activation of NMDA receptors (NMDARs) during this late excitatory phase contributed to the duration of individual neuronal depolarizations and delayed recovery of extracellular slow potential changes. Selectively targeting the NMDAR activation following SD onset (by delayed pressure application of a competitive NMDAR antagonist) significantly decreased the duration of cellular depolarizations. Recovery of dendritic Ca(2+) elevations following SD were also sensitive to delayed NMDA antagonist application. Partial inhibition of neuronal energy metabolism converted SD into an irrecoverable event with persistent Ca(2+) overload and membrane compromise. Delayed NMDAR block was sufficient to prevent these acute injurious events in metabolically compromised neurons. These results identify a significant contribution of a late component of SD that could underlie neuronal injury in pathological circumstances.

The relevance of individual genetic background and its role in animal models of epilepsy

Growing evidence has indicated that genetic factors contribute to the etiology of seizure disorders. Most epilepsies are multifactorial, involving a combination of additive and epistatic genetic variables. However, the genetic factors underlying epilepsy have remained unclear, partially due to epilepsy being a clinically and genetically heterogeneous syndrome. Similar to the human situation, genetic background also plays an important role in modulating both seizure susceptibility and its neuropathological consequences in animal models of epilepsy, which has too often been ignored or not been paid enough attention to in published studies. Genetic homogeneity within inbred strains and their general amenability to genetic manipulation have made them an ideal resource for dissecting the physiological function(s) of individual genes. However, the inbreeding that makes inbred mice so useful also results in genetic divergence between them. This genetic divergence is often unaccounted for but may be a confounding factor when comparing studies that have utilized distinct inbred strains. The purpose of this review is to discuss the effects of genetic background strain on epilepsy phenotypes of mice, to remind researchers that the background genetics of a knockout strain can have a profound influence on any observed phenotype, and outline the means by which to overcome potential genetic background effects in experimental models of epilepsy.

Excessive Na+/H+ exchange in disruption of dendritic Na+ and Ca2+ homeostasis and mitochondrial dysfunction following in vitro ischemia

Neuronal dendrites are vulnerable to injury under diverse pathological conditions. However, the underlying mechanisms for dendritic Na(+) overload and the selective dendritic injury remain poorly understood. Our current study demonstrates that activation of NHE-1 (Na(+)/H(+) exchanger isoform 1) in dendrites presents a major pathway for Na(+) overload. Neuronal dendrites exhibited higher pH(i) regulation rates than soma as a result of a larger surface area/volume ratio. Following a 2-h oxygen glucose deprivation and a 1-h reoxygenation, NHE-1 activity was increased by ∼70-200% in dendrites. This elevation depended on activation of p90 ribosomal S6 kinase. Moreover, stimulation of NHE-1 caused dendritic Na(+)(i) accumulation, swelling, and a concurrent loss of Ca(2+)(i) homeostasis. The Ca(2+)(i) overload in dendrites preceded the changes in soma. Inhibition of NHE-1 or the reverse mode of Na(+)/Ca(2+) exchange prevented these changes. Mitochondrial membrane potential in dendrites depolarized 40 min earlier than soma following oxygen glucose deprivation/reoxygenation. Blocking NHE-1 activity not only attenuated loss of dendritic mitochondrial membrane potential and mitochondrial Ca(2+) homeostasis but also preserved dendritic membrane integrity. Taken together, our study demonstrates that NHE-1-mediated Na(+) entry and subsequent Na(+)/Ca(2+) exchange activation contribute to the selective dendritic vulnerability to in vitro ischemia.

Reduced size of the dendritic tree does not protect Purkinje cells from excitotoxic death

Purkinje cell loss by excitotoxic damage is a typical finding in many cerebellar diseases. One important aspect of this high sensitivity of Purkinje cells to excitotoxic death might be the enormous size of their dendritic tree, with a high load of excitatory glutamate receptors. We have studied whether reduction in the size of the dendritic tree might confer resistance against excitotoxic death to Purkinje cells. We have grown Purkinje cells in organotypic cerebellar slice cultures under chronic activation of metabotropic glutamate receptors or of protein kinase C. Both treatments strongly reduced dendritic tree size. After this treatment, cells were exposed to the glutamate receptor agonist AMPA, which has a strong excitotoxic effect on Purkinje cells. We found that Purkinje cells with small dendritic trees were as sensitive to AMPA exposure as untreated control cells with large dendritic trees. Immunostaining against vesicular glutamate transporter 1 revealed that the small dendritic trees were densely covered by glutamatergic terminals. Our results indicate that the expansion of the dendritic tree and the total number of AMPA receptors per neuron do not play a major role in determining the susceptibility of Purkinje cells to excitotoxic death.

Searching for new animal models of Alzheimer's disease

The pathophysiology of chronic neurodegenerative diseases, as Alzheimer's diseases, has remained inaccessible till recently. But this situation is changing quickly. In the past decades, genes causing familiar forms of the disease have been identified and provided the genetic framework for the emerging amyloid hypothesis. On the basis of these findings, engineered mouse models have been developed and have allowed the understanding of crucial information about the pathogenic process. Certain observations obtained by transgenic mice, however, do not easily fit with the simplest version of the amyloid hypothesis. Even if there are transgenic lines that offer robust and relatively faithful reproductions of a subset of Alzheimer's disease's features, a mouse model that recapitulates all aspects of the disease has not yet been produced. Several still not completely known factors combine to produce highly variability across transgenic mouse models. Discrepancies in neuropathology and behaviour between transgenic mouse models and human Alzheimer's disease, and among different transgenic-lines, suggest caution in the interpretation of the results. Here we try to analyze critically some of the information provided by transgenic mice but ascertaining which elements of the neuropathological and behavioural phenotype of these various strains of transgenic mice are relevant to that observed in Alzheimer's disease continues to be a challenge.

Contributions of Ca2+ and Zn2+ to spreading depression-like events and neuronal injury

The phenomenon of spreading depression (SD) involves waves of profound neuronal and glial depolarization that spread throughout brain tissue. Under many conditions, tissue recovers full function after SD has occurred, but SD-like events are also associated with spread of injury following ischemia or trauma. Initial large cytosolic Ca2+ increases accompany all forms of SD, but persistently elevated Ca2+ loading is likely responsible for neuronal injury following SD in tissues where metabolic capacity is insufficient to restore ionic gradients. Ca2+ channels are also involved in the propagation of SD, but the channel subtypes and cation fluxes differ significantly when SD is triggered by different types of stimuli. Ca2+ influx via P/Q type channels is important for SD generated by localized application of high K+ solutions. In contrast, SD-like events recorded in in vitro ischemia models are not usually prevented by Ca2+ removal, but under some conditions, Zn2+ influx via L-type channels contributes to SD initiation. This review addresses different roles of Ca2+ in the initiation and consequences of SD, and discusses recent evidence that selective chelation of Zn2+ can be sufficient to prevent SD under circumstances that may have relevance for ischemic injury.

Selective vulnerability of neurons in primary cultures and in neurodegenerative diseases

Primary neuronal cultures are commonly used to dissect the molecular and cellular mechanisms that underlie human brain diseases. Neurons dissociated from an embryonic brain and grown in culture dishes are almost by definition different from those residing inside a living brain. Not only are the individual cells stripped of their normal chemical and physical contacts, but the cellular composition of the cultures (the ratio of cell types) can be affected by many intrinsic and extrinsic factors, including brain region, neuronal birthday, gender, genetic background and in vitro age. Changes in any of these factors may have a strong impact on the outcome of the experiment. In a recent study, Romito-DiGiacomo et al. /54/ demonstrated that when neurons were harvested from murine embryonic cortex, the typical protocol favored cells that were just finishing cell division at the time of harvest. By taking advantage of the fact that the date of the final cell division (birthday) of a neuron correlates with its final position in the cortical plate they were able to assay deeper layer neurons (layers V-VI) separately from the more superficial layers (layers II-III). They reported that while the superficial cells were sensitive to the toxic effect of beta-amyloid, the deeper layer neurons were virtually resistant to death in its presence. The findings recapitulate selective vulnerability in the neocortex of Alzheimer's disease. This is a beautiful example of how to turn the apparent weakness of primary cultures into strength through experimental design and data interpretation. Selective vulnerability is a common feature of neurodegenerative disease, thus it is critical to use the right primary culture. Do you know what is in your culture?

Animal models of neurodegenerative diseases

Animal models of neurodegenerative disease are excellent tools for studying pathogenesis and therapies including cellular transplantation. In this chapter, we describe different models of Huntington's disease and Parkinson's disease, stereotactic surgery (used in creation of lesion models and transplantation) and finally transplantation studies in these models.

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.

Variation in Galr1 expression determines susceptibility to exocitotoxin-induced cell death in mice

Inbred strains of mice differ in their susceptibility to excitotoxin-induced cell death, but the genetic basis of individual variation in differential susceptibility is unknown. Previously, we identified a highly significant quantitative trait locus (QTL) on chromosome 18 that influenced susceptibility to kainic acid-induced cell death (Sicd1). Comparison of susceptibility to seizure-induced cell death between reciprocal congenic lines for Sicd1 and parental background mice indicates that genes influencing this trait were captured in both strains. Two positional gene candidates, Galr1 and Mbp, map to 55 cM, where the Sicd1 QTL had been previously mapped. Thus, this study was undertaken to determine if Galr1 and/or Mbp could be considered as candidate genes. Genomic sequence comparison of these two functional candidate genes from the C57BL/6J (resistant at Sicd1) and the FVB/NJ (susceptible at Sicd1) strains showed no single-nucleotide polymorphisms. However, expression studies confirmed that Galr1 shows significant differential expression in the congenic and parental inbred strains. Galr1 expression was downregulated in the hippocampus of C57BL/6J mice and FVB.B6-Sicd1 congenic mice when compared with FVB/NJ or B6.FVB-Sicd1 congenic mice. A survey of Galr1 expression among other inbred strains showed a significant effect such that 'susceptible' strains showed a reduction in Galr1 expression as compared with 'resistant' strains. In contrast, no differences in Mbp expression were observed. In summary, these results suggest that differential expression of Galr1 may contribute to the differences in susceptibility to seizure-induced cell death between cell death-resistant and cell death-susceptible strains.

Relevance of transgenic mouse models to human Alzheimer disease

During the past 2 decades, the elucidation of susceptibility and causative genes for Alzheimer disease as well as proteins involved in the pathogenic process has greatly facilitated the development of genetically altered mouse models. These models have played a major role in defining critical disease-related mechanisms and in evaluating novel therapeutic approaches, with many treatments currently in clinical trial owing their origins to studies initially performed in mice. This review discusses the utility of transgenic mice as a research tool and their contributions to our understanding of Alzheimer disease.

Contribution of Na(+)/Ca(2+) exchange to excessive Ca(2+) loading in dendrites and somata of CA1 neurons in acute slice

Multiple Ca(2+) entry routes have been implicated in excitotoxic Ca(2+) loading in neurons and reverse-operation of sodium-calcium exchangers (NCX) has been shown to contribute under conditions where intracellular Na(+) levels are enhanced. We have investigated effects of KB-R7943, an inhibitor of reverse-operation NCX activity, on Ca(2+) elevations in single CA1 neurons in acute hippocampal slices. KB-R7943 had no significant effect on input resistance, action potential waveform, or action potential frequency adaptation, but reduced L-type Ca(2+) entry in somata. Nimodipine was therefore included in subsequent experiments to prevent complication from effects of L-type influx on evaluation of NCX activity. NMDA produced transient primary Ca(2+) increases, followed by propagating secondary Ca(2+) increases that initiated in apical dendrites. KB-R7943 had no significant effect on primary or secondary Ca(2+) increases generated by NMDA. The Na(+)/K(+) ATPase inhibitor ouabain (30 microM) produced degenerative Ca(2+) overload that was initiated in basal dendrites. KB-R7943 significantly reduced initial Ca(2+) increases and delayed the propagation of degenerative Ca(2+) loads triggered by ouabain, raising the possibility that excessive intracellular Na(+) loading can trigger reverse-operation NCX activity. A combination of NMDA and ouabain produced more rapid Ca(2+) overload, that was contributed to by NCX activity. These results suggest that degenerative Ca(2+) signaling can be triggered by NMDA in dendrites, before intracellular Na(+) levels become sufficient to reverse NCX activity. However, since Na(+)/K(+) ATPase inhibition does appear to produce significant reverse-operation NCX activity, this additional Ca(2+) influx pathway may operate in ATP-deprived CA1 neurons and play a role in ischemic neurodegeneration.

Modulation of GABAA receptors in cerebellar granule neurons by ethanol: a review of genetic and electrophysiological studies

Cerebellar granule neurons (CGNs) receive inhibitory input from Golgi cells in the form of phasic and tonic currents that are mediated by postsynaptic and extrasynaptic gamma-aminobutyric acid type A (GABAA) receptors, respectively. Extrasynaptic receptors are thought to contain alpha6betaxdelta subunits. Here, we review studies on ethanol (EtOH) modulation of these receptors, which have yielded contradictory results. Although studies with recombinant receptors expressed in Xenopus oocytes indicate that alpha6beta3delta receptors are potently enhanced by acute exposure to low (>or=3 mM) EtOH concentrations, this effect was not observed when these receptors were expressed in Chinese hamster ovary cells. Slice recordings of CGNs have consistently shown that EtOH increases the frequency of phasic spontaneous inhibitory postsynaptic currents (sIPSCs), as well as the tonic current amplitude and noise. However, there is a lack of consensus as to whether EtOH directly acts on extrasynaptic receptors or modulates them indirectly; that is, via an increase in spillover of synaptically released GABA. It was recently demonstrated that an R to Q mutation of amino acid 100 of the alpha6 subunit increases the effect of EtOH on both sIPSCs and tonic current. These electrophysiological findings have not been reproducible in our hands. Moreover, it was shown the alpha6-R100Q mutation enhances sensitivity to the motor-impairing effects of EtOH in outbred Sprague-Dawley rats, but this was not observed in a line of rats selectively bred for high sensitivity to EtOH-induced motor alterations (Alcohol Non-Tolerant rats). We conclude that currently there is insufficient evidence conclusively supporting a direct potentiation of extrasynaptic GABAA receptors following acute EtOH exposure in CGNs.

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.

CaMKII translocation requires local NMDA receptor-mediated Ca2+ signaling

Excitatory synaptic transmission and plasticity are critically modulated by N-methyl-D-aspartate receptors (NMDARs). Activation of NMDARs elevates intracellular Ca(2+) affecting several downstream signaling pathways that involve Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). Importantly, NMDAR activation triggers CaMKII translocation to synaptic sites. NMDAR activation failed to induce Ca(2+) responses in hippocampal neurons lacking the mandatory NMDAR subunit NR1, and no EGFP-CaMKIIalpha translocation was observed. In cells solely expressing Ca(2+)-impermeable NMDARs containing NR1(N598R)-mutant subunits, prolonged NMDA application elevated internal Ca(2+) to the same degree as in wild-type controls, yet failed to translocate CaMKIIalpha. Brief local NMDA application evoked smaller Ca(2+) transients in dendritic spines of mutant compared to wild-type cells. CaMKIIalpha mutants that increase binding to synaptic sites, namely CaMKII-T286D and CaMKII-TT305/306VA, rescued the translocation in NR1(N598R) cells in a glutamate receptor-subtype-specific manner. We conclude that CaMKII translocation requires Ca(2+) entry directly through NMDARs, rather than other Ca(2+) sources activated by NMDARs. Together with the requirement for activated, possibly ligand-bound, NMDARs as CaMKII binding partners, this suggests that synaptic CaMKII accumulation is an input-specific signaling event.

Inbred mouse strains as a tool to analyze hippocampal neuronal loss after brain injury: a stereological study

Traumatic brain injury (TBI) damages the hippocampus both in experimental animal models and in humans. In particular, the mechanical injury in combination with the genetic susceptibility to injury may result in neuronal loss from the hippocampus. This report explores the time-course of neuronal loss in the four primary subregions of the mouse hippocampus after a lateral fluid percussion injury (FPI) to the brain, and how subtle genetic differences between C57BL/6J and C57BL/10J mouse strains influence the extent and time course of neuronal loss. Using design-based stereological procedures, our results indicate negligible neuronal loss ipsilateral to the injury at 2 days postinjury in C57BL/6J mice, whereas a significant number (30-40%) of neurons are lost across all subregions of the hippocampus (dentate, hilus, area CA3, and area CA1) by 1 week, which does not appear to progress at 1 month, compared to sham. Additionally, neuronal counts after lateral FPI in a genetically similar, yet kainic acid-sensitive, mouse strain (C57BL/10J) showed no statistically significant differences in neuron number compared to the C57BL/6J strain in response to brain injury. Hippocampal neuronal loss after lateral FPI and its consequent circuit disruption may depend more on factors related to the mechanics and secondary consequences of the injury, as opposed to subtle genetic variations between inbred mouse strains. The loss of neurons appears to be restricted to the first week post-injury, and the remaining neurons may serve as a substrate for recovery.

GABAergic protection of hippocampal pyramidal neurons against glutamate insult: deficit in young animals compared to adults

Hypoxia-ischemia (HI) injury in neonatal animals leads to selective regional loss of neurons including CA1 and CA3 pyramidal neurons of the hippocampus as well as nonlethal pathologies. Glutamate-receptor over-activation and Ca2+ influx are involved in these neonatal changes. We examined glutamate-evoked Ca2+ responses in neonatal (PN 7-13) and young adult (PN 21-27) CA1 pyramidal neurons in acute slices from rats. In neonates, transient exposure to glutamate produced large Ca2+ increases throughout neurons, including distal dendrites (primary Ca2+ responses). Repeated exposures resulted in sustained Ca2+ increases in apical dendrites (secondary Ca2+ responses) that were independent of continued glutamate exposure. These responses propagated and invaded the soma, resulting in irrecoverably high Ca2+ elevations. In neurons from adults, identical glutamate exposure evoked primary Ca2+ responses only in somata and proximal apical dendrites. Repeated glutamate exposures in the adult neurons also led to secondary Ca2+ responses, but they arose in the peri-somatic region and then spread outward to distal apical dendrites, again without recovery. More stimuli were required to initiate secondary responses in neurons from adult versus neonates. Block of GABAA receptors in adults caused the primary and secondary responses to revert to the spatial pattern seen in the neonates and greatly increased their vulnerability to glutamate. These findings suggest that neurodegenerative secondary Ca2+ events may be important determinants of susceptibility to HI injury in the developing CNS and that immature CA1 neurons may be more susceptible to excitotoxic injury due at least in part to insufficient development of GABAergic inputs to their dendrites.

Alcohol is a potent stimulant of immature neuronal networks: implications for fetal alcohol spectrum disorder

Ethanol consumption during development affects the maturation of hippocampal circuits by mechanisms that are not fully understood. Ethanol acts as a depressant in the mature CNS and it has been assumed that this also applies to immature neurons. We investigated whether ethanol targets the neuronal network activity that is involved in the refinement of developing hippocampal synapses. This activity appears during the growth spurt period in the form of giant depolarizing potentials (GDPs). GDPs are generated by the excitatory actions of GABA and glutamate via a positive feedback circuit involving pyramidal neurons and interneurons. We found that ethanol potently increases GDP frequency in the CA3 hippocampal region of slices from neonatal rats. It also increased the frequency of GDP-driven Ca2+ transients in pyramidal neurons and increased the frequency of GABA(A) receptor-mediated spontaneous postsynaptic currents in CA3 pyramidal cells and interneurons. The ethanol-induced potentiation of GABAergic activity is probably the result of increased quantal GABA release at interneuronal synapses but not enhanced neuronal excitability. These findings demonstrate that ethanol is a potent stimulant of developing neuronal circuits, which might contribute to the abnormal hippocampal development associated with fetal alcohol syndrome and alcohol-related neurodevelopmental disorders.

Susceptibility to excitotoxic and metabolic striatal neurodegeneration in the mouse is genotype dependent

Previously, we had reported that hippocampal susceptibility to the neurotoxic effects of excitotoxin administration is strain dependent [Schauwecker and Steward, Proc. Natl. Acad. Sci. U.S.A. 94 (1997) 4103]. However, it has been unclear whether strain-related gene products may play a similar role in providing protection against drugs that produce striatal lesions. The present series of experiments sought to elucidate whether genetic background alters neuronal viability within the striatum following metabolic or excitotoxic injury. Thus, we have examined the effect of mouse strain on susceptibility to striatal injury using well-characterized animal models of Huntington's disease by examining whether C57BL/6 mice, previously identified as resistant to excitotoxin-induced hippocampal cell death, are resistant to quinolinate, malonate, and 3-nitropropionic acid (3-NP). Intrastriatal injection of either malonate or quinolinate and systemic administration of 3-NP resulted in significantly smaller striatal lesions in C57BL/6 mice as compared to FVB/N mice, previously identified as susceptible to hippocampal excitotoxic injury. The existence of an animal strain with decreased resistance to striatal lesions suggests that there are mediating factors involved in the preferential vulnerability of the striatum to neurotoxic lesioning. The identification of these factors could provide strategies for therapeutic intervention in Huntington's disease.

An in vitro model of hippocampal sharp waves: regional initiation and intracellular correlates

During slow wave sleep and consummatory behaviors, electroencephalographic recordings from the rodent hippocampus reveal large amplitude potentials called sharp waves. The sharp waves originate from the CA3 circuitry and their generation is correlated with coherent discharges of CA3 pyramidal neurons and dependent on activities mediated by AMPA glutamate receptors. To model sharp waves in a relatively large hippocampal circuitry in vitro, we developed thick (1 mm) mouse hippocampal slices by separating the dentate gyrus from the CA2/CA1 areas while keeping the functional dentate gyrus-CA3-CA1 connections. We found that large amplitude (0.3-3 mV) sharp wave-like field potentials occurred spontaneously in the thick slices without extra ionic or pharmacological manipulation and they resemble closely electroencephalographic sharp waves with respect to waveform, regional initiation, pharmacological manipulations, and intracellular correlates. Through measuring tissue O2, K+, and synaptic and single cell activities, we verified that the sharp wave-like potentials are not a consequence of anoxia, nonspecific elevation of extracellular K+ and dissection-related tissue damage. Our data suggest that a subtle but crucial increase in the CA3 glutamatergic activity effectively recruits a population of neurons thus responsible for the generation of the sharp wave-like spontaneous field potentials in isolated hippocampal circuitry.

Expression of apoptosis inhibitor protein Mcl1 linked to neuroprotection in CNS neurons

Mcl1 is a Bcl2-related antiapoptotic protein originally isolated from human myeloid leukemia cells. Unlike Bcl2, expression has not been reported in CNS neurons. We isolated Mcl1 in a direct screen for candidate modifier genes of neuronal vulnerability by differential display of mRNAs upregulated following prolonged seizures in two mouse strains with contrasting levels of hippocampal cell death. Mcl1 is widely expressed in neurons, and transcription is rapidly induced in both strains. In resistant C57Bl/6J mice, Mcl1 protein levels remain persistently elevated in hippocampal pyramidal neurons after seizures, but fall rapidly in C3H/HeJ hippocampus, coinciding with extensive neuronal apoptosis. DNA damage and caspase-mediated cell death were strikingly increased in Mcl1-deficient mice when compared to +/+ littermates after similar seizures. We identify Mcl1 as a neuronal gene responsive to excitotoxic insult in the brain, and link relative levels of Mcl1 expression to inherited differences in neuronal thresholds for apoptosis.

Neurosteroid paradoxical enhancement of paired-pulse inhibition through paired-pulse facilitation of inhibitory circuits in dentate granule cells

Neurosteroids are produced in the brain independently of peripheral endocrine glands to act locally in the nervous system. They exert potent promnesic effects and play significant roles in mental health-related disorders. In part, neurosteroids act by affecting ligand-gated ion channels and metabotropic receptors through rapid non-genomic processes. We have previously demonstrated that neurosteroids also affect synaptic transmission presynaptically in the CA1 region of the hippocampus. Here we describe the effects of the most abundant neurosteroid in the rodent brain, pregnenolone sulfate (PregS), on signal processing in the dentate subfield of the hippocampus. We show that PregS acts presynaptically at low concentrations (300 nM) to enhance paired-pulse facilitation (PPF) in perforant pathway terminals on dentate granule cells. Similar effects were found with two steroid sulfatase inhibitors demonstrating a potential contribution of endogenous steroids to dentate synaptic plasticity. This enhanced presynaptic facilitation paradoxically increases paired-pulse inhibition (PPI) at short interpulse intervals. Based on these data, a model of dentate gyrus circuit interactions is proposed for the presynaptic action of PregS on the filtering dynamics of the dentate subfield at frequencies similar to those of the endogenous signals from the entorhinal cortex. These modeling studies are consistent with experimental measurements demonstrating positive modulation by PregS at low frequencies and negative modulation at high frequencies. These studies show an important role for the presynaptic action of neurosteroids in modulating input signals to the hippocampus.

Size does matter: generation of intrinsic network rhythms in thick mouse hippocampal slices

Rodent hippocampal slices of < or = 0.5 mm thickness have been widely used as a convenient in vitro model since the 1970s. However, spontaneous population rhythmic activities do not consistently occur in this preparation due to limited network connectivity. To overcome this limitation, we develop a novel slice preparation of 1 mm thickness from adult mouse hippocampus by separating dentate gyrus from CA3/CA1 areas but preserving dentate-CA3-CA1 connectivity. While superfused in vitro at 32 or 37 degrees C, the thick slice exhibits robust spontaneous network rhythms of 1-4 Hz that originate from the CA3 area. Via assessing tissue O2, K+, pH, synaptic, and single-cell activities of superfused thick slices, we verify that these spontaneous rhythms are not a consequence of hypoxia and nonspecific experimental artifacts. We suggest that the thick slice contains a unitary circuitry sufficient to generate intrinsic hippocampal network rhythms and this preparation is suitable for exploring the fundamental properties and plasticity of a functionally defined hippocampal "lamella" in vitro.

Genetic background markedly influences vulnerability of the hippocampal neuronal organization in the "twitcher" mouse model of globoid cell leukodystrophy

The twitcher mouse is well known as a naturally occurring authentic mouse model of human globoid cell leukodystrophy (GLD; Krabbe disease) due to genetic deficiency of lysosomal galactosylceramidase. The twitcher mice used most commonly are on the C57BL/6J background. We generated twitcher mice that were on the mixed background of C57BL/6J and 129SvEv, the standard strain for production of targeted mutations. Twitcher mice on the mixed background were smaller and had a shorter lifespan than were those on the C57BL/6J background. Many twitcher mice on the mixed background developed generalized seizures around 30 days that were never seen in twitcher mice on the C57BL/6J background. Neuropathologically, although the degree of the typical demyelination with infiltration of macrophages was similar in the central and peripheral nervous systems, in both strains, marked neuronal cell death was observed only in twitcher mice on the mixed background. In the hippocampus, the neuronal cell death occurred prominently in the CA3 region in contrast to the relatively well-preserved CA1 and CA2 areas. This neuropathology has never been seen in twitcher mice on the C57BL/6J background. Biochemically, the brain of twitcher mice on the mixed background showed much greater accumulation of lactosylceramide. Genetic background must be carefully taken into consideration when phenotype of mutant mice is evaluated, particularly because most targeted mutants are initially on a mixed genetic background and gradually moved to a pure background. These findings also suggest an intriguing possibility of important function of some sphingolipids in the hippocampal neuronal organization and maintenance.

Caffeine releasable stores of Ca2+ show depletion prior to the final steps in delayed CA1 neuronal death

In addition to their role in signaling, Ca2+ ions in the endoplasmic reticulum also regulate important steps in protein processing and trafficking that are critical for normal cell function. Chronic depletion of Ca2+ in the endoplasmic reticulum has been shown to lead to cell degeneration and has been proposed as a mechanism underlying delayed neuronal death following ischemic insults to the CNS. Experiments here have assessed the relative content of ryanodine receptor-gated stores in CA1 neurons by measuring cytoplasmic Ca2+ increases induced by caffeine. These measurements were performed on CA1 neurons, in slice, from normal gerbils, and compared with responses from this same population of neurons 54-60 h after animals had undergone a standard ischemic insult: 5-min bilateral occlusion of the carotid arteries. The mean amplitude of responses in the postischemic population were less than one-third of those in control or sham-operated animals, and 35% of the neurons from postischemic animals showed very small responses that were approximately 10% of the control population mean. Refilling of these stores after caffeine challenges was also impaired in postischemic neurons. These observations are consistent with our earlier finding that voltage-gated influx is sharply reduced in postischemic in CA1 neurons and the hypothesis that the resulting depletion in endosomal Ca2+ is an important cause of delayed neuronal death.

Protection provided by cyclosporin A against excitotoxic neuronal death is genotype dependent

PURPOSE

Previous studies have shown that the immunosuppressant cyclosporin A (CsA), a specific blocker of the mitochondrial permeability transition (MPT) pore, can dramatically ameliorate the selective neuronal necrosis resulting from ischemia-reperfusion, traumatic brain injury, and N-methyl-d-aspartate (NMDA)-evoked neurotoxicity. The purpose of this study was to determine whether two different immunosuppressants, CsA and FK-506, could ameliorate the neuronal damage observed after kainate-induced seizures in strains that are differentially susceptible to excitotoxin-induced cell death.

METHODS

Excitotoxin-resistant (C57BL/6) or -susceptible (FVB/N) mice were administered kainate alone (30 mg/kg), CsA alone (5, 10, or 20 mg/kg), or one of the immunosuppressants (CsA, 5 mg/kg or 10 mg/kg; FK-506, 0.5 mg/kg) followed by kainate. After drug administration, mice were monitored continuously for the onset and extent of seizure activity. After a survival of 7 days, animals were assessed for hippocampal damage.

RESULTS

Whereas CsA alone induced no epileptogenic effects and both immunosuppressants were without effect on the induction of kainate-induced seizures, administration of CsA to excitotoxin-susceptible mice (FVB/N) virtually eliminated neuronal cell death. In contrast, induction of neuronal cell death was evident when CsA was administered to excitotoxin-resistant mice (C57BL/6). Administration of FK-506, another commonly used immunosuppressant, which lacks an effect on the MPT, had no effect on modification of susceptibility to kainate-induced cell death in either strain.

CONCLUSIONS

As our data show differential protection of hippocampal neurons against excitotoxic cell death by pretreatment with CsA, these results suggest that strain-dependent differences in mitochondrial integrity and function may exist.

Functional significance of cannabinoid-mediated, depolarization-induced suppression of inhibition (DSI) in the hippocampus

A number of recent studies have demonstrated that a well-known form of short-term plasticity at hippocampal GABAergic synapses, called depolarization-induced suppression of inhibition (DSI), is in fact mediated by the retrograde actions of endocannabinoids released in response to depolarization of the postsynaptic cells. These studies suggest that endogenous cannabinoids may play an important role in regulating inhibitory tone in the mammalian CNS. Despite the widespread interest and potential physiological importance of DSI, many questions regarding the physiological relevance of DSI remain. To that end, this study set out to define the specific limiting conditions that could elicit DSI at GABAergic synapses in CA1 hippocampal pyramidal neurons and to determine if DSI could be elicited with pulse trains that mimic hippocampal cell-firing patterns that occur in vivo. Whole cell recordings from hippocampal neurons under voltage-clamp configuration were made in rat hippocampal slices. Spontaneous and evoked gamma-aminobutyric acid-A (GABAA) receptor-mediated inhibitory postsynaptic currents (sIPSCs and eIPSCs, respectively) were recorded prior to and following depolarization of CA1 hippocampal pyramidal cells. Depolarizing voltage pulses were shaped to evoke currents in QX-314-treated cells similar to those accompanying single spontaneous voltage-clamped action potentials recorded from the soma. Attempts were made to elicit DSI with trains of these pulses that mimicked hippocampal cell firing patterns in vivo, for instance, when animals traverse place fields or are performing a short-term memory task. DSI could not be elicited by such pulse trains or by a number of other combinations of behaviorally specific firing parameters. The minimum duration of depolarization necessary to elicit DSI in hippocampal neurons determined by paired-pulse manipulation was 50 -75 ms at a critical interval of 20 -30 ms between pulse pairs. Under the conditions tested, the normal firing patterns of hippocampal neurons that occur in vivo do not appear to elicit DSI.

Alcohol potently inhibits the kainate receptor-dependent excitatory drive of hippocampal interneurons

Kainate receptors (KA-Rs) are members of the glutamate-gated family of ionotropic receptors, which also includes N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors. KA-Rs are important modulators of interneuron excitability in the CA1 region of the hippocampus. Activation of these receptors enhances interneuron firing, which robustly increases spontaneous inhibitory postsynaptic currents in pyramidal neurons. We report here that ethanol (EtOH) potently inhibits this KA-R-mediated effect at concentrations as low as those that can be achieved in blood after the ingestion of just 1-2 drinks (5-10 mM). Pressure application of kainate, in the presence of AMPA and NMDA receptor antagonists, evoked depolarizing responses in interneurons that triggered repetitive action potential firing. EtOH potently inhibited these responses to a degree that was sufficient to abolish action potential firing. This effect appears to be specific for KA-Rs, as EtOH did not affect action potential firing triggered by AMPA receptor-mediated depolarizing responses. Importantly, EtOH inhibited interneuron action potential firing in response to KA-R activation by synaptically released glutamate, suggesting that our findings are physiologically relevant. KA-R-dependent modulation of glutamate release onto pyramidal neurons was not affected by EtOH. Thus, EtOH increases excitability of pyramidal neurons indirectly by inhibiting the KA-R-dependent drive of gamma-aminobutyric acid (GABA)ergic interneurons. We postulate that this effect may explain, in part, some of the paradoxical excitatory actions of this widely abused substance. The excitatory actions of EtOH may be perceived as positive by some individuals, which could contribute to the development of alcoholism.

NAD(P)H fluorescence imaging of postsynaptic neuronal activation in murine hippocampal slices

We examined mechanisms contributing to stimulus-evoked changes in NAD(P)H fluorescence as a marker of neuronal activation in area CA1 of murine hippocampal slices. Three types of stimuli (electrical, glutamate iontophoresis, bath-applied kainate) produced biphasic fluorescence changes composed of an initial transient decrease ("initial component," 1-3%), followed by a longer-lasting transient increase ("overshoot," 3-8%). These responses were matched by inverted biphasic flavin adenine dinucleotide (FAD) fluorescence transients, suggesting that these transients reflect mitochondrial function rather than optical artifacts. Both components of NAD(P)H transients were abolished by ionotropic glutamate receptor block, implicating postsynaptic neuronal activation as the primary event involved in generating the signals, and not presynaptic activity or reuptake of synaptically released glutamate. Spatial analysis of the evoked signals indicated that the peak of each component could arise in different locations in the slice, suggesting that there is not always obligatory coupling between the two components. The initial NAD(P)H response showed a strong temporal correspondence to intracellular Ca+ increases and mitochondrial depolarization. However, despite the fact that removal of extracellular Ca2+ abolished neuronal cytosolic Ca2+ transients to exogenous glutamate or kainate, this procedure did not reduce slice NAD(P)H responses evoked by either of these agonists, implying that mechanisms other than neuronal mitochondrial Ca2+ loading underlie slice NAD(P)H transients. These data show that, in contrast to previous proposals, slice NAD(P)H transients in mature slices do not reflect neuronal Ca2+ dynamics and demonstrate that these signals are sensitive indicators of both the spatial and temporal characteristics of postsynaptic neuronal activation in these preparations.

Mouse strain differences in kainic acid sensitivity, seizure behavior, mortality, and hippocampal pathology

Genetic influences contribute to susceptibility to seizures and to excitotoxic injury, but it is unclear if/how these susceptibilities are linked. This study assessed the impact of genetic background on mouse strain seizure susceptibility, seizure phenotype, mortality, and hippocampal histopathology. A subcutaneous (s.c.) kainic acid multiple injection protocol was developed. Five mouse strains were tested: a and b) C57BL/6J and 129/SvJ, strains commonly used in gene targeting experiments; c) C3HeB/FeJ, a strain with reported sensitivity to the convulsant effects of kainic acid (KA); d) 129/SvEms, a strain reportedly susceptible to hippocampal excitotoxic cell death; and e) a mixed genetic background strain (129/SvJXC57BL/6J) from which targeted gene deletion experiments have been carried out. Histopathological features were examined at early (7-10 day), delayed (2-4 month), and late (6-13 month) time points.Mouse background strains can be genetically segregated based on excitotoxin sensitivity, seizure phenotype, mortality, and hippocampal histopathology. When injected with KA, C3HeB/FeJ and C57BL/6J strains were resistant to cell death and synaptic reorganization despite severe behavioral seizures, while 129/SvEms mice developed marked pyramidal cell loss and mossy fiber sprouting despite limited seizure activity. The mixed background 129/SvJXC57BL/6J group exhibited features of both parental strains. In the mouse strains tested, the duration or severity of seizure activity was not predictive of subsequent hippocampal pyramidal cell death and/or synaptic reorganization. Unlike rats, mice exhibiting prolonged high-grade KA-induced seizure activity did not develop subsequent spontaneous behavioral seizures.

Genetic influences on secondary degeneration and wound healing following spinal cord injury in various strains of mice

Various inbred strains of mice exhibit dramatic differences in sensitivity to excitotoxic cell death induced by systemic injections of kainic acid (KA). The present study evaluates whether the same strains are also differentially sensitive to secondary degeneration after spinal cord injury, in which excitotoxic cell death is thought to play a pathogenic role. Spinal cord crush injuries were produced at T9 in two inbred strains that are resistant to KA-induced excitotoxic cell death (C57Bl/6 and Balb/c) and four strains that are sensitive (CD-1, FVB/N, 129T2 Sv/EMS, and C57Bl/10). The spinal cord was prepared for light microscopy at intervals from 1 to 56 days postinjury, and the area of damaged tissue (termed lesion size) and amount of cavitation were determined by quantitative image analysis. Lesion size increased between 1 and 7 days in all strains and then decreased steadily in a wound-healing process that occurs uniquely in mice. The extent of cavitation also gradually decreased from 7 to 56 days in all strains. Although lesion area and cavitation decreased in all strains, there were significant differences in lesion size and cavitation across strains. Specifically, lesion areas in the KA-sensitive strains FVB/N, 129T2 Sv/EMS, and CD-1 were significantly larger at 56 days postinjury than in the KA-resistant strains C57Bl/6 and Balb/c. We conclude that the genetic differences that confer resistance and sensitivity to KA-induced neurotoxicity also modify the secondary degenerative processes that occur after spinal cord injury, so that resistance to excitotoxic injury leads to smaller overall lesions and a more effective wound-healing response.

Characterization of neuronal death induced by focally evoked limbic seizures in the C57BL/6 mouse

Research into the molecular mechanisms of epileptic brain injury is hampered by the resistance of key mouse strains to seizure-induced neuronal death evoked by systemically administered excitotoxins such as kainic acid. Because C57BL/6 mice are extensively employed as the genetic background for transgenic/knockout modeling in cell death research but are seizure resistant, we sought to develop a seizure model in this strain characterized by injury to the hippocampal CA subfields. Adult male C57BL/6 mice underwent focally evoked seizures induced by intraamygdala microinjection of kainic acid. Kainic acid (KA) effectively elicited ipsilateral CA3 pyramidal neuronal death within a narrow dose range of 0.1-0.3 microg, with mortality < 10%. With employment of the most consistent (0.3 microg) dose, seizures were terminated 15, 30, 60, or 90 min after KA by diazepam. Damage was largely restricted to the ipsilateral CA3 subfield of the hippocampus, but injury was also consistent within CA1, suggesting that this mouse model better reflects the hippocampal neuropathology of human temporal lobe epilepsy than does the rat, in which CA1 is typically spared. Confirming this CA1 injury as seizure specific and not a consequence of ischemia, we used laser-Doppler flowmetry to determine that cerebral perfusion did not significantly change (97% to 118%) over control. Degenerating cells were > 95% neuronal as determined by neuron-specific nuclear protein (NeuN) counterstaining of terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeled (TUNEL) brain sections. Furthermore, TUNEL-positive cells often exhibited the morphological features of apoptosis, and small numbers were positive for cleaved caspase-3. These data establish a mouse model of focally evoked seizures in the C57BL/6 strain associated with a restricted pattern of apoptotic neurodegeneration within the hippocampal subfields that may be applied to research into the molecular basis of neuronal death after seizures.

GluR2-dependent properties of AMPA receptors determine the selective vulnerability of motor neurons to excitotoxicity

AMPA receptor-mediated excitotoxicity has been implicated in the selective motor neuron loss in amyotrophic lateral sclerosis. In some culture models, motor neurons have been shown to be selectively vulnerable to AMPA receptor agonists due to Ca(2+) influx through Ca(2+)-permeable AMPA receptors. Because the absence of GluR2 in AMPA receptors renders them highly permeable to Ca(2+) ions, it has been hypothesized that the selective vulnerability of motor neurons is due to their relative deficiency in GluR2. However, conflicting evidence exists about the in vitro and in vivo expression of GluR2 in motor neurons, both at the mRNA and at the protein level. In this study, we quantified electrophysiological properties of AMPA receptors, known to be dependent on the relative abundance of GluR2: sensitivity to external polyamines, rectification index, and relative Ca(2+) permeability. Cultured rat spinal cord motor neurons were compared with dorsal horn neurons (which are resistant to excitotoxicity) and with motor neurons that survived an excitotoxic insult. Motor neurons had a higher sensitivity to external polyamines, a lower rectification index, and a higher relative Ca(2+) permeability ratio than dorsal horn neurons. These findings confirm that motor neurons are relatively deficient in GluR2. The AMPA receptor properties correlated well with each other and with the selective vulnerability of motor neurons because motor neurons surviving an excitotoxic event had similar characteristics as dorsal horn neurons. These data indicate that the relative abundance of GluR2 in functional AMPA receptors may be a major determinant of the selective vulnerability of motor neurons to excitotoxicity in vitro.

A fundamental oscillatory state of isolated rodent hippocampus

Population neuronal rhythms of various frequencies are observed in the rodent hippocampus during distinct behavioural states. However, the question of whether the hippocampus exhibits properties of spontaneous rhythms and population synchrony in isolation has not been definitively answered. To address this, we developed a novel preparation for studying neuronal rhythms in a relatively large hippocampal tissue in vitro. We isolated the whole hippocampus from mice up to 28 days postnatal age, removing the dentate gyrus while preserving the functional CA3-to-CA1 connections. Placing the hippocampal isolate in a perfusion chamber for electrophysiological assessment extracellular recordings from the CA1 revealed rhythmic field potential of 0.5 to </= 4 Hz that occurred spontaneously and propagated along the ventro-dorsal hippocampal axis. We provide convergent evidence, via measurements of extracellular pH and K(+), recordings of synaptic and intracellular activities and morphological assessments, verifying that these rhythms were not the consequence of hypoxia. Data obtained via simultaneous extracellular and patch clamp recordings suggest that the spontaneous rhythms represent a summation of GABAergic IPSPs originating from pyramidal neurons, which result from synchronous discharges of GABAergic inhibitory interneurons. Similar spontaneous field rhythms were also observed in the hippocampal isolate prepared from young gerbils and rats. Based on these data, we postulate that the spontaneous rhythms represent a fundamental oscillatory state of the hippocampal circuitry isolated from extra-hippocampal inputs.