Swope Supernova Survey 2017a (SSS17a), the optical counterpart to a gravitational wave source.

On 17 August 2017, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo interferometer detected gravitational waves (GWs) emanating from a binary neutron star merger, GW170817. Nearly simultaneously, the Fermi and INTEGRAL (INTErnational Gamma-Ray Astrophysics Laboratory) telescopes detected a gamma-ray transient, GRB 170817A. At 10.9 hours after the GW trigger, we discovered a transient and fading optical source, Swope Supernova Survey 2017a (SSS17a), coincident with GW170817. SSS17a is located in NGC 4993, an S0 galaxy at a distance of 40 megaparsecs. The precise location of GW170817 provides an opportunity to probe the nature of these cataclysmic events by combining electromagnetic and GW observations.

Electromagnetic evidence that SSS17a is the result of a binary neutron star merger.

Eleven hours after the detection of gravitational wave source GW170817 by the Laser Interferometer Gravitational-Wave Observatory and Virgo Interferometers, an associated optical transient, SSS17a, was identified in the galaxy NGC 4993. Although the gravitational wave data indicate that GW170817 is consistent with the merger of two compact objects, the electromagnetic observations provide independent constraints on the nature of that system. We synthesize the optical to near-infrared photometry and spectroscopy of SSS17a collected by the One-Meter Two-Hemisphere collaboration, finding that SSS17a is unlike other known transients. The source is best described by theoretical models of a kilonova consisting of radioactive elements produced by rapid neutron capture (the r-process). We conclude that SSS17a was the result of a binary neutron star merger, reinforcing the gravitational wave result.

Early spectra of the gravitational wave source GW170817: Evolution of a neutron star merger.

On 17 August 2017, Swope Supernova Survey 2017a (SSS17a) was discovered as the optical counterpart of the binary neutron star gravitational wave event GW170817. We report time-series spectroscopy of SSS17a from 11.75 hours until 8.5 days after the merger. Over the first hour of observations, the ejecta rapidly expanded and cooled. Applying blackbody fits to the spectra, we measured the photosphere cooling from [Formula: see text] to [Formula: see text] kelvin, and determined a photospheric velocity of roughly 30% of the speed of light. The spectra of SSS17a began displaying broad features after 1.46 days and evolved qualitatively over each subsequent day, with distinct blue (early-time) and red (late-time) components. The late-time component is consistent with theoretical models of r-process-enriched neutron star ejecta, whereas the blue component requires high-velocity, lanthanide-free material.

Light curves of the neutron star merger GW170817/SSS17a: Implications for r-process nucleosynthesis.

On 17 August 2017, gravitational waves (GWs) were detected from a binary neutron star merger, GW170817, along with a coincident short gamma-ray burst, GRB 170817A. An optical transient source, Swope Supernova Survey 17a (SSS17a), was subsequently identified as the counterpart of this event. We present ultraviolet, optical, and infrared light curves of SSS17a extending from 10.9 hours to 18 days postmerger. We constrain the radioactively powered transient resulting from the ejection of neutron-rich material. The fast rise of the light curves, subsequent decay, and rapid color evolution are consistent with multiple ejecta components of differing lanthanide abundance. The late-time light curve indicates that SSS17a produced at least ~0.05 solar masses of heavy elements, demonstrating that neutron star mergers play a role in rapid neutron capture (r-process) nucleosynthesis in the universe.

Normal and epilepsy-associated pathologic function of the dentate gyrus.

The dentate gyrus plays critical roles both in cognitive processing, and in regulation of the induction and propagation of pathological activity. The cellular and circuit mechanisms underlying these diverse functions overlap extensively. At the cellular level, the intrinsic properties of dentate granule cells combine to endow these neurons with a fundamental reluctance to activate, one of their hallmark traits. At the circuit level, the dentate gyrus constitutes one of the more heavily inhibited regions of the brain, with strong, fast feedforward and feedback GABAergic inhibition dominating responses to afferent activation. In pathologic states such as epilepsy, a number of alterations within the dentate gyrus combine to compromise the regulatory properties of this circuit, culminating in a collapse of its normal function. This epilepsy-associated transformation in the fundamental properties of this critical regulatory hippocampal circuit may contribute both to seizure propensity, and cognitive and emotional comorbidities characteristic of this disease state.

Structure-function analysis of SAP97, a modular scaffolding protein that drives dendrite growth.

Activation of AMPA receptors assembled with the GluA1 subunit can promote dendrite growth in a manner that depends on its direct binding partner, SAP97. SAP97 is a modular scaffolding protein that has at least seven recognizable protein-protein interaction domains. Several complementary approaches were employed to show that the dendrite branching promoting action of full length SAP97 depends on ligand(s) that bind to the PDZ3 domain. Ligand(s) to PDZ1, PDZ2 and I3 domains also contribute to dendrite growth. The ability of PDZ3 ligand(s) to promote dendrite growth depends on localization at the plasma membrane along with GluA1 and SAP97. These results suggest that the assembly of a multi-protein complex at or near synapses is vital for the translation of AMPA-R activity into dendrite growth.

Role of the NR2A/2B subunits of the N-methyl-D-aspartate receptor in glutamate-induced glutamic acid decarboxylase alteration in cortical GABAergic neurons in vitro.

The vulnerability of brain neuronal cell subpopulations to neurologic insults varies greatly. Among cells that survive a pathological insult, for example ischemia or brain trauma, some may undergo morphological and/or biochemical changes that may compromise brain function. The present study is a follow-up of our previous studies that investigated the effect of glutamate-induced excitotoxicity on the GABA synthesizing enzyme glutamic acid decarboxylase (GAD65/67)'s expression in surviving DIV 11 cortical GABAergic neurons in vitro [Monnerie and Le Roux, (2007) Exp Neurol 205:367-382, (2008) Exp Neurol 213:145-153]. An N-methyl-D-aspartate receptor (NMDAR)-mediated decrease in GAD expression was found following glutamate exposure. Here we examined which NMDAR subtype(s) mediated the glutamate-induced change in GAD protein levels. Western blotting techniques on cortical neuron cultures showed that glutamate's effect on GAD proteins was not altered by NR2B-containing diheteromeric (NR1/NR2B) receptor blockade. By contrast, blockade of triheteromeric (NR1/NR2A/NR2B) receptors fully protected against a decrease in GAD protein levels following glutamate exposure. When receptor location on the postsynaptic membrane was examined, extrasynaptic NMDAR stimulation was observed to be sufficient to decrease GAD protein levels similar to that observed after glutamate bath application. Blocking diheteromeric receptors prevented glutamate's effect on GAD proteins after extrasynaptic NMDAR stimulation. Finally, NR2B subunit examination with site-specific antibodies demonstrated a glutamate-induced, calpain-mediated alteration in NR2B expression. These results suggest that glutamate-induced excitotoxic NMDAR stimulation in cultured GABAergic cortical neurons depends upon subunit composition and receptor location (synaptic vs. extrasynaptic) on the neuronal membrane. Biochemical alterations in surviving cortical GABAergic neurons in various disease states may contribute to the altered balance between excitation and inhibition that is often observed after injury.

Effects of status epilepticus on hippocampal GABAA receptors are age-dependent.

Long-term GABA(A) receptor alterations occur in hippocampal dentate granule neurons of rats that develop epilepsy after status epilepticus in adulthood. Hippocampal GABA(A) receptor expression undergoes marked reorganization during the postnatal period, however, and the effects of neonatal status epilepticus on subsequent GABA(A) receptor development are unknown. In the current study, we utilize single cell electrophysiology and antisense mRNA amplification to determine the effect of status-epilepticus induced by lithium-pilocarpine in postnatal day 10 rat pups on GABA(A) receptor subunit expression and function in hippocampal dentate granule neurons. We find that rats subjected to lithium-pilocarpine-induced status epilepticus at postnatal day 10 show long-term GABA(A) receptor changes including a two-fold increase in alpha1 subunit expression (compared with lithium-injected controls) and enhanced type I benzodiazepine augmentation that are opposite of those seen after status epilepticus in adulthood and may serve to enhance dentate gyrus inhibition. Further, unlike adult rats, postnatal day 10 rats subjected to status epilepticus do not become epileptic. These findings suggest age-dependent differences in the effects of status epilepticus on hippocampal GABA(A) receptors that could contribute to the selective resistance of the immature brain to epileptogenesis.

Specific proteolysis of the NR2 subunit at multiple sites by calpain.

The NMDA subtype of glutamate receptor plays an important role in the molecular mechanisms of learning, memory and excitotoxicity. NMDA receptors are highly permeable to calcium, which can lead to the activation of the calcium-dependent protease, calpain. In the present study, the ability of calpain to modulate NMDA receptor function through direct proteolytic digestion of the individual NMDA receptor subunits was examined. HEK293t cells were cotransfected with the NR1a/2A, NR1a/2B or NR1a/2C receptor combinations. Cellular homogenates of these receptor combinations were prepared and digested by purified calpain I in vitro. All three NR2 subunits could be proteolyzed by calpain I while no actin or NR1a cleavage was observed. Based on immunoblot analysis, calpain cleavage of NR2A, NR2B and NR2C subunits was limited to their C-terminal region. In vitro calpain digestion of fusion protein constructs containing the C-terminal region of NR2A yielded two cleavage sites at amino acids 1279 and 1330. Although it has been suggested that calpain cleavage of the NMDA receptor may act as a negative feedback mechanism, the current findings demonstrated that calpain cleavage did not alter [(125)I]MK801 binding and that receptors truncated to the identified cleavage sites had peak intracellular calcium levels, (45)Ca uptake rates and basal electrophysiological properties similar to wild type.

gamma-Aminobutyric acid(A) receptor subunit expression predicts functional changes in hippocampal dentate granule cells during postnatal development.

Profound alterations in the function of GABA occur over the course of postnatal development. Changes in GABA(A) receptor expression are thought to contribute to these differences in GABAergic function, but how subunit changes correlate with receptor function in individual developing neurons has not been defined precisely. In the current study, we correlate expression of 14 different GABA(A) receptor subunit mRNAs with changes in the pharmacological properties of the receptor in individual hippocampal dentate granule cells over the course of postnatal development in rat. We demonstrate significant developmental differences in GABA(A) receptor subunit mRNA expression, including greater than two-fold lower expression of alpha1-, alpha4- and gamma2-subunit mRNAs and 10-fold higher expression of alpha5-mRNA in immature compared with adult neurons. These differences correlate both with regional changes in subunit protein level and with alterations in GABA(A) receptor function in immature dentate granule cells, including two-fold higher blockade by zinc and three-fold lower augmentation by type-I benzodiazepine site modulators. Further, we find an inverse correlation between changes in GABA(A) receptor zinc sensitivity and abundance of vesicular zinc in dentate gyrus during postnatal development. These findings suggest that developmental differences in subunit expression contribute to alterations in GABA(A) receptor function during postnatal development.

Zinc-induced augmentation of excitatory synaptic currents and glutamate receptor responses in hippocampal CA3 neurons.

Zinc is found throughout the CNS at synapses co-localized with glutamate in presynaptic terminals. In particular, dentate granule cells' (DGC) mossy fiber (MF) axons contain especially high concentrations of zinc co-localized with glutamate within vesicles. To study possible physiological roles of zinc, visualized slice-patch techniques were used to voltage-clamp rat CA3 pyramidal neurons, and miniature excitatory postsynaptic currents (mEPSCs) were isolated. Bath-applied zinc (200 microM) enhanced median mEPSC peak amplitudes to 153.0% of controls, without affecting mEPSC kinetics. To characterize this augmentation further, rapid agonist application was performed on perisomatic outside-out patches to coapply zinc with glutamate extremely rapidly for brief (1 ms) durations, thereby emulating release kinetics of these substances at excitatory synapses. When zinc was coapplied with glutamate, zinc augmented peak glutamate currents (mean +/- SE, 116.6 +/- 2.8% and 143.8 +/- 9.8% of controls at 50 and 200 microM zinc, respectively). This zinc-induced potentiation was concentration dependent, and pharmacological isolation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated currents (AMPAR currents) gave results similar to those observed with glutamate application (mean, 115.0 +/- 5.4% and 132.5 +/- 9.1% of controls at 50 and 200 microM zinc, respectively). Inclusion of the AMPAR desensitization blocker cyclothiazide in the control solution, however, abolished zinc-induced augmentation of glutamate-evoked currents, suggesting that zinc may potentiate AMPAR currents by inhibiting AMPAR desensitization. Based on the results of the present study, we hypothesize that zinc is a powerful modulator of both excitatory synaptic transmission and glutamate-evoked currents at physiologically relevant concentrations. This modulatory role played by zinc may be a significant factor in enhancing excitatory neurotransmission and could significantly regulate function at the mossy fiber-CA3 synapse.

Epilepsy-associated plasticity in gamma-aminobutyric acid receptor expression, function, and inhibitory synaptic properties.

Although epilepsy is fundamentally a circuit phenomenon, the most basic manifestation of the hyperexcitability characteristic of epilepsy must be evident at the level of a single neuron. Furthermore, in the future, manipulations of surviving neurons within the epileptic focus will constitute one of the best therapeutic targets for intervention to cure this devastating disease. Therefore, the more that can be learned about epileptogenic alterations in this population of surviving focal neurons, the more potential avenues for therapeutic intervention will emerge. This chapter has summarized one aspect of postsynaptic neuronal function that undergoes dramatic alterations in the epileptic brain: the properties of inhibitory neurotransmitter (i.e., GABA) receptors in surviving focal neurons. GABARs in these neurons undergo significant alterations in their function and pharmacology, which appear to be mediated, at least in part, by alterations in the transcriptional production of GABAR subunits. These GABAR alterations fulfill many of the requirements for an epileptogenic mechanism: they are consistent with the hyperexcitability characteristic of epilepsy; the changes develop prior to the onset of recurrent spontaneous seizures; and the elevated zinc sensitivity of epileptic GABARs combined with epilepsy-associated mossy fiber sprouting (a zinc "delivery mechanism") can account for the existence of a prolonged latent period. Although GABAR alterations in DGCs of the epileptic hippocampus may be consistent with hyperexcitability and therefore contribute to epileptogenesis, many other processes undoubtedly also contribute, including (but not limited to) neuronal loss, circuit rearrangements, alterations in other membrane proteins, and birth of new neurons. Assuming any single change is both necessary and sufficient to fully account for epilepsy is undoubtedly an oversimplification. The initial precipitating events associated with the subsequent development of epilepsy are often traumatic events and associated with changes in many processes in widespread areas of the brain. Some of these processes may contribute to excitability changes, some may resist the development of epilepsy, and some may be unrelated to epileptogenesis. Characterizing the critical processes initiated during epileptogenesis remains an important and challenging research endeavor for the foreseeable future.

Protracted postnatal development of inhibitory synaptic transmission in rat hippocampal area CA1 neurons.

In the CNS, inhibitory synaptic function undergoes profound transformation during early postnatal development. This is due to variations in the subunit composition of subsynaptic GABA(A) receptors (GABA(A)Rs) at differing developmental stages as well as other factors. These include changes in the driving force for chloride-mediated conductances as well as the quantity and/or cleft lifetime of released neurotransmitter. The present study was undertaken to investigate the nature and time course of developmental maturation of GABAergic synaptic function in hippocampal CA1 pyramidal neurons. In neonatal [postnatal day (P) 1-7] and immature (P8-14) CA1 neurons, miniature inhibitory postsynaptic currents (mIPSCs) were significantly larger, were less frequent, and had slower kinetics compared with mIPSCs recorded in more mature neurons. Adult mIPSC kinetics were achieved by the third postnatal week in CA1 neurons. However, despite this apparent maturation of mIPSC kinetics, significant differences in modulation of mIPSCs by allosteric agonists in adolescent (P15-21) neurons were still evident. Diazepam (1-300 nM) and zolpidem (200 nM) increased the amplitude of mIPSCs in adolescent but not adult neurons. Both drugs increased mIPSC decay times equally at both ages. These differential agonist effects on mIPSC amplitude suggest that in adolescent CA1 neurons, inhibitory synapses operate differently than adult synapses and function as if subsynaptic receptors are not fully occupied by quantal release of GABA. Rapid agonist application experiments on perisomatic patches pulled from adolescent neurons provided additional support for this hypothesis. In GABA(A)R currents recorded in these patches, benzodiazepine amplitude augmentation effects were evident only when nonsaturating GABA concentrations were applied. Furthermore nonstationary noise analysis of mIPSCs in P15-21 neurons revealed that zolpidem-induced mIPSC augmentation was not due to an increase in single-channel conductance of subsynaptic GABA(A)Rs but rather to an increase in the number of open channels responding to a single GABA quantum, further supporting the hypothesis that synaptic receptors may not be saturated during synaptic function in adolescent neurons. These data demonstrate that inhibitory synaptic transmission undergoes a markedly protracted postnatal maturation in rat CA1 pyramidal neurons. In the first two postnatal weeks, mIPSCs are large in amplitude, are slow, and occur infrequently. By the third postnatal week, mIPSCs have matured kinetically but retain distinct responses to modulatory drugs, possibly reflecting continued immaturity in synaptic structure and function persisting through adolescence.

Mossy fiber zinc and temporal lobe epilepsy: pathological association with altered "epileptic" gamma-aminobutyric acid A receptors in dentate granule cells.

Temporal lobe epilepsy is associated with circuit rearrangements within the hippocampus. Mossy fibers sprout and pathologically innervate the inner molecular layer of the dentate gyrus, providing a recurrent excitatory pathway not present in the control brain. In addition to releasing glutamate, these recurrent collaterals also release zinc, which can accumulate in high concentrations in the extracellular space. Accompanying these dentate gyrus circuit rearrangements are alterations in the subunit expression patterns and pharmacology of gamma-aminobutyric acid A (GABAA) receptors in dentate granule cells. In normal, control granule cells, GABAA receptors are zinc insensitive as a result of high levels of expression of the alpha1 subunit in these cells. In epileptic brain, expression of alpha1 subunits decreases and expression of alpha4 and delta subunits increases, leading to the assembly of GABAA receptors that are exquisitely zinc sensitive. This temporal and spatial association of the expression of zinc-sensitive GABAA receptors and the emergence of a zinc-delivery system unique to the epileptic hippocampus has led to the formulation of an hypothesis that suggests that zinc release during repetitive activation of the dentate gyrus may lead to a catastrophic failure of inhibition under conditions mediating seizure initiation. This could contribute to the limbic hyperexcitability characteristic of temporal lobe epilepsy.

Cellular actions of topiramate: blockade of kainate-evoked inward currents in cultured hippocampal neurons.

PURPOSE: This study was undertaken to evaluate the effects of topiramate (TPM) on excitatory amino acid-evoked currents. METHODS: Kainate and N-methyl-D-aspartate (NMDA) were applied to cultured rat hippocampal neurons by using a concentration-clamp apparatus to selectively activate the AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid)/kainate and NMDA receptor subtypes, respectively. The evoked membrane currents were recorded by using perforated-patch whole-cell voltage-clamp techniques. RESULTS: TPM partially blocked kainate-evoked currents with an early-onset reversible phase (phase I) and a late-onset phase (phase II) that occurred after a 10- to 20-min delay and did not reverse during a 2-h washout period. Application of dibutyryl cyclic adenosine monophosphate (cAMP; 2 mM) during washout after phase II block enhanced reversal, with the kainate current amplitude being restored by approximately 50%. Phase II but not phase I block was prevented by prior application of okadaic acid (1 microM), a broad-spectrum phosphatase inhibitor, suggesting that phase II block may be mediated through interactions with intracellular intermediaries that alter the phosphorylation state of kainate-activated channels. Topiramate at 100 microM blocked kainate-evoked currents by 90% during phase II, but had no effect on NMDA-evoked currents. The median inhibitory concentration (IC50) values for phase I and II block of kainate currents were 1.6 and 4.8 microM, respectively, which are within the range of free serum levels of TPM in patients. CONCLUSIONS: The specific blockade of the kainate-induced excitatory conductance is consistent with the ability of TPM to reduce neuronal excitability and could contribute to the anticonvulsant efficacy of this drug.

Effects of topiramate on sustained repetitive firing and spontaneous recurrent seizure discharges in cultured hippocampal neurons.

PURPOSE: In this study, we examined the effects of topiramate (TPM) on the electrophysiologic properties of cultured rat hippocampal pyramidal neurons. METHODS: Whole-cell current-clamp recording techniques were used to determine the effects of TPM on sustained repetitive firing (SRF), spontaneous epileptiform-burst firing, and spontaneous recurrent seizures (SRS). RESULTS: Topiramate at therapeutic concentrations (10-100 microM) significantly decreased or abolished SRF in a dose-dependent and partially reversible manner. When transiently exposed to a medium in which Mg2+ is omitted, hippocampal neurons in culture develop SRS ("epilepsy") and epileptiform discharges. Application of TPM at concentrations ranging from 10 to 100 microM to cells displaying seizure activity caused a concentration-dependent decrease in the number of action potentials within a burst and in the average duration of epileptiform activity. Both effects were partially reversed during a 5- to 30-min drug washout period. CONCLUSIONS: These effects on the electrophysiologic properties of cultured neurons are consistent with the concept that TPM exerts modulatory effects on voltage-dependent Na+ and/or Ca2+ conductances responsible for the generation and propagation of action potentials. Topiramate also may inhibit synaptic conductances responsible for transmission of epileptiform discharges.

Regionally selective blockade of GABAergic inhibition by zinc in the thalamocortical system: functional significance.

The thalamocortical (TC) system is a tightly coupled synaptic circuit in which GABAergic inhibition originating from the nucleus reticularis thalami (NRT) serves to synchronize oscillatory TC rhythmic behavior. Zinc is colocalized within nerve terminals throughout the TC system with dense staining for zinc observed in NRT, neocortex, and thalamus. Whole cell voltage-clamp recordings of GABA-evoked responses were conducted in neurons isolated from ventrobasal thalamus, NRT, and somatosensory cortex to investigate modulation of the GABA-mediated chloride conductance by zinc. Zinc blocked GABA responses in a regionally specific, noncompetitive manner within the TC system. The regional levels of GABA blockade efficacy by zinc were: thalamus > NRT > cortex. The relationship between clonazepam and zinc sensitivity of GABA(A)-mediated responses was examined to investigate possible presence or absence of specific GABA(A) receptor (GABAR) subunits. These properties of GABARs have been hypothesized previously to be dependent on presence or absence of the gamma2 subunit and seem to display an inverse relationship. In cross-correlation plots, thalamic and NRT neurons did not show a statistically significant relationship between clonazepam and zinc sensitivity; however, a statistically significant correlation was observed in cortical neurons. Spontaneous epileptic TC oscillations can be induced in vitro by perfusion of TC slices with an extracellular medium containing no added Mg(2+). Multiple varieties of oscillations are generated, including simple TC burst complexes (sTBCs), which resemble spike-wave discharge activity. A second variant was termed a complex TC burst complex (cTBC), which resembled generalized tonic clonic seizure activity. sTBCs were exacerbated by zinc, whereas cTBCs were blocked completely by zinc. This supported the concept that zinc release may modulate TC rhythms in vivo. Zinc interacts with a variety of ionic conductances, including GABAR currents, N-methyl-D-aspartate (NMDA) receptor currents, and transient potassium (A) currents. D-2-amino-5-phosphonovaleric acid and 4-aminopyridine blocked both s- and cTBCs in TC slices. Therefore NMDA and A current-blocking effects of zinc are insufficient to explain differential zinc sensitivity of these rhythms. This supports a significant role of zinc-induced GABAR modulation in differential TC rhythm effects. Zinc is localized in high levels within the TC system and appears to be released during TC activity. Furthermore application of exogenous zinc modulates TC rhythms and differentially blocks GABARs within the TC system. These data are consistent with the hypothesis that endogenously released zinc may have important neuromodulatory actions impacting generation of TC rhythms, mediated at least in part by effects on GABARs.

Basic mechanisms of status epilepticus.

This chapter reviews two main aspects of the basic mechanisms of status epilepticus--acute factors, which are important in inducing status epilepticus in an in vitro brain slice model of status epilepticus, and the acute and chronic epileptogenic consequences of status epilepticus. Status epilepticus is difficult to produce in vitro in normal extracellular medium. This suggests that seizure-terminating mechanisms are normally quite robust. To produce long- duration, self-sustained epileptic discharges in vitro, we have found it necessary to include reciprocally connected entorhinal cortex with our hippocampal slices. Doing so closes the normal excitatory limbic loop in the brain. We found incorporation of the full loop in our brain-slice preparations necessary to bring about epileptic discharges of long duration that fit the definition of status epilepticus. Reentrant activation from distant sites may be necessary for maintenance of status epilepticus-like activity of long duration. Similar requirements may exist for generalized tonic-clonic status epilepticus discharges, but as yet no data support or refute this hypothesis. There are both acute and chronic consequences of an episode of status epilepticus. Acute consequences are alterations in membrane potential and membrane properties of hippocampal pyramidal cells accompanied by alterations in neurotransmitter-activated conductances and receptor expression. Some of these acute alterations in receptor and transmembrane iongradient associated with status epilepticus may be critically involved in the development of drug resistance during the late stages of status epilepticus. Long-term consequences of status epilepticus in the limbic system include alterations in patterns of expression of neurotransmitter receptors and in the function of excitatory and inhibitory synapses, cell loss, and circuit rearrangements within the limbic system. An episode of status epilepticus that involves the limbic system clearly elicits brain damage, at least among adult animals. This brain damage can contribute to the development of epilepsy, or a condition of recurrent, spontaneous seizures. Conversely, development of an epileptic condition enhances the susceptibility of the limbic system to trigger status epilepticus discharges.

Human neuronal gamma-aminobutyric acid(A) receptors: coordinated subunit mRNA expression and functional correlates in individual dentate granule cells.

gamma-Aminobutyric acid(A) receptors (GABARs) are heteromeric proteins composed of multiple subunits. Numerous subunit subtypes are expressed in individual neurons, which assemble in specific preferred GABAR configurations. Little is known, however, about the coordination of subunit expression within individual neurons or the impact this may have on GABAR function. To investigate this, it is necessary to profile quantitatively the expression of multiple subunit mRNAs within individual cells. In this study, single-cell antisense RNA amplification was used to examine the expression of 14 different GABAR subunit mRNAs simultaneously in individual human dentate granule cells (DGCs) harvested during hippocampectomy for intractable epilepsy. alpha4, beta2, and delta-mRNA levels were tightly correlated within individual DGCs, indicating that these subunits are expressed coordinately. Levels of alpha3- and beta2-mRNAs, as well as epsilon- and beta1-mRNAs, also were strongly correlated. No other subunit correlations were identified. Coordinated expression could not be explained by the chromosomal clustering of GABAR genes and was observed in control and epileptic rats as well as in humans, suggesting that it was not species-specific or secondary to epileptogenesis. Benzodiazepine augmentation of GABA-evoked currents also was examined to determine whether levels of subunit mRNA expression correlated with receptor pharmacology. This analysis delineated two distinct cell populations that differed in clonazepam modulation and patterns of alpha-subunit expression. Clonazepam augmentation correlated positively with the relative expression of alpha1- and gamma2-mRNAs and negatively with alpha4- and delta-mRNAs. These data demonstrate that specific GABAR subunit mRNAs exhibit coordinated control of expression in individual DGCs, which has significant impact on inhibitory function.

Chronic epileptogenic cellular alterations in the limbic system after status epilepticus.

Status epilepticus (SE) is associated with both acute and permanent pathological sequellae. One common long term consequence of SE is the subsequent development of a chronic epileptic condition, with seizures frequently originating from and involving the limbic system. Following SE, many studies have demonstrated selective loss of neurons within the hilar region of the dentate gyrus, CA1 and CA3 pyramidal neurons. Selective loss of distinct subpopulations of interneurons throughout the hippocampus is also frequently evident, although interneurons as a whole are selectively spared relative to principal cells. Accompanying this loss of neurons are circuit rearrangements, the most widely studied being the sprouting of dentate granule cell (DGC) axons back onto the inner molecular layer of the dentate gyrus, termed mossy fiber sprouting. Less studied are the receptor properties of the surviving neurons within the epileptic hippocampus following SE. DGCs in epileptic animals exhibit marked alterations in the functional and pharmacological properties of gamma-aminobutyric acid (GABA) receptors. DGCs have a significantly elevated density of GABA(A) receptors in chronically epileptic animals. In addition, the pharmacological properties of GABA(A) receptors in post-SE epileptic animals are quite different compared to controls. In particular, GABA(A) receptors in DGCs from epileptic animals show an enhanced sensitivity to blockade by zinc, and a markedly altered sensitivity to modulation by benzodiazepines. These pharmacological differences may be due to a decreased expression of alpha1 subunits of the GABA(A) receptor relative to other alpha subunits in DGCs of post-SE epileptic animals. These GABA(A) receptor alterations precede the onset of spontaneous seizures in post-SE DGCs, and so are temporally positioned to contribute to the process of epileptogenesis in the limbic system. The presence of zinc sensitive GABA receptors combined with the presence of zinc-containing "sprouted" mossy fiber terminals innervating the proximal dendrites of DGCs in the post-SE epileptic hippocampus prompted the development of the hypothesis that repetitive activation of the DG in the epileptic brain could result in the release of zine. This zinc in turn may diffuse to and block "epileptic" zinc-sensitive GABA(A) receptors in DGCs, leading to a catastrophic failure of inhibition and concomitant enhanced seizure propensity in the post-SE epileptic limbic system.