Changes in mIPSCs and sIPSCs after kainate treatment: status epilepticus-induced neuronal loss or direct activation of kainate receptors?
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Animals↗
Biomedical subjects
F Edward Dudek
Publications and source records attributed to F Edward Dudek.
The use of radiotelemetry to evaluate electrographic seizures in rats with kainate-induced epilepsy.
Temporal lobe epilepsy in humans is a chronic condition with a highly variable temporal evolution. Animal models of this disorder have been developed to recapitulate many of the characteristics seen in humans with temporal lobe epilepsy. These animal models generate chronic spontaneous electrographic and motor seizures with a progressive increase in frequency over many months. In order to understand the underlying cellular and molecular mechanisms driving epileptogenesis, a practical means for accurately assessing seizure progression over this extended time period must be devised. In this report, we describe the use of a three-channel radiotelemetry system to record spontaneous electrographic interictal "spikes" and seizure activity from the cortical surface and the two hippocampi. This approach has allowed continuous recording before, during, and several months after kainate-induced status epilepticus. The important advantages of this approach are the potential for long-term continuous electrographic recording with comparatively unrestricted behavior; the disadvantages include increased cost, surgical difficulty and lower frequency-response in the recordings.
Action Potentials↗
Epilepsy and reproductive disorders: the role of the gonadotropin-releasing hormone network.
Individuals with temporal lobe epilepsy have an increased incidence of reproductive dysfunction. The comorbidity may be due to the acute effects of the seizures, the chronic effects of the epilepsy, and/or the use of antiepileptic drugs on the gonadotropin-releasing hormone network and the hypothalamic-pituitary-gonadal axis. This review provides a brief overview of evidence from experimental animal and clinical studies exploring the basis for epilepsy-associated reproductive abnormalities.
Animals↗
Efficient unsupervised algorithms for the detection of seizures in continuous EEG recordings from rats after brain injury.
Long-term EEG monitoring in chronically epileptic animals produces very large EEG data files which require efficient algorithms to differentiate interictal spikes and seizures from normal brain activity, noise, and, artifact. We compared four methods for seizure detection based on (1) EEG power as computed using amplitude squared (the power method), (2) the sum of the distances between consecutive data points (the coastline method), (3) automated spike frequency and duration detection (the spike frequency method), and (4) data range autocorrelation combined with spike frequency (the autocorrelation method). These methods were used to analyze a randomly selected test set of 13 days of continuous EEG data in which 75 seizures were imbedded. The EEG recordings were from eight different rats representing two different models of chronic epilepsy (five kainate-treated and three hypoxic-ischemic). The EEG power method had a positive predictive value (PPV, or true positives divided by the sum of true positives and false positives) of 18% and a sensitivity (true positives divided by the sum of true positives and false negatives) of 95%, the coastline method had a PPV of 78% and sensitivity of 99.59, the spike frequency method had a PPV of 78% and a sensitivity of 95%, and the autocorrelation method yielded a PPV of 96% and a sensitivity of 100%. It is possible to detect seizures automatically in a prolonged EEG recording using computationally efficient unsupervised algorithms. Both the quality of the EEG and the analysis method employed affect PPV and sensitivity.
Algorithms↗
Noise of the slowly inactivating Na current in suprachiasmatic nucleus neurons.
Slow depolarization induces a riluzole-sensitive persistent Na current (INa,P) in several types of neurons and a pharmacologically similar slowly inactivating Na current (INa,S) in rat suprachiasmatic nucleus neurons. In isolated suprachiasmatic nucleus neurons, INa,S fluctuations were analyzed to characterize the Na channel responsible for INa,S. The single-channel current near resting potential was -0.53+/-0.04 pA in an external solution containing 151 mM Na+ and 5 mM Na+ in the patch pipette. Tetrodotoxin completely blocked INa,S and single-channel current noise; 25 microM riluzole also suppressed INa,S and current noise by about 80% without a significant effect on mean single-channel current. The data suggest that single-channel noise is due to the opening of channels mediating INa,S with a conductance of about 3.4 pS.
Animals↗
Changes in mIPSCs and sIPSCs after kainate treatment: evidence for loss of inhibitory input to dentate granule cells and possible compensatory responses.
How inhibition is altered after status epilepticus and the role of inhibition during epileptogenesis remain unsettled issues. The present study examined acute (4-7 days) and chronic (>3 mo) changes of GABA(A) receptor-mediated inhibitory synaptic input to dentate granule cells after kainate-induced status epilepticus. Whole cell patch-clamp techniques were used to record spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs) in the presence of 6,7-dinitroquinoxaline-2,3-dione and dl-2-amino-5-phosphonopentanoic acid to block glutamatergic excitatory synaptic transmission. In both groups, mean sIPSC frequency of dentate granule cells from the saline- and kainate-treated rats was not significantly different. However, mIPSC frequency from the kainate-treated rats of both groups was approximately 30% lower than that of the respective saline controls. The mean amplitude of sIPSCs and mIPSCs from kainate-treated rats was not reduced in either the acute or chronic groups. The mean 10-90% rise time of IPSCs was not altered in kainate-treated rats, but the decay time constant was slightly longer than in controls, and the charge transfer 4-7 days after kainate treatment was significantly larger. The similar reduction of mIPSC frequency (i.e., approximately 30%) in the two groups of kainate-treated rats suggests a decreased inhibitory input to dentate granule cells (presumably due to a partial loss of inhibitory interneurons that innervate them) without recovery during epileptogenesis. The lack of effect on sIPSC frequency and the decreased mIPSC frequency in both groups suggests a possible compensatory increase in firing rate of interneurons, which may involve a hypothetical reduction of inhibitory input to the remaining interneurons.
Analysis of Variance↗
5-HT1B receptor-mediated presynaptic inhibition of GABA release in the suprachiasmatic nucleus.
The suprachiasmatic nucleus (SCN) receives a dense serotonergic innervation that modulates photic input to the SCN via serotonin 1B (5-HT1B) presynaptic receptors on retinal glutamatergic terminals. However, the majority of 5-HT1B binding sites in the SCN are located on nonretinal terminals and most axonal terminals in the SCN are GABAergic. We therefore tested the hypothesis that 5-HT1B receptors might also be located on SCN GABAergic terminals by examining the effects of the highly selective 5-HT1B receptor agonist CP-93,129 on SCN miniature inhibitory postsynaptic currents (mIPSCs). Whole cell patch-clamp recordings of mIPSCs were obtained from rat and mouse SCN neurons in hypothalamic slices. Using CsCl-containing microelectrodes with QX314, we isolated mPSCs that were sensitive to the GABAA receptor antagonist, bicuculline. Bath application of CP-93,129 (1 microM) decreased the frequency of mIPSCs by an average of 22% (n = 7) in rat SCN neurons and by an average of 30% (n = 8) in mouse SCN neurons with no clear effect on mIPSC amplitude. In mice lacking functional 5-HT1B receptors, CP-93,129 (1 microM) had no clear effect on the frequency or the amplitude of mIPSCs recorded in any of the cells tested (n = 4). The decrease in the frequency of mIPSCs of SCN neurons produced by the selective 5-HT1B receptor agonist CP-93,129 is consistent with the interpretation that 5-HT1B receptors are located on GABA terminals in the SCN and that 5-HT inhibits GABA release via a 5-HT1B presynaptic receptor-mediated mechanism.
Animals↗
Use of chronic epilepsy models in antiepileptic drug discovery: the effect of topiramate on spontaneous motor seizures in rats with kainate-induced epilepsy.
PURPOSE: Potential antiepileptic drugs (AEDs) are typically screened on acute seizures in normal animals, such as those induced in the maximal electroshock and pentylenetet-razole models. As a proof-of-principle test, the present experiments used spontaneous epileptic seizures in kainate-treated rats to examine the efficacy of topiramate (TPM) with a repeated-measures, crossover protocol. METHODS: Kainic acid was administered in repeated low doses (5 mg/kg) every hour until each Sprague-Dawley rat experienced convulsive status epilepticus for >3 h. Six 1-month trials (n = 6-10 rats) assessed the effects of 0.3-100 mg/kg TPM on spontaneous seizures. Each trial involved six pairs of TPM and saline-control treatments administered as intraperitoneal injections on alternate days with a recovery day between each treatment day. Data analysis included a log transformation to compensate for the asymmetric distribution of values and the heterogeneous variances, which appeared to arise from clustering of seizures. RESULTS: A significant effect of TPM was observed for 12 h (i.e., two 6-h periods) after a 30-mg/kg injection, and full recovery from the drug effect was complete within 43 h. TPM exerted a significant effect at doses of 10, 30, and 100 mg/kg, and the effects of TPM (0.3-100 mg/kg) were dose dependent. CONCLUSIONS: These data suggest that animal models with spontaneous seizures, such as kainate- and pilocarpine-treated rats, can be used efficiently for rapid testing of AEDs with a repeated-measures, crossover protocol. Furthermore, the results indicate that this design allows both dose-effect and time-course-of-recovery studies.
Animals↗
Detection of increased local excitatory circuits in the hippocampus during epileptogenesis using focal flash photolysis of caged glutamate.
PURPOSE: Local synaptic circuits, particularly recurrent excitation, are hypothesized to contribute to the generation and synchronization of epileptiform activity. The present study tested whether local excitatory circuits in the hippocampus are increased in an animal model of temporal lobe epilepsy, and thus may contribute to epileptic seizures. METHODS: Rats were given hourly injections of kainic acid to induce status epilepticus, which led to chronic epilepsy with spontaneous recurrent seizures. Whole-cell recording was performed in hippocampal slices, and focal flash photolysis of caged glutamate was used to detect local excitatory circuits. RESULTS: In the dentate gyrus of rats with kainate-induced epilepsy and mossy fiber sprouting, focal stimulations with caged glutamate at many different sites in the granule cell layer consistently evoked repetitive excitatory postsynaptic currents (EPSCs) in normal medium and prolonged bursts of action potentials in bicuculline; these responses were not observed in similarly treated slices from control rats. In CA1, focal flash photolysis of caged glutamate in stratum pyramidale revealed significantly more excitatory connections between CA1 pyramidal cells in rats with kainate-induced epilepsy than saline-treated control animals. CONCLUSION: Focal flash photolysis of caged glutamate revealed that new local excitatory circuits are formed in both the dentate gyrus and CA1 area of rats with kainate-induced epilepsy, which supports the hypothesis that the progressive formation of new local excitatory circuits occurs in many locations during epileptogenesis.
Animals↗
Do interictal spikes drive epileptogenesis?
Interictal spikes are periodic, very brief bursts of neuronal activity that are observed in the electroencephalogram of patients with chronic epilepsy. These spikes are useful diagnostically, but we do not know why they are so strongly associated with the spontaneous seizures that characterize chronic epilepsy. Interictal spikes appear before the first spontaneous seizures in animal models of acquired epilepsy, and spikes are sufficient to induce long-term changes in synaptic connections between neurons. Thus, spikes may guide the development of the neuronal circuits that initiate spontaneous seizures. If so, then attempts to prevent or cure epilepsy may best be directed at spikes rather than seizures.
Action Potentials↗
Electrophysiological evidence using focal flash photolysis of caged glutamate that CA1 pyramidal cells receive excitatory synaptic input from the subiculum.
The hippocampus sends efferent fibers to the subiculum, which projects to the entorinal cortex. Previous studies suggest that the hippocampal CA1 area may receive a projection back from the subiculum. This hypothesis was tested using whole cell recording from CA1 pyramidal cells while subicular neurons were selectively stimulated with focal flash photolysis of caged glutamate, which avoids stimulation of fibers of passage. Control experiments showed that focal flash stimulations caused direct glutamate-mediated depolarizations and bursts of action potentials in the recorded CA1 pyramidal cells, but only when the stimulation targeted the somatodendritic regions of a neuron, not the axons. To block GABA(A)-mediated inhibition and isolate local excitatory circuits, bicuculline was applied to minislices containing only the isolated CA1 area and the subiculum. Of 24 CA1 pyramidal cells, 25% (6 of 24) consistently generated repetitive excitatory postsynaptic currents (EPSCs) in response to flash stimulation in the subiculum. The responsive neurons were located 200-500 microm from the distal end of CA1 and 400-1,100 microm from the stimulation sites in subiculum, suggesting excitatory synaptic projections from the subicular neurons to CA1 pyramidal cells. This study provides new electrophysiological evidence that CA1 pyramidal cells receive excitatory synaptic input from the subiculum. Thus a reciprocal excitatory synaptic circuit connects the subiculum and the CA1 area in the normal adult rat.
Action Potentials↗
Increased excitatory synaptic activity and local connectivity of hippocampal CA1 pyramidal cells in rats with kainate-induced epilepsy.
Formation of local excitatory circuits may contribute to epileptogenesis. We tested the hypothesis that epileptogenesis is associated with increased recurrent excitation in the hippocampal CA1 area of rats with kainate-induced epilepsy. Whole cell recordings were obtained during focal flash photolysis of caged glutamate, which served as a focal excitant to activate local pyramidal cells and to study possible connections between neurons. Kainate-treated rats with spontaneous seizures were studied months after status epilepticus and were compared with saline-injected control rats. Experiments were done in isolated CA1 minislices and in bicuculline to block GABA(A) receptors. Spontaneous excitatory postsynaptic currents (sEPSCs) were present in 42% of the CA1 pyramidal cells from controls and 62% from kainate-treated rats. The frequency of sEPSCs in the kainate group was significantly higher than that in the control group, but mean amplitude was not different. Flash photolysis of caged glutamate on the somatodendritic area of CA1 pyramidal neurons caused a burst of action potentials. Local excitatory connections between CA1 pyramidal cells were found in 4 of 48 neurons (8%) in slices from control animals, but in significantly more neurons (12 of 37; 32%) from rats with kainate-induced epilepsy exhibited interconnections (P < 0.001). Photoactivation of glutamate on recorded CA1 pyramidal cells in the kainate group sometimes caused afterdischarges, but not in controls. The kainate-treated rats with pyramidal cells that responded to photostimulaltion with repetitive EPSCs appeared to have experienced more severe seizures. These data provide new electrophysiological evidence for the formation of recurrent excitatory circuits in the CA1 area of rats with kainate-induced epilepsy.
Animals↗
Update on connexins and gap junctions in neurons and glia in the mammalian nervous system.
Among the 20 proposed members of the connexin family of proteins that form gap junctional intercellular communication (GJIC) channels in mammalian tissues, over half are reported to be expressed in the nervous system. There have been conflicting observations, however, concerning the particular connexins expressed by astrocytes, oligodendrocytes, Schwann cells and neurons. Identification of the several connexin proteins at gap junctions between each neuronal and glial cell type is essential for the rational design of investigations into the functions of GJIC between glial cells and into the functional contributions of electrical and "mixed" (chemical plus electrical) synapses to communication between neurons in the mammalian nervous system. In this report, we provide a summary of recent findings regarding the localization of connexins in gap junctions between glial cells and between neurons. Attention is drawn to technical considerations involved in connexin localization by light and electron microscope immunohistochemistry and to limitations of physiological methods and approaches currently used to analyze neuronal and glial coupling. Early physiological studies that provided evidence for the presence of gap junctions and electrical synapses in isolated regions of the mammalian brain and spinal cord are reexamined in light of recent evidence for widely expressed neuron-specific connexins and for the existence of several newly discovered types of gap junctions linking neurons.
Animals↗
Epilepsy and synaptic reorganization in a perinatal rat model of hypoxia-ischemia.
PURPOSE: One of the potential consequences of perinatal hypoxia-ischemia (H-I) is the development of epilepsy, and synaptic reorganization in the hippocampus has been associated with epilepsy after an injury. We tested the hypothesis that perinatal H-I will induce spontaneous motor seizures, hippocampal lesions, and synaptic reorganization in the dentate gyrus. METHODS: The right common carotid artery of 7-day-old rats was permanently ligated, and the rats were placed for 120 min into a chamber filled with 8% oxygen (37 degrees C). Animals were directly observed for chronic motor seizures for 7 to 24 months after the H-I insult. RESULTS: Nearly half of the rats (i.e., eight of 20) were seen to have spontaneous motor seizures after the H-I injury. The ipsilateral hippocampi from both the rats with seizures and the rats not seen to have seizures had hippocampal lesions and increased amounts of Timm stain in the inner molecular layer (IML) compared with controls. The contralateral hippocampi from the rats with seizures, but not the hippocampi from the rats not seen to have seizures, had significantly increased amounts of Timm stain in the IML. CONCLUSIONS: These results suggest that perinatal H-I can induce epilepsy, ipsilateral hippocampal lesions, and mossy fiber sprouting in the lesioned and contralateral hippocampus.
Animals↗
GluR5 Kainate Receptors and Topiramate: A New Site of Action for Antiepileptic Drugs?
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Journal Article↗
Hyperpolarization-activated Cation Channels in Human and Experimental Epilepsy: Do They Play a Role in Epileptogenesis?
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Journal Article↗
Mechanism of irregular firing of suprachiasmatic nucleus neurons in rat hypothalamic slices.
The mechanisms of irregular firing of spontaneous action potentials in neurons from the rat suprachiasmatic nucleus (SCN) were studied in hypothalamic slices using cell-attached and whole cell recording. The firing pattern of spontaneous action potentials could be divided into regular and irregular, based on the interspike interval (ISI) histogram and the membrane potential trajectory between action potentials. Similar to previous studies, regular neurons had a firing rate about >3.5 Hz and irregular neurons typically fired about <3.5 Hz. The ISI of irregular-firing neurons was a linear function of the sum of inhibitory postsynaptic potentials (IPSPs) between action potentials. Bicuculline (10-30 microM) suppressed IPSPs and converted an irregular pattern to a more regular firing. Bicuculline also depolarized SCN neurons and induced bursting-like activity in some SCN neurons. Gabazine (20 microM), however, suppressed IPSPs without depolarization, and also converted irregular activity to regular firing. Thus GABAA receptor-mediated IPSPs appear responsible for irregular firing of SCN neurons in hypothalamic slices.
Action Potentials↗
Riluzole-sensitive slowly inactivating sodium current in rat suprachiasmatic nucleus neurons.
The persistent (i.e., slowly inactivating) fraction of the Na current (I(Na,P)) regulates excitability of CNS neurons. In isolated rat suprachiasmatic nucleus (SCN) neurons with a ramp-type voltage-clamp protocol, we have studied the properties of a robust current that has the general properties of I(Na,P) but exhibits a slow inactivation (I(Na,S)). The time dependence of the development of the inactivation was also studied by clamping of the membrane potential at different levels: time constants ranging from approximately 50 to approximately 700 ms, depending on the voltage level, were revealed. The I(Na,S) (50-150 pA) was present in both spontaneously active and silent neurons. The neurons exhibited I(Na,S) without visible rundown during approximately 1-h recordings. I(Na,S) had a threshold between -65 and -60 mV and was maximal at about -45 mV. Tetrodotoxin (TTX; 1 microM) completely and reversibly blocked I(Na,S). Riluzole, an effective blocker of I(Na,P), inhibited reversibly I(Na,S) with an EC(50) of 1-2 microM. Microapplication of 10 microM riluzole during either extracellular or intracellular recording suppressed spontaneous activity in isolated SCN neurons. In the slice preparation, bath application of 20 microM riluzole resulted in decreased firing rate or complete suppression of spontaneous activity in some neurons (9/14) but had no effect on other neurons (5/14). In riluzole-resistant neurons in cell-attached experiments, low-amplitude current spikes were present in 1 microM TTX. We concluded that I(Na,S) is ubiquitously expressed by all SCN neurons and that this current is a necessary but not sufficient depolarizing component of the mechanism for spontaneous firing.
Animals↗