6-cyano-7-nitroquinoxaline-2-3-dione and Epilepsy--Absence

The effects of a single and multiple doses of ginkgolide A, B, C, and bilobalide, active components of Ginkgo biloba extract (EGb 761), on absence seizures were investigated in male WAG/Rij rats, a genetic animal model of absence epilepsy. Furthermore, the interactions of ginkgolide A together with NMDA receptor antagonist MK-801, AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), or L-type calcium channel blocker nicardipine were studied to figure out how ginkgolide A affects spike-wave discharges (SWDs) in the brain. The experiments were done using 6-8-month-old male WAG/Rij rats with infusion cannula and EEG electrode implanted. Ginkgolide A, B, C, and bilobalide were administered intraperitoneally for 7 days at a dose of 6 mg/kg. In interaction groups, 6 μg ginkgolide A was injected intracerebroventricularly in combination with MK-801 (10 μg), CNQX (1 μg), and nicardipine (50 μg) for 7 days. EEG was recorded from animals at the baseline, first dose, and seventh dose periods for 4 h. Ginkgolide A (p = .028), C (p = .046), and bilobalide (p = .043) significantly increased the frequency of SWDs in WAG/Rij rats. Ginkgolide A injected into the lateral ventricle with MK-801 (p = .046), CNQX (p = .043), and nicardipine (p = .046) significantly increased the number of SWDs after seventh dose. Finally, the EGb 761-related increase in absence epilepsy was determined to be caused by ginkgolide A, C, and bilobalide. All three receptor antagonists/channel blockers do not inhibit the pro-absence effect of ginkgolide A. The findings revealed that ginkgolide A's pro-absence effect is mediated by brain circuits other than ionotropic glutamate receptors or L-type calcium channels.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Bilobalides; Disease Models, Animal; Dizocilpine Maleate; Electroencephalography; Epilepsy, Absence; Excitatory Amino Acid Antagonists; Ginkgolides; Male; Nicardipine; Rats; Seizures

The intergeniculate leaflet (IGL) of the thalamus is a retinorecipient structure implicated in orchestrating circadian rhythmicity. The IGL network is highly GABAergic and consists mainly of neuropeptide Y-synthesising and enkephalinergic neurons. A high density of GFAP-immunoreactive astrocytes has been observed in the IGL, with a probable function in guarding neuronal inhibition. Interestingly, putatively enkephalinergic IGL neurons generate action potentials with an infra-slow oscillatory (ISO) pattern in vivo in urethane anesthetised Wistar rats, under light-on conditions only. Absence epilepsy (AE) is a disease characterised by spike-wave discharges present in the encephalogram, directly caused by hypersynchronous thalamo-cortical oscillations. Many pathologies connected with the arousal system, such as abnormalities in sleep architecture and an insufficient brain sleep-promoting system accompany the epileptic phenotype. We hypothesise that disturbances in the function of biological clock structures, controlling this rhythmic physiological process, may be responsible for the observed pathomechanism. To test this hypothesis, we performed an in vitro patch-clamp study on WAG/Rij rats, a well-validated genetic model of AE, in order to assess dampened GABAergic synaptic transmission in the IGL expressed as a lower IPSC amplitude and reduced sIPSC frequency. Moreover, our in vivo extracellular recordings showed higher firing rate of ISO IGL neurons with an abnormal reaction to a change in constant illumination (maintenance of rhythmic neuronal activity in darkness) in the AE model. Additional immunohistochemical experiments indicated astrogliosis in the area of the IGL, which may partially underlie the observed changes in inhibition. Altogether, the data presented here show for the first time the disinhibition of IGL neurons in a model of AE, thereby proposing the possible involvement of circadian-related brain structures in the epileptic phenotype.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Age Factors; Animals; Animals, Newborn; Disease Models, Animal; Epilepsy, Absence; Excitatory Amino Acid Antagonists; GABA Agents; Geniculate Bodies; Inhibitory Postsynaptic Potentials; Male; Nerve Net; Neural Inhibition; Neuropeptide Y; Rats; Rats, Mutant Strains; Rats, Wistar; Sodium Channel Blockers; Tetrodotoxin; Valine

Overexpression of GABA(B)R1a receptors in mice (R1a(+)) results in an atypical absence seizure phenotype characterized by 3- to 6-Hz slow spike-and-wave discharges (SSWDs), reduced synaptic plasticity, and cognitive impairment. Here we tested the hypothesis that increased R1 expression causes atypical absence epilepsy and is not subunit specific. GABA(B)R1b receptors were overexpressed in mouse forebrain (R1b(+)) and confirmed by immunoblot and (3)H-CGP54626A autoradiography. The R1b(+) mice showed a reduction in hippocampal long-term potentiation and GABA(A) receptor-mediated inhibitory postsynaptic currents. R1b(+) mice manifested an electrographic, pharmacological, and behavioral phenotype consistent with atypical absence seizures, though less robust than R1a(+) in terms of SSWD duration and severity of cognitive impairment. These results suggest that abnormal GABA(B)R1b function plays a lesser role in the development of atypical absence epilepsy.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Animals, Newborn; Autoradiography; Cognition Disorders; Disease Models, Animal; Electric Stimulation; Electroencephalography; Epilepsy, Absence; Excitatory Amino Acid Antagonists; Hippocampus; In Vitro Techniques; Inhibitory Postsynaptic Potentials; Long-Term Potentiation; Maze Learning; Mice; Mice, Transgenic; Neurons; Organophosphorus Compounds; Patch-Clamp Techniques; Phenotype; Protein Binding; Receptors, GABA-B; Tritium; Valine

The tottering (tg) mice have a mutation in the CaV2.1 (P/Q-type) voltage-dependent Ca2+ channel alpha(1)2.1 subunit gene. tg mice show not only cerebellar ataxia but also absence epilepsy, which begins at approximately 3 weeks of age and persists throughout life. Similarities in EEG and sensitivity to antiepileptic drugs suggest that tg mice are a good model for human absence epilepsy. Although imbalance between excitatory and inhibitory activity in the thalamocortical network is thought to contribute to the pathogenesis of absence epilepsy, the effect of the mutation on thalamocortical synaptic responses remains unknown. Here we showed imbalanced impairment of inhibitory synaptic responses in tg mice using brain slice preparations. Somatosensory thalamocortical projection makes not only monosynaptic glutamatergic connections but also disynaptic GABAergic connections, which mediate feedforward inhibition, onto layer IV neurons. In tg mice, IPSC amplitudes recorded from layer IV pyramidal cells of the somatosensory cortex in response to thalamic stimulation became disproportionately reduced compared with EPSC amplitudes at later developmental stages (postnatal days 21-30). Similar results were obtained by local stimulation of layer IV pyramidal neurons. However, IPSC reduction was not seen in layer V pyramidal neurons of epileptic tg mice or in layer IV pyramidal neurons of younger tg mice before the onset of epilepsy (postnatal days 14-16). These results showed that the feedforward inhibition from the thalamus to layer IV neurons of the somatosensory cortex was severely impaired in tg mice and that the impairment of the inhibitory synaptic transmission was correlated to the onset of absence epilepsy.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Afferent Pathways; Age Factors; Animals; Calcium Channel Blockers; Calcium Channels, N-Type; Disease Models, Animal; Electric Stimulation; Electroencephalography; Epilepsy, Absence; Evoked Potentials; Excitatory Postsynaptic Potentials; Homeostasis; Lidocaine; Mice; Mice, Inbred C57BL; Mice, Neurologic Mutants; omega-Agatoxin IVA; omega-Conotoxin GVIA; Patch-Clamp Techniques; Picrotoxin; Pyramidal Cells; Somatosensory Cortex; Synaptic Transmission; Thalamic Nuclei; Valine

Human and experimental studies indicate that molecular genetic changes in GABA(A) receptors may underlie the expression of spike-and-waves discharges (SWDs) occurring during absence seizures. However, the full spectrum of the genetic defects underlying these seizures has only been partially elucidated, the expression and functional profiles of putative abnormal protein(s) within the thalamocortical network are undefined, and the pathophysiological mechanism(s) by which these proteins would lead to absence paroxysms are poorly understood. Here we investigated GABA(A) inhibitory postsynaptic currents (IPSCs) in key thalamocortical areas, i.e., the somatosensory cortex, ventrobasal thalamus (VB) and nucleus reticularis thalami (NRT), in preseizure genetic absence epilepsy rats from Strasbourg (GAERS), a well-established genetic model of typical absence seizures that shows no additional neurological abnormalities, and compared their properties to age-matched non-epileptic controls (NECs). Miniature GABA(A) IPSCs of VB and cortical layers II/III neurons were similar in GAERS and NEC, whereas in GAERS NRT neurons they had 25% larger amplitude, 40% faster decay. In addition, baclofen was significantly less effective in decreasing the frequency of NRT mIPSCs in GAERS than in NEC, whereas no difference was observed for cortical and VB mIPSCS between the two strains. Paired-pulse depression was 45% smaller in GAERS NRT, but not in VB, and was insensitive to GABA(B) antagonists. These results point to subtle, nucleus-specific, GABA(A) receptor abnormalities underlying SWDs of typical absence seizures rather than a full block of these receptors across the whole thalamocortical network, and their occurrence prior to seizure onset suggests that they might be of epileptogenic significance.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Baclofen; Epilepsy, Absence; Excitatory Postsynaptic Potentials; GABA Agonists; GABA Antagonists; GABA-A Receptor Antagonists; gamma-Aminobutyric Acid; Immunohistochemistry; In Vitro Techniques; Intralaminar Thalamic Nuclei; Membrane Potentials; Organophosphorus Compounds; Phosphinic Acids; Propanolamines; Rats; Receptors, GABA-A; Somatosensory Cortex; Synapses; Synaptic Transmission; Ventral Thalamic Nuclei

Mutations in GABA-A receptor subunits have been reported in a number of idiopathic generalized epilepsies including childhood absence epilepsy. One of these mutations is located within a high-affinity benzodiazepine-binding domain, and clonazepam is clinically used as an anti-absence drug. The intrathalamic loop consisting of the GABAergic neurons of the nucleus reticularis thalami (NRT) and the thalamocortical (TC) neurons of sensory thalamic nuclei plays an essential role in spike and wave discharges. In a well-established genetic model of absence epilepsy (Genetic Absence Epilepsy rat from Strasbourg, GAERS), systemic injections of benzodiazepines have been shown to suppress spike-and-waves discharges. The aim of this study, therefore, was to determine whether the sensitivity of GABAergic synaptic currents to clonazepam in NRT and TC neurons was different in GAERS and non-epileptic control (NEC) rats. In both pre-seizure GAERS and NEC clonazepam (100 nM) had no effect on the mIPSCs recorded from TC neurons while it increased the decay time constant of the mIPSCs recorded in NRT neurons by a similar amount in GAERS (54.5+/-5%) and NEC (50.7+/-5%). Similar results have been obtained in the presence of 100 microM Cd2+, showing that the effect of clonazepam did not occur via modulation of voltage-activated Ca2+ currents. These results are relevant to understand that in GAERS, the clonazepam anti-absence actions cannot be fully explained by the enhancement of the intra-NRT inhibition and the modulation of the GABAergic synaptic currents in other brain areas, in particular the cortex, must be taken into consideration.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Action Potentials; Animals; Anticonvulsants; Cadmium; Clonazepam; Disease Models, Animal; Drug Interactions; Electric Stimulation; Epilepsy, Absence; Excitatory Amino Acid Antagonists; Female; gamma-Aminobutyric Acid; In Vitro Techniques; Male; Neural Inhibition; Neurons; Patch-Clamp Techniques; Rats; Rats, Mutant Strains; Synapses; Thalamus

Use-dependent depression of inhibitory postsynaptic potentials was investigated with intracellular recordings and the paired-pulse paradigm in rat neocortical neurons in vitro. Pairs of stimuli invariably reduced the second inhibitory postsynaptic potential-A (GABA(A) receptor-mediated inhibitory postsynaptic potential) of a pair; at interstimulus intervals of 500 ms, the amplitude of the second inhibitory postsynaptic potential-A was considerably smaller than the first (36.2 +/- 6.2%, n= 17). Decreasing the interstimulus interval reduced the second inhibitory postsynaptic potential-A further and with interstimulus intervals shorter than 330 ms the compound excitatory postsynaptic potential-inhibitory postsynaptic potential response reversed from a hyperpolarizing to a depolarizing response. The depression of the inhibitory postsynaptic potential-A exhibited a maximum at interstimulus intervals near 150 ms and recovered with a time constant of 282 +/- 96.2 ms. Elimination of excitatory transmission by the application of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D(-)-2-amino-5-phosphonovaleric acid yielded an essentially unaltered time-course of paired-pulse depression (maximum depression near 150 ms, time constant of recovery 232 +/- 98 ms). The polarity change of the compound excitatory postsynaptic potential response at shorter interstimulus intervals was abolished in the presence of CNQX and D(- )-2-amino-5-phosphonovaleric acid. CNQX and D(-)-2-amino-5-phosphonovaleric acid also reduced the apparent depolarizing shift of the reversal potential between the first and second inhibitory postsynaptic potential-A from about 6 mV to less than 2 mV. Application of the GABA(B) receptor antagonist CGP 55845A in the presence of CNQX and (-)-2-amino-5-phosphonovaleric acid abolished the inhibitory postsynaptic potential-B and paired-pulse depression. Under these conditions, the amplitude of the second inhibitory postsynaptic potential was, on average, about 90% of the first, i.e. reduced by about 10%. The second inhibitory postsynaptic potential-A was approximately constant at interstimulus intervals between 100 and 500 ms. It is concluded that paired-pulse depression of cortical inhibition is predominantly mediated by presynaptic GABA(B) receptors of GABAergic interneurons. The abolition of net inhibition at interstimulus intervals near 330 ms may facilitate spread of excitation and neuronal synchrony during repetitive cortical activation near 3 Hz. This u

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Action Potentials; Animals; Delta Rhythm; Epilepsy, Absence; Excitatory Amino Acid Antagonists; Excitatory Postsynaptic Potentials; GABA Antagonists; GABA-B Receptor Antagonists; Neocortex; Neural Inhibition; Organ Culture Techniques; Phosphinic Acids; Presynaptic Terminals; Propanolamines; Pyramidal Cells; Rats; Rats, Sprague-Dawley; Receptors, GABA-B; Stimulation, Chemical

Genetic absence epilepsy rats from Strasbourg (GAERS) have non-convulsive generalized seizures associated with spike-wave (SW) discharges, which are due to a hyperexcitable state of the thalamo-cortico circuits involving the reticular thalamic nucleus (nRt). Investigation of the primary genetically-determined defect responsible for GAERS epilepsy revealed the following abnormalities: (1) increased effectiveness of AMPA receptors dependent glutamate-mediated transmission; (2) impairment of GABA-mediated transmission in the neocortex; (3) increased amplitude of the voltage-dependent low-threshold Ca2(+)-current (I(T)) in the nRt. The maturational profile of these abnormalities supports the conclusion that the abnormality in the I(T) current in the nRt is the primary genetically-determined defect, which may secondarily induce the other changes found in the neocortex and thalamus of GAERS.

Topics: 2-Amino-5-phosphonovalerate; 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Cerebral Cortex; Disease Models, Animal; Electric Stimulation; Epilepsy, Absence; gamma-Aminobutyric Acid; Neural Pathways; Pyramidal Cells; Rats; Receptors, N-Methyl-D-Aspartate; Synaptic Transmission; Thalamic Nuclei; Thalamus

The effects on seizures, EEG and behavior of the non-NMDA receptor antagonist CNQX (6-cyano-7-nitroquinoxaline-2,3-dione), were studied in the WAG/Rij rat with absence epilepsy. Intracerebroventricular injections (10, 50, and 100 nmol/5 microliters CNQX) showed that CNQX decreases the number of spike wave discharges in a dose-dependent way. Coinjection of CNQX (100 nmol/5 microliters) and AMPA (0.1 pmol/5 microliters), kainic acid (0.01 nmol/5 microliters) or NMDA (50 pmol/5 microliters) attenuated the CNQX response, indicating that CNQX acts on both non-NMDA and NMDA receptors. The observed effects appear to be specific manipulations of the epilepsy not mediated by behavioral changes.

Topics: 6-Cyano-7-nitroquinoxaline-2,3-dione; Animals; Azasteroids; Dose-Response Relationship, Drug; Electroencephalography; Epilepsy, Absence; Female; Injections, Intraventricular; Kainic Acid; Male; N-Methylaspartate; Postural Balance; Pregnanediones; Quinoxalines; Rats; Rats, Inbred Strains; Receptors, N-Methyl-D-Aspartate