The Effect of Calcium Channel Antagonists on Spontaneous and Evoked Epileptiform Activity in the Rat Neocortex In Vitro

Calcium influx through voltage-activated calcium channels may play a crucial role in the propagation and maintenance of seizure activity. We have examined the contribution of various types of calcium currents to epileptogenesis by studying the effects of various calcium channel blockers on epileptiform activity. N-methyl-d-aspartate receptor-mediated epileptiform activity was induced by removal of magnesium ions superfusing the cortex, or by low-frequency stimulation of the underlying white matter. CoCl2, CdCl2 and omega-conotoxin, acting at the N- and L-type calcium channels, significantly reduced epileptiform activity. L-channel antagonists nifedipine and verapamil, and the agonist BAY K 8644, increased spontaneous bursting in cortical wedges, but had no effect upon evoked activity. The T-channel blocker NiCl2 had variable effects on epileptiform activity, whereas phenytoin consistently reduced such activity. These results suggest that calcium influx underlying epileptiform activity in the rat neocortex may occur at least partially via the activation of the N-type calcium channel. However, contributions from other calcium channel types cannot be excluded.     

Motor and Electrocorticographic Epileptic Activity Induced by Bicuculline in Developing Rats

Motor seizures were induced by intraperitoneally (i.p.) injected bicuculline in 270 rats aged 7, 12, 18, 25, or 90 days. Bicuculline was able to elicit both minimal (clonic) and major (tonic-clonic) seizures in all age groups, but in 7-day-old rats minimal seizures were only noted exceptionally. CD50s (for major seizures) ranged from 2.48 to 2.85 mg/kg in the three younger groups and increased to approximately 7 mg/kg in 25- and 90-day-old rats. An intravenous (i.v.) administration of bicuculline in 67 rats, 18 and 25 days old, caused identical CD50s in these groups, indicating that the difference that occurs with an i.p. administration is due to pharmacokinetic reasons. Electrocorticographic (ECoG) studies in acute experiments as well as in young rats with implanted electrodes demonstrated general principles of the development of EEG: an increase in frequency of individual elements, in generalization of the epileptic activity, in synchronization of activity among various cortical regions, and in the correlation between ECoG and motor phenomena. An exception occurred as an age-related phenomenon: rhythmic activity of the spike-and-wave type. This activity appeared in 18-day-old and older rats and was invariably accompanied by "freezing" of the animals.     

Induction of Epileptiform Activity by Temperature Elevation in Hippocampal Slices from Young Rats: An In Vitro Model for Febrile Seizures?

Extracellular field potential recordings were performed in the CA1 subfield of hippocampal slices obtained from Wistar rats aged 2-38 days. When the brain tissue was maintained at 35 degrees-36 degrees C (values obtained in the tissue chamber well), single-shock orthodromic stimuli elicited a response in the stratum pyramidale that consisted of a single population spike. In contrast, when the temperature in the well was increased to levels greater than 38.2 degrees C for periods of 5-15 min, the same type of stimuli elicited an epileptiform response characterized by a 250- to 600-ms-long, positive-going field potential with superimposed, multiple, negative-going population spikes. This potential resembled the epileptiform response recorded in the hippocampal slice in the presence of convulsants such as penicillin or bicuculline. Once the temperature was restored to control values (i.e., 35 degrees-36 degrees C) after induction of epileptiform activity, the abnormal response could be observed for less than or equal to 2 h. In some experiments (approximately one third of the successful trials), spontaneous epileptiform discharges appeared during and persisted after the increase in temperature. The ability of the hyperthermic period to induce epileptiform changes was age dependent: Epileptiform activity outlasting the period of temperature elevation was not observed in slices obtained from rats aged less than 4 days or greater than 28 days. Our data show that epileptiform activity can be induced by a transient increase in temperature and that the age of the animals from which slices are obtained plays an important role in the appearance of this phenomenon.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of the Fastigial Nucleus in Generalized Seizures as Demonstrated by GABA Agonist Microinjections

The cerebellum is electrically and metaboli cally active during seizures. Numerous studies have also shown that cerebellar electrical stimulation and lesions of the cerebellar cortex or nuclei influence seizure thresh old, but there are significant contradictions, with different effects observed even in investigations using the same species and similar seizure types and experimental ma nipulations. Discrete intracerebral microinjection of neu roactive agents has been used to characterize the way in which other brain regions control seizures, but has not been applied to the cerebellar systems. This approach has advantages because effects are restricted to specific re ceptors and spare passing axons; experimental variables also can be simply specified and reproduced. We used this method to characterize the role of the cerebellar nu clei in seizures and to determine if observed effects could be reproduced with different agents at different doses. Effects of bilateral control microinjections in the fastigial (medial) cerebellar nucleus were compared with different doses of the GABA_A agonist piperidine-4-sulfonic acid and the GABA_B agonist (?)baclofen (Bf). Soon after in jection, the animals were ataxic. After 4 min, seizures were induced by timed continuous intravenous (i.v.) bicu culline (BIC) infusion. Both GABA agonists produced significant reductions in myoclonic, clonic, and tonic sei zure thresholds. Injections just dorsal or anterior to this nucleus and bilateral dentate (lateral) nucleus injections had little effect on seizures. These results demonstrate that the cerebellar system does control seizures, but does not provide support for the early concept that cerebellar stimulation and systemic phenytoin block seizures through inhibition of cerebellar nuclei secondary to Pur kinje cell activation.     

Epileptiform propagation patterns mediated by NMDA and non-NMDA receptors in rat neocortex

PURPOSE: The neocortex can generate various forms of epileptiform activity, including one that depends on N-methyl-D-aspartate (NMDA)-type glutamate receptors (NMDARs), and another dependent on non-NMDA-type (AMPA) glutamate receptors (AMPARs). Previous work in vitro suggests that both forms of activity are initiated by neurons of layer 5, but the spatial patterns of horizontal propagation have been studied only for the AMPAR form. We have tested the hypothesis that both types of epileptiform activity spread via common pathways in one cortical layer, suggesting that lamina-specific intervention might selectively interrupt both. METHODS: Slices of rat somatosensory cortex were maintained in vitro and treated with the gamma-aminobutyric acid type A (GABA(A))-receptor antagonist picrotoxin. Single all-or-none epileptiform discharges were evoked with an electrical stimulus, and extracellular microelectrodes were used to track the vertical and lateral spread of the discharges. RESULTS: In both high and low concentrations of picrotoxin, the non-NMDAR antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) completely blocked propagation, whereas the NMDAR antagonist D-2-amino-5-phosphonovaleric acid (DAPV) only shortened the duration of discharges. When extracellular [Mg2+] was reduced in the presence of picrotoxin and CNQX, NMDAR-dependent epileptiform discharges could be initiated. NMDAR-dependent discharges spread at about one fifth the conduction velocity of AMPAR-dependent events. Analysis of spatiotemporal field-potential patterns suggested that both NMDAR- and AMPAR-mediated propagation involved early activity in layers 5 and 6, followed by larger-amplitude activity in upper cortical layers along the path of propagation. CONCLUSIONS: Our results imply that a common pathway mediates the propagation of these two forms of epileptiform activity, and suggests that lamina-specific surgical intervention might maximize anticonvulsant effect while minimally disrupting cortical function.     

The Effect of Hypoxia on the Epileptiform Activities Induced by Magnesium-Free Medium in Rat Brain Slices

Abstract: In order to understand the mechanisms underlying the seizure generation, the present study has investigated the effect of hypoxia on the transition between seizure and interictal bursting. Bathing rat brain slices of the hippocampus and entorhinal cortex in magnesium-free medium elicits electrographic seizures. However, they are eventually replaced by the interictal bursts. It has previously been shown that the interictal bursts, arising in the hippocampal area CA3, are propagated to and disrupt the seizure generation in the entorhinal cortex. In this report we demonstrate that bypoxia promotes the seizure reappearance in the entorhind cortex by suppressing the interictal bursts in CA3.     

Induction of Fictive Locomotion by Sulphur-containing Amino Acids in an In Vitro Newborn Rat Preparation

The role of the sulphur-containing amino acids (SAAs) in the initiation of fictive locomotion was tested in an isolated spinal cord preparation from newborn rats. These substances were bath-applied and the fictive locomotion was recorded in the lumbar ventral roots. It emerged from this study that all the compounds tested could trigger an organized pattern (alternating left and right bursts of activity) with a dose-dependent response. However, specific frequency and concentration ranges were observed with each of these SAAs. Moreover, a clear-cut difference between d and l isomers in the ability of the SAAs to induce this activity was observed: the SAAs of the d -forms were found to be generally more potent than those of the l -forms. The effects of the SAAs were found to be mediated by both NMDA and non-NMDA receptors, since they were blocked in a dose-dependent manner by the specific antagonists of these receptors. Moreover, it was observed that β- p -chlorophenylglutamic acid, an uptake inhibitor of homocysteic acid (HCA), potentiated the effect of exogenously applied HCA, which supports the idea that HCA may act as a transmitter. The sulphuric and non-sulphuric amino acids were also classified in their order of potency. The most potent compound turned out to be d -homocysteine sulphinic acid, while d -cysteine sulphinic acid was the least potent. It also emerged that the maximal frequencies obtained with SAAs and excitatory amino acids were in the same range, which might correspond to the maximal limits of this system.