Sustained Limbic Seizures Research Articles

Hippocampal granule cells express glutamic acid decar☐ylase-67 after limbic seizures in the rat

Temporal lobe epilepsy is the most common form of epilepsy. Decreased GABA-ergic inhibition has been suggested as one cause of hyperexcitability. On the other hand, increased expression of glutamic acid decarboxylase, the rate-limiting enzyme of GABA synthesis, has been found in interneurons of the hippocampus in patients with temporal lobe epilepsy and in rats after kainic acid-induced limbic seizures, indicating increased GABA-ergic transmission. Here we report differential expression of two genes encoding different molecular forms of glutamic acid decarboxylase (GAD), GAD65 and GAD67, after kainic acid-induced seizures in the rat. There is a rapid but transient elevation of GAD67 mRNA levels in granule cells 6-24 h after kainic acid injection, followed by enhanced GAD immunoreactivity in the terminal field of mossy fibers. In interneurons in the hilus of the dentate gyrus, a sustained and progressing increase in the expression of both GAD65 and GAD67 messenger RNA occurs. These observations indicate that consitutively glutamatergic mossy fibers may be capable of synthetizing and utilizing the inhibitory transmitter GABA in sustained limbic seizures. Enhanced expression of glutamic acid decarboxylases within interneurons and in granule cells/mossy fibers suggest augmented GABA-ergic neurotransmission supporting selfprotective, anticonvulsive mechanisms in limbic epilepsy.

The glycine/NMDA receptor partial agonist D-cycloserine blocks kainate-induced seizures in rats. Comparison with MK-801 and diazepam

Systemic administration of kainic acid in the rat results in the development of a characteristic excitotoxic syndrome, consisting of automatisms (wet dog shakes, WDS), sustained limbic seizures and brain damage. Since kainate increases the release of excitatory amino acid neurotransmitters such as glutamate, this syndrome is thought to be due, at least in part, to excessive activation of glutamate receptors, particularlyof theN-methyl- D-aspartate (NMDA) subtype. We examined the effect of D-cycloserine, a partial agonist for the NMDA receptor-associated glycine binding site, in the kainate model of limbic seizures in rats. For comparison, the uncompetitive NMDA antagonist MK-801 (dizocilpine) and the GABAmimetic anticonvulsant diazepam were used. D-Cycloserine exerted a potent, dose-dependent and long-lasting anticonvulsant effect against kainate-induced seizures. At 160 mg/kg, seizures were almost completely suppressed by D-cycloserine over a 3 h observation period. No adverse effects were observed at anticonvulsant doses of D-cycloserine. In contrast to its potent effect on kainate-induced seizures, D-cycloserine did not significantly alter the number of automatisms (WDS) determined after kainate. MK-801, 0.3 mg/kg, also markedly reduced seizure severity in response to kainate, but this anticonvulsant effect was accompanied by marked motor impairment. Similarly, diazepam, 5 mg/kg, significantly attenuated kainate-induced seizures but marked ataxia was observed at this dosage. In contrast to D-cycloserine, both MK-801 and diazepam reduced WDS behaviour caused by kainate. The data demonstrate that pharmacological manipulation of the strychnine-insensitive glycine site is a powerful means of protecting against kainate-induced seizures.

Substantia nigra regulates action of antiepileptic drugs

The cholinergic agonist pilocarpine triggers sustained limbic seizures in rodents. Pilocarpine seizures were blocked by systemic administration of benzodiazepines, barbiturates, valproate and trimethadione, while diphenylhydantoin did not affect, and ethosuximide increased the susceptibility of rats to such seizures. This pattern of action antiepileptic drugs is characteristic for pilocarpine seizures and different from other rodent models of epilepsy. Although the anatomical substrates in the forebrain involved in the expression of anticonvulsant activity are unknown, the basal ganglia are believed to be essential for the motor expression of pilocarpine seizures. Bilateral microinjections into the substantia nigra, a major output station of the basal ganglia, of midazolam (ED 50 38.5 nmol; range 29–52 nmol), phenobarbital (ED 50 16 nmol; range 7–39 nmol) and trimethadione (ED 50 30 nmol; range 16–56 nmol) protected rats against pilocarpine seizures (380 mg/kg i.p.). Diphenylhydantoin (up to 100 nmol) remained inactive, while ethosuximide (ED 50 38 nmol; range 22–65.5 nmol) reduced the threshold for pilocarpine seizures, converting subconvulsant doses of pilocarpine (200 mg/kg i.p.) into convulsant ones. The profiles of action of antiepileptic drugs on pilocarpine seizures were similar following intranigral and systemic administration. These observations suggest that the substantia nigra may mediate some actions of antiepileptic drugs.

Effects of morphine and naloxone on pilocarpine-induced convulsions in rats

Systemic administration of morphine hydrochloride (MF; 5–80 mg/kg; i.p.) in rats enhanced the epileptogenic potential of pilocarpine hydrochloride (PIL) in a dose-dependent manner. PIL, 100 mg/kg; i.p., which did not result in convulsions by itself, produced sustained limbic seizures and epileptic brain damage in MF-pretreated rats. MF-induced enhancement of PIL neurotoxicity was blocked by naloxone hydrochloride (NAL; 2 and 10 mg/kg; i.p.). Administration of NAL (2–20 mg/kg) prior to PIL in the dose of 380 mg/kg moderately decreased the incidence of convulsions, brain damage and lethal toxicity produced by this agent. These results support the hypothesis that opiate mechanisms are involved in the maintenance of the threshold for propagation of seizure activity within limbic circuits.

Cholinomimetics produce seizures and brain damage in rats.

Microinjections of the cholinergic agonists, carbachol and bethanechol, either into the amygdala or into the dorsal hippocampus produced sustained limbic seizures and brain damage in rats. Systemic administration of pilocarpine in rats resulted in a sequence of convulsive disorders and widespread brain damage as well. Scopolamine prevented the development of convulsive activity and brain damage produced by cholinomimetics. These results suggest that the excessive stimulation of cholinergic muscarinic receptors can lead to limbic seizures and brain damage. It is postulated that muscarinic cholinergic mechanisms are linked to the etiology of temporal lobe epilepsy and epileptic brain damage.

“Epileptic” brain damage in rats induced by sustained electrical stimulation of the perforant path. II. Ultrastructural analysis of acute hippocampal pathology

Sustained electrical stimulation of the perforant path evokes granule cell population spikes and epileptiform discharges, abolishes recurrent inhibition in the granule cell layer and induces a reproducible pattern of hippocampal damage (see preceding paper, this volume, for electrophysiological and light microscopic findings). Electron microscopic findings described here reveal that the hippocampal damage is identical in pattern and cytopathological detail to that associated with sustained limbic seizures induced by chemical convulsants such as kainic acid, folic acid and dipiperidinoethane. Acutely swollen dendritic segments distributed in a laminar pattern corresponding closely with the termination of putative glutamate or aspartate-containing fibers, including those of the perforant path, were a conspicuous finding. Cell bodies of CA1 and CA3 pyramidal neurons and various interneurons in the hilus and elsewhere displayed degenerative changes ranging from mild to severe. Both the dendritic and somal degenerative changes closely resemble the “excitotoxic” type of damage that the putative transmitters glutamate and aspartate are known to cause. It is proposed, therefore, that sustained electrical stimulation of the perforant path results in excessive synaptic release and accumulation of glutamate (or aspartate) at numerous dendrosomal receptors in the hippocampus with consequent degeneration of the dendrosomal structures housing these receptors. Early excitotoxic effects on interneurons that mediate recurrent inhibition may play an important role in the observed loss of recurrent inhibition and in the evolution of subsequent excitotoxic degeneration in the hippocampus.

Systemic cholinergic agents induce seizures and brain damage in lithium-treated rats.

Administration of pilocarpine or physostigmine to rats treated with lithium chloride produced sustained limbic seizures, widespread brain damage, and increased concentrations of D-myo-inositol-1-phosphate (a metabolite of the phosphoinositides, lipids involved in membrane receptor function) in the brain. The syndrome was preventable with atropine. The physostigmine doses and concentrations of blood lithium that caused the syndrome are similar to those considered appropriate for psychiatric chemotherapy.

Seizure-related brain damage induced by cholinergic agents.

Distinctive acute brain damage involving limbic and related brain regions develops in adult rats following sustained limbic seizures induced by systemic administration of kainic acid or dipiperidinoethane (DPE) or by intra-amygdaloid injection of kainic acid or folic acid. This seizure-brain damage (S-BD) syndrome is of particular interest because it tends to parallel the type of seizures and brain damage seen in human temporal lobe epilepsy. We have observed that DPE induces the S-BD syndrome by systemic but not intra-mygdaloid injection, whereas an oxidized DPE derivative which structurally resembles the cholinergic agonist oxotremorine is effective when injected into the amygdala. Prompted by this finding, we injected known acetylcholine (ACh) agonists and cholinesterase (ChE) inhibitors into the rat amygdala and found that either class of agent reproduces this type of S-BD syndrome. These and related findings suggest that ACh mechanisms might have a more important role in human epilepsy and epileptic brain damage than has generally been appreciated.

Intraamygdaloid morphine produces seizures and brain damage in rats

Behavioral and neuropathological responses to increasing doses of morphine hydrochloride (10–75 μg) administered into the rat amygdala were studied. Unilateral microinjections of morphine in doses of 40 and 75 μg produced a sequence of behavioral alterations including staring spells, gustatory automatisms, wet dog shakes, motor limbic seizures and limbic status epilepticus. Lower doses of morphine (10 and 20 μg) showed different threshold for these behavioral phenomena but a similar time course of development. Histological examination of frontal forebrain sections revealed widespread, apparently seizure-mediated pattern of brain damage. Neuropathological alterations were observed in the olfactory cortex, thalamus, neocortex, hippocampal formation and amygdaloid complex. Pretreatment of animals with diazepam (10 mg/kg i.p.) prevented the development of sustained limbic seizures and brain damage caused by morphine, while pretreatment with naloxone hydrochloride (2–20 mg/kg i.p.) failed to affect morphine-induced convulsant activity and brain damage. These results may suggest that morphine elaborates sustained limbic seizures and widespread brain damage by mechanism underlying the antagonism of inhibitory amino acid neurotransmission and opioid receptors do not seem to be involved.

Naloxone blocks morphine enhancement of kainic acid neurotoxicity

Kainic acid (KA) is a potent convulsant which, when administered subcutaneously, induces sustained limbic seizures and a pattern of limbic brain damage that is thought to be seizure-mediated. Diazepam suppresses and morphine enhances both the seizures and brain damage induced by KA. Here we show that morphine enhancement of KA neurotoxicity is blocked in a dose-dependent manner by subcutaneous pretreatment with naloxone. These and related findings support the hypothesis that morphine enhances the seizure-linked neurotoxicity of KA by an opiate specific action at certain limbic receptor sites where opiates suppress GABAergic activity, thereby lowering the threshold for propagation of seizure activity in limbic circuits.

Kainate-like neurotoxicity of folates.

Kainic acid (KA) is one of the most powerful of a group of ‘excitotoxic’ analogues of the putative neurotransmitter glu-tamate (Glu) whose neurotoxicity may involve an excitatory mechanism mediated through glutamergic postsynaptic receptors1–8. The finding9 that neural membranes have specific sites where KA binds quite firmly and that Glu inhibits such binding very weakly, however, raises the possibility that KA and Glu receptors may be separate and distinct (see also refs 10, 11). It is in any case known that the neurotoxic properties of K A and Glu are not identical. Thus, when injected into the amygdala, both Glu and KA destroy local neurones but only KA induces sustained limbic seizures and an apparently seizure-mediated pattern of extra-amygdaloid brain damage12–14. Ruck et al.15, having recently found that the folic acid derivative, methyl-tetrahydrofolate (MTHF), competes powerfully for KA binding sites on rat cerebellar membranes and mimics KA in depolarizing frog spinal neurones, proposed that MTHF may be an endogenous neuromodulator with both excitatory and neurotoxic properties. We have therefore injected MTHF directly into the amygdala of the adult rat and found that at a rather high dose (300 nmol), it reproduces the specific component of KA neurotoxicity that Glu fails to reproduce, namely the limbic seizure/brain damage syndrome. We have also found that folic acid itself (pteroyl-L-glutamic acid, PGA) and one of its reduced derivatives (N-5-formyltetrahydrofolate, FTHF) are substantially more powerful than MTHF in reproducing this syndrome.

Sustained limbic seizures induced by intraamygdaloid kainic acid in the baboon: Symptomatology and neuropathological consequences.

In Papio papio baboons chronically prepared for cortical and deep electroencephalographic recording, injection of kainic acid into the amygdala (7 animals) or temporal pole (2 animals) gave rise to focal epileptic discharges lasting 15 to 150 hours. Electrographically, the seizure activity spread ipsilaterally and contralaterally within the limbic system but did not become generalized. The principal associated motor signs were arrest of movement and oral automatisms. Histological examination after two to ten days demonstrated lesions (neuron loss and gliosis) at the injection site that varied according to the amount of kainic acid injected (2 to 68 microgram). "Remote" lesions occurred in the ipsilateral hippocampus (end-folium and Sommer sector) and neocortex (occipital and frontal regions). The hippocampal lesions were comparable to those previously described as consequent to status epilepticus.