Time-dependent pro- and anticonvulsant effects of cysteamine on the development and expression of amygdaloid kindled seizures

The time-dependent pro- and anticonvulsant effects of cysteamine, a depletor of somatostatin, were investigated on the development and expression of amygdaloid kindled seizures. Acute administration of cysteamine (25-400 mg/kg, i.p.) produced a dose-dependent potentiation of kindled seizures when evoked 4 h after the drug. However, the seizures initiated 1 day after drug administration were dose-dependently suppressed. Furthermore, elicitation of seizures 4 h after cysteamine enhanced its anticonvulsant effects at 1 day after the drug, causing a parallel left shift of the dose-response curve. Since it has been reported that somatostatin is released during generalized seizures, the seizures given 4 h after cysteamine may encourage the somatostatin depletion by cysteamine and thereby potentiate its later anticonvulsant effects. The repeated administration of cysteamine (100 mg/kg, i.p.) during kindling development strongly retarded the development of generalized seizures but not the development of focal seizures or of afterdischarges in the amygdala. In contrast to the acute experiments, kindling stimulation given 4 h after each cysteamine treatment did not augment the blocking effect on kindling development. These data indicate that chronic cysteamine treatment has a strong inhibitory effect on the development of amygdaloid kindling.     

Effects of Dextromethorphan, a Nonopioid Antitussive, on Development and Expression of Amygdaloid Kindled Seizures

The effects of dextromethorphan (DM), a nonopioid antitussive and a functional N-methyl-D-aspartate (NMDA) antagonist, on expression and development of amygdaloid kindled seizures were examined. The maximum anticonvulsant effect of DM (30 mg/kg) on fully kindled seizures appeared within 30 min of administration and lasted for at least 2 h. DM decreased, in a dose-dependent manner [10-70 mg/kg, intraperitoneally (i.p.)], the severity of kindled seizures 30 min after injection, but the estimated ED50 was 3 times higher than the previously reported value for maximal electroshock convulsions. Furthermore, the high dose (70 mg/kg), while suppressing kindled seizures, produced myoclonus which coincided with EEG spike activity in the amygdala and the cortex. When tested on the development of kindling, 30 mg/kg DM retarded the growth of afterdischarge in the amygdala and the cortex, but had no effect on the development of behavioral seizures. DM 60 mg/kg accelerated development of kindling and produced spontaneous seizures. These results indicate that DM, unlike other NMDA antagonists, has a narrow therapeutic window as an anticonvulsant on kindled seizures and that higher doses may potentiate the kindling process.     

Development of amygdaloid kindling in histidine decarboxylase-deficient and histamine H1 receptor-deficient mice

PURPOSE: This study attempted to clarify the role of histamine or histamine H1 receptors in the development of amygdaloid kindling by using histidine decarboxylase (HDC)-deficient and histamine H1 receptor (H1R)-deficient mice. METHODS: Under pentobarbital anesthesia, mice were fixed to a stereotaxic apparatus, and bipolar electrodes were implanted into the right amygdala. Electrodes were connected to a miniature receptacle, which was embedded in the skull with dental cement. A bipolar electroencephalogram was recorded; bipolar stimulation of the amygdala was applied every day with a constant-current stimulator and continued until a generalized convulsion was obtained. RESULTS: The development of amygdaloid kindling in HDC-deficient and H1R-deficient mice was significantly accelerated compared with that in their respective wild-type mice. In addition, the afterdischarge (AD) duration and generalized seizure duration in HDC-deficient and H1R-deficient mice were prolonged. Intraperitoneal injection of histidine resulted in an inhibition of amygdaloid kindled seizures in wild-type mice at doses that caused an increase in the histamine contents of the brain. However, no significant effect was observed with histidine in H1R-deficient mice at the same dose. CONCLUSIONS: These findings suggest that histaminergic mechanisms through H1 receptors play a crucial role not only in amygdaloid kindled seizures but also in the development of amygdaloid kindling.     

Lasting secondary antiepileptogenesis induced by cingulate kindling

PURPOSE: To test the validity of a hypothesis that kindling-induced enduring kindling inhibition of the homotopic secondary site is not a primate species-specific effect but occurs also in feline species. METHODS: Five mature male cats were kindled at the anterior (AC) and posterior (PC) cingulate cortex followed by secondary-site kindling and primary-site retest. RESULTS: Kindling was characterized by a marked seizure-stage instability and a progressive afterdischarge threshold elevation in all the animals. A protracted nonconvulsive seizure stage was followed by convulsive evolution and rapid generalization. At the secondary site, positive transfer effect was absent, and kindling did not occur. Primary-site retest activated kindled seizure within three stimulations. Recurrent spontaneous seizures, identical to the primary-site kindled seizure, occurred in two AC-kindled animals. Subsequent to this, a markedly elevated afterdischarge threshold also was found at some distant cortical and subcortical sites, to which the afterdischarge propagated. Kindling of the amygdala ipsilateral to the kindled AC or PC was tardy, with no positive transfer effect. However, it was associated with afterdischage threshold reduction. CONCLUSIONS: Cingulate kindling-induced enduring kindling inhibition at a homotopic secondary site is not a primate species-specific effect and occurs also in feline species. It is not limited to a homotopic site and represents a lasting secondary antiepileptogenesis, presumably due to enhanced intrinsic inhibitory mechanism(s) available to the mammalian brain. A selective afterdischarge threshold elevation concurrently observed is suspected to be a feline species-specific effect.     

Anticonvulsant action of a non-competitive antagonist of NMDA receptors (MK-801) in the kindling model of epilepsy

Anticonvulsant action of MK-801, a novel non-competitive antagonist of N-methyl-d-aspartate (NMDA) receptors, was investigated in the kindling model of epilepsy in rats. The results obtained were as follows. (1) Both the seizure stage and afterdischarge duration of previously kindled seizures from the amygdala were significantly suppressed following systemic injection of MK-801 (0.25–4 mg/kg) in a dose-dependent manner. The maximum effects were observed between 2 and 4 h after the injection. (2) The MK-801 also showed significant anticonvulsant effedts on kindled seizures from the frontal cortex and the ventral and dorsal hippocampus. The efficacy however, significantly differed between these kindled sites. (3) Daily treatment of MK-801 (0.25 and 1 mg/kg) prior to each electrical stimulation of the amygdala significantly retarded kindling seizure development and increased the total amount of afterdischarge (accumulated AD) required to reach the first stage 5 seizure. During drug sessions of 1 mg/kg MK-801 for 19 days, all rats showed only partial seizures and the growth of afterdischarge was strongly prevented. (4) Pretreatment with reserpine did not antagonize the anticonvulsant effects of MK-801 on previously kindled seizures from the amydala, suggesting that the effects may not be mediated by catecholaminergic systems. These results indicate that MK-801 has potent anticonvulsant actions on kindled seizures from both limbic and cortical foci, the NMDA system may play a critical role in the seizure-triggering mechanism of kindling. The possible application of NMDA antagonists in clinical epilepsy is suggested.     

Effect of transient hippocampal inhibition on amygdaloid kindled seizures and amygdaloid kindling rate

In this study the effect of transient inhibition of the CA1 region of the dorsal hippocampus by lidocaine on amygdala kindling rate and amygdaloid kindled seizures was investigated. In experiment 1, rats were divided into four groups. In group 1, animals were implanted only with a tripolar electrode into the amygdala but in groups 2-4, two guide cannulae were also implanted into the CA1 regions of the dorsal hippocampi. Animals were stimulated daily to be kindled. In groups 3 and 4, saline or 2% lidocaine (1 microl/2 min) was also injected respectively into the hippocampus, 5 min before each stimulation. Results obtained showed that amygdala kindling rate and the number of stimulations to receive from stage 4 to stage 5 seizure were significantly increased in group 4. In experiment 2, lidocaine (1% and 2%) was infused (1 microl/2 min) into the hippocampus of amygdala kindled rats bilaterally and animals were stimulated at 5, 15 and 30 min after drug injection. Twenty four h before lidocaine injection, saline was also infused (1 microl/2 min) into the hippocampus as control. Obtained results showed that afterdischarge duration was reduced 5 min after lidocaine (1% and 2%) injection. Stage 5 seizure duration was also decreased 5 and 15 min after 2% lidocaine. Thus, it may be suggested that in amygdala kindling, activation of the hippocampal CA1 region has a role in seizure acquisition and seizure severity so that inhibition of this region results in decreasing of seizure severity and retards amygdala kindling rate.     

Kindling of the Visual Cortex in Cats: Comparison with Amygdaloid Kindling

Abstract: Kindling of the primary visual cortex (VC) was compared with that of the amygdala in cats. VC kindling was basically similar to kindling of the amygdala in that daily electrical stimulation can lead to the development of a generalized convulsion in most subjects, a growth of afterdischarges in their configuration and duration, and a reduction of the afterdischarge threshold. The kindling response of the VC differed from that of the amygdala in a number of respects, i.e., a high afterdischarge threshold, a different pattern of behavioral seizure development, an abrupt growth of electroclinical seizures coincident with the onset of a generalized convulsion, an intersubject variability in seizure susceptibility, and a marked seizure instability. In VC kindling the afterdischarge propagation into the amygdala was not observed until the generalized convulsion developed, and the early involvement of afterdischarge was seen in the pulvinar, lateral geniculate body, and superior colliculus. These data suggest that a neural mechanism different from amygdaloid kindling may participate in VC kindling, and that the sub-cortical structures of the visual system are involved in the preferential pathway for a seizure generalization from the VC.     

Common epileptic pathway in amygdaloid bicuculline and electrical kindling demonstrated by transferability

The transferability between amygdaloid bicuculline kindling and electrical kindling was studied in rats. Electrically kindled animals exhibited stage 5 generalized seizures on the first injection of bicuculline, in contrast to the 3–7 injections required in controls. Similarly, bicuculline-kindled animals reached stage 5 after significantly fewer electrical stimulations than controls; two-thirds of the bicuculline-kindled animals began in stage 1 but skipped stages 3 and 4 on their way to stage 5. These data indicate that a common generalization mechanism exists in both types of kindling, but the process of progression from local seizure to generalized seizure may be immature in bicuculline kindling.     

Effects of hypoglycemia on kindling seizures in suckling rats

The effects of insulin-induced hypoglycemia on epileptic disorders of suckling rats were examined using an amygdala kindling model. Kindling stimulations were conducted at 16 and 17 days of age with electrodes implanted in the amygdala 2 days earlier. In 18-day-old kindled rats, which acquired generalized behavioral seizures (stages 4 and 5; Moshé's score) by kindling stimulations, the duration of afterdischarge and behavioral seizures evoked by the stimulation at a threshold intensity to produce a generalized seizure was significantly prolonged after an injection of insulin (25 U/kg, i.p.). The prolongation was not observed in kindled rats that exhibited normal blood glucose concentrations after the application of saline or insulin together with glucose. There were no apparent changes in the amplitude of the afterdischarge, the score of behavioral seizure stages, or the generalized seizure threshold. A similar, marked prolongation of afterdischarge and behavioral seizures following the application of insulin, as in the kindled rats, was also observed during the course of the kindling acquisition without accelerating the development of kindling seizure scores. These results indicate that insulin-induced hypoglycemia easily increases the risk of prolonged seizures in immature brain without precipitating the secondarily generalizing mechanism.     

Deep prepyriform cortex kindling and its relation to amygdala kindling in the rat

Kindling of the deep prepyriform cortex (DPC), a recently identified sensitive site to chemoconvulsants, was compared to kindling of the amygdala in rats. Although the two kindled sites were very similar in their initial responses, electroencephalographic changes during kindling stimulation, kindling seizure development, and growth of afterdischarge, they differed in that the deep prepyriform cortex showed a significantly lower final threshold, a significantly higher frequency of afterdischarge and a significantly shorter latency to bilateral forelimb clonus than the amygdala. Significant bidirectional transfer was found during secondary kindling of either of the two sites, suggesting a close interrelation between them. Rhythmic synchronous discharge was directly induced in the deep prepyriform cortex by ipsilateral amygdala stimulation, but not in the amygdala by ipsilateral cortical stimulation. Local application of both muscimol (0.45 nM) and 2-amino-5-phosphonovaleric acid (25 nM) into the cortical site reduced the afterdischarge duration of amygdala-kindled seizures by approximately 20% without changing the type or duration of the motor seizure. We suggest that the deep prepyriform cortex is “downstream” from the amygdala in the preferential routes of afterdischarge propagation, and that, although the deep prepyriform cortical neurocircuits of GABA and excitatory amino acids may be involved in the maintenance of amygdala-kindled, self-sustained afterdischarge, they are not essential in the expression of amygdala-kindled motor convulsions.     

The anticonvulsant action of nafimidone on kindled amygdaloid seizures in rats

The anticonvulsant effectiveness of nafimidone (1-[2-naphthoylmethyl]imidazole hydrochloride) was evaluated in the kindled amygdaloid seizure model in rats. Nafimidone (3.1-120 mg/kg i.p.) was evaluated at 30 min in previously kindled rats using both threshold (20 microA increments) and supranthreshold (400 microA) paradigms. Nafimidone (25-50 mg/kg) significantly reduced supranthreshold elicited afterdischarge length and seizure severity only at doses with some prestimulation toxicity. The maximum anticonvulsant effectiveness for the 25 mg/kg i.p. dose of nafimidone was seen between 15 and 30 min utilizing a suprathreshold kindling paradigm. Nafimidone did not significantly elevate seizure thresholds at the doses tested; however, nafimidone (3.1-50 mg/kg) reduced the severity and afterdischarge duration of threshold elicited seizures in a non-dose response manner. Drug-induced electroencephalographic spikes were seen in both cortex and amygdala in most kindled rats receiving 100-120 mg/kg i.p. within 30 min of dosing before electrical stimulation. The frequency of spike and wave complexes increased in most of these animals leading to drug-induced spontaneous seizures and death in approximately 25% before electrical stimulation. This study has demonstrated that although nafimidone can modify both threshold and suprathreshold elicited kindled amygdaloid seizures, it lacks significant specificity in this model of epilepsy.     

Chemical kindling by muscarinic amygdaloid stimulation in the rat

507 Holtzman rats received injections, through chemitrodes chronically implanted into the basolateral amygdala, of 0.2–1 μl of sterile isotonic solution containing nanomolar quantities of cholinergic muscarinic agonists and/or antagonists. The bulk of the injected solution diffused only a short distance as judged by autoradiography. Once daily injections of 2.7 nmoles of carbamylcholine, an initially subconvulsive dose, kindled the progressive development of epileptic seizures similar to those seen in electrical amygdaloid kindling. This response was dependent on dose and on interstimulus interval, and once established persisted at least 8 weeks without further stimulation. Spontaneous seizures were observed in some fully kindled animals. No kindling-specific changes were seen by light microscopy. Muscarine (3 nmol) and the active (+), but not the inactive (−), isomer of acetyl-β-methylcholine also kindled seizures. The action of (+)-acetyl-β-methylcholine was potentiated by the cholinesterase inhibitor physostigmine. The muscarinic antagonists atropine and quinuclidinyl benzylate (QNB) blocked kindling by carbamylcholine or muscarine. Atropine, QBN and scopoamine greatly reduced agonist-induced seizures in previously kindled rats. Highly significant transfer effects were observed between muscarinic agonists, i.e. muscarine-kindled rats had widespread seizures on their first carbamylcholine exposure and vice versa. Kindled animals had a lowered seizure threshold for muscarinic agonists. Dibutyryl cyclic GMP produced seizures but no kindling. Those results demonstrate that in this model the stimulation of a group of muscarinic cholinergic synapses is both necessary and sufficient to induce a kindled state characterized by both evoked and spontaneous seizures, and support the view that epilepsy can be acquired and expressed transsynaptically.     

Interactions between chemical and electrical kindling of the rat amygdala

Holtzman rats were implanted with a chemitrode into the left basolateral amygdala, which could then be stimulated electrically (400 μA, 1 s, AC) or chemically by injection of carbachol (1 μl, 2.7 nmoles, sterile, isotonic). Group A received a daily injection of carbachol and developed kindled seizures. Group B received carbachol mixed with equimolar atropine, which blocked seizures and kindling. After 20 injections, both groups were stimulated electrically once a day and kindled at similar rates. Two additional groups received electrical or sham stimulation, followed by carbachol kindlind. No transfer effects were observed. Four additional groups received 27 nmoles of atropine through the chemitrode, followed 15 min later by electrical stimulation, sham stimulation, carbachol injection or saline injection, respectively. Atropine completely blocked carbachol kindling but did not alter the rate of electrical kindling. No difference in the number of QNB binding sites was observed in the amygdala of rats sacrificed two weeks after full electrical kindling. The lack of interaction between electrical and carbachol kindling and the failure of atropine to block electrical kindling of the amygdala suggest that the activation of local muscarinic synapses, while essential for carbachol kindling, is not required for electrical kindling of the rat amygdala.     

Proconvulsant actions of lindane: effects on afterdischarge thresholds and durations during amygdaloid kindling in rats

Daily exposure to lindane, 5 mg/kg, produced a number of consistent changes in rats undergoing amygdaloid kindling. Lindane decreased the number of afterdischarge-evoking stimulations required to produce the fully kindled state, increased the length of each successive afterdischarge during kindling and increased the severity of each convulsive response during kindling. However, lindane exposure did not change the total seconds of afterdischarge activity required to create the fully kindled state. In this study the enhancement of kindling rates by lindane stemmed primarily from an increase in the duration of epileptiform activity following each amygdaloid stimulation. These proconvulsant effects cannot be attributed to a lowering of seizure thresholds by lindane. Thresholds evaluated at different times during and after completion of kindling were either unaffected or actually elevated by lindane exposure. The length and severity of seizures evoked in fully kindled subjects by amygdaloid stimulation were much less affected by lindane exposure. This indicates that the most useful period for demonstrating proconvulsant activity with the kindling seizure model is during the dynamic phase of seizure development. Once the process has reached completion, seizures are maximally expressed, and their duration is controlled by processes not as susceptible to lindane exposures.     

Effects of Intraamygdaloid Injection of Taurine and Valyltaurine on Amygdaloid Kindled Seizure in Rats

Abstract: Antiepileptic effects of intracerebral injections of taurine and valyltaurine were examined in amygdaloid kindled rats. The effects were assessed whether the animals can evoke generalized seizures by a 10,μA higher stimulation intensity than triggering thresholds. In all fully-kindled animals that have received intraamygdaloid injection of 500 nmol taurine, the kindled seizure was completely abolished. Such a significant seizure suppression (p < 0.05) was observed 12–24 h after the taurine injection. Valyltaurine (500 nmol) also suppressed the seizure in 60% of animals tested, but the effect was not statistically significant. The results indicate that taurine may effectively suppress epileptic seizures when it acts directly at the stimulation site amygdala.     

Aminophylline and kindled seizures

The effects of aminophylline on amygdaloid and cortically kindled rats was studied. Rats implanted with chronic amygdaloid electrodes received either saline or 150 mg/kg, i.p., aminophylline 20 min prior to their first stimulation. On the first stimulation, aminophylline-treated rats had dramatically longer afterdischarge durations and more severe seizure ranks. When fully kindled, the animals were retested with saline or aminophylline. Again, the aminophylline-treated animals had longer afterdischarge durations than the saline-treated rats. In a second experiment, fully amygdaloid kindled rats were pretreated with various doses of aminophylline and stimulated 20 min later. With suprathreshold stimulation, a dose-dependent increase was noted in the afterdischarge duration. During seizure threshold determinations, aminophylline pretreatment markedly prolonged afterdischarge durations without significantly changing seizure severity or threshold. When animals were treated with the adenosine agonist, 2-chloroadenosine, prior to kindled amygdaloid stimulation, the elicited afterdischarge was shortened. The effect was antagonized where treatment with both 2-chloroadenosine and aminophylline occurred prior to amygdaloid stimulation. Rats with neocortical electrodes were also exposed to various doses of aminophylline while in a stable, partially developed kindled stage and again when fully kindled. At both stages, afterdischarge duration was increased by aminophylline in a dose-dependent manner. The partially developed, cortically kindled animals were more responsive to aminophylline than were those fully kindled and they tended to have greater increases in afterdischarge duration and seizure rank. These data demonstrate that aminophylline acts to prolong afterdischarges elicited at various stages of kindling from both amygdaloid and cortical sites.     

Facilitation of Amygdala Kindling Development and Kindled Seizures by Metaphit

Summary: The effect of metaphit (a phencyclidine analogue with an acylating isothiocyanate) on kindling development and kindled seizures from amygdala was investigated in rats pretreated once with metaphit. Administration of a single dose of metaphit (10 or 20 mg/kg intraperitoneally i.p.) 4 h before the first electrical stimulation of the amygdala did not in itself induce seizures, but greatly facilitated development of behavioral seizures during kindling. This effect persisted throughout the whole process of electrical amygdala kindling without further dosing. In contrast, metaphit only transiently and modestly increased the growth of afterdischarge (AD) duration. In kindled rats, pretreatment with a single dose of metaphit (20 mg/kg) 8 h before the test stimulation reduced the threshold current required to elicit a stage 5 seizure and shortened the latency for bilateral forelimb clonus (BFC) without changing AD duration or BFC duration. The facilitation of kindling development and kindled seizures may be due to an excessive excitatory transmission by metaphit in the limbic seizure circuitry.     

Effect of low-intensity 50-Hz magnetic fields on kindling acquisition and fully kindled seizures in rats

The possibility that chronic exposure to power-line frequency (50 Hz) magnetic fields (MFs) might affect the acquisition or characteristics of focal and generalized seizures in amygdala kindled rats was studied. Acute, short-lasting 50-Hz MF exposure of fully kindled rats at either 1 or 100 μT had no effect on afterdischarge threshold (ADT) or seizure parameters recorded at ADT. In the chronic experiments, rats with electrodes implanted in the basolateral amygdala were exposed to a 50-Hz, 100-μT (1 Gauss) MF or to a sham field condition before and after onset of daily electrical stimulations over the whole period of kindling development. The focal seizure threshold (ADT) was determined before and after kindling development in MF exposed and sham exposed rats. Pre-kindling ADT was significantly increased by MF exposure. Exposed rats needed about the same number of stimulations to kindle than sham exposed rats, but the cumulative afterdischarge duration to reach criterion (i.e., a stage 5 seizure) was significantly reduced in MF exposed animals. Post-kindling ADT was similar in the two groups, but MF exposed rats showed a significantly higher threshold for generalized seizures. The data indicate that chronic exposure of rats to a 50-Hz, 100-μT MF exerts weak inhibitory effects on some seizure parameters of the kindling model.     

Potent and long-lasting anticonvulsant effects of 1-naphthylacetyl spermine, an analogue of Joro spider toxin, against amygdaloid kindled seizures in rats

The anticonvulsant effect of 1-naphthylacetyl spermine (1-NA-Spm), an analogue of Joro spider toxin, against amygdaloid kindled seizures was studied in rats. 1-NA-Spm (10, 20 and 40 μg/rat) dose-dependently improved kindled seizures and shortened the afterdischarge duration 30 min after the administration. The anticonvulsant effect was observed even one day after the drug, and then gradually disappeared within 4 days. The present findings demonstrate that 1-NA-Spm acts as a potent and long-acting anticonvulsant against amygdaloid kindled seizures, and also suggest, together with the previous findings, that the calcium-permeable AMPA receptors, which are selectively antagonized by 1-NA-Spm, play a critical role in the seizure generation mechanism of amygdaloid kindling.     

Kindling in the seizure-prone and seizure-resistant Mongolian gerbil

The kindling response in the amygdala of each of two strains of Mongolian gerbil bred for resistance to and suceptibility to stress-precipitated seizures was assessed. Seizure-prone gerbils had a lower afterdischarge threshold, a longer initial afterdischarge duration, and required fewer sessions to develop generalized kindled convulsions. Following kindling, the precipitated seizure pattern was exacerbated in both groups. These results suggest that the naturally occurring and kindled seizure states may depend in part on a common neural mechanism in this species.