Evidence for a norepinephrine-dependent brain-stem substrate in the development of kindling antagonism

Spectral analysis of bilateral or alternate-site kindling-induced afterdischarges in the rabbit hippocampi

Kindling is one of the popular animal models of temporal lobe epilepsy. In the present study following the previous results obtained using unilateral hippocampal kindling (UK), we performed spectral analysis of bilateral or alternate-site kindling-induced afterdischarges (ADs) in the rabbit hippocampi. Eight and ten adult rabbits were used for bilateral kindling (BK) and alternate-site kindling (AK), respectively. Kindling stimuli consisted of a train of biphasic pulses (1ms duration each) of 50Hz for 1s, with suprathreshold intensity for AD. The stimulations were applied simultaneously to the bilateral hippocampi in the BK and were delivered to the right and left hippocampus once every 24h in the AK. Motor responses were classified into five stages according to the conventional criteria. All animals in BK as well as AK developed stage 5 convulsions. This contrasts to the result of UK (kindled: 50%; incomplete: 50%). We normalized power spectral density (PSD) and monitored the changes in the proportion of lower frequency band component (LFB: 0-9Hz) and the higher frequency band (HFB: 12-30Hz). BK animals showed a significantly large decrement (0.5 times, p<0.01) in LFB component at the final stage compared to the initial stage, but a very large increment (4.7 times) in HFB component. Likewise, AK animals exhibited a significantly large decrement (0.6 times, p<0.01) in LFB component at the final stage, but a very large increment (3.6 times) in HFB component. Correlation analyses were performed between the HFB component and AD duration, interictal discharge frequency, and behavioral stages during kindling progression. Very strong positive correlations were found in both kindling animals. Chronological spectral analysis of seizure discharges, resulting in a pattern of LFB decrement accompanied by HFB increment, is a convenient tool to investigate epileptic disorders and diagnose epileptic states.

Fast Fourier transformation analysis of kindling-induced afterdischarge in the rabbit hippocampus

Kindling is a widely used animal model of intractable temporal lobe epilepsy. In the present study, we performed fast Fourier transformation (FFT) analysis of kindling-induced afterdischarge (AD) in the rabbit hippocampus. Ten adult rabbits were used. Kindling stimulation to the right hippocampus was delivered as a train of biphasic pulses (1 ms duration each) of 50 Hz for 1s, with suprathreshold intensity for AD. Motor responses were classified into five stages according to the conventional criteria. Of 10 animals, five developed stage 5 convulsions with a mean of 21 stimulations (kindled (K) group), while the remaining five animals did not (incomplete kindling (IK) group). We standardized each ratio of power spectral density of lower frequency band component (LFB: 0-9 Hz) and the higher frequency band (HFB: 12-30 Hz) in the initial stage as 1.0. The IK group exhibited small decrements (0.99 and 0.94 times) in LFB and HFB components at the final stage. In contrast, the K group exhibited a significantly (p<0.05) large decrement (0.49 times) in the LFB component and a very large increment (4.45 times) of HFB component at the final stage. Correlation analyses were performed between alteration of power spectral density ratio of the HFB component and AD duration, interictal discharge frequency, and behavioral stage during kindling progression. Fairly strong positive correlations were found in all cases in the K group. FFT analysis of kindling-induced AD demonstrated an important role of the HFB component: enhancement of the HFB component is associated with kindled stage, while decrement of it is associated with incomplete kindling stage. These findings suggest that FFT analysis of stimulus-induced and spontaneous seizure discharges is useful for examination of the progression of epileptic disorders.

Kindling and status epilepticus models of epilepsy: rewiring the brain

This review focuses on the remodeling of brain circuitry associated with epilepsy, particularly in excitatory glutamate and inhibitory GABA systems, including alterations in synaptic efficacy, growth of new connections, and loss of existing connections. From recent studies on the kindling and status epilepticus models, which have been used most extensively to investigate temporal lobe epilepsy, it is now clear that the brain reorganizes itself in response to excess neural activation, such as seizure activity. The contributing factors to this reorganization include activation of glutamate receptors, second messengers, immediate early genes, transcription factors, neurotrophic factors, axon guidance molecules, protein synthesis, neurogenesis, and synaptogenesis. Some of the resulting changes may, in turn, contribute to the permanent alterations in seizure susceptibility. There is increasing evidence that neurogenesis and synaptogenesis can appear not only in the mossy fiber pathway in the hippocampus but also in other limbic structures. Neuronal loss, induced by prolonged seizure activity, may also contribute to circuit restructuring, particularly in the status epilepticus model. However, it is unlikely that any one structure, plastic system, neurotrophin, or downstream effector pathway is uniquely critical for epileptogenesis. The sensitivity of neural systems to the modulation of inhibition makes a disinhibition hypothesis compelling for both the triggering stage of the epileptic response and the long-term changes that promote the epileptic state. Loss of selective types of interneurons, alteration of GABA receptor configuration, and/or decrease in dendritic inhibition could contribute to the development of spontaneous seizures.

Limbic epileptogenicity, cell loss and axonal reorganization induced by audiogenic and amygdala kindling in wistar audiogenic rats (WAR strain)

Audiogenic seizures are a model of generalized tonic-clonic brainstem-generated seizures. Repeated induction of audiogenic seizures, in audiogenic kindling (AuK) protocols, generates limbic epileptogenic activity. The present work evaluated associations between permanence of AuK-induced limbic epileptogenicity and changes in cell number/gluzinergic terminal reorganization in limbic structures in Wistar audiogenic rats (WARs). Additionally, we evaluated histological changes after only amygdala kindling (AmK) and only AuK, and longevity of permanence of AuK-induced limbic epileptogenicity, up to 160 days. WARs and Wistar non-susceptible rats were submitted to AuK (80 stimuli) followed by both 50 days without acoustic stimulation and AmK (16 stimuli), only AmK and only AuK. Cell counting and gluzinergic terminal reorganization were assessed, respectively, by using Nissl and neo-Timm histochemistries, 24 h after the last AmK stimulus. Evaluation of behavioral response to a single acoustic stimulus after AuK and up to 160 days without acoustic stimulation was done in another group. AuK-induced limbic epileptogenicity developed in parallel with a decrease in brainstem-type seizure severity during AuK. AmK was facilitated after AuK. Permanence of AuK-induced limbic epileptogenicity was associated with cell loss only in the rostral lateral nucleus of amygdala. Roughly 20 generalized limbic seizures induced by AuK were neither associated with hippocampal cell loss nor mossy fiber sprouting (MFS). AmK developed with cell loss in hippocampal and amygdala nuclei but not MFS. Main changes of gluzinergic terminals after kindling protocols were observed in amygdala, perirhinal and piriform cortices. AuK and AuK-AmK induced a similar number and type of seizures, higher than in AmK. AmK and AuK-AmK were associated with broader cell loss than AuK. Data indicate that permanent AuK-induced limbic epileptogenicity is mainly associated to gluzinergic terminal reorganization in amygdala but not in the hippocampus and with no hippocampal cell loss. Few AmK-induced seizures are associated to broader and higher cell loss than a higher number of AuK-induced seizures.

Epileptogenesis induced by alternate-site kindling in bilateral hippocampi

PURPOSE

Alternate-site kindling (AK), which has been known to induce so-called kindling antagonism, was performed in the bilateral hippocampi to reveal neural mechanisms underlying hippocampal kindling.

METHODS

Ten adult rabbits were used. Daily kindling stimulation consisted of a 1- s train of 50 pulses (pulse duration, 1 ms) of 80 to 200 microA (base-to-peak), which was higher than the afterdischarge (AD) threshold. The concurrent alternating stimulations were delivered to the right and left hippocampus once every 24 h.

RESULTS

All animals developed a stage 5 convulsion with a mean of 28.1 +/- 3.3 (mean +/- SEM) stimulations. The right and left hippocampus received 14.8 +/- 1.7 and 14.6 +/- 1.6 stimulations, respectively. Behavioral stages induced by stimulation of the right or left hippocampus evolved to generalized seizures along a similar course. Kindling antagonism was not observed. The two sides showed similar increases in AD duration, and similar chronologic changes in interictal discharge (IID) frequency. Simple A-type IID and complex types of IID appeared at higher rates, whereas simple B-type IID remained at a relatively low level.

CONCLUSIONS

The present AK procedure did not induce kindling antagonism, but it induced progression of kindling manifestations. The origin of simple B-type IID is known to be in the contralateral side, and its intracellular counterpart corresponds to a sequence of small depolarization followed by large hyperpolarization, suggesting that plastic changes in the feed-forward inhibitory system play an important role in hippocampal kindling.

Prenatal morphine exposure induces age- and sex-dependent changes in seizure susceptibility

1. Prenatal exposure to morphine induces long-term alterations in seizure susceptibility, which are age-, sex-, and seizure model-specific. 2. Adult male and female rats exposed prenatally to morphine show decreased susceptibility to GABA-regulated seizures. 3. Prenatally morphine-exposed, adult male rats are more sensitive to excitatory amino acid receptor-mediated seizures than control males, control females, or morphine-exposed females.

Simultaneous kindling of the bilateral hippocampi: an advanced model for epilepsy research

PURPOSE

To examine whether simultaneous kindling of bilateral hippocampi [bilateral kindling (BK)] could accelerate the achievement of seizures by the breakdown of kindling antagonism or decelerate the achievement of seizures by its enhancement.

METHODS

The hippocampi of 17 adult rabbits were simultaneously kindled bilaterally according to Goddard's method.

RESULTS

All animals developed stage 5 convulsions after a mean of 28 stimulations. Six animals showed spontaneous seizure discharges. Afterdischarge duration increased abruptly during the early period of kindling, but thereafter it gradually progressed. Chronological analyses of interictal discharges (IIDs) demonstrated that simple as well as complex types of IIDs increased their frequencies during BK.

CONCLUSIONS

Compared with unilaterally kindled animals, the BK procedure significantly increased the percentage of animals that successfully kindled (100% vs. 59%; p < 0.01), whereas it significantly decelerated the kindling progression (28 days vs. 19 days; p < 0.02). We conclude that the BK procedure represents potentiation of both excitatory and inhibitory mechanisms. Although the reason why such an antagonistic relationship between them breaks down is still unknown, the BK provides an advanced animal model to study the pathogenic mechanisms of kindling and to screen anticonvulsants.

Alternate-site kindling in the guinea-pig results in accelerated seizure progression and generalization

In rats, the concurrent alternate elicitation of epileptiform activity in two forebrain structures can result in both the rapid production of severe seizures and the development of fully generalized seizures in one (dominant) site, while arresting the progress of seizure activity at intermediate stages in the other (suppressed) site. The latter phenomenon is known as kindling antagonism. In this study, we examined alternate-site kindling in the guinea-pig as they fail to express fully generalized (stage 5) convulsions during single-site kindling. We assessed both seizure stage and afterdischarge duration following inter-hemispheric alternate-site kindling stimulation of the amygdala and medial septal areas. Alternating-site kindling of the medial septal and amygdaloid areas bypassed the normal inhibitory mechanisms in some guinea-pigs, enabling them to reach a stage 5 seizure. Furthermore, alternate-site kindled guinea-pigs demonstrated three (absolute, relative, and mutual) types of kindling antagonism. Guinea-pig kindling as a model of human partial epilepsy is discussed.

Experimental models of epilepsy in young animals

Seizures occur more frequently early in life. Some of these early seizures may eventually become epilepsy. Others are reactive seizures due to excessive environmental stimuli that, in any other age group, might not have elicited a similar response. To understand the developmental aspects of seizures and epilepsy in humans, it is important to study these processes in animals of equivalent ages. In this paper, we describe several animal models of developmental seizures, including their electroclinical manifestations and their validity in respect to human epileptic syndromes. There are several factors that may account for the increased seizure susceptibility of the immature brain, including the delayed development of effective systems or synaptic networks that are involved in the suppression of seizures. A better insight of the basic pathophysiology of seizures as a function of age in animal models will lead to the development of new therapeutic approaches for the treatment of age-specific epileptic disorders in humans.

Convulsions induced by methyl beta-carboline-3-carboxylate in mice: effects of preceding saline injections

The convulsant effects of a high (5 mg/kg intraperitoneally, i.p.) dose of benzodiazepine (BZD) receptor ligand methyl beta-carboline-3-carboxylate (beta-CCM), whether or not preceded by administration of two lower doses of beta-CCM (0.5 and 1 mg/kg i.p.) or of saline were studied in nine inbred mouse strains. In five of the strains (A/J, BALB/cBy, C3H/HeJ, CBA/H, and DBA/2J), neither saline nor preceding injections of beta-CCM had any effect on subsequent reactivity to the subsequent convulsant dose. In the other 4 strains, such injections induced either tolerance (CPB-K, NZB), or sensitization (C57BL/6J, XLII), whatever the compound subsequently administered (beta-CCM or saline). In these strains, the data rule out any tolerance or sensitization effect due to beta-CCM, but suggest that such effects could be due to injection itself.

Kindling antagonism: mapping of susceptible sites

Previous research has shown that concurrent alternating stimulation of paired limbic sites culminates in kindling of generalized seizures from 1 (dominant) site, whereas the other (suppressed) site supports only focal or partial seizures. This phenomenon has been referred to as kindling antagonism, and it has been proposed that antagonism reflects an arrest of kindling, which is therefore viewed as a non-continuous stepwise process. We have attempted to replicate these important observations in adult rats stimulated in various combinations of forebrain sites. Kindling antagonism was displayed by rats stimulated in the amygdala and the septal area, in the bilateral amygdala, the septal area and the splenium of the corpus callosum, and the amygdala and the cingulate cortex. We also found that antagonism between the amygdala and septal area as well as electrographic and behavioral correlates of alternating stimulation were sensitive to the hemispheric relation of the electrodes and to the order in which the sites received initial stimulations. That is, rats that carried ipsilateral amygdaloid and septal electrodes were less likely to display antagonism when the amygdala was the first site stimulated. On the other hand, we failed to obtain antagonism from rats stimulated in other limbic pairs (e.g. entorhinal cortex and septal areas.

Evidence for the interaction of brainstem systems mediating seizure expression in kindling and electroconvulsive shock seizure models

Amygdala kindling was observed to increase significantly the proportion of rats that exhibited tonic hindlimb extension in response to corneal electroshock stimulation. Mechanical brainstem lesions which abolished electroshock-induced tonic hindlimb extension failed to alter either the expression of fully generalized kindled seizures or the development of amygdala kindled seizures. Results suggest that while kindling can alter the sensitivity of brainstem systems involved in the expression of tonic hindlimb extension, these same systems are not necessary for either the development or expression of amygdala kindled seizures.

Suppression of amygdala kindling with massed stimulation: effect of noradrenaline antagonists

Afterdischarge (AD) triggered by brief, daily stimulation of the amygdala progressively increases in complexity and duration and, over days, develops into generalized convulsions. This progression, called kindling, is delayed by noradrenaline (NA). When brief stimulation of the amygdala occurs too frequently (massed), there is a suppression of AD growth and little evidence of kindling. Previously we showed that depletion of NA before massed amygdala stimulation prevented the suppression of AD growth described above, and readily precipitated generalized seizures. In the present report, we examined the role of NA in maintaining this suppression of AD growth, after it was well established. We showed that suppression of AD development during the first 15 massed stimulations (interstimulus interval of 5 min) was reduced by subsequent injection of the NA alpha 2 antagonist, yohimbine, with most rats exhibiting occasional generalized convulsions. Conversely, rats exposed to the beta antagonist, propranolol, like controls, not only showed suppressed AD growth, but also elevated AD thresholds. Three weeks later, only a small positive transfer to daily kindling was observed in all groups. We conclude that alpha 2 NA receptors help maintain suppression of AD growth induced by massed stimulation of the amygdala, while beta receptors provide only a small proepileptic influence. These results and those from the 'rapid' kindling model (Lothman et al., Brain Research, 360 (1985) 83-91) are compared, and related to NA receptor subtype variations in the amygdala and hippocampus.