Transfer of epileptogenesis between perirhinal cortex and amygdala induced by electrical kindling

An interesting feature of the kindling phenomenon relates to the finding that kindling established in one region of the brain may reduce the number of stimulations required to establish the phenomenon in a second region. It has been proposed that this ‘transfer' phenomenon reflects the extent to which seizures arising in two distinct regions share common underlying mechanisms. The perirhinal cortex (PRC) is currently receiving considerable attention with regard to its possible role in epileptogenesis. Although the role of this region in limbic seizures is unclear, the existence of reciprocal connections between the PRC and amygdala provides a possible neural substrate through which these two regions may influence one another. On the basis of this connectivity, one might expect a transfer between PRC kindling and amygdaloid kindling. Using kindling transfer, the present study was formulated to determine the nature of the relationship between electrical kindling of the PRC and amygdala. Animals previously kindled from the PRC to a cortico-generalised level displayed significantly more advanced behavioural seizures during the early stages of amygdaloid kindling than either controls or those partially kindled. This suggests that primary PRC kindling may facilitate amygdaloid access to systems responsible for the generation of motor seizures. Thus, in terms of kindling, the PRC and amygdala appear to be functionally related, with generalised seizures elicited from the PRC and amygdala sharing, at some level, common underlying mechanisms. Finally, the finding that seizures kindled from the dorsal component of the PRC tended to exhibit characteristics which were quite distinct from those elicited by ventral PRC kindling suggests that these two subregions may have different kindling characteristics and/or different patterns of connectivity with the amygdaloid complex.     

Positive transfer of audiogenic kindling to electrical hippocampal kindling in rats.

Audiogenic seizures in genetically susceptible rodents are provoked by intense acoustic stimulations which result in a tonic seizure associated with a short flattening of the EEG. These seizures have been shown to involve primarily brainstem structures. Daily exposure to sound for 30-40 days produced a permanent change in the evoked seizure with development of facial myoclonias, rearing and falling, or of tonic-clonic seizures accompanied by high amplitude cortical spike-and-wave discharges. Kindled audiogenic seizures appear similar to seizures kindled from amygdala or hippocampus, suggesting that repeated auditory stimulations cause a progressive propagation of the epileptic discharge toward limbic structures. To verify this hypothesis, the behavioral and EEG development of electrical hippocampal kindling has been studied in 7 non epileptic controls (NE), 8 acoustic susceptible (AS), and 8 audiogenic kindled rats (KAS). The behavioral and EEG development of the electrical hippocampal kindling was similar in the AS and the NE rats. However, 2 animals in the AS group but no controls exhibited behavioral running and bouncing during the course of hippocampal kindling. In the KAS group, the hippocampal kindling was clearly facilitated as compared to NE and AS: behavioral stage greater than or equal to 5 was reached in a mean of 4 stimulations in KAS versus 30 and 22 stimulations respectively in NE and AS groups. This positive transfer phenomenon suggests that during kindling of audiogenic seizures, epileptic discharge spreads from the brainstem to the forebrain and progressively involves the hippocampus.     

Two types of neuroplasticities in the kindling phenomenon.

We stimulated the dorsal hippocampus of the rat with 2 Hz electrical stimulation to induce kindling seizures. As we reported previously using cats, pulse-number threshold (PNT), the number of stimulating-pulses required for the triggering of epileptic afterdischarge, decreased profoundly in the initial stage of the kindling process and the behavioral seizure stage (BSS) developed in the later stage. After the completion of kindling, a 4 week interval elevated PNT significantly compared to the value at the completion of kindling, whereas BSS showed no regression. These results suggest that there are two types of neuroplasticities which are independent of each other in the kindling phenomenon, one is the early-short type which is involved in the susceptibility of epileptic focus in hippocampus and the other is the late-long type which is involved in the full propagation of kindled seizures to the extra-limbic area.     

Seizure susceptibility in aged rats--pentylenetetrazol-induced seizures and amygdaloid kindling seizures]

Seizure susceptibility in male aged rats (21-25 mo. age) was investigated by pentylenetetrazol (PTZ)-induced seizures and amygdaloid kindling seizures. Male adult rats (3-4 mo. of age) were used as the control group. During 60 min. after the administration of PTZ (70 mg/kg, s.c.), the aged rats had a higher incidence and a significantly longer duration of generalized convulsion than the control group. On the other hand, the kindling rate of the aged rats was significantly slower than that of the control. The aged rats remained longer at stage 1-2 indicative of partial seizures. However, the aged rats skipped stage 3 on their way to stage 4-5 (generalized seizures), and showed a more violent stage 5 seizure than the control group. They had a longer duration of afterdischarges (AD) through the kindling process with a significantly faster propagation of AD to the contralateral amygdala as compared with the control group. The present study suggests that aged rats are prone to convulsion, while they have a difficulty in acquisition of epileptogenesis.     

Development of spontaneous seizures over extended electrical kindling: I. Electrographic, behavioral, and transfer kindling correlates

The present study was aimed at evaluating an extended kindling model of spontaneous epilepsy. Behavioral and electrographic responses to repeated kindling of either the perforant path or amygdala were monitored for up to 300 trials. Kindling initially led to generalized convulsions equivalent to the level 5 seizure on the rating scale developed by Racine. The evoked seizures became progressively more complex with additional kindling, which was described by a 10-stage classification system. The highest stage (stage 10) was achieved when the kindling stimulation evoked two or more bouts of level 5 seizures combined with running and jumping fits. These more complex seizures developed over the course of amygdala, but not perforant path kindling. Electrographic seizures from both the amygdala and dentate gyrus increased in duration and amplitude during the early phase of kindling, but did not correlate with motor seizure development beyond level 5. During the late phase of kindling, the dentate gyrus afterdischarge amplitude decreased and became dissociated from the behavioral seizures. Manifestations of spontaneously recurring seizures were seen in the majority of animals, but spontaneous seizures of level 4 or greater were observed in only five rats. The second part of this study examined kindling transfer effects, the efficacy of kindling a new site after the completion of the initial (in this case extended) kindling protocol. The effect depended on both primary and secondary site location. When the amygdala served as primary site, perforant path transfer was complete in some animals but absent in others. No transfer occurred in the opposite direction, from the perforant path to the amygdala. Finally, transfer effects in the dentate gyrus, which was tested as tertiary site, were complete. Previous studies have found weaker transfer effects in the dentate when kindling to the standard stage 5 level.     

Amygdala kindling in juvenile rats following neonatal administration of 6-hydroxydopamine

The role of catecholamines in mediating the acquisition of amygdala-kindled seizures was investigated in juvenile rats administered intracisternal injections of 6-hydroxydopamine (6-OHDA) on postnatal days 1 and 2. Amygdala kindling was initiated on day 28, using stimulations delivered each hour through two consecutive stage V seizures. The 6-OHDA treatment resulted in a 53% increase in the overall rate of kindling in juvenile rats. This acceleration was confined primarily to the early phases of kindling in that the 6-OHDA-treated rats skipped the early kindling stages, and the later stages of kindling were unaffected. These findings support evidence from adult rats that catecholamines play a role in initially limiting the spread of seizure activity during kindled seizure acquisition; however, when the seizures have begun to generalize, the ability of catecholaminergic systems to inhibit seizure spread diminishes.     

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.     

Long-term changes in sensitivity to GABA in dorsal raphe neurons following amygdala kindling

The present experiments were undertaken to evaluate GABA sensitivity in dorsal raphe neurons following amygdala-kindled seizures. Dorsal raphe neurons of amygdala-kindled rats exhibited significant subsensitivity to GABA as measured electrophysiologically 3 or 4 weeks after the last stage 5 seizure. Amygdala stimulation with currents which did not produce kindled seizures did not produce subsensitivity to GABA. The subsensitivity observed after kindling was equivalent in magnitude to that observed following chronic diazepam treatment. However, exposure of fully kindled rats to chronic diazepam did not further decrease the sensitivity of dorsal raphe neurons to GABA. Additionally, while subsensitivity to GABA was reversed by bath application of the benzodiazepine antagonist, Ro 15-1788, in chronic diazepam-treated rats, it had no effect on GABA subsensitivity in fully kindled rats. These findings suggest a decrease in GABA sensitivity within the dorsal raphe might reflect long-term neuronal changes associated with kindled seizures. These data also suggest that the decrease in GABA sensitivity of dorsal raphe neurons following chronic diazepam may involve different mechanisms from those observed after amygdala kindling.     

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.     

The relation between extent of dorsal hippocampal kindling and delayed-match-to-place performance in the Morris water maze

Dorsal hippocampal kindling impairs subsequent performance on spatial tasks. The relation between this effect and the extent of kindling achieved prior to testing has not been clearly established. Thus, the present study investigated the effects of dorsal hippocampal kindling on performance of a delayed-match-to-place (DMTP) task in the Morris water maze by assessing performance after each of series of different points in the kindling process including 1, 6, 11, and 16 afterdischarges, 1 stage 1 seizure, and 1 stage 5 seizure. We found that kindling produced a deficit that was apparent very early into kindling in terms of both direct swim (by 1 AD) and escape distance (by 6 ADs) measures but that did not clearly change in severity with further kindling. These results illustrate that kindling of even a few localized hippocampal seizures can disrupt spatial cognition and suggest that the mechanisms mediating memory disruption either do not change substantially as kindling progresses or that compensatory processes are engaged across training that mitigate any further kindling-related deteriorations in performance.     

双侧海马同时点燃与单侧海马点燃建立复杂部分性癫痫模型的对照研究

目的　观察双侧海马同时点燃时 ,是因破坏了点燃的拮抗作用而加速达到痫性发作 ,还是因点燃的拮抗作用的增强而延缓达到痫性发作。方法　按照Goddard方法将 2 0只成年Warster兔的双侧海马同时点燃 ,并与单侧海马点燃组作对照。结果　双侧海马同时点燃组 (BK)所有兔在平均 31次刺激后均出现 5期惊厥 ,8只兔显示有自发性痫性放电。在点燃早期 ,后放电 (AD)持续时间突然增加 ,此后逐渐增加。与单侧点燃 (UK)动物相比 ,双侧点燃法能显著提高动物点燃的成功率 (10 0 %对 5 5 % ,P <0 .0 1) ,但也显著减缓了点燃的进程 (31天 / 2 0天 ,P <0 .0 1)。结论　BK方法表现为神经元兴奋和抑制机制都增强。BK对点燃的致痫机制和筛选抗惊厥药物研究提供了一种更先进的复杂部分性癫痫动物模型。     

Neuropathological profile of the pentylenetetrazol (PTZ) kindling model

ABSTRACT

Introduction: There are three phases of seizure developing in pentylenetetrazol (PTZ)-induced kindling animal model: (i) pre-kindling phase; (ii) kindling phase or after animals are fully kindled; (iii) post-kindling phase with non-provoked spontaneous recurrent seizures. The aims of this review were to summarize the progress over time of the electroencephalographic features and neuropathological alterations in kindled PTZ treated animals.

Materials and methods: Keywords relevant to PTZ kindling were used to a guide a literature search on Pubmed, Medline and Cochrane Library.

Results: Clonic seizures induced PTZ at kindling phase led to a strong c-Fos expression in the hippocampus. Although, decline hippocampal neuron and metabolism disturbances were detected at pre-kindlig phase. Repeated PTZ induced seizures alter the GABA-mediated inhibition and glutamate-mediated excitation, which may contribute to increased seizure susceptibility. Similar to chemical animal models such as the pilocarpine and the kainic acid models, mossy fiber sprouting, hippocampal damage, and glucose hypometabolism had been seen after PTZ induced seizures.

Conclusion: PTZ kindling model may improve understanding of the seizures development provided that the differences existing between the phases of kindling model are taken into account.     

Amygdala kindling elevates plasma vasopressin

Acute and chronic effects of epilepsy on endocrine function are known to occur in humans with partial seizures of limbic origin and in animals with limbic kindled seizures. The amygdala, a component of the limbic system, has dense hypothalamic connections and amygdala stimulation in monkeys and cats result in vasopressin release. In the present study we sought to determine if amygdala stimulation in the rats results in an immediate acute release of vasopressin and to determine if acute or chronic changes occur in vasopressin release in the fully kindled animal. Plasma vasopressin, osmolality and hematocrit were measured in blood samples drawn from rats with implanted venous catheters before and after stimulation and at different stages of kindling. Low-frequency (15 Hz) electrical stimulation of the amygdala was followed by an immediate, 3-fold increase in plasma vasopressin concentration. Moreover, although the 60 Hz kindling stimulus did not result in a significant immediate rise in plasma vasopressin prior to kindling, after kindling to stage 5 seizures the 60 Hz kindling stimulus resulted in seizures and a significant immediate rise in plasma vasopressin. In addition, we found that kindling was followed by a significant, though modest, rise in the resting plasma vasopressin without an accompanying change in osmolality or hematocrit. We conclude that kindling results in a persistent alteration in the vasopressinergic neuroendocrine system.     

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.     

Pharmacological prophylaxis in the kindling model of epilepsy.

Review of antiepileptic drug assessment to date by means of the kindling model of epilepsy suggests that it fills a gap that is evident in standard screening methods such as maximum electroshock or pentylenetetrazol screening tests. However, in order to obtain a comprehensive profile of antiepileptic drugs regarding prophylaxis of developing seizures and treatment of well-established seizures through the kindling preparation, it is desirable to have the following: (1) standardization of kindling techniques, (2) examination of drug effects on developing as opposed to developed seizures, (3) use of a variety of animal species, involving different functional brain sites, and (4) monitoring of plasma levels of the drug administered. It is envisaged that judicious use of the kindling preparation might also enable us to gain some insight into the mechanisms by which drugs produce their prophylactic or therapeutic effect.     

Expression of connexin 30 and connexin 32 in hippocampus of rat during epileptogenesis in a kindling model of epilepsy

Objective Understanding the molecular and cellular mechanisms underlying epileptogenesis yields new insights into potential therapies that may ultimately prevent epilepsy.Gap junctions(GJs) create direct intercellular conduits between adjacent cells and are formed by hexameric protein subunits called connexins(Cxs).Changes in the expression of Cxs affect GJ communication and thereby could modulate the dissemination of electrical discharges.The hippocampus is one of the main regions involved in epileptogenesis and has a wide network of GJs between different cell types where Cx30 is expressed in astrocytes and Cx32 exists in neurons and oligodendrocytes.In the present study,we evaluated the changes of Cx30 and Cx32 expression in rat hippocampus during kindling epileptogenesis.Methods Rats were stereotaxically implanted with stimulating and recording electrodes in the basolateral amygdala,which was electrically stimulated once daily at afterdischarge threshold.Expression of Cx30 and Cx32,at both the mRNA and protein levels, was measured in the hippocampus at the beginning,in the middle(after acquisition of focal seizures),and at the end(after establishment of generalized seizures) of the kindling process,by real-time PCR and Western blot.Results Cx30 mRNA expression was upregulated at the beginning of kindling and after acquisition of focal seizures.Then it was downregulated when the animals acquired generalized seizures.Overexpression of Cx30 mRNA at the start of kindling was consistent with the respective initial protein increase.Thereafter,no change was found in protein abundance during kindling.Regarding Cx32,mRNA expression decreased after acquisition of generalized seizures and no other significant change was detected in mRNA and protein abundance during kindling.Conclusion We speculate that Cx32 GJ communication in the hippocampus does not contribute to kindling epileptogenesis.The Cx30 astrocytic network localized to perivascular regions in the hippocampus is,however,overexpressed at the initiation of kindling to clear excitotoxic molecules from the milieu.     

Ro 15-1788 suppresses the development of kindling through the benzodiazepine receptor

beta-Carboline (norharman) has been shown to produce kindled seizures when given systemically for long periods of time. The expression of the kindled seizure activity can be blocked by ligands of the benzodiazepine receptor suggesting the receptor as a site of vulnerability in the kindling mechanism. The present data show that Ro 15-1788, a receptor antagonist, suppresses the development of kindled seizures as demonstrated by the delayed appearance of each behavioral stage and the decreased severity of symptoms within each stage. Animals treated with Ro 15-1788 still expressed lower behavioral stages when Ro 15-1788 was eliminated from the trials indicating that this compound suppresses the very process of kindling itself and not just the expression of the kindled seizures. Ro 15-1788 given with norharman causes an increase in Bmax of [3H]flunitrazepam binding to the benzodiazepine receptor in cortex. It is not known if this increase is instrumental in lowering the kindling rate.     

Increased susceptibility to hippocampal and amygdala kindling following intrahippocampal kainic acid

The effects of unilateral intrahippocampal injection of kainic acid, a potent neuroexcitant and neurotoxin, on subsequent susceptibility to kindling of the contralateral hippocampus or contralateral amygdala were investigated in albino rats. At the chosen doses (0.20 to 1.25 micrograms dissolved in physiologic saline), the kainic acid-induced lesion was confined to the injected hippocampus and in two cases the ipsilateral entorhinal cortex; never were there contralateral lesions. Approximately 2 to 6 weeks post-injection, each animal received daily afterdischarge-producing electrical stimulations until stage 5 kindled limbic seizures occurred. Kindling in pretreated animals was significantly accelerated compared with controls; the hippocampal kindling rate decreased from 13.2 stimulations to 3.7, the amygdala kindling rate from 7.8 stimulations to 3.0. Many treated animals had first-stimulation stage 5 seizures, compared with none for controls. Importantly, this facilitation of kindling was not reversed by suppression of the acute, induced seizures with the anticonvulsants, diazepam and phenobarbital, which have repeatedly been demonstrated to effectively suppress limbic kindling. Such results, considered together with findings from the literature, suggest that partial kindling does not occur during kainic acid-induced seizures, and that the observed susceptibility to kindling and other epileptogenic agents subsequent to kainic acid treatment may in fact be related to neurophysiologic and neurochemical consequences of kainic acid-induced lesions.     

Audiogenic kindling and secondary subcortico-cortical epileptogenesis: Behavioral correlates and electrographic features

Human epilepsy is usually considered to result from cortical pathology, but animal studies show that the cortex may be secondarily involved in epileptogenesis, and cortical seizures may be triggered by extracortical mechanisms. In the audiogenic kindling model, recurrent subcortical (brainstem-driven) seizures induce secondary epileptic activation of the cortex. The present review focuses on behavioral and electrographic features of the subcortico-cortical epileptogenesis: (1) behavioral expressions of traditional and mild paradigms of audiogenic kindling produced by full-blown (generalized) and minimal (focal) audiogenic seizures, respectively; (2) electrographic manifestations of secondary epileptic activation of the cortex — cortical epileptic discharge and cortical spreading depression; and (3) persistent individual asymmetry of minimal audiogenic seizures and secondary cortical events produced by their repetition. The characteristics of audiogenic kindling suggest that this model represents a unique experimental approach to studying cortical epileptogenesis and network aspects of epilepsy.This article is part of a Special Issue entitled "Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic".     

Frontal lobe and kindling in the rat.

To test the hypothesis that the cortex participates in amygdaloid kindling in rats, bilateral aspiration lesions were made in various cortical areas in rats prior to kindling. Lesions in orbital cortex (on the dorsal lip of the rhinal sulcus) or prefrontal cortex (area 10) significantly retarded the rate of amygdaloid kindling; lesions in motor cortex, anterior cingulate cortex, or visual cortex were without effect. Detailed analysis indicated that the orbital lesioned and frontal-lesioned rats kindled relatively normally up to the second-last stage of amygdaloid kindling, in which stage they perseverated significantly longer than the controls and the other lesioned rats. These results suggest that areas of the frontal lobe participate in the elaboration and generalization of amygdaloid seizures in rats. Although retarded in rate, kindling nonetheless occurred in the lesioned rats, indicating that these cortical areas are not essential for the development of amygdaloid seizures.