The effects of various lesions and knife-cuts on septal and amygdala kindling in the rat

Principal neurons in the olfactory cortex mediate bidirectional modulation of seizures

Although the piriform cortex (PC) has been previously implicated as a critical node for seizure generation and propagation, the underlying neural mechanism has remained unclear. Here, we found increased excitability in PC neurons during amygdala kindling acquisition. Optogenetic or chemogenetic activation of PC pyramidal neurons promoted kindling progression, whereas inhibition of these neurons retarded seizure activities induced by electrical kindling in the amygdala. Furthermore, chemogenetic inhibition of PC pyramidal neurons alleviated the severity of kainic acid-induced acute seizures. These results demonstrate that PC pyramidal neurons bidirectionally modulate seizures in temporal lobe epilepsy, providing evidence for the efficacy of PC pyramidal neurons as a potential therapeutic target for epileptogenesis. KEY POINTS: While the piriform cortex (PC) is an important olfactory centre critically involved in olfactory processing and plays a crucial role in epilepsy due to its close connection with the limbic system, how the PC regulates epileptogenesis is largely unknown. In this study, we evaluated the neuronal activity and the role of pyramidal neurons in the PC in the mouse amygdala kindling model of epilepsy. PC pyramidal neurons are hyperexcited during epileptogenesis. Optogenetic and chemogenetic activation of PC pyramidal neurons significantly promoted seizures in the amygdala kindling model, whereas selective inhibition of these neurons produced an anti-epileptic effect for both electrical kindling and kainic acid-induced acute seizures. The results of the present study indicate that PC pyramidal neurons bidirectionally modulate seizure activity.

Alteration of Excitation/Inhibition Imbalance in the Hippocampus and Amygdala of Drug-Resistant Epilepsy Patients Treated with Acute Vagus Nerve Stimulation

An imbalance between excitation (E) and inhibition (I) in the brain has been identified as a key pathophysiology of epilepsy over the years. The hippocampus and amygdala in the limbic system play a crucial role in the initiation and conduction of epileptic seizures and are often referred to as the transfer station and amplifier of seizure activities. Existing animal and imaging studies reveal that the hippocampus and amygdala, which are significant parts of the vagal afferent network, can be modulated in order to generate an antiepileptic effect. Using stereo-electroencephalography (SEEG) data, we examined the E/I imbalance in the hippocampus and amygdala of ten drug-resistant epilepsy children treated with acute vagus nerve stimulation (VNS) by estimating the 1/f power slope of hippocampal and amygdala signals in the range of 1-80 Hz. While the change in the 1/f power slope from VNS-BASE varied between different stimulation amplitudes and brain regions, it was more prominent in the hippocampal region. In the hippocampal region, we found a flatter 1/f power slope during VNS-ON in patients with good responsiveness to VNS under the optimal stimulation amplitude, indicating that the E/I imbalance in the region was improved. There was no obvious change in 1/f power slope for VNS poor responders. For VNS non-responders, the 1/f power slope slightly increased when the stimulation was applied. Overall, this study implies that the regulation of E/I imbalance in the epileptic brain, especially in the hippocampal region, may be an acute intracranial effect of VNS.

Hyperexcitability: From Normal Fear to Pathological Anxiety and Trauma

Hyperexcitability in fear circuits is suggested to be important for development of pathological anxiety and trauma from adaptive mechanisms of fear. Hyperexcitability is proposed to be due to acquired sensitization in fear circuits that progressively becomes more severe over time causing changing symptoms in early and late pathology. We use the metaphor and mechanisms of kindling to examine gains and losses in function of one excitatory and one inhibitory neuropeptide, corticotrophin releasing factor and somatostatin, respectively, to explore this sensitization hypothesis. We suggest amygdala kindling induced hyperexcitability, hyper-inhibition and loss of inhibition provide clues to mechanisms for hyperexcitability and progressive changes in function initiated by stress and trauma.

Amygdaloid complex anatomopathological findings in animal models of status epilepticus

Temporal lobe epileptic seizures are one of the most common and well-characterized types of epilepsies. The current knowledge on the pathology of temporal lobe epilepsy relies strongly on studies of epileptogenesis caused by experimentally induced status epilepticus (SE). Although several temporal lobe structures have been implicated in the epileptogenic process, the hippocampal formation is the temporal lobe structure studied in the greatest amount and detail. However, studies in human patients and animal models of temporal lobe epilepsy indicate that the amygdaloid complex can be also an important seizure generator, and several pathological processes have been shown in the amygdala during epileptogenesis. Therefore, in the present review, we systematically selected, organized, described, and analyzed the current knowledge on anatomopathological data associated with the amygdaloid complex during SE-induced epileptogenesis. Amygdaloid complex participation in the epileptogenic process is evidenced, among others, by alterations in energy metabolism, circulatory, and fluid regulation, neurotransmission, immediate early genes expression, tissue damage, cell suffering, inflammation, and neuroprotection. We conclude that major efforts should be made in order to include the amygdaloid complex as an important target area for evaluation in future research on SE-induced epileptogenesis. This article is part of the Special Issue "NEWroscience 2018".

Effective connectivity among the hippocampus, amygdala, and temporal neocortex in epilepsy patients: A cortico-cortical evoked potential study

OBJECTIVE

Mesial temporal lobe epilepsy (MTLE) is one of the most common types of intractable epilepsy. The hippocampus and amygdala are two crucial structures of the mesial temporal lobe and play important roles in the epileptogenic network of MTLE. This study aimed to explore the effective connectivity among the hippocampus, amygdala, and temporal neocortex and to determine whether differences in effective connectivity exist between MTLE patients and non-MTLE patients.

METHODS

This study recruited 20 patients from a large cohort of drug-resistant epilepsy patients, of whom 14 were MTLE patients. Single-pulse electrical stimulation (SPES) was performed to acquire cortico-cortical evoked potentials (CCEPs). The root mean square (RMS) was used as the metric of the magnitude of CCEP to represent the effective connectivity. We then conducted paired and independent sample t-tests to assess the directionality of the effective connectivity.

RESULTS

In both MTLE patients and non-MTLE patients, the directional connectivity from the amygdala to the hippocampus was stronger than that from the hippocampus to the amygdala (P < 0.01); the outward connectivity from the amygdala to the cortex was stronger than the inward connectivity from the cortex to the amygdala (P < 0.01); the amygdala had stronger connectivity to the neocortex than the hippocampus (P < 0.01). In MTLE patients, the neocortex had stronger connectivity to the hippocampus than to the amygdala (P < 0.01). No significant differences in directional connectivity were noted between the two groups.

CONCLUSIONS

A unique effective connectivity pattern among the hippocampus, amygdala, and temporal neocortex was identified through CCEPs analysis. This study may aid in our understanding of physiological and pathological networks in the brain and inspire neurostimulation protocols for neurological and psychiatric disorders.

Commonalities in epileptogenic processes from different acute brain insults: Do they translate?

The most common forms of acquired epilepsies arise following acute brain insults such as traumatic brain injury, stroke, or central nervous system infections. Treatment is effective for only 60%-70% of patients and remains symptomatic despite decades of effort to develop epilepsy prevention therapies. Recent preclinical efforts are focused on likely primary drivers of epileptogenesis, namely inflammation, neuron loss, plasticity, and circuit reorganization. This review suggests a path to identify neuronal and molecular targets for clinical testing of specific hypotheses about epileptogenesis and its prevention or modification. Acquired human epilepsies with different etiologies share some features with animal models. We identify these commonalities and discuss their relevance to the development of successful epilepsy prevention or disease modification strategies. Risk factors for developing epilepsy that appear common to multiple acute injury etiologies include intracranial bleeding, disruption of the blood-brain barrier, more severe injury, and early seizures within 1 week of injury. In diverse human epilepsies and animal models, seizures appear to propagate within a limbic or thalamocortical/corticocortical network. Common histopathologic features of epilepsy of diverse and mostly focal origin are microglial activation and astrogliosis, heterotopic neurons in the white matter, loss of neurons, and the presence of inflammatory cellular infiltrates. Astrocytes exhibit smaller K+ conductances and lose gap junction coupling in many animal models as well as in sclerotic hippocampi from temporal lobe epilepsy patients. There is increasing evidence that epilepsy can be prevented or aborted in preclinical animal models of acquired epilepsy by interfering with processes that appear common to multiple acute injury etiologies, for example, in post-status epilepticus models of focal epilepsy by transient treatment with a trkB/PLCγ1 inhibitor, isoflurane, or HMGB1 antibodies and by topical administration of adenosine, in the cortical fluid percussion injury model by focal cooling, and in the albumin posttraumatic epilepsy model by losartan. Preclinical studies further highlight the roles of mTOR1 pathways, JAK-STAT3, IL-1R/TLR4 signaling, and other inflammatory pathways in the genesis or modulation of epilepsy after brain injury. The wealth of commonalities, diversity of molecular targets identified preclinically, and likely multidimensional nature of epileptogenesis argue for a combinatorial strategy in prevention therapy. Going forward, the identification of impending epilepsy biomarkers to allow better patient selection, together with better alignment with multisite preclinical trials in animal models, should guide the clinical testing of new hypotheses for epileptogenesis and its prevention.

The piriform cortex and human focal epilepsy

It is surprising that the piriform cortex, when compared to the hippocampus, has been given relatively little significance in human epilepsy. Like the hippocampus, it has a phylogenetically preserved three-layered cortex that is vulnerable to excitotoxic injury, has broad connections to both limbic and cortical areas, and is highly epileptogenic – being critical to the kindling process. The well-known phenomenon of early olfactory auras in temporal lobe epilepsy highlights its clinical relevance in human beings. Perhaps because it is anatomically indistinct and difficult to approach surgically, as it clasps the middle cerebral artery, it has, until now, been understandably neglected. In this review, we emphasize how its unique anatomical and functional properties, as primary olfactory cortex, predispose it to involvement in focal epilepsy. From recent convergent findings in human neuroimaging, clinical epileptology, and experimental animal models, we make the case that the piriform cortex is likely to play a facilitating and amplifying role in human focal epileptogenesis, and may influence progression to epileptic intractability.

Consequences of pilocarpine-induced status epilepticus in immunodeficient mice

Systemic injection of pilocarpine in rodents induces status epilepticus (SE) and reproduces the main characteristics of temporal lobe epilepsy (TLE). Different mechanisms are activated by SE contributing to cell death and immune system activation. We used BALB/c nude mice, a mutant that is severely immunocompromised, to characterize seizure pattern, neurochemical changes, cell death and c-Fos activation secondarily to pilocarpine-induced SE. The behavioral seizures were less severe in BALB/c nude than in BALB/c wild type mice. However, nude mice presented more tonic-clonic episodes and higher mortality rate during SE. The c-Fos expression was most prominent in the caudate-putamen, CA3 (p<0.05), dentate gyrus, entorhinal cortex (p<0.001), basolateral nucleus of amygdala (p<0.01) and piriform cortex (p<0.05) of BALB/c nude mice than of BALB/c. Besides, nude mice subjected to SE presented high number of Fluorojade-B (FJB) stained cells in the piriform cortex, amygdala (p<0.05) and hilus (p<0.001) in comparison with BALB/c mice. A significant increase in the level of glutamate and GABA was found in the hippocampus and cortex of BALB/c mice presenting SE in comparison to controls. However, the level of glutamate was higher in the brains of BALB nude mice than in the brains of BALB/c wild type mice, while the levels of GABA were unchanged. These results indicate that the brains of immunodeficient nude mice are more vulnerable to the deleterious effects of pilocarpine-induced SE as they present intense activation, increased glutamate levels and more cell death.

Characterization of spontaneous recurrent epileptiform discharges in hippocampal-entorhinal cortical slices prepared from chronic epileptic animals

Epilepsy, a common neurological disorder, is characterized by the occurrence of spontaneous recurrent epileptiform discharges (SREDs). Acquired epilepsy is associated with long-term neuronal plasticity changes in the hippocampus resulting in the expression of spontaneous recurrent seizures. The purpose of this study is to evaluate and characterize endogenous epileptiform activity in hippocampal-entorhinal cortical (HEC) slices from epileptic animals. This study employed HEC slices isolated from a large series of control and epileptic animals to evaluate and compare the presence, degree and localization of endogenous SREDs using extracellular and whole cell current clamp recordings. Animals were made epileptic using the pilocarpine model of epilepsy. Extracellular field potentials were recorded simultaneously from areas CA1, CA3, dentate gyrus, and entorhinal cortex and whole cell current clamp recordings were obtained from CA3 neurons. All regions from epileptic HEC slices (n=53) expressed SREDs, with an average frequency of 1.3Hz. In contrast, control slices (n=24) did not manifest any SREDs. Epileptic HEC slices demonstrated slow and fast firing patterns of SREDs. Whole cell current clamp recordings from epileptic HEC slices showed that CA3 neurons exhibited paroxysmal depolarizing shifts associated with these SREDs. To our knowledge this is the first significant demonstration of endogenous SREDs in a large series of HEC slices from epileptic animals in comparison to controls. Epileptiform discharges were found to propagate around hippocampal circuits. HEC slices from epileptic animals that manifest SREDs provide a novel model to study in vitro seizure activity in tissue prepared from epileptic animals.

Diazepam administration after prolonged status epilepticus reduces neurodegeneration in the amygdala but not in the hippocampus during epileptogenesis

An episode of status epilepticus (SE), if left untreated, can lead to death, or brain damage with long-term neurological consequences, including the development of epilepsy. The most common first-line treatment of SE is administration of benzodiazepines (BZs). However, the efficacy of BZs in terminating seizures is reduced with time after the onset of SE; this is accompanied by a reduced efficacy in protecting the hippocampus against neuronal damage, and is associated with impaired function and internalization of hippocampal GABA(A) receptors. In the present study, using Fluoro-Jade C staining, we found that administration of diazepam to rats at 3 h after the onset of kainic acid-induced SE, at a dose sufficient to terminate SE, had no protective effect on the hippocampus, but produced a significant reduction in neuronal degeneration in the amygdala, piriform cortex, and endopiriform nucleus, examined on days 7-9 after SE. Thus, in contrast to the hippocampus, the amygdala and other limbic structures are responsive to neuroprotection by BZs after prolonged SE, suggesting that GABA(A) receptors are not significantly altered in these structures during SE.

Pathological alterations in GABAergic interneurons and reduced tonic inhibition in the basolateral amygdala during epileptogenesis

An acute brain insult such as traumatic head/brain injury, stroke, or an episode of status epilepticus can trigger epileptogenesis, which, after a latent, seizure-free period, leads to epilepsy. The discovery of effective pharmacological interventions that can prevent the development of epilepsy requires knowledge of the alterations that occur during epileptogenesis in brain regions that play a central role in the induction and expression of epilepsy. In the present study, we investigated pathological alterations in GABAergic interneurons in the rat basolateral amygdala (BLA), and the functional impact of these alterations on inhibitory synaptic transmission, on days 7 to 10 after status epilepticus induced by kainic acid. Using design-based stereology combined with glutamic acid decarboxylase (GAD) 67 immunohistochemistry, we found a more extensive loss of GABAergic interneurons compared to the loss of principal cells. Fluoro-Jade C staining showed that neuronal degeneration was still ongoing. These alterations were accompanied by an increase in the levels of GAD and the alpha1 subunit of the GABA(A) receptor, and a reduction in the GluK1 (previously known as GluR5) subunit, as determined by Western blots. Whole-cell recordings from BLA pyramidal neurons showed a significant reduction in the frequency and amplitude of action potential-dependent spontaneous inhibitory postsynaptic currents (IPSCs), a reduced frequency but not amplitude of miniature IPSCs, and impairment in the modulation of IPSCs via GluK1-containing kainate receptors (GluK1Rs). Thus, in the BLA, GABAergic interneurons are more vulnerable to seizure-induced damage than principal cells. Surviving interneurons increase their expression of GAD and the alpha1 GABA(A) receptor subunit, but this does not compensate for the interneuronal loss; the result is a dramatic reduction of tonic inhibition in the BLA circuitry. As activation of GluK1Rs by ambient levels of glutamate facilitates GABA release, the reduced level and function of these receptors may contribute to the reduction of tonic inhibitory activity. These alterations at a relatively early stage of epileptogenesis may facilitate the progress towards the development of epilepsy.

Anticonvulsant impact of lesions in the ventrolateral forebrain of rats challenged with soman

Mapping of trigger sites and/or propagation pathways for soman-induced seizures may provide clues for the designing of anticonvulsant drugs. In the present study, anticonvulsant efficacy against soman intoxication (1.3 x LD50) was examined in rats with either lesion of the perirhinal cortex, posterior piriform cortex, entorhinal cortex, hippocampal region, or amygdala. The results showed that prevention of convulsions or increased latency to onset of convulsions was ensured in rats with perirhinal or piriform cortical lesions, whereas anticonvulsant effects were not achieved in rats with damage to the entorhinal cortex, hippocampal region, or amygdala. The results from the present study suggest that critical structures for induction of seizures after soman exposure are located in the ventrolateral aspect of the forebrain. This suggestion is in compliance with convulsant reactions to microinfusions of soman or VX into ventrolateral brain structures and increased neuronal activity in corresponding structures revealed by c-fos staining in response to soman. Furthermore, results from studies of kindling, lesions, and microinfusion of chemoconvulsants in experimental epilepsy also imply that the perirhinal and piriform cortices are critically involved in seizure control.

Pathology and pathophysiology of the amygdala in epileptogenesis and epilepsy

Acute brain insults, such as traumatic brain injury, status epilepticus, or stroke are common etiologies for the development of epilepsy, including temporal lobe epilepsy (TLE), which is often refractory to drug therapy. The mechanisms by which a brain injury can lead to epilepsy are poorly understood. It is well recognized that excessive glutamatergic activity plays a major role in the initial pathological and pathophysiological damage. This initial damage is followed by a latent period, during which there is no seizure activity, yet a number of pathophysiological and structural alterations are taking place in key brain regions, that culminate in the expression of epilepsy. The process by which affected/injured neurons that have survived the acute insult, along with well-preserved neurons are progressively forming hyperexcitable, epileptic neuronal networks has been termed epileptogenesis. Understanding the mechanisms of epileptogenesis is crucial for the development of therapeutic interventions that will prevent the manifestation of epilepsy after a brain injury, or reduce its severity. The amygdala, a temporal lobe structure that is most well known for its central role in emotional behavior, also plays a key role in epileptogenesis and epilepsy. In this article, we review the current knowledge on the pathology of the amygdala associated with epileptogenesis and/or epilepsy in TLE patients, and in animal models of TLE. In addition, because a derangement in the balance between glutamatergic and GABAergic synaptic transmission is a salient feature of hyperexcitable, epileptic neuronal circuits, we also review the information available on the role of the glutamatergic and GABAergic systems in epileptogenesis and epilepsy in the amygdala.

Neuronal structures involved in the induction and propagation of seizures caused by nerve agents: Implications for medical treatment

In epilepsy research, studies have been made to identify brain areas critical for triggering and/or controlling propagated seizure activity. The purpose of the present study was to focus on a similar approach in nerve agent research by reviewing relevant literature to map potential trigger sites and propagation pathways for seizures. The piriform cortex and medial septal area emerge as prime target areas for soman-induced seizures. The cholinergic hyperactivation in the latter structures seems to induce increased glutamatergic activity in the piriform, entorhinal, and perirhinal cortices along with the hippocampal region. For prophylactic or early treatment, mapping of muscarinic subreceptors in the piriform cortex and medial septum would be guiding for designing anticholinergic drugs with optimal properties. Sustained seizures governed by glutamatergic over-activity may primarily be terminated by drugs with optimal glutamatergic antagonism primarily in the piriform, entorhinal, and perirhinal cortices. Studies of radiolabeled ligands to map subreceptors may provide specification of wanted drug properties to guide the choice among existing agents or to synthesize novel ones.

Mechanisms regulating GABAergic inhibitory transmission in the basolateral amygdala: implications for epilepsy and anxiety disorders

The amygdala, a temporal lobe structure that is part of the limbic system, has long been recognized for its central role in emotions and emotional behavior. Pathophysiological alterations in neuronal excitability in the amygdala are characteristic features of certain psychiatric illnesses, such as anxiety disorders and depressive disorders. Furthermore, neuronal excitability in the amygdala, and, in particular, excitability of the basolateral nucleus of the amygdala (BLA) plays a pivotal role in the pathogenesis and symptomatology of temporal lobe epilepsy. Here, we describe two recently discovered mechanisms regulating neuronal excitability in the BLA, by modulating GABAergic inhibitory transmission. One of these mechanisms involves the regulation of GABA release via kainate receptors containing the GluR5 subunit (GluR5KRs). In the rat BLA, GluR5KRs are present on both somatodendritic regions and presynaptic terminals of GABAergic interneurons, and regulate GABA release in an agonist concentration-dependent, bidirectional manner. The relevance of the GluR5KR function to epilepsy is suggested by the findings that GluR5KR agonists can induce epileptic activity, whereas GluR5KR antagonists can prevent it. Further support for an important role of GluR5KRs in epilepsy comes from the findings that antagonism of GluR5KRs is a primary mechanism underlying the antiepileptic properties of the anticonvulsant topiramate. Another mechanism regulating neuronal excitability in the BLA by modulating GABAergic synaptic transmission is the facilitation of GABA release via presynaptic alpha1A adrenergic receptors. This mechanism may significantly underlie the antiepileptic properties of norepinephrine. Notably, the alpha1A adrenoceptor-mediated facilitation of GABA release is severely impaired by stress. This stress-induced impairment in the noradrenergic facilitation of GABA release in the BLA may underlie the hyperexcitability of the amygdala in certain stress-related affective disorders, and may explain the stress-induced exacerbation of seizure activity in epileptic patients.

Temporal sequence of ictal discharges propagation in the corticolimbic basal ganglia system during amygdala kindled seizures in freely moving rats

We used a multiple channel, single unit recording technique to investigate the neural activity in different corticolimbic and basal ganglia regions in freely moving rats before and during generalized amygdala kindled seizures. Neural activity was recorded simultaneously in the sensorimotor cortex (Ctx), hippocampus, amygdala, substantia nigra pars reticulata (SNr) and the subthalamic nucleus (STN). We observed massive synchronized activity among neurons of different brain regions during seizure episodes. Neurons in the kindled amygdala led other regions in synchronized firing, revealed by time lags of neurons in other regions in crosscorrelogram analysis. While there was no obvious time lag between Ctx and SNr, the STN and hippocampus did lag behind the Ctx and SNr in correlated firing. Activity in the amygdala and SNr contralateral to the kindling stimulation site lagged behind their ipsilateral counterparts. However, no time lag was found between the kindling and contralateral sides of Ctx, hippocampus and STN. Our data confirm that the amygdala is an epileptic focus that emits ictal discharges to other brain regions. The observed temporal pattern indicates that ictal discharges from the amygdala arrive first at Ctx and SNr, and then spread to the hippocampus and STN. The simultaneous activation of both sides of the Ctx suggests that the neocortex participates in kindled seizures as a unisonant entity to provoke the clonic motor seizures. Early activation of the SNr (before the STN and hippocampus) points to an important role of the SNr in amygdala kindled seizures and supports the view that different SNr manipulations may be effective ways to control seizures.

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.

Secondary generalization of hippocampal kindled seizures in rats: examining the role of the piriform cortex

A primary feature of epilepsy is the potential for focal seizures to recruit distant structures and generalize into convulsions. Key to understanding generalization is to identify critical structures facilitating the transition from focal to generalized seizures. In kindling, development of a primary site leads progressively to secondarily generalized convulsions. In addition, subsequent kindling of a secondary site results in rapid kindling from that site, presumably because of its facilitated access to the primary kindled network. Here, we investigated the role of the piriform cortex in convulsive generalization from a secondary site kindled in the hippocampus after primary site amygdala kindling. In a necessarily complicated design, rats initially experienced forebrain commissurotomy to lateralize the experiment to one hemisphere. Then the amygdala was kindled and, 3 weeks later, it was electrically-triggered into status epilepticus, which destroyed the ipsilateral piriform cortex. This experience occurred several days before secondary site kindling of the dorsal hippocampus. In rats with complete piriform cortex loss, there was no disruption in kindling or convulsive seizure expression from the hippocampus. However, when damage also involved parts of the perirhinal, insular and entorhinal cortices, convulsive expression was blocked. Although other evidence suggests that piriform lesions affect generalization of primary site kindling, the present study shows that they do not alter secondary site kindling in the dorsal hippocampus. The additional involvement of parahippocampal cortical areas in convulsive expression suggests an important functional association between these cortical regions and the hippocampus in seizure propagation and clinical expression.

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.

Neural growth, neural damage and neurotrophins in the kindling model of epilepsy

Do seizures change the brain? Studies on the kindling model--a widely used animal model of epilepsy--suggest that they do. Dr. Racine, one of the pioneers in the kindling field, describes the basic phenomena of kindling, and discusses the possible roles of cell growth and cell death in this model.

Kindling: some old and some new

A brief review of kindling is provided, which highlights some important points of historical interest often overlooked by researchers. These points include the fact that the original rating scale of convulsive seizures presented by Racine 'EEG Clin. Neurophysiol 32 (1972) 281'. was based on amygdala kindling, and may not be applicable to kindling from other sites. The functional anatomy of these convulsive seizures was similarly addressed. Also emphasized was the observation that kindling results ultimately in spontaneous seizures, seemingly identical to those seen in models of status epilepticus (SE), and can provide a unique perspective on those seizures because of its controlled natural history and minimal brain damage. Much of the recent work described here focused on genetic susceptibility versus resistance to kindling, as witnessed by the Fast and Slow kindling rat strains. The results of those studies indicated substantial strain differences in GABAergic function in different limbic structures associated with GABA(A) subunit expression, spontaneous miniature inhibitory postsynaptic currents (mIPCs) and behavioral comorbidities. We concluded the review with our recent attempt to discover consistent and unique gene profile differences associated with the different seizure predispositions of the Fast and Slow kindling rat strains.

Anticonvulsant effect of bilateral injection of N6-cyclohexyladenosine into the CA1 region of the hippocampus in amygdala-kindled rats

In this study the role of adenosine A(1) receptors of CA1 region of the hippocampus on amygdala-kindled seizures was investigated in rats. Results obtained showed that in kindled animals, bilateral injection of N(6)-cyclohexyladenosine (CHA), an adenosine A(1) receptor agonist, at doses of 0.1, 1 and 10 microM into the CA1 region of the hippocampus significantly decreased the afterdischarge duration and stage 5 seizure duration and increased the latency to stage 4 seizure, but there were no changes in seizure stage. Also, bilateral injection of 1,3-dimethyl-8-cyclopenthylxanthine (CPT), an adenosine A(1) receptor antagonist, at doses of 0.5 and 1 microM into the CA1 region of the hippocampus could not produce any changes in the seizure parameters. Intrahippocampal pretreatment of CPT (1 microM) before CHA (0.1 and 1 microM), reduced the effects of CHA on seizure parameters significantly. Thus, it may be suggested that CA1 region of the hippocampus plays an important role in spreading seizure spikes from the amygdala to other brain regions and activation of adenosine A(1) receptors in this region, participates in anticonvulsant effects of adenosine agonists.

A device for stereotactic transection of fiber bundles in rats

We have designed a device for stereotactic transection of fiber bundles in experiments using rats. Here, we present our assessment of its features relative to those of conventional methods. The instrument consists of a stainless steel cannula and a thin inner wire with a hook at one end and a hilt at the other. The hook can be extended or withdrawn freely by pushing or pulling the hilt. Lesions were evaluated in 12 male Wistar rats, after two targets, the anterior commissure (n=6) and the caudate-putamen (n=6), were transected. After the cannula was introduced into the target, the inner hook was extended in an anterior direction orthogonally to the transverse plane. Next, the entire device was pulled back along the insertion path to transect the neuronal fibers. Then the inner hook was withdrawn into the cannula and the entire device was removed. Seven days later, brains were removed for histologic processing. Microscopic examination demonstrated a slit like gap produced by transection at the target; the lesions were infiltrated by microglia and surrounded by gliosis. Adjacent regions were minimally damaged. The path of the cannula demonstrated only minimal gliosis. Unlike conventional methods, this device permits precise transection of deep fiber bundles with minimal damage to surrounding brain tissue.

Hippocampal transection attenuates kainic acid-induced amygdalar seizures in rats

Since the dorsal and ventral hippocampus in the rat may act differently from one another in limbic seizures, we studied effects of orthogonal transection between the dorsal and ventral hippocampus upon kainic acid-induced amygdalar seizures. A total of 26 rats were divided into three groups. Ten rats underwent transection using a modified wire knife (transection group); 16 others were untransection group (n=10) and controls (n=6). All the rats then underwent stereotactic implantation of electrodes in the left amygdala (LA), left dorsal hippocampus (LdH), left ventral hippocampus (LvH), and the left sensorimotor cortex (LCx). A stainless steel cannula also was introduced into the LA. Rats except controls later received 1.0 microg of kainic acid (KA) via the cannula. Controls received phosphate buffer solution alone. In the untransection group, multiple spike discharges in the LA immediately propagated concurrently to the LvH and LdH. Propagation involved the LCx to become status epilepticus 1 to 2 h after KA injection. Seizures, characterized by mastication, salivation, facial twitching, forelimb clonus, and sometimes rearing and falling, lasted 1 to 2 days. Microscopic examination revealed severe neuronal cell damage in the LA, LvH, and LdH. In the transection group, multiple spike discharges initiated from the LA and were propagated to LvH, but LdH as well as LCx involvement was slight. Status epilepticus involved only the LA and LvH 1 to 2 h following KA injection. Seizures subsided within 24 h, showing no ictal manifestations except for aggressiveness. Overall, seizures were weak and transient compared with those in controls. Histologically, hippocampal neuronal damage was slight, but damage to amygdalar neurons was similar to that in untransection group. No electroclinical and histological changes were seen in controls. These results indicated that connections between the dorsal and ventral hippocampus are important for full development of KA-induced amygdalar seizures.

Bilateral lesions of the central but not anterior or posterior parts of the piriform cortex retard amygdala kindling in rats

The piriform cortex is thought to be involved in temporal lobe seizure propagation, such as that occurring during kindling of the amygdala or hippocampus. A number of observations suggested that the circuits of the piriform cortex might act as a critical pathway for limbic seizure discharges to assess motor systems, but direct evidence for this suggestion is scarce. Furthermore, the piriform cortex is not a homogeneous structure, which complicates studies on its role in limbic epileptogenesis. We have previously reported data indicating that the central part of the piriform cortex might be particularly involved during amygdala kindling. In order to further evaluate the role of different parts of the piriform cortex during kindling development, we bilaterally destroyed either the central, anterior or posterior piriform cortex by microinjections of ibotenate two weeks before onset of amygdala kindling. Lesions of the anterior piriform cortex hardly affected kindling acquisition, except that fewer animals exhibited stage 3 (unilateral forelimb) seizures compared to sham controls. Lesions of the central piriform cortex significantly retarded kindling, which was due to a decreased progression from stage 3 to stage 4/5 seizures, i.e. the lesioned rats needed significantly longer for the acquisition of generalized clonic seizures in the late stages of kindling development. Lesions of the posterior piriform cortex did not significantly affect kindling development. The data demonstrate that different parts of the piriform cortex mediate qualitatively different effects on amygdala kindling. The central piriform cortex seems to be a neural substrate involved in the continuous development of kindling from stage 3 to stages 4/5, indicating that this part of the piriform cortex may have preferred access, either directly or indirectly, to structures capable of supporting generalized kindled seizure expression.

Basal forebrain neurons suppress amygdala kindling via cortical but not hippocampal cholinergic projections in rats

Intraventricular administration of the immunotoxin 192 IgG-saporin in rats has been shown to cause a selective loss of cholinergic afferents to the hippocampus and cortical areas, and to facilitate seizure development in hippocampal kindling. Here we demonstrate that this lesion also accelerates seizure progression when kindling is induced by electrical stimulations in the amygdala. However, whereas intraventricular 192 IgG-saporin facilitated the development of the initial stages of hippocampal kindling, the same lesion promoted the late stages of amygdala kindling. To explore the role of various parts of the basal forebrain cholinergic system in amygdala kindling, selective lesions of the cholinergic projections to either hippocampus or cortex were produced by intraparenchymal injections of 192 IgG-saporin into medial septum/vertical limb of the diagonal band or nucleus basalis, respectively. Cholinergic denervation of the cortical regions caused acceleration of amygdala kindling closely resembling that observed after the more widespread lesion induced by intraventricular 192 IgG-saporin. In contrast, removal of the cholinergic input to the hippocampus had no effect on the development of amygdala kindling. These data indicate that basal forebrain cholinergic neurons suppress kindling elicited from amygdala, and that this dampening effect is mediated via cortical but not hippocampal projections.

The parahippocampal cortices and kindling

The piriform and perirhinal cortices are parahippocampal structures with strong connections to limbic structures, including the amygdala and hippocampus, as well as other parahippocampal structures such as the entorhinal cortex. In this paper, we present results, based on anatomical, physiological, and kindling studies, that suggest that the perirhinal and piriform cortices might be very important in the secondary generalization of limbic seizures, particularly those with convulsive expression. These kindling data further suggest that the progressive lowering of afterdischarge thresholds in the parahippocampal structures, due to insult and/or genetic predisposition, might provide the neural basis for the clinical presentation of temporal lobe epilepsy.

Effect of dipotassium clorazepate on amygdaloid-kindling and comparison between amygdaloid- and hippocampal-kindled seizures in rats

We examined the effect of dipotassium clorazepate (7-chloro-1, 3-dihydro-2-oxo-5-phenyl-1H-1, 4-benzodiazepine-3-carboxylate potassium hydroxide), an antianxiety drug, on amygdaloid kindling and compared its effects for 7 successive days on amygdaloid- versus hippocampal-kindled seizures, using the rat kindling model of epilepsy. Dipotassium clorazepate at 5 mg/kg significantly delayed amygdaloid kindling. The contralateral cortical after-discharge duration in the dipotassium clorazepate-treated group was significantly shorter than the after-discharge duration in the amygdala in the first seven stimulations, whereas it was significantly shorter only in the first three stimulations in the control group, indicating that dipotassium clorazepate suppressed the spread of seizure activity from focus to contralateral cortex. Dipotassium clorazepate suppressed amygdaloid-kindled seizures at 2 and 5 mg/kg, while 1 mg/kg or more suppressed hippocampal-kindled seizures. Thus, differences in effective dosages in both amygdaloid- and hippocampal-kindled seizures may suggest a difference in the neuronal mechanisms involved in this kindling.

Effect of intraperitoneal and intrahippocampal (CA1) 2-chloroadenosine in amygdaloid kindled rats

Effects of intraperitoneal and intrahippocampal 2-chloroadenosine and caffeine were examined in fully kindled amygdaloid rats. Intraperitoneal administration of 2-chloroadenosine (5 and 10 mg/kg) decreased afterdischarge duration, stage 5 seizure duration and prolonged time taken to reach stage 4 seizure. Only the 10 mg/kg dose induced a significant reduction in seizure stage. Intraperitoneal administration of caffeine (50 mg/kg) increased both afterdischarge duration and stage 5 seizure duration but did not significantly alter other parameters. Intrahippocampal microinfusion of 2-chloroadenosine (1 mM) or caffeine (2 mM) did not alter any of the measured seizure parameters. Intraperitoneal but not intrahippocampal pretreatment of animals with caffeine (50 mg/kg and 2 mM, respectively) blocked the anticonvulsant effects induced by intraperitoneal administration of 2-chloroadenosine. It may therefore be concluded that the adenosine A1 receptors of the CA1 region of the hippocampus do not play a role in mediating the anticonvulsant effects of intraperitoneally administered 2-chloroadenosine in amygdaloid kindled rats.

The role of the piriform cortex in kindling

In epilepsy research, there is growing interest in the role of the piriform cortex (PC) in the development and maintenance of limbic kindling and other types of limbic epileptogenesis leading to complex partial seizures, i.e. the most common type of seizures in human epilepsy. The PC ("primary olfactory cortex") is the largest area of the mammalian olfactory cortex and receives direct projections from the olfactory bulb via the lateral olfactory tract (LOT). Beside the obvious involvement in olfactory perception and discrimination, the PC, because of its unique intrinsic associative fiber system and its various connections to and from other limbic nuclei, has been implicated in the study of memory processing, spread of excitatory waves, and in the study of brain disorders such as epilepsy with particular emphasis on the kindling model of temporal lobe epilepsy with complex partial seizures. The interest in the kindling model is based primarily on the following observations. (1) The PC contains the most susceptible neural circuits of all forebrain regions for electrical (or chemical) induction of limbic seizures. (2) During electrical stimulation of other limbic brain regions, broad and large afterdischarges can be observed in the ipsilateral PC, indicating that the PC is activated early during the kindling process. (3) The interictal discharge, which many consider to be the hallmark of epilepsy, originates in the PC, independent of which structure serves as the kindled focus. (4) Autoradiographic studies of cerebral metabolism in rat amygdala kindling show that, during focal seizures, the area which exhibits the most consistent increase in glucose utilization is the ipsilateral paleocortex, particularly the PC. (5) During the commonly short initial afterdischarges induced by stimulation of the amygdala at the early stages of kindling, the PC is the first region that exhibits induction of immediate-early genes, such as c-fos. (6) The PC is the most sensitive brain structure to brain damage by continuous or frequent stimulation of the amygdala or hippocampus. (7) Amygdala kindling leads to a circumscribed loss of GABAergic neurons in the ipsilateral PC, which is likely to explain the increase in excitability of PC pyramidal neurons during kindling. (8) Kindling of the amygdala or hippocampus induces astrogliosis in the PC, indicating neuronal death in this brain region. Furthermore, activation of microglia is seen in the PC after amygdala kindling. (9) Complete bilateral lesions of the PC block the generalization of seizures upon kindling from the hippocampus or olfactory bulb. Incomplete or unilateral lesions are less effective in this regard, but large unilateral lesions of the PC and adjacent endopiriform nucleus markedly increase the threshold for induction of focal seizures from stimulation of the basolateral amygdala (BLA) prior to and after kindling, indicating that the PC critically contributes to regulation of excitability in the amygdala. (10) Potentiation of GABAergic neurotransmission in the PC markedly increases the threshold for induction of kindled seizures via stimulation of the BLA, again indicating a critical role of the PC in regulation of seizure susceptibility of the amygdala. Microinjections of NMDA antagonists or sodium channel blockers into the PC block seizure generalization during kindling development. (11) Neurophysiological studies on the amygdala-PC slice preparation from kindled rats showed that kindling of the amygdala induces long-lasting changes in synaptic efficacy in the ipsilateral PC, including spontaneous discharges and enhanced susceptibility to evoked burst responses. The epileptiform potentials in PC slice preparations from kindled rats seem to originate in neuron at the deep boundary of PC. Spontaneous firing and enhanced excitability of PC neurons in response to kindling from other sites is also seen in vivo, substantiating the fact that kindling induces long-lasting changes in the PC c

Perirhinal cortex involvement in limbic kindled seizures

Investigations into the anatomical substrate of temporal lobe epilepsy have yielded a number of important observations regarding the involvement of the piriform and perirhinal cortical areas in temporal lobe seizure propagation. Although early reports indirectly suggested that the circuits of the piriform cortex might act as a critical conduit for limbic seizure discharges to access motor systems, recent reports more strongly implicate the perirhinal cortex in this process. In the following report, we provide a brief summary of the earlier work involving the piriform cortex and its potential involvement in kindled limbic seizures. This is followed then by the results of several recent in vivo and in vitro electrophysiological studies that ascribe a critical importance for the perirhinal cortex in convulsive limbic seizures. Finally, since our anatomical studies indicated that the perirhinal cortex densely innervates the frontal motor cortex, we examined the involvement of this latter region in amygdala kindled seizures using the reversible functional lesion of cortical spreading depression. Based on these findings we suggest that the circuits of the perirhinal cortex may be important in the amplification and distribution of temporal lobe seizure discharges, providing access to structures that are capable of driving a convulsive response.

Efferent projections of the anterior perirhinal cortex in the rat

Because convulsive seizures develop very rapidly from kindling sites in the anterior perirhinal cortex, we studied perirhinal efferents by using the anterograde tracer Phaseolus vulgaris leucoagglutinin (PhAL). PhAL injections into the anterior perirhinal cortex labelled a prominent network of fibers within the frontal cortex that was most dense within layers I and II and layer VI. As individual PhAL injection sites within the perirhinal cortex were restricted to one or two adjacent laminae, we were able to determine that layer V was the main source of the perirhinofrontal projection. This was confirmed by frontal cortex injections of the retrograde tracer Fluorogold (FG). Other cortical areas with densely labelled fibers following perirhinal PhAL injections included the agranular insular, infralimbic, orbital, parietal, and entorhinal cortices. Moderate to mild fiber labelling was also noted in the posterior piriform, temporal and occipital cortices, and the claustrum. Subcortical labelling was seen in the nucleus accumbens; fundus striati; basal and lateral amygdala nuclei; the "acoustic thalamus"; and the central grey. Several of these cortical and subcortical projections were bilateral. The different laminar origin of these perirhinal efferents is discussed. These results confirmed our prediction of extensive direct projections from the anterior perirhinal cortex to the frontal cortex in the rat. The significance of this projection is discussed with special reference to the anatomical basis of convulsive limbic seizures.

Fyn tyrosine kinase is required for normal amygdala kindling

To identify specific genes involved with epileptogenesis kindling was examined in mice carrying mutations engineered by gene targeting. Amygdala kindling was tested in mice with a null-mutation in the Fyn tyrosine kinase gene, a mutation that raises the threshold for the induction of long-term potentiation in the hippocampus. The fyn mutants had a normal threshold, duration and stability of epileptiform after-discharge, which is crucial for kindling. Despite the normal after-discharge, fyn mutants showed a striking retardation in the rate of kindling. Once the kindled state was established in fyn mutants it remained stable. This implicates a Fyn-dependent biochemical pathway in the induction but not the maintenance of normal amygdala kindling. fyn is the first gene identified to be required for normal epileptogenesis.

Susceptibility of different cell layers of the anterior and posterior part of the piriform cortex to electrical stimulation and kindling: comparison with the basolateral amygdala and "area tempestas"

Several lines of evidence suggest that the piriform cortex functions as a generator in the development and propagation of forebrain (limbic type) seizures, particularly in the kindling model of epilepsy. It is, however, not clear where, within the rather large piriform cortex region, the generator resides, and how much tissue is involved. Highly sensitive loci to chemical or electrical stimulation have been described both in the deep anterior and posterior parts of the piriform cortex. Furthermore, data from piriform cortex slice preparations indicated that epileptiform potentials originate in deep structures, particularly the endopiriform nucleus that underlies the piriform cortex. In the present study, in rats, we implanted stimulation and recording electrodes in various rostrocaudal locations of the piriform cortex and endopiriform nucleus, including the "area tempestas", i.e. a structure in the anterior part of the piriform cortex previously proposed to be critically involved in the generation of convulsive seizures of limbic origin. Within the piriform cortex, electrodes were aimed at different cellular layers of this structure. For comparison, additional animals received electrodes in different parts of the basolateral amygdala. A total of 19 different locations was obtained in this way. The susceptibility of these locations to electrical stimulation was characterized by determining the threshold for induction of afterdischarges. The afterdischarge threshold was lowest in layer III of the posterior piriform cortex and some locations in the endopiriform nucleus, whereas amygdala and "area tempestas" displayed higher values. In several animals, particularly those with electrodes in layer III of the posterior piriform cortex, spontaneous spiking was seen in prestimulation recordings, whereas this was never observed in recordings from the amygdala. Subsequent kindling by repeated stimulation of the various locations demonstrated marked differences in afterdischarge threshold reduction and kindling rate. The most marked decreases in afterdischarge threshold were seen in locations within layer III of the piriform cortex, whereas several other locations, including the "area tempestas", exhibited only moderate decreases or no decrease at all. In contrast to previous observations with only few locations in the piriform cortex region, the posterior piriform cortex was not in general slower to kindle than the anterior piriform cortex, although some locations in the posterior piriform cortex exhibited significantly lower kindling rates than the amygdala. The highest kindling rate was seen in the dorsal endopiriform nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Development and pharmacological suppression of secondary afterdischarges in the hippocampus of amygdala-kindled rats

The development and spread of afterdischarges in the ipsilateral limbic system during amygdala kindling, a model of complex partial seizures, was studied in male and female rats. Kindling stimulation was performed in the basolateral amygdala, and afterdischarges were recorded from the stimulation electrode and electrodes in the nucleus accumbens, the posterior piriform cortex and the ventral hippocampus, all implanted on the right side of the brain. All structures showed primary afterdischarges already after the first stimulation, indicating a close anatomical and physiological connection to the epileptogenic focus. The development of robust secondary afterdischarges, which occurred after the end of the primary afterdischarges in the amygdala and which always originated in the hippocampus but also spread to one or more of the other recording sites, is described. The secondary afterdischarges initially occurred after about nine kindling stimulations in both male and female rats, and were associated with an increase in primary afterdischarge duration and a progression from focal to motor seizures. In order to test the effect of common antiepileptic drugs on the secondary afterdischarges, a group of female rats were treated with valproate, carbamazepine or phenytoin. All drugs suppressed the secondary afterdischarges, although they had a different anticonvulsant efficacy on motor seizures and afterdischarge duration after amygdala stimulation. While valproate and carbamazepine dose-dependently reduced all parameters of the kindled seizure, including the secondary afterdischarges in the hippocampus, phenytoin suppressed the secondary afterdischarges also in the absence of any anticonvulsant effect, suggesting that recurrent hippocampal activation is not crucial for the kindled state.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential regional and time course increases in thyrotropin-releasing hormone, neuropeptide Y and enkephalin mRNAs following an amygdala kindled seizure

Previous studies have shown that neuropeptide mRNA expression is altered in the dentate gyrus, and pyriform, entorhinal and perirhinal cortices following amygdala kindling. However, because rats were kindled every day and some mRNA alterations last longer than 24 h, a true measure of the alterations induced by a single seizure was confounded by the previous day's seizure. To circumvent this problem, rats were fully kindled, had six days without stimulation, and then were given one more seizure. Rats were sacrificed either 4 h, 24 h or 4 days after this last seizure. The levels of mRNAs for TRH, NPY and ENK were measured in the dentate gyrus and limbic cortices. Four hours after a seizure, TRH and NPY mRNAs were maximally increased in the dentate gyrus granule layer, but returned to baseline levels by 24 h. In contrast, 4 h after a seizure, TRH and NPY mRNAs were not, or only slightly, increased in the pyriform, entorhinal and perirhinal cortices, but significantly elevated 24 h after a seizure. ENK mRNA was increased both 4 and 24 h after a seizure in the pyriform, entorhinal and perirhinal cortices but showed no increases in the dentate gyrus at any time. By 4 days, peptide mRNA levels returned to baseline, except for ENK mRNA in the pyriform cortex. These results demonstrate a non-uniform and complex pattern of peptide mRNA expression following an amygdala kindled seizure. They further suggest that regional and time course differences in gene transcription and expression may be important factors in understanding both the transient, adaptive anticonvulsant and longer lasting proconvulsant effects of these neuropeptides.

Persistent physiological effects caused by a single pentylenetetrazol induced seizure in neonatal rats

A single seizure was induced by pentylenetetrazol (PTZ; 150 mg/kg i.p.) in 1-day-old and 21-day-old rats; control littermates were given saline (i.p.) injections. In vitro recordings were made in hippocampal slices derived from adult (2-3.5-month-old) rats. The population responses in CA1, CA3 and dentate gyrus (DG) were recorded following double-pulse stimulation of Schaffer collateral (CA1 stratum radiatum, for CA1 and CA3 recordings) and perforant path (for DG recordings). Paired-pulse stimuli at an interpulse interval (IPI) of 10-200 ms and intensity of 1.5, 2 or 4 times the stimulus threshold were used. PTZ given on day 1 resulted in a highly significant increase in the paired-pulse facilitation (PPF) of the population EPSP, but not of the population spike, in CA1 at all stimulus intensities. In the DG, PPF of both the population EPSP and population spike was found at 1.5 x threshold intensity. PTZ given on day 21 decreased PPF of the population EPSP and spike in CA1 and had no significant effect in the DG. No significant difference was found in CA3 responses after seizures on day 1 or day 21. The slices from seized and control animals were not different in their stimulus thresholds or response to a single pulse. It is concluded that a single neonatal PTZ-induced seizure had long-lasting physiological consequences which depend on the age of seizure.(ABSTRACT TRUNCATED AT 250 WORDS)

The NMDA antagonist MK-801 suppresses behavioral seizures, augments afterdischarges, but does not block development of perforant path kindling

The role of N-methyl-D-aspartate (NMDA) in the development and expression of kindled seizures was assessed using a crossover design. Rats were stimulated once daily in the perforant path for 10 consecutive days 30 min following daily administration of saline or the NMDA antagonist MK-801 (1.0 mg/kg) (phase I). Five to 10 days elapsed prior to an additional 10 stimulations with the drug treatments reversed (phase II). A separate group of animals was stimulated following saline administration in both phases of the study. MK-801 produced a significant increase in afterdischarge (AD) threshold and a suppression of behavioral seizure development during the first 10 stimulations. However, upon removal of the drug, an immediate increase in seizure stage and the number of animals displaying generalized seizure signs (clonic component) was observed. Paradoxically, MK-801 also produced an increase in mean AD duration in the perforant path and dentate gyrus over the first 10 stimulations. Upon reversal of the dose treatments in phase II of the study, AD duration increased in animals treated with MK-801 for the first time, and decreased in animals taken off MK-801 and stimulated drug free. The augmentation in AD associated with MK-801 was partially attributed to an increase in secondary or rebound AD. Rebound ADs occurred more frequently, had a decreased latency and longer duration in drugged compared to control animals, irrespective of the phase of the study. These data indicate that MK-801 possesses anticonvulsant properties with respect to behavioral seizure, and is less effective as an antiepileptogenic agent-i.e., significant kindling development occurred with MK-801 in the absence of overt behavioral expression of the kindled response. A dissociation between seizure stage and AD duration suggests that independent mechanisms may control the electrographic and behavioral indices of kindling.

The effect of kindling beyond the 'stage 5' criterion on paired-pulse depression and hilar cell counts in the dentate gyrus

Recent experiments have indicated that recurrent inhibition in the dentate gyrus, as measured with paired-pulse tests, is reduced following the induction of status epilepticus. Also, a loss of cells in the hilus has been reported, and it has been suggested that the two effect might be related. In this experiment, we have monitored paired-pulse depression and counted cells in the hilus in animals that have been kindled well beyond the typical stage 5 criterion. Responses evoked in the dentate gyrus by paired-pulse stimulation of the perforant path were monitored before and after kindling of the perforant path. One group of animals served as controls and received no kindling stimulations. Another group was kindled to 4 stage 5 seizures and then allowed to recover for 2 months. A third group was kindled to 44 stage 5 seizures and then allowed to recover for at least 5 weeks. Paired-pulse tests were taken at 1 week intervals during the kindling and recovery phases. Paired-pulse inhibition increased during kindling, peaked after 4 stage 5 seizures, remained enhanced throughout the additional 40 stage 5 seizures, and recovered towards baseline over a period of about 5 weeks. Upon completion of this phase of the experiment, cell counts were taken in the hilar regions of the Nissl stained brain sections. There was a significant reduction in number of cells in the tissue from kindled animals, compared to controls, but there was no significant difference between the 2 kindled groups.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of lesions of the posterior piriform cortex on amygdala kindling in the rat

The piriform cortex (PC) is thought to be critically involved in the genesis of forebrain (limbic type) seizures, including limbic kindled seizures. More recent studies have shown that the posterior PC is particularly sensitive to kindling stimulation, suggesting that the posterior PC contains specific generating sites which may be important for the stepwise progression of kindling. In the present experiments, we used microinjections of ibotenate to study the effect of selective lesions of the posterior PC on amygdala kindling in rats. Large unilateral lesions of the posterior PC and adjacent endopiriform nucleus markedly decreased the susceptibility of the ipsilateral basolateral amygdala to electrical stimulation, thus indicating that the posterior PC may normally contribute to regulation of physiologic excitability in amygdala. During kindling, rats with large lesions of the PC stayed longer in the initial phase of kindling (stage 1) than sham-lesioned controls, consistent with involvement of the posterior PC in the early stages of seizure propagation during kindling acquisition. However, the PC lesions were not capable of blocking or even severely retarding kindling. Following kindling development, rats with large lesions of the posterior PC had significantly higher focal seizure thresholds than kindled rats without lesion or rats with only small PC lesions, which suggests that the posterior PC is involved in the mechanisms which are responsible for the marked increase in seizure susceptibility induced by kindling. Taken together, the data substantiate that PC structures play a facilitatory role in kindling.

Regulation of neurotrophin and trkA, trkB and trkC tyrosine kinase receptor messenger RNA expression in kindling

Levels of messenger RNA for nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, and the tyrosine kinase receptors trkA, trkB and trkC have been studied using in situ hybridization in the rat brain 2 h and four weeks after kindling-induced seizures. Epileptiform activity evoked by hippocampal stimulation and exceeding 70 s lead to a concomitant and transient increase of brain- derived neurotrophic factor, nerve growth factor, trkB and trkC messenger RNA expression in dentate granule cells after both focal and generalized seizures. Brain-derived neurotrophic factor messenger RNA levels were also increased bilaterally in the CA1-CA3 regions, amygdala and the piriform, entorhinal, perirhinal, retrosplenial and temporal cortices after generalized seizures. The magnitude of the increases was similar throughout the development of kindling and in the fully kindled brain. No changes of trkA messenger RNA were observed. In amygdalar kindling, elevated brain-derived neurotrophic factor messenger RNA levels developed more rapidly in the amygdala-piriform cortex than after stimulation in the hippocampus but changes in the hippocampal formation were only seen in few animals. Intraventricular 6-hydroxydopamine or a bilateral fimbria-fornix lesion did not alter basal expression or seizure-evoked changes in messenger RNA levels for neurotrophins or trk receptors but increased the number of animals exhibiting elevated levels after the first stimulation, probably due to a prolongation of seizure activity. Both in sham-operated and fimbria-fornix-lesioned rats seizure activity caused a marked reduction of neurotrophin-3 messenger RNA levels in dentate granule cells. The results indicate that activation of the brain-derived neurotrophic factor gene, at least in dentate granule cells, is an "all-or-none" type of response and dependent on the duration but not the severity of seizures or the stage of kindling epileptogenesis. Changes in brain-derived neurotrophic factor, nerve growth factor, neurotrophin-3 and trkB and trkC were observed concomitantly in the dentate gyrus, which suggests that seizure activity sets in motion a cascade of genomic events possibly mediated via a common mechanism. Since altered messenger RNA levels outside hippocampus were detected only for brain-derived neurotrophic factor, neurotrophin and trk gene expression in these regions seems to be regulated differently.

Are differences in dorsal hippocampal kindling related to amygdala-piriform area excitability?

It has been suggested that several structures associated with the amygdala-piriform (A-P) area are important, if not critical, for convulsive generalization of limbic seizures. In experiment 1, when examining the development of convulsive seizures kindled from the dorsal hippocampus (cornus ammonis; DH), a broad range of kindling rates was observed. Independent of electrode location within the hippocampus, kindling rates were correlated positively with both local and, more dramatically, distant excitability (measured by the duration of the primary and secondary hippocampal afterdischarges, respectively) at all stages of epileptogenesis. Because kindling rates were bimodally distributed, we bisected the distribution into 'faster' and 'slower' kindling rats. Here we examined the magnitude of both electrophysiological and behavioral differences between 'faster' and 'slower' rats and their changes over time. The 'faster' rats had longer primary and secondary afterdischarge (AD) durations than 'slower' rats throughout all stages of kindling. With the appearance of generalized convulsions, the 'faster' rats showed longer latencies to clonus onset, with longer clonus and AD durations than 'slower' rats. Also, the generalized convulsions of 'faster' rats appeared during a much enlarged secondary AD period, while 'slower' rats convulsed during primary AD. In both groups, convulsions were invariably associated with increased discharge in A-P associated structures. We interpreted the differences between 'faster' and 'slower' DH rats to reflect genetic differences in excitability in both local and A-P associated structures. If the DH kindling profile of the 'faster' rats differed from 'slower' rats largely because of naturally greater excitability in A-P associated structures, then experimentally increased excitability in those structures (via amygdala kindling) in a random sample of rats should duplicate much of the 'faster' DH kindling profile. In experiment 2, this outcome was observed. In conclusion, we suggest that either natural or induced differences in the excitability of A-P associated structures affect both the genesis and the profile of convulsive generalization of limbic kindled seizures.

The infusion of an NMDA antagonist into perirhinal cortex suppresses amygdala-kindled seizures

The seizure-modulating role of N-methyl-D-aspartate (NMDA) receptors located in several limbic areas was investigated. Amygdala-kindled rats were microinfused with the selective NMDA-receptor antagonist 2-amino-5-phosphonovalerate (APV, 1 microliter, 70 nmol) or artificial cerebrospinal fluid (ACSF) applied through a cannula located in either the amygdala or perirhinal, pyriform or deep prepyriform cortices. APV infused into the stimulation site raised the threshold for seizure generation. Surprisingly, APV infused into perirhinal cortex, but not into other regions, also dramatically suppressed behavioural seizures and afterdischarges (AD) elicited 5 min after the infusion. If stimulus intensities were markedly elevated however, the seizure suppression was overcome. This latter effect was reversible and repeatable, as seizures and AD were reliably reinstated when these animals were stimulated after infusion with ACSF. A similar effect, whereby perirhinal infusions blocked seizure activity, was also demonstrated in an animal kindled from the olfactory bulb and in one kindled from the perforant path. These results suggest that NMDA receptors located in the perirhinal cortex may play a major role in the modulation of AD activity elicited from more distal brain regions. Furthermore, activation of perirhinal cortex may be a critical requirement for the generation of amygdala-stimulated AD in the kindled animal.

EEG frequency and time domain mapping study of the cortical projections of temporal lobe amygdala afterdischarge during kindling in the cat

EEG frequency and time domain color maps were computed during amygdala kindling in cats. The pattern of the amygdala afterdischarge (AM/AD) propagation to the cortex was assessed as kindling evolved. Our results show that the AM/AD has 4 components that coincide with the activation of certain cortical areas during specific behavioral stages. The pattern of the cortical projection follows an asymmetrical temporo-fronto-occipital direction, the ipsilateral temporal lobe being the first activated zone, followed by the ipsilateral and contralateral prefrontal areas. The contralateral temporal activation is a late phenomenon. We conclude that the electrographic and behavioral manifestations of this model of complex partial epilepsy are asymmetrical during the whole process, including the convulsive stage.

Long-lasting enhancement of metabotropic excitatory amino acid receptor-mediated polyphosphoinositide hydrolysis in the amygdala/pyriform cortex of deep prepiriform cortical kindled rats

We have previously demonstrated that ibotenate (IBO)-stimulated polyphosphoinositide (PPI) hydrolysis is increased for a long period in the amygdala/pyriform cortex (AM/PC) of amygdala (AM)- and hippocampal (HIPP)-kindled rats. This finding indicates that enhanced function of the PPI-coupled excitatory amino acid (EAA) receptor may be associated with the long-lasting seizure susceptibility of kindling. The present study further examined PPI hydrolysis induced by trans-ACPD, a selective agonist of the metabotropic EAA receptor, as well as by IBO in brain slices of rats kindled from the deep prepiriform cortex (DPC). IBO-stimulated accumulation of [3H]inositol monophosphate ([3H]InsP) was significantly increased in the AM/PC by 162 (P less than 0.0001), 130 (P less than 0.005) and 81% (P less than 0.03) at 24 h, 7 days and 28 days, respectively, after the last kindled seizure, whereas it was increased significantly only at 24 h after the last seizure in the HIPP and did not change at any time in the limbic forebrain (LFB). The IBO-stimulated accumulation of [3H]InsP was significantly increased by 55% (P less than 0.01) in the AM/PC of partially kindled rats reaching an average stage of 3.7, but not in the AM/PC of those remaining at stage 1, 7 days after the last kindled seizure. Trans-ACPD-stimulated PPI hydrolysis was significantly increased in the AM/PC of DPC-kindled rats by 65 (P less than 0.05) and 45% (P less than 0.005) at 7 and 28 days, respectively, after the last kindled seizure. Cis-ACPD-stimulated PPI hydrolysis was also significantly increased in the AM/PC of DPC-kindled rats by 45 (P less than 0.03) and 30% (P less than 0.04) at 7 and 28 days, respectively, after the last seizure. There was no increase in trans-ACPD- or cis-ACPD-stimulated PPI hydrolysis in the HIPP or LFB. These results further confirm our previous studies showing that the metabotropic EAA receptor-stimulated PPI hydrolysis exhibited a long-lasting increase in the AM/PC irrespective of the primary stimulation site for kindling.