The mnemonic effects of kindling

Dynamic electrical synapses rewire brain networks for persistent oscillations and epileptogenesis

One of the very fundamental attributes for telencephalic neural computation in mammals involves network activities oscillating beyond the initial trigger. The continuing and automated processing of transient inputs shall constitute the basis of cognition and intelligence but may lead to neuropsychiatric disorders such as epileptic seizures if carried so far as to engross part of or the whole telencephalic system. From a conventional view of the basic design of the telencephalic local circuitry, the GABAergic interneurons (INs) and glutamatergic pyramidal neurons (PNs) make negative feedback loops which would regulate the neural activities back to the original state. The drive for the most intriguing self-perpetuating telencephalic activities, then, has not been posed and characterized. We found activity-dependent deployment and delineated functional consequences of the electrical synapses directly linking INs and PNs in the amygdala, a prototypical telencephalic circuitry. These electrical synapses endow INs dual (a faster excitatory and a slower inhibitory) actions on PNs, providing a network-intrinsic excitatory drive that fuels the IN-PN interconnected circuitries and enables persistent oscillations with preservation of GABAergic negative feedback. Moreover, the entities of electrical synapses between INs and PNs are engaged in and disengaged from functioning in a highly dynamic way according to neural activities, which then determine the spatiotemporal scale of recruited oscillating networks. This study uncovers a special wide-range and context-dependent plasticity for wiring/rewiring of brain networks. Epileptogenesis or a wide spectrum of clinical disorders may ensue, however, from different scales of pathological extension of this unique form of telencephalic plasticity.

Epilepsy and its neurobehavioral comorbidities: Insights gained from animal models

It is well established that epilepsy is associated with numerous neurobehavioral comorbidities, with a bidirectional relationship; people with epilepsy have an increased incidence of depression, anxiety, learning and memory difficulties, and numerous other psychosocial challenges, and the occurrence of epilepsy is higher in individuals with those comorbidities. Although the cause-and-effect relationship is uncertain, a fuller understanding of the mechanisms of comorbidities within the epilepsies could lead to improved therapeutics. Here, we review recent data on epilepsy and its neurobehavioral comorbidities, discussing mainly rodent models, which have been studied most extensively, and emphasize that clinically relevant information can be gained from preclinical models. Furthermore, we explore the numerous potential factors that may confound the interpretation of emerging data from animal models, such as the specific seizure induction method (e.g., chemical, electrical, traumatic, genetic), the role of species and strain, environmental factors (e.g., laboratory environment, handling, epigenetics), and the behavioral assays that are chosen to evaluate the various aspects of neural behavior and cognition. Overall, the interplay between epilepsy and its neurobehavioral comorbidities is undoubtedly multifactorial, involving brain structural changes, network-level differences, molecular signaling abnormalities, and other factors. Animal models are well poised to help dissect the shared pathophysiological mechanisms, neurological sequelae, and biomarkers of epilepsy and its comorbidities.

Kindling of the basolateral or central nucleus of the amygdala increases suboptimal choice in a rat gambling task and increases motor impulsivity in risk-preferring animals

Impairments in decision making under uncertainty, as measured by the Iowa Gambling Task (IGT), are observed in persons suffering from temporal lobe epilepsy (TLE), in which seizures originate in the amygdala and hippocampal formations. Gambling disorder is also more prevalent in this population. Individuals with amygdala damage show similar deficits in decision-making, as do rats with lesions restricted to the basolateral amygdala (BLA) performing an analogous rat gambling task (rGT), yet whether hyperstimulation of the BLA impacts risky decision-making has yet to be demonstrated. We therefore investigated whether kindling of the BLA affected rGT performance. In this task, sugar pellet profits are maximised through consistent selection of options associated with smaller per-trial gains but shorter punishing time-outs. Just as in the IGT, subjects must avoid the risky options, as penalties are disproportionately high despite the higher reward available. Most rats adopt the optimal strategy, but some instead make high numbers of risky, disadvantageous choices. Once stable choice preferences had been established on-task, sixteen male Long Evans rats were implanted unilaterally with a bipolar electrode targeting the BLA and stimulated twice daily until three stage five seizures had been elicited. The electrodes revealed to be nearly evenly places in the BLA and the Central Nucleus of the Amygdala (CeA). Kindling transiently increased choice of the option paired with the smallest reward but also the lowest level of punishment- a risk-averse, but suboptimal, choice. Risk-preferring rats also made more premature responses, a marker of motor impulsivity, and were faster to make a choice, whereas these variables were unaffected in optimal decision-makers. These data suggest epileptiform activity originating within the amygdala can impair choice and promote impulsivity, at least in some individuals.

Long-lasting disruption of spatial memory by GABAB receptor antagonist induced seizures

The objective of this project was to test whether a drug-induced model of temporal lobe seizures, namely seizures induced by a gamma aminobutyric acid (GABA_B) receptor antagonist, CGP35348, result in long-term disruption of hippocampal memory function. Seizures were induced in experimental rats by intracerebroventricular (i.c.v.) injection of CGP35348 (0.64 μmol in 3 μL) for three consecutive days; control rats received no injection. Rats were first trained to criterion on an open radial arm maze (RAM) with 4 of the 8 arms baited, then received seizure and control treatment, and tested again on the RAM during the first week (days 1-5) and fourth week (days 22-29) after the last injection. An initial i.c.v. CGP35348 injection induced a mean of 4.4 seizures in the hippocampus, often accompanied with stages 3-5 convulsions, and sometimes with jumping; three daily CGP35348 injections induced 10.4 ± 1.8 (n = 7 rats) seizures in total. In two separate experiments, seizure-treated rats performed worse than control rats in working memory (WM) during both the 1st and 4th weeks after seizures. Reference memory (RM) deficit during the 1st week after seizures was observed in only one experiment in which RM was acquired >2 weeks ago. The memory deficits were not accompanied by gross neuronal loss in the hippocampus. In conclusion, i.c.v. injection of a GABA_B receptor antagonist in adult rats induced brief, multiple, focal hippocampal seizures that induced deficits in spatial memory for up to 4 weeks.

Corneal kindled C57BL/6 mice exhibit saturated dentate gyrus long-term potentiation and associated memory deficits in the absence of overt neuron loss

Memory deficits have a significant impact on the quality of life of patients with epilepsy and currently no effective treatments exist to mitigate this comorbidity. While these cognitive comorbidities can be associated with varying degrees of hippocampal cell death and hippocampal sclerosis, more subtle changes in hippocampal physiology independent of cell loss may underlie memory dysfunction in many epilepsy patients. Accordingly, animal models of epilepsy or epileptic processes exhibiting memory deficits in the absence of cell loss could facilitate novel therapy discovery. Mouse corneal kindling is a cost-effective and non-invasive model of focal to bilateral tonic-clonic seizures that may exhibit memory deficits in the absence of cell loss. Therefore, we tested the hypothesis that corneal kindled C57BL/6 mice exhibit spatial pattern processing and memory deficits in a task reliant on DG function and that these impairments would be concurrent with physiological remodeling of the DG as opposed to overt neuron loss. Following corneal kindling, C57BL/6 mice exhibited deficits in a DG-associated spatial memory test - the metric task. Compatible with this finding, we also discovered saturated, and subsequently impaired, LTP of excitatory synaptic transmission at the perforant path to DGC synapse. This saturation of LTP was consistent with evidence suggesting that perforant path to DGC synapses in kindled mice had previously experienced LTP-like changes to their synaptic weights: increased postsynaptic depolarizations in response to equivalent presynaptic input and significantly larger amplitude AMPA receptor mediated spontaneous EPSCs. Additionally, there was evidence for kindling-induced changes in the intrinsic excitability of DGCs: reduced threshold to population spikes under extracellular recording conditions and significantly increased membrane resistances observed in DGCs. Importantly, quantitative immunohistochemical analysis revealed hippocampal astrogliosis in the absence of overt neuron loss. These changes in spatial pattern processing and memory deficits in corneal kindled mice represent a novel model of seizure-induced cognitive dysfunction associated with pathophysiological remodeling of excitatory synaptic transmission and granule cell excitability in the absence of overt cell loss.

The PTZ kindling mouse model of epilepsy exhibits exploratory drive deficits and aberrant activity amongst VTA dopamine neurons in both familiar and novel space

Recurrent seizures that define epilepsy are often accompanied by psychosocial problems and cognitive deficits with incompletely understood aetiology. We therefore used the pentylenetetrazol (PTZ) kindling model of epilepsy in mice to examine potential seizure-associated neuropathologies, focusing on motivation, memory and novel-environment-induced activation of midbrain dopaminergic neurons. In addition to recurrent seizures, we found that PTZ kindling led to a strong suppression of novelty-driven exploration while largely sparing fear-driven exploration. The deficits in exploratory drive may be relevant for other cognitive impairments since reduced unassisted rearing in a learning arena correlated with poorer spatial memory of object location. Using c-Fos immunofluorescence as a marker of neuronal activity, we observed that dopamine neurons within the ventral tegmental area (VTA) of PTZ kindled mice demonstrate hyperactivity at baseline and hypoactivity in response to a novel environment compared to saline-injected cagemate controls. These data extend previous findings of PTZ kindling-mediated disruptions of hippocampal processes important for novel environment recognition and learning by demonstrating PTZ kindling also induces motivational deficits that are associated with reduced stimulus-evoked activation of VTA dopamine neurons. More broadly, these data help understand the aetiology of complex behavioural changes in the PTZ kindling model, and may assist in the development of superior diagnoses and treatments for epilepsy.

Accelerated cognitive decline in a rodent model for temporal lobe epilepsy

OBJECTIVE

Cognitive impairment is frequently observed in patients with temporal lobe epilepsy. It is hypothesized that cumulative seizure exposure causes accelerated cognitive decline in patients with epilepsy. We investigated the influence of seizure frequency on cognitive decline in a rodent model for temporal lobe epilepsy.

METHODS

Neurobehavioral assessment was performed before and after surgery, after the induction of self-sustaining limbic status epilepticus (SSLSE), and in the chronic phase in which rats experienced recurrent seizures. Furthermore, we assessed potential confounders of memory performance.

RESULTS

Rats showed a deficit in spatial working memory after the induction of the SSLSE, which endured in the chronic phase. A progressive decline in recognition memory developed in SSLSE rats. Confounding factors were absent. Seizure frequency and also the severity of the status epilepticus were not correlated with the severity of cognitive deficits.

SIGNIFICANCE

The effect of the seizure frequency on cognitive comorbidity in epilepsy has long been debated, possibly because of confounders such as antiepileptic medication and the heterogeneity of epileptic etiologies. In an animal model of temporal lobe epilepsy, we showed that a decrease in spatial working memory does not relate to the seizure frequency. This suggests for other mechanisms are responsible for memory decline and potentially a common pathophysiology of cognitive deterioration and the occurrence and development of epileptic seizures. Identifying this common denominator will allow development of more targeted interventions treating cognitive decline in patients with epilepsy. The treatment of interictal symptoms will increase the quality of life of many patients with epilepsy.

Effect of low frequency electrical stimulation on seizure-induced short- and long-term impairments in learning and memory in rats

Kindled seizures can impair learning and memory. In the present study the effect of low-frequency electrical stimulation (LFS) on kindled seizure-induced impairment in spatial learning and memory was investigated and followed up to one month. Animals were kindled by electrical stimulation of hippocampal CA1 area in a semi-rapid manner (12 stimulations per day). One group of animals received four trials of LFS at 30s, 6h, 24h, and 30h following the last kindling stimulation. Each LFS trial was consisted of 4 packages at 5min intervals. Each package contained 200 monophasic square wave pulses of 0.1ms duration at 1Hz. The Open field, Morris water maze, and novel object recognition tests were done 48h, 1week, 2weeks, and one month after the last kindling stimulation respectively. Kindled animals showed a significant impairment in learning and memory compared to control rats. LFS decreased the kindling-induced learning and memory impairments at 24h and one week following its application, but not at 2week or 1month after kindling. In the group of animals that received the same 4 trials of LFS again one week following the last kindling stimulation, the improving effect of LFS was observed even after one month. Obtained results showed that application of LFS in fully kindled animals has a long-term improving effect on spatial learning and memory. This effect can remain for a long duration (one month in this study) by increasing the number of applied LFS.

Issues related to symptomatic and disease-modifying treatments affecting cognitive and neuropsychiatric comorbidities of epilepsy

Many symptoms of neurologic or psychiatric illness--such as cognitive impairment, depression, anxiety, attention deficits, and migraine--occur more frequently in people with epilepsy than in the general population. These diverse comorbidities present an underappreciated problem for people with epilepsy and their caregivers because they decrease quality of life, complicate treatment, and increase mortality. In fact, it has been suggested that comorbidities can have a greater effect on quality of life in people with epilepsy than the seizures themselves. There is increasing recognition of the frequency and impact of cognitive and behavioral comorbidities of epilepsy, highlighted in the 2012 Institute of Medicine report on epilepsy. Comorbidities have also been acknowledged, as a National Institutes of Health (NIH) Benchmark area for research in epilepsy. However, relatively little progress has been made in developing new therapies directed specifically at comorbidities. On the other hand, there have been many advances in understanding underlying mechanisms. These advances have made it possible to identify novel targets for therapy and prevention. As part of the International League Against Epilepsy/American Epilepsy Society workshop on preclinical therapy development for epilepsy, our working group considered the current state of understanding related to terminology, models, and strategies for therapy development for the comorbidities of epilepsy. Herein we summarize our findings and suggest ways to accelerate development of new therapies. We also consider important issues to improve research including those related to methodology, nonpharmacologic therapies, biomarkers, and infrastructure.

Stress, epilepsy, and psychiatric comorbidity: how can animal models inform the clinic?

Psychiatric complaints afflict many patients with epilepsy, and these contribute significantly to the impaired quality of life experienced by sufferers of this common group of neurological conditions. Psychiatric disorders in epilepsy patients are under-diagnosed and under-treated. Moreover, evidence suggests that the psychiatric disorders may act as risk factors for some types of epilepsy and exacerbate disease progression in established cases, promoting the case for a bidirectional relationship between epilepsy and psychopathology. While cause and effect relationships can be difficult to establish in human studies, appropriate animal models provide valuable tools with which to study the interactions between epilepsy and stress-related disorders. Indeed, many epilepsy models exhibit behavioral phenotypes which are reflective of psychiatric disorders, and, conversely, stressful environments appear to promote a vulnerability to developing epilepsy. This review summarizes this research area, exploring the behavioral phenotypes in animal models of epilepsy and then examining the influence of stressful environments on susceptibility to seizures and epilepsy. The ultimate goal of this line of research is to be able to translate these findings to humans. Understanding the relationships between epilepsy and associated psychiatric disorders will facilitate effective treatment of mood disorders in epilepsy, inform about the pathophysiology of each individually, and potentially open up novel therapeutic disease-modifying strategies for patients with epilepsy.

Reduction of vesicle-associated membrane protein 2 expression leads to a kindling-resistant phenotype in a murine model of epilepsy

Our previous work has correlated permanent alterations in the rat neurosecretory machinery with epileptogenesis. Such findings highlighted the need for a greater understanding of the molecular mechanisms underlying epilepsy so that novel therapeutic regimens can be designed. To this end, we examined kindling in transgenic mice with a defined reduction of a key element of the neurosecretory machinery: the v-SNARE (vesicle-bound SNAP [soluble NSF attachment protein] receptor), synaptobrevin/vesicle-associated membrane protein 2 (VAMP2). Initial analysis of biochemical markers, which previously displayed kindling-dependent alterations in rat hippocampal synaptosomes, showed similar trends in both wild-type and VAMP2(+/-) mice, demonstrating that kindled rat and mouse models are comparable. This report focuses on the effects that a ~50% reduction of synaptosomal VAMP2 has on the progression of electrical kindling and on glutamate release in hippocampal subregions. Our studies show that epileptogenesis is dramatically attenuated in VAMP2(+/-) mice, requiring both higher current and more stimulations to reach a fully kindled state (two successive Racine stage 5 seizures). Progression through the five identifiable Racine stages was slower and more variable in the VAMP2(+/-) animals compared with the almost linear progression seen in wild-type littermates. Consistent with the expected effects of reducing a major neuronal v-SNARE, glutamate-selective, microelectrode array (MEA) measurements in specific hippocampal subregions of VAMP2(+/-) mice showed significant reductions in potassium-evoked glutamate release. Taken together these studies demonstrate that manipulating the levels of the neurosecretory machinery not only affects neurotransmitter release but also mitigates kindling-induced epileptogenesis.

Regional changes in gene expression after limbic kindling

Repeated electrical stimulation results in development of seizures and a permanent increase in seizure susceptibility (kindling). The permanence of kindling suggests that chronic changes in gene expression are involved. Kindling at different sites produces specific effects on interictal behaviors such as spatial cognition and anxiety, suggesting that causal changes in gene expression might be restricted to the stimulated site. We employed focused microarray analysis to characterize changes in gene expression associated with amygdaloid and hippocampal kindling. Male Long-Evans rats received 1 s trains of electrical stimulation to either the amygdala or hippocampus once daily until five generalized seizures had been kindled. Yoked control rats carried electrodes but were not stimulated. Rats were euthanized 14 days after the last seizures, both amygdala and hippocampus dissected, and transcriptome profiles compared. Of the 1,200 rat brain-associated genes evaluated, 39 genes exhibited statistically significant expression differences between the kindled and non-kindled amygdala and 106 genes exhibited statistically significant differences between the kindled and non-kindled hippocampus. In the amygdala, subsequent ontological analyses using the GOMiner algorithm demonstrated significant enrichment in categories related to cytoskeletal reorganization and cation transport, as well as in gene families related to synaptic transmission and neurogenesis. In the hippocampus, significant enrichment in gene expression within categories related to cytoskeletal reorganization and cation transport was similarly observed. Furthermore, unique to the hippocampus, enrichment in transcription factor activity and GTPase-mediated signal transduction was identified. Overall, these data identify specific and unique neurochemical pathways chronically altered following kindling in the two sites, and provide a platform for defining the molecular basis for the differential behaviors observed in the interictal period.

The differential effects of protein synthesis inhibition on the expression and reconsolidation of pentylenetetrazole kindled seizures

This work attempted to answer the question whether the central processes engaged in the memory formation and the epilepsy development are governed by the overlapping mechanisms. The effects of the protein synthesis inhibitor cycloheximide (CHX) were examined on the expression and reconsolidation of pentylenetetrazole (PTZ) - induced kindled seizures and for comparative purposes, on the reconsolidation of conditioned fear response (conditioned freezing). It was found that post-test intracerebroventricular administration of CHX (125microg/5microl) significantly attenuated the expression of a conditioned fear response examined 24h later. Thus, inhibition of de novo brain protein synthesis interfered with the reconsolidation of a conditioned response. CHX given at the same dose repeatedly to fully kindled rats immediately after three consecutive sessions of PTZ-induced seizures (35mg/kg ip) did not modify the strength of convulsions. On the other hand, CHX significantly attenuated the strength of convulsions when the drug was administered 1h before the PTZ injection, which occurred every second day for three consecutive sessions. However, when CHX was omitted in a consecutive session, PTZ induced a fully developed expression of tonic-clonic convulsions, thereby indicating that CHX-induced changes in seizure intensity were transitory. Western Blot analysis confirmed that CHX potently inhibited PTZ-induced protein synthesis (c-Fos) in the rat brain, examined 60min after CHX and PTZ administration. The present findings suggest that the mechanisms underlying kindling are resistant to modification, even under the influence of protein synthesis inhibitors, and that there are important differences between the processes of learning and kindling seizures.

Cortical kindling induces elevated levels of AMPA and GABA receptor subunit mRNA within the amygdala/piriform region and is associated with behavioral changes in the rat

Cortical kindling causes alterations within the motor cortex and results in long-standing motor deficits. Less attention has been directed to other regions that also participate in the epileptiform activity. We examined if cortical kindling could induce changes in excitatory and inhibitory receptor subunit mRNA in the amygdala/piriform regions and if such changes are associated with behavioral deficits. After cortical kindling, amygdala/piriform regions were dissected to analyze mRNA levels of NMDA, AMPA, and GABA receptor subunits using reverse transcription polymerase chain reaction, or rats were subjected to a series of behavioral tests. Kindled rats had significantly greater amounts of GluR1 and GluR2 AMPA receptor mRNA, and alpha1 and alpha2 GABA receptor subunit mRNA, compared with sham controls, which was associated with greater anxiety-like behaviors in the elevated plus maze and reduced freezing behaviors in the fear conditioning task. In summary, cortical kindling produces dynamic receptor subunit changes in regions in addition to the seizure focus.

Effects of pentylenetetrazol-induced brief convulsive seizures on spatial memory and fear memory

Previous studies have demonstrated that in the pentylenetetrazol (PTZ) kindling model, recurrent seizures either impair or have no effect on learning and memory. However, the effects of brief seizures on learning and memory remain unknown. Here, we found that a single injection of a convulsive dose of PTZ (50 mg/kg, ip) induced brief seizures in Sprague-Dawley rats. Administration of PTZ before training impaired the acquisition of spatial memory in the Morris water maze (MWM) and fear memory in contextual fear conditioning. However, the administration of PTZ immediately after training did not affect memory consolidation in either task. These findings suggest that brief seizures have different effects on acquisition and consolidation of spatial and fear memory.

Amygdaloid kindling is anxiogenic but fails to alter object recognition or spatial working memory in rats

Kindling in rats produces enduring behavioral changes that parallel the psychobehavioral disturbances frequently accompanying temporal lobe epilepsy. Some evidence suggests that the site of kindling is an important determinant of the type of behavioral changes observed following kindling, although this variable has not been systematically investigated. In the present experiments, the effects of amygdaloid kindling were assessed on a battery of behavioral tests we used previously to assess the effects of kindling in dorsal hippocampus or perirhinal cortex. Three generalized seizures were kindled with stimulation in or near the basolateral amygdala. One week later, rats were tested successively on measures of anxiety, activity, object recognition memory, and spatial working memory over a period of 3 weeks. Amygdaloid kindling produced increased anxiety, but spared all other behaviors assessed. This pattern of results is partially distinct from the previously described effects of perirhinal cortical kindling, which increases anxiety but also impairs object recognition memory, and is completely distinct from dorsal hippocampal kindling, which selectively increases activity and impairs spatial working memory. The observations suggest that kindling of distinct highly interconnected temporal lobe sites produces distinct patterns of behavioral comorbidity. The underlying mechanisms are thus most likely localized to intrinsic circuits at the site of seizure origination.

Repeated seizures lead to altered skilled behaviour and are associated with more highly efficacious excitatory synapses

People with epilepsy have a high incidence of interictal behavioural problems that appear to be related to the location of their seizure focus. This study investigated a novel test of the hypotheses that repeated seizures result in behavioural deficits and altered performance during the interictal state, and that those behaviours are related to the presence of more highly efficacious excitatory synapses. We tested these hypotheses by first repeatedly eliciting seizures with electric current through indwelling electrodes in the corpus callosum at the level of the caudal forelimb area of sensorimotor neocortex in the rat. We then assessed learned skilled behaviours that primarily utilize the forelimbs on tasks that are sensitive to the functional integrity of that structure. We observed both behavioural deficits and altered kinematic performance in rats that experienced repeated neocortical seizures relative to an electrode-implanted control group. From a separate set of rats, tissue was prepared for quantification of thickness and excitatory synaptic subtypes from neocortical layer V. We observed significantly increased numbers of perforated synapses that make their connections directly onto the dendritic shaft at 3 weeks following the last seizure. Altered reaching behaviours are likely due to neural reorganization in the neocortex including more efficacious synapses.

Kindling of basolateral amygdala but not ventral hippocampus or perirhinal cortex disrupts sensorimotor gating in rats

The neural mechanisms mediating prepulse inhibition (PPI) appear to have relevance to neurological and psychiatric disorders. Patients with temporal lobe epilepsy exhibit psychotic symptoms and disrupted PPI, therefore the present experiments examined the consequences of seizures induced by kindling on PPI. Rats were chronically implanted with an electrode into the basolateral amygdala, perirhinal cortex, or ventral hippocampus and stimulated twice daily until 3 fully generalized, class 5 seizures were elicited. Kindling of basolateral amygdala, but not perirhinal cortex or ventral hippocampus, disrupted PPI when testing began 2min, but not 48h, following the elicitation of the third class 5 seizure. Startle amplitudes were unaffected by kindling. These results suggest that the anatomical origin of seizures is an important factor in determining their potentially disruptive effects on PPI.

Kindling increases aversion to saccharin in taste aversion learning

Kindling is a model in which an initially subconvulsive electrical stimulation of certain brain areas eventually develops a generalized seizure that produces behavioral and long term neuronal changes. In the present study we evaluated if kindling can modify conditioning taste aversion (CTA). In this paradigm animals acquire aversion to saccharin when it is presented as the conditioned stimulus (CS) followed by an injection of lithium chloride (LiCl) that induces a gastric irritation as the unconditioned stimulus (US). Male Wistar rats were implanted with bipolar electrodes aimed at the right amygdala (AMG) or at the right insular cortex (IC). The animals were stimulated daily until they reached stages 2-4 (intermediate) or until kindling was fully established (three consecutive stage 5 seizures). At least two weeks after kindling stimulation had ceased the animals were deprived of water for 24 h and given 10-min drinking sessions twice a day for 4 days. On day 5 (morning session) tap water was replaced by saccharin solution (0.1%), 20 min later the animals were injected with LiCl (7.5 ml/kg i.p., 0.2 M) to induce gastric malaise or taste aversion. After three more days of baseline consumption, water was substituted by a fresh 0.1% saccharin solution to test the aversion. AMG-kindling delayed the extinction of CTA. Animals with kindling in the IC had a higher retention than the sham kindling group; that is, they drank significantly less saccharin solution than the other groups. The results of the present experiment show that local modification of brain function induced by kindling stimulation can prolong the aversive effects of CTA.

Intrahippocampal injection of endothelin-1 in immature rats results in neuronal death, development of epilepsy and behavioral abnormalities later in life

The direct injection of endothelin-1 (ET-1) into brain parenchyma was recently suggested as a suitable model of stroke. The present study was designed to assess whether intrahippocampal injection of ET-1 in immature rats causes neurodegeneration and immediate seizures, and results in impairment of motor development, cognitive decline, epilepsy and chronic hippocampal lesion. ET-1 was injected unilaterally into the dorsal hippocampus in doses of 20 or 40 pmol at the age of 12 (P12) or 25 (P25) days. Video-electroencephalographic monitoring performed during 100 min after the injection of ET-1 demonstrated the development of convulsive epileptic seizures in 75-100% of animals of individual age-and-dose groups. Long-term behavioral follow-up did not reveal impairment of motor development in any dose-and-age group. At 2 months after ET-1 injection, impairment of spatial memory occurred only in rats with 40 pmol of ET-1 at P12. At 3 months after ET-1 injection spontaneous electrographic seizures occurred in 62.5-100% animals of both ages with no relation to the dose used. Seizures were always non-convulsive. The total seizure duration per 24 h was higher in the P12 than the P25 group, suggesting more severe epilepsy. The extent of the hippocampal lesion increased with the dose of ET-1 and was significantly higher in the P12 than the P25 group. The severity of the ET-1-induced lesion correlated positively with total seizure duration per 24 h at both ages. Our results document that early intrahippocampal injection of ET-1 results in lesion development and both immediate seizures and chronic epilepsy in either age group. Cognitive impairment occurred only in rats with ET-1 injection at P12.

Behavioral seizure correlates in animal models of epilepsy: a road map for assay selection, data interpretation, and the search for causal mechanisms

A broad spectrum of learning/memory, social interaction, and affective behavioral measures serve as functional correlates for neurobiological changes in seizure-prone animals as well as in epileptic clinical populations. The utility of such measures is demonstrated by their ability to distinguish anomalous characteristics in developing organisms predisposed to seizure onset, as well as to discriminate prior seizure history in organisms with established pathology. For instance, typical findings that generalize across species suggest that seizure-experienced organisms exhibit a variety of deficits in cognitive function as well as inappropriate social neglect and aggression. Behavioral testing batteries have also proven useful in assessing neural mechanisms for seizure induction, subcortical neural circuits, and neuropeptide modulators, for example, as well as in identifying neural pathology resulting from prior seizure activity. However, the wanton application of behavioral tests can also produce false positives in the identification of seizure-related disorders unless alternative performance and motivational hypotheses are discounted effectively. Accordingly, the present review attempts to provide the reader interested in behavioral phenotyping and characterization of seizure-prone rats and mice with a roadmap for rational selection, implementation, and interpretation of data from behavior assays while highlighting potential successes and pitfalls inherent in employing functional correlates of brain activity using animal models of epilepsy.

Kindling-induced changes in plasticity of the rat amygdala and hippocampus

Temporal lobe epilepsy (TLE) is often accompanied by interictal behavioral abnormalities, such as fear and memory impairment. To identify possible underlying substrates, we analyzed long-term synaptic plasticity in two relevant brain regions, the lateral amygdala (LA) and the CA1 region of the hippocampus, in the kindling model of epilepsy. Wistar rats were kindled through daily administration of brief electrical stimulations to the left basolateral nucleus of the amygdala. Field potential recordings were performed in slices obtained from kindled rats 48 h after the last induced seizure, and in slices from sham-implanted and nonimplanted controls. Kindling resulted in a significant impairment of long-term potentiation (LTP) in both the LA and the CA1, the magnitude of which was dependent on the number of prior stage V seizures. Saturation of CA1-LTP, assessed through repeated spaced delivery of high-frequency stimulation, occurred at lower levels in kindled compared to sham-implanted animals, consistent with the hypothesis of reduced capacity of further synaptic strengthening. Furthermore, theta pulse stimulation elicited long-term depression in the amygdala in nonimplanted and sham-implanted controls, whereas the same stimulation protocol stimulation caused LTP in kindled rats. In conclusion, kindling differentially affects the magnitude, saturation, and polarity of LTP in the CA1 and LA, respectively, most likely indicating an activity-dependent mechanism in the context of synaptic metaplasticity.

Hippocampal CA1 kindling but not long-term potentiation disrupts spatial memory performance

Long-term synaptic enhancement in the hippocampus has been suggested to cause deficits in spatial performance. Synaptic enhancement has been reported after hippocampal kindling that induced repeated electrographic seizures or afterdischarges (ADs) and after long-term potentiation (LTP) defined as synaptic enhancement without ADs. We studied whether repeated stimulations that gave LTP or ADs resulted in spatial performance deficits on the radial arm maze (RAM) and investigated the minimal number of ADs required for such deficits. Three experimental groups were run as follows: (1) 5 hippocampal ADs in 1 d (5-AD group), (2) 10 hippocampal ADs in 2 d (10-AD group), and (3) 12 -frequency primed-burst stimulations (PBSs) in 2 d in order to induce LTP without ADs (LTP group). Each experimental group was run together with a control group during the same time period. Rats were first trained in a spatial task on a radial arm maze with four of the eight arms baited, then given control or experimental treatment, and maze performance was tested in the first week (1-4 d) and fourth week (22-25 d) after treatment. Basal dendritic population excitatory postsynaptic potentials (pEPSPs) and medial perforant path (MPP)-evoked dentate gyrus population spike and polysynaptic CA1 excitation were recorded before and after experimental and control treatment. Spatial memory errors, in particular reference memory errors, were significantly higher in the 10-AD kindled group than any other group on the first and fourth week after treatment. Spatial memory errors were not significantly different in the 5-AD and LTP groups as compared with any control groups at any time. Basal dendritic pEPSP in CA1 was enhanced for about 1 wk after 12 PBSs, 10 ADs, or 5 ADs, while the dentate gyrus population spike and CA1 polysynaptic excitation evoked by MPP was increased for up to 4 wk after 10 ADs, but not 12 PBSs. Thus, distributed alteration of multiple synaptic transmission in the entorhinal-hippocampal circuit, but not LTP at the basal dendritic synapses in CA1, may disrupt spatial performance after 10 hippocampal ADs.

Studies of the reproduction of long-term memory during exposure to kainic Acid

The effects of the neurotoxin kainic acid on a food-producing habit were studied in Wistar rats in an experimental chamber. Single doses of kainic acid at a subconvulsive dose (8 mg/kg, i.p.) were found to impair the habit, onset of impairment being delayed by several weeks rather than immediate. Conversely, administration of the neurotoxin at the convulsive dose (10 mg/kg) impaired reproduction of the habit with onset within several hours after treatment and persistence for periods of up to 10 days, though this dose did not prevent the acquisition of a new food-procuring habit. These defects in the reproduction of long-term memory traces were explained in terms of the characteristics of the effects of kainic acid on the hippocampal system, which is the most susceptible to systemic administration of the neurotoxin.

Anterior perirhinal cortex kindling produces long-lasting effects on anxiety and object recognition memory

Temporal lobe epilepsy (TLE) is frequently accompanied by memory impairments and, although their bases are unknown, most research has focused on the hippocampus. The present study investigated the importance of another medial temporal lobe structure, the perirhinal cortex (Prh), in changes in memory in TLE using kindling as a model. Rats were kindled twice daily with anterior Prh stimulation until three fully generalized seizures were evoked. Beginning 7 days later and on successive days, rats were tested in an elevated plus maze, a large circular open field, an open field object exploration task and a delayed-match-to-place task in a water maze in order to assess anxiety-related and exploratory behaviour, object recognition memory and spatial cognition. Kindling increased anxiety-related behaviour in both the elevated plus and open field mazes and disrupted spontaneous object recognition but spared all other behaviours tested. These results are consistent with other findings indicating a greater role for the Prh in object memory and emotional behaviour than in spatial memory and contrast with the selective disruption of spatial memory produced by dorsal hippocampal kindling. The site-selectivity of the behavioural disruptions produced by kindling indicates that such effects are probably mediated by changes particular to the site of seizure initiation rather than to changes in the characteristic circuitry activated by limbic seizure generalization. Further investigation of the behavioural effects of Prh kindling may be useful for studying the mechanisms of mnemonic and affective dysfunction associated with TLE and offer insights into bases for variability in such dysfunction across patients.

Conditioned effects of kindling three different sites in the hippocampal complex of the rat

Rats received kindling stimulations to the perirhinal cortex (PRh), ventral hippocampus (VH), or dorsal hippocampus (DH) in 1 environment and an equivalent number of sham stimulations in a 2nd environment. The PRh-kindled rats displayed rapid kindling and a swift emergence of conditioned interictal defensiveness. In contrast, the VH- and DH-kindled rats displayed much slower kindling and slow or no conditioning, respectively. No effects of conditioning on the convulsions, comparable with those associated with amygdala kindling, were observed. These results establish the generality of some of the previously reported kindling-related conditioned effects, confirm the site specificity of some of these effects, and suggest that the convulsions, rather than the stimulations, function as the unconditioned stimuli for the conditioning of interictal behavior.

Learning and memory

Learning and memory processes are thought to underlie a variety of human psychiatric disorders, including generalised anxiety disorder and post-traumatic stress disorder. Basic research performed in laboratory animals may help to elucidate the aetiology of the respective diseases. This chapter gives a short introduction into theoretical and practical aspects of animal experiments aimed at investigating acquisition, consolidation and extinction of aversive memories. It describes the behavioural paradigms most commonly used as well as neuroanatomical, cellular and molecular correlates of aversive memories. Finally, it discusses clinical implications of the results obtained in animal experiments in respect to the development of novel pharmacotherapeutic strategies for the treatment of human patients.

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.

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.

Do seizures cause irreversible cognitive damage? Evidence from animal studies

Data from experimental models provide evidence that both prolonged and brief seizures can cause irreversible impairment in spatial and emotional learning and memory. Factors related to the severity of the behavioral impairments include genetic background, age at the time of the epileptogenic insult, extent of brain lesion, location of seizure focus, seizure duration, seizure number, brain reserve, and environmental and social living conditions. Further, as in humans, the interval between the last seizure and behavioral testing as well as treatment with antiepileptic drugs can affect the test results.

Amygdala-kindling induces alterations in neuronal density and in density of degenerated fibers

Kindling is characterized by a progressive intensification of seizure activity culminating in generalized seizures following repeated administration of an initially subconvulsive electrical or chemical stimulus. Since it is known that epilepsy induces morphological alterations in the limbic system, we examined the neuropathological consequences of kindling with a sensitive silver-staining method for the visualization of damaged neurons and Nissl staining for the estimation of the neuronal densities in different limbic areas. Wistar rats implanted with electrodes in the left basolateral nucleus were stimulated until 15 consecutive stage V seizures (scale of Racine). Amygdala-kindled animals had reduced cell density in the amygdala and increased density of fragments of degenerated axons. Reduced neuronal density and the occurrence of degenerated axons in kindled animals were more prominent in the ipsilateral than in the contralateral hemisphere. In addition, more degenerated axons were found in cortical structures of kindled than sham-operated animals. These results indicate that kindling induced morphological alterations that were not restricted to either the ipsilateral hemisphere or the stimulated region. These morphological changes might be responsible for the emotional and behavioral disturbances that can accompany epilepsy.

Environmental Enrichment Facilitates Amygdala Kindling but Reduces Kindling-Induced Fear in Male Rats

The purpose of this experiment was to determine the effect of prior environmental enrichment on the acquisition of kindling and the expression of kindling-induced fear. Sixty male rats were housed either in an enriched environment or in isolation, starting immediately after weaning. As adults, they were subjected to either 50 amygdala-kindling stimulations or sham stimulations, followed by testing in an unfamiliar open field. The kindled-enriched rats acquired the kindled state more quickly than did the kindled-isolated rats, but they also showed less fear in the open field than did the kindled-isolated rats. These results suggest that environmental enrichment has differential effects on kindling acquisition and its behavioral consequences.

Anticipating the Attack: Temporal Conditioning During Amygdala Kindling in Rats

The present study showed that amygdala-kindled rats use short-interval timing superimposed on phase or ordinal timing to predict when a convulsion will occur. In 2 experiments, rats received 1 stimulation and 1 sham stimulation each day, always at the same times (conditioned stimulus [CS]+ and CS- times, respectively) and 150 s after rats had been placed in the testing chamber (the preadministration interval). As kindling progressed, the rats displayed more defensive behavior at the CS+ time than at the CS- time. Then, a stimulation-free peak-procedure test was conducted: At the CS+ time, but not at the CS- time, defensive behavior increased progressively as the 150-s preadministration interval elapsed, and then it gradually declined.

Kindling suppresses primed-burst-induced long-term potentiation in hippocampal CA1

In a previous study, we showed that partial hippocampal kindling in rats, a model of temporal lobe epilepsy, reduced the efficacy of presynaptic GABA(B) receptors in the CA1 area of hippocampal slices. In this study, we show that long-term potentiation (LTP) induced by theta-frequency primed bursts was suppressed in kindled as compared to control rats, but not in the presence of the GABA(B) receptor antagonists CGP35348 or CGP55845A. This is original evidence that LTP is suppressed by pathophysiological downregulation of GABA(B) autoreceptors. Control of postsynaptic inhibition by presynaptic GABA(B) receptors may provide a compensatory mechanism for controlling paroxysmal activity, with a side effect of blocking synaptic plasticity.

Conditioning of a flavor aversion in rats by amygdala kindling

Rats received 30 stimulations and 30 sham stimulations (the lead was attached to the subjects but no current was delivered) to the left basolateral amygdala in a quasirandom sequence. Stimulations were preceded by the presentation of 1 flavored solution conditional stimulus (CS+); sham stimulations were preceded by the presentation of another flavored solution, CS-. As kindled motor seizures developed, the rats began to consume significantly less of the CS+ than the CS-. Moreover, at the end of the experiment, the rats consumed significantly less of the CS+ than the CS- during a 20-min conditioned flavor preference test in which both solutions were available simultaneously. These findings confirm and extend the recent report that interictal changes in defensive behavior can be conditioned by amygdalar kindling.

Conditioned effects of kindling

Mild periodic electrical stimulation to any one of many brain sites leads to the development and progressive intensification of elicited motor seizures. Since its discovery in 1969, this kindling phenomenon has been widely studied both as a model of epileptogenesis and as a form of neuroplasticity, and recently there has been increasing interest in kindling as a model of the interictal (i.e. between-seizures) changes in emotionality that accompany certain forms of epilepsy. Despite the extensive use of the kindling model, little consideration has been given to the role played by the cues regularly associated with the delivery of the kindling stimulations. However, we have recently demonstrated that cues associated with the standard kindling protocol (e.g. the stimulation environment) produce conditioned effects on both the motor seizures and interictal behavior of rats and that some kindling sites, such as the amygdala, produce conditioned interictal behaviors that are defensive in nature. The implications that these findings have for the study of interictal behavioral changes in particular and to kindling research in general are discussed.

Impaired fear conditioning but enhanced seizure sensitivity in rats given repeated experience of withdrawal from alcohol

Repeated experience of withdrawal from chronic alcohol treatment increases sensitivity to seizures. It has been argued by analogy that negative affective consequences of withdrawal also sensitize, but repeated experience of withdrawal from another sedative-hypnotic drug, diazepam, results in amelioration of withdrawal anxiety and aversiveness. We tested whether giving rats repeated experience of withdrawal from alcohol altered their ability to acquire a conditioned emotional response (CER). Male Hooded Lister rats were fed a nutritionally complete liquid diet as their only food source. Different groups received control diet, or diet containing 7% ethanol. Rats receiving ethanol diet were fed for either 24 days (Single withdrawal, SWD), or 30 days, with two periods of 3 days, starting at day 11, and 21, in which they received control diet (Repeated withdrawal, RWD). All rats were fed lab chow at the end of their liquid diet feeding period. Starting 12 days after the final withdrawal, groups of Control, SWD and RWD rats were given pentylenetetrazole (PTZ; 30 mg/kg, i.p.) three times a week, and scored for seizures. The occurrence of two successive Stage 5 seizures was taken as the criterion for full PTZ kindling. Other groups of control, SWD and RWD rats were trained to operate levers to obtain food, and were then exposed, in a fully counterbalanced design, to light and tone stimuli which predicted unavoidable footshock (CS+), or which had no consequences (CS-). Rats consumed approximately 17.5 g/kg/day of ethanol, resulting in blood alcohol levels of approximately 100 mg/dL. Repeated administration of PTZ resulted in increasing seizure scores. RWD rats achieved kindling criterion faster than either Control or SWD rats. No differences were seen in the groups in flinch threshold to footshock (0.3 mA). At a shock intensity of 0.35 mA, Control, but not RWD or SWD rats showed significant suppression to the CS+ CS- presentation did not affect response rates. The three groups differed in their response to pairing the CS+ with increasing shock levels, the Controls remaining more sensitive to the CS+. SWD rats showed significant suppression of lever pressing during CS+ presentations only at 0.45 and 0.5 mA, and RWD rats only at 0.5 mA. Giving rats repeated experience of withdrawal from chronic ethanol results in increased sensitivity to PTZ kindling, but reduces their ability to acquire a CER. Withdrawal kindling of sensitivity to anxiogenic events does not seem to occur under circumstances which give rise to kindling of seizure sensitivity.

Up-regulation of D3 dopaminergic receptor mRNA in the core of the nucleus accumbens accompanies the development of seizures in a genetic model of absence-epilepsy in the rat

The basal ganglia system is thought to play a key role in the control of absence-seizures and there is ample evidence that epileptic seizures modify brain dopamine function. We recently reported that local injections of dopamine D1 or D2 agonists in the core of the nucleus accumbens suppressed absence-seizures in a spontaneous, genetic rodent model of absence-epilepsy whereas injections of D1 or D2 antagonists had aggravating effects. These findings raised the possibility that the dopaminergic system may be altered in absence-epilepsy prone rats. Therefore, we studied by in situ hybridization histochemistry the expression of pre- and postsynaptic components of the dopaminergic system in this strain of rats. When compared to non-epileptic control rats, epileptic rats displayed no change in the expression of mRNAs coding for the neuronal dopaminergic markers (tyrosine hydroxylase, membraneous and vesicular dopamine transporters). In addition, there was no difference between the two strains concerning the expression of the dopamine receptor transcripts D1, D2 and D5. In adult absence-epilepsy prone rat with an overt epileptic phenotype, however, an elevated level of D3 mRNA expression was observed in neurons of the core of the nucleus accumbens (+23% increase in silver grain density compared to non-epileptic control rats). D3 transcripts were not increased in juvenile epileptic rats without seizures. These findings suggests that up-regulation of D3 receptor mRNA is part of the epileptic phenotype in absence-epilepsy prone rats. Its localization in the core of the nucleus accumbens bears close resemblance to the dopamine-sensitive antiepileptic sites in ventral striatum and further support the involvement of ventral structures of the basal ganglia system in the control of absence-seizures.

Dorsal hippocampal kindling produces a selective and enduring disruption of hippocampally mediated behavior

Kindling produces enduring neural changes that are subsequently manifest in enhanced susceptibility to seizure-evoking stimuli and alterations in some types of behavior. The present study investigated the effects of dorsal hippocampal (dHPC) kindling on a variety of behaviors to clarify the nature of previously reported effects on spatial task performance. Rats were kindled twice daily with dHPC stimulation until three fully generalized seizures were evoked. Beginning 7 d later and on successive days, rats were tested in an elevated plus maze, a large circular open field, an open field object exploration task, and a delayed-match-to-place (DMTP) task in a water maze to assess anxiety-related and activity-related behavior (tasks 1 and 2), object recognition memory (task 3), and spatial cognition (task 4). Kindling disrupted performance on the DMTP task in a manner that was not delay dependent and produced a mild enhancement of activity-related behaviors in the open field task but not the elevated plus maze. All other aspects of testing were spared. These findings indicate that dHPC kindling produces enduring and selective effects on behavior that are consistent with a restricted disruption of hippocampally mediated functions. Possible bases for these effects are changes in local NMDA receptor function and/or changes in local inhibition, which might alter the optimal conditions for experience-dependent induction of intrahippocampal plasticity. This preparation may be useful for studying the mechanisms of mnemonic dysfunction associated with temporal lobe epilepsy and may offer unique insights into the mechanisms underlying normal hippocampal function.