Enhanced rate of kindling after prolonged electrode implantation into the amygdala of rats

Development of spontaneous recurrent seizures accompanied with increased rates of interictal spikes and decreased hippocampal delta and theta activities following extended kindling in mice

Interictal epileptiform discharges refer to aberrant brain electrographic signals between seizures and feature intermittent interictal spikes (ISs), sharp waves, and/or abnormal rhythms. Recognition of these epileptiform activities by electroencephalographic (EEG) examinations greatly aids epilepsy diagnosis and localization of the seizure onset zone. ISs are a major form of interictal epileptiform discharges recognized in animal models of epilepsy. Progressive changes in IS waveforms, IS rates, and/or associated fast ripple oscillations have been shown to precede the development of spontaneous recurrent seizures (SRS) in various animal models. IS expressions in the kindling model of epilepsy have been demonstrated but IS changes during the course of SRS development in extended kindled animals remain to be detailed. We hence addressed this issue using a mouse model of kindling-induced SRS. Adult C57 black mice received twice daily hippocampal stimulations until SRS occurrence, with 24-h EEG monitoring performed following 50, 80, and ≥ 100 stimulations and after observation of SRS. In the stimulated hippocampus, increases in spontaneous ISs rates, but not in IS waveforms nor IS-associated fast ripples, along with decreased frequencies of hippocampal delta and theta rhythms, were observed before SRS onset. Comparable increases in IS rates were further observed in the unstimulated hippocampus, piriform cortex, and entorhinal cortex, but not in the unstimulated parietal cortex and dorsomedial thalamus. These data provide original evidence suggesting that increases in hippocampal IS rates, together with reductions in hippocampal delta and theta rhythms are closely associated with development of SRS in a rodent kindling model.

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.

Fit for purpose application of currently existing animal models in the discovery of novel epilepsy therapies

Animal seizure and epilepsy models continue to play an important role in the early discovery of new therapies for the symptomatic treatment of epilepsy. Since 1937, with the discovery of phenytoin, almost all anti-seizure drugs (ASDs) have been identified by their effects in animal models, and millions of patients world-wide have benefited from the successful translation of animal data into the clinic. However, several unmet clinical needs remain, including resistance to ASDs in about 30% of patients with epilepsy, adverse effects of ASDs that can reduce quality of life, and the lack of treatments that can prevent development of epilepsy in patients at risk following brain injury. The aim of this review is to critically discuss the translational value of currently used animal models of seizures and epilepsy, particularly what animal models can tell us about epilepsy therapies in patients and which limitations exist. Principles of translational medicine will be used for this discussion. An essential requirement for translational medicine to improve success in drug development is the availability of animal models with high predictive validity for a therapeutic drug response. For this requirement, the model, by definition, does not need to be a perfect replication of the clinical condition, but it is important that the validation provided for a given model is fit for purpose. The present review should guide researchers in both academia and industry what can and cannot be expected from animal models in preclinical development of epilepsy therapies, which models are best suited for which purpose, and for which aspects suitable models are as yet not available. Overall further development is needed to improve and validate animal models for the diverse areas in epilepsy research where suitable fit for purpose models are urgently needed in the search for more effective treatments.

The bumetanide prodrug BUM5, but not bumetanide, potentiates the antiseizure effect of phenobarbital in adult epileptic mice

OBJECTIVE

The loop diuretic bumetanide has been reported to potentiate the antiseizure activity of phenobarbital in rodent models of neonatal seizures, most likely as a result of inhibition of the chloride importer Na-K-Cl cotransporter isoform 1 (NKCC1) in the brain. In view of the intractability of neonatal seizures, the preclinical findings prompted a clinical trial in neonates on bumetanide as an add-on to phenobarbital, which, however, had to be terminated because of ototoxicity and lack of efficacy. We have recently shown that bumetanide penetrates only poorly into the brain, so that we developed lipophilic prodrugs such as BUM5, the N,N-dimethylaminoethylester of bumetanide, which penetrate more easily into the brain and are converted to bumetanide.

METHODS

In the present study, we used a new strategy to test whether BUM5 is more potent than bumetanide in potentiating the antiseizure effect of phenobarbital. Adult mice were made epileptic by pilocarpine, and the antiseizure effects of bumetanide, BUM5, and phenobarbital alone or in combination were determined by the maximal electroshock seizure threshold test.

RESULTS

In nonepileptic mice, only phenobarbital exerted seizure threshold-increasing activity, and this was not potentiated by the NKCC1 inhibitors. In contrast, a marked potentiation of phenobarbital by BUM5, but not bumetanide, was determined in epileptic mice.

SIGNIFICANCE

Thus, bumetanide is not capable of potentiating phenobarbital's antiseizure effect in an adult mouse model, which, however, can be overcome by using the prodrug BUM5. These data substantiate that BUM5 is a promising tool compound for target validation and proof-of-concept studies on the role of NKCC1 in brain diseases.

Impact of injury location and severity on posttraumatic epilepsy in the rat: role of frontal neocortex

Human posttraumatic epilepsy (PTE) is highly heterogeneous, ranging from mild remitting to progressive disabling forms. PTE results in simple partial, complex partial, and secondarily generalized seizures with a wide spectrum of durations and semiologies. PTE variability is thought to depend on the heterogeneity of head injury and patient's age, gender, and genetic background. To better understand the role of these factors, we investigated the seizures resulting from calibrated fluid percussion injury (FPI) to adolescent male Sprague-Dawley rats with video electrocorticography. We show that PTE incidence and the frequency and severity of chronic seizures depend on the location and severity of FPI. The frontal neocortex was more prone to epileptogenesis than the parietal and occipital, generating earlier, longer, and more frequent partial seizures. A prominent limbic focus developed in most animals, regardless of parameters of injury. Remarkably, even with carefully controlled injury parameters, including type, severity, and location, the duration of posttraumatic apnea and the age and gender of outbred rats, there was great subject-to-subject variability in frequency, duration, and rate of progression of seizures, indicating that other factors, likely the subjects' genetic background and physiological states, have critical roles in determining the characteristics of PTE.

Distribution of GABAergic neurons in the striatum of amygdala-kindled rats: An immunohistochemical and in situ hybridization study

A large body of experimental evidence suggests that the basal ganglia circuitry may be part of a remote control system modulating the spread of epileptic seizures. In the kindling model of temporal lobe epilepsy, this endogenous inhibitory control mechanism seems to be impaired. Neurochemical and neurophysiological studies have indicated that the activity of the GABAergic projection from the striatum to the substantia nigra pars reticulata is reduced in kindled rats, but the exact mechanisms involved in this observation are not known. Possible explanations include a kindling-induced loss of striatal GABAergic projection neurons to the substantia nigra or enhanced inhibition of these neurons by GABAergic interneurons. In the present experiments, the GABAergic system of the striatum (caudate-putamen) of amygdala-kindled rats and controls was studied immunohistochemically with a monoclonal antibody to GABA and with nonisotopic in situ hybridization with cRNA probes selective for glutamic acid decarboxylase 65 (GAD65) and GAD67, respectively. Compared to sham controls, an increased density of neurons heavily labeled for GAD67 mRNA was observed in the anterior striatum of kindled rats when cells were counted 6 weeks after the last kindled seizure. This subgroup of striatal GABAergic neurons has been suggested previously to correspond to the medium-sized aspiny interneurons in the striatum, indicating that kindling is associated with an increased activity of these neurons. Our previous finding of reduced GAD and GABA levels in synaptosomes isolated from the substantia nigra of kindled rats together with the present observation of increased density of GABAergic striatal interneurons in such rats suggest that kindling affects the regulation of the GABAergic projections from the striatum to the substantia nigra rather than directly damaging GABAergic neurons in the striatum.

Epilepsy induced by extended amygdala-kindling in rats: lack of clear association between development of spontaneous seizures and neuronal damage

Most patients with temporal lobe epilepsy (TLE), the most common type of epilepsy, show pronounced loss of neurons in limbic brain regions, including the hippocampus, amygdala, and parahippocampal regions. Hippocampal damage in patients with TLE is characterized by extensive neuronal loss in the CA3 and CA1 sectors and the hilus of the dentate gyrus. There is a long and ongoing debate on whether this type of hippocampal damage, referred to as hippocampal sclerosis, is the cause or consequence of TLE. Furthermore, hippocampal damage may contribute to the progressive features of TLE. The present study was designed to determine whether development of spontaneous recurrent seizures (SRS) after extended kindling of the amygdala in rats is associated with neuronal damage. The kindling model of TLE was chosen because previous studies have shown that only part of the rats develop SRS after extended kindling, thus allowing to compare the brain pathology of rats that received the same number of amygdala stimulation but did or did not develop SRS. For extended kindling, rats were stimulated twice daily 3-5 days a week for up to about 280 stimulations. During long-term EEG/video monitoring, SRS were observed in 50% of the rats over the period of extended kindling. SRS often started with myoclonic jerks or focal seizures and subsequently progressed into secondarily generalized seizures, so that the development of SRS recapitulated the earlier kindling of elicited seizures. No obvious neurodegeneration was observed in the CA1 and CA3 sectors of the hippocampus, the amygdala, parahippocampal regions or thalamus. A significant bilateral reduction in neuronal density was determined in the dentate hilus after extended kindling, but this reduction in hilar cell density did not significantly differ between rats with and without observed SRS. Determination of the total number of hilar neurons and of hilar volume indicated that the reduced neuronal density in the dentate hilus was due to expansion of hilar area but not to neuronal damage. The data demonstrate that extended kindling does not cause any hippocampal damage resembling hippocampal sclerosis, but that SRS develop in the absence of such damage.

Kindling causes persistent in vivo changes in firing rates and glutamate sensitivity of central piriform cortex neurons in rats

The present experiments were undertaken to study whether amygdala kindling induces persistent alterations in the functional status of neurons of the central piriform cortex, a subregion of the piriform cortex identified previously as a site involved in the kindling process. Extracellular, single-unit recordings of piriform cortex neurons were made in anesthetized fully kindled rats at an interval of at least five weeks after the last seizure. Electrode implanted but not kindled rats served as sham controls. An additional group of non-implanted rats was used as naive controls. Spontaneously firing piriform cortex neurons were characterized in all groups by smooth, sharp, biphasic (i.e. positive/negative) action potentials with a duration of 0.8-1.8 ms, and were primarily located at the border between piriform cortex layers II and III. In kindled rats, neurons in the central piriform cortex exhibited a significantly higher firing rate compared to controls. Based on median group values, the increase in basal activity in kindled rats averaged about 90%. The responsiveness of piriform cortex neurons to neurotransmitters was tested by microiontophoretic application of glutamate, N-methyl-D-aspartate and GABA. Piriform cortex neurons of kindled rats exhibited a significantly lower responsiveness to the excitatory effect of glutamate than naive controls. A lowered glutamate responsiveness was also seen in sham controls. No significantly altered transmitter sensitivities of piriform cortex neurons from kindled rats were seen with N-methyl-D-aspartate or GABA. The data indicate that amygdala kindling causes persistent interictal changes in both basal activity and glutamate responsiveness of central piriform cortex neurons which could contribute to the abnormal hyperexcitability characteristic of kindling.

Effect of depth electrode implantation with or without subsequent kindling on GABA turnover in various rat brain regions

Kindling is a chronic model of epilepsy characterized by a progressive increase in response to the same regularly applied electrical stimulus. The biological basis of the kindling phenomenon requires to be determined, but several studies indicate that impairment of GABAergic inhibition may be involved. In the present experiments, GABA turnover was determined in vivo by the GABA aminotransferase (GABA-T) inhibition method in 13 brain regions in three groups of rats: (1) a group which was kindled via electrical stimulation of intra-amygdala electrodes and was sacrificed 36 days after the last fully kindled seizure for neurochemical determinations; (2) a group of implanted but non-stimulated rats (sham control group) in which neurochemical measurements were done at the same time after electrode implantation as in the kindled group; and (3) a group of non-implanted, naive control rats. Regional GABA levels were determined after vehicle injection as well as 30 and 90 min after administration of aminooxyacetic acid (AOAA) at a dose which completely inhibits GABA-T. Compared to naive controls, prolonged electrode implantation in the amygdala induced a significant reduction of AOAA-induced GABA accumulation in amygdala, hippocampus, piriform cortex, olfactory bulb, frontal cortex, striatum, hypothalamus, tectum, and cerebellar cortex. In view of the GABA hypothesis of kindling, reduced GABA turnover in response to electrode implantation would suggest that the implantation per se exerts a pro-kindling effect, which was recently demonstrated in rats with intraamygdala electrodes. However, amygdala kindling itself appeared to antagonize the effect of electrode implantation in most regions. Thus, although, compared to naive controls, the predominant change in kindled rats was a decrease in GABA turnover, this decrease was less marked than in sham controls. In thalamus and brainstem kindling markedly increased GABA turnover above the levels determined in both naive and sham controls, possibly in response to impaired postsynaptic GABAergic function. The data indicate that both electrode implantation and kindling significantly alter regional GABA turnover, which might contribute to the pathophysiology of the kindling phenomenon. Furthermore, the data substantiate that the choice of adequate controls is critical in neurochemical and functional studies on the kindling phenomenon.

Chronic electrode implantation entails epileptiform field potentials in rat hippocampal slices, similarly to amygdala kindling

Evoked field potentials were recorded in the CA3, CA1 and dentate gyrus (DG) of hippocampal slices from amygdala kindled, non-stimulated amygdala electrode-implanted, and non-implanted age-matched rats to evaluate the consequences on hippocampal neuronal networks of kindling stimulation versus electrode implantation. No overt modification of field potentials was detected in either the CA1 or the DG areas. In contrast, a very significant increase in the occurrence of repetitive population spikes evoked by single stimuli was detected in the CA3 area in slices from both amygdala kindled and non-stimulated amygdala implanted rats. The epileptiform pattern of CA3 field potentials was at least as well expressed in implanted non-stimulated, as in kindled rats, suggesting that electrode implantation has a major contribution to this marker of epileptogenesis.

Bidirectional transfer between electrical and flurothyl kindling in mice: evidence for common processes in epileptogenesis

PURPOSE

This study sought to determine whether there was a transfer of seizure susceptibility between two models of epileptogenesis, electrical kindling and a newly described model of flurothyl kindling. In this study, we determined the effects of preexposure to one kindling agent on the seizure responsiveness to the other.

METHODS

Mice were divided into three groups: (a) six mice (FLK) were kindled with flurothyl, rechallenged with flurothyl after a 28-day incubation phase, implanted with olfactory bulb (OB) electrodes, and electrically kindled; (b) six mice (ELK) were implanted with OB electrodes, electrically kindled to six stage 5 seizures, and given one flurothyl trial 3 days later and a second flurothyl trial after a 28-day incubation period; and (c) six mice (IMP) were implanted with OB electrodes, tested with flurothyl at the same times as the ELK group, and later electrically kindled.

RESULTS

Mice that were previously kindled with flurothyl (FLK) had significantly faster electrical kindling rates to one stage 5 seizure or to six stage 5 seizures compared with animals in the ELK and IMP groups. Mice that were previously exposed to either electrical kindling or flurothyl kindling had significantly diminished latencies to generalized seizure onset (flurothyl-induced seizure thresholds) either before or after a 28-day incubation period compared with the IMP control mice. In addition, both the FLK and ELK groups had significantly increased percentages of mice expressing forebrain-brainstem seizures, compared with the IMP group, following either rechallenge with flurothyl after a 28-day incubation or focal electrical kindling.

CONCLUSIONS

These findings indicate a near-complete bidirectional transfer between these electrical and flurothyl kindling models. Mice that were previously exposed to either electrical or flurothyl kindling have increased seizure susceptibilities and altered seizure phenotypes when exposed to the other seizure paradigm. Overall, these studies indicate that previous seizures are the critical determinant of the bidirectional transfer of seizure susceptibility observed, and not the electrical or pharmacologic properties of the original kindling agent. Finally, the observation of near identity in transfer characteristics between electrical and flurothyl kindling models suggests that the proepileptogenic processes initiated by exposure to either model are similar.

Electrical but not chemical kindling increases sensitivity to some phencyclidine-like behavioral effects induced by the competitive NMDA receptor antagonist D-CPPene in rats

We have previously reported that a competitive N-methyl-D-aspartate (NMDA) receptor antagonist, DL-[E]-2-amino-4-methyl-5-phosphono-3-pentenoic acid (CGP 37849), produces stereotyped behaviors and hyperlocomotion in amygdala kindled rats at doses which do not induce such phencyclidine (PCP)-like behaviors in nonkindled rats, indicating that kindling predisposes rats to such adverse effects of competitive NMDA receptor antagonists. From these data we predicted that epileptic patients may exhibit a hypersensitivity to PCP-like adverse effects of competitive NMDA receptor antagonists, which was subsequently confirmed in a clinical trial with D-CPPene (SDZ EAA-494; 3-(2-carboxypiperazine-4-yl)propenyl-1-phosphonate). For further exploration of the functional alterations in NMDA receptor responsiveness produced by kindling, we studied whether the enhanced susceptibility of amygdala-kindled rats to PCP-like adverse effects of CGP 37849 is also observed with D-CPPene. Furthermore, we determined whether the enhanced susceptibility of kindled rats to such adverse effects occurs only after relatively short intervals following the last seizure, as used in our previous study, or is a more permanent phenomenon. For this purpose, we compared adverse effects in kindled rats not only with naive (non-implanted) controls, as done in our previous study, but used electrode-implanted nonkindled rats as an additional control to assess the possible bias of mere electrode-implantation. In addition, we studied whether the enhanced susceptibility to NMDA receptor antagonists of electrically kindled rats is also present in chemically kindled animals. In some experiments, the PCP-like uncompetitive NMDA receptor antagonist MK-801 (dizocilpine) was included for comparison. In amygdala kindled rats, D-CPPene produced significantly more stereotyped behaviors than in electrode-implanted or naive nonkindled controls. The enhanced sensitivity of electrically kindled rats to PCP-like stereotypies induced by D-CPPene was observed both 7 and 180 days after the last kindled seizure, indicating a long-lasting if not permanent hypersensitivity to these adverse effects. In addition, more intense circling was observed in amygdala kindled rats, whereas hyperlocomotion only tended to be more intense after D-CPPene in kindled rats. These alterations in D-CPPene-induced behaviors were not observed after chemical kindling with pentylenetetrazole, but D-CPPene induced significantly less hypothermia in chemically kindled rats both 7 and 70 days after the last seizure. The data demonstrate that kindling produces long-lasting alterations in some adverse effects of D-CPPene, substantiating that epileptogenesis as initiated by kindling renders the brain more susceptible to PCP-like behavioral side effects of competitive NMDA receptor antagonists.

Does prolonged implantation of depth electrodes predispose the brain to kindling?

Chronically implanted depth electrodes are widely used for the study of electrical signals generated in deep cerebral locations and for electrical stimulation of such locations. Although the effects of lesions resulting from electrode implantation are generally considered minimal, some reports have shown lasting neurochemical, histological, and behavioral alterations in response to such implantation. Furthermore, there is some evidence that prolonged electrode implantation may decrease the seizure threshold of the implanted region and increases the rate of kindling from this region. This prompted us to undertake a study on different periods of post-surgical delay to onset of electrical stimulation and subsequent characteristics of kindling development. Rats were implanted with a bipolar electrode in the basolateral amygdala, and the threshold for induction of focal paroxysmal activity (afterdischarge threshold, ADT) was determined after post-surgical recovery periods of either 1, 2, 4, or 8 weeks. The animals were then kindled by daily administration of an electrical stimulus until all rats exhibited fully kindled seizures. In fully kindled rats, the ADT was redetermined. Compared to animals with 1 week of electrode implantation, the pre-kindling ADT was significantly lower in rats with 2 and 4 weeks of electrode implantation, but returned towards the 1 week values at 8 weeks. An enhanced kindling rate was seen when kindling stimulations were started after 4 and 8 weeks of electrode implantation. Despite the marked differences in pre-kindling ADT, the post-kindling ADT was similar in the groups with 1, 2, or 4 weeks but significantly lower in the group with 8 weeks post-surgical delay to onset of testing. The data suggest that prolonged implantation of a bipolar electrode into a sensitive region of the limbic system predisposes the brain to kindling. Based on previous observation of iron deposits induced by electrode implantation and the epileptogenic effect of iron in cortical and limbic regions, we propose that the present observations are due to deposition of iron from hemoglobin destruction in local microhemorrhages caused by the implantation.

Kindling increases the sensitivity of rats to adverse effects of certain antiepileptic drugs

Development of novel antiepileptic drugs (AEDs) requires determining the margin between the desired anticonvulsant effect and undesired adverse effects (AE) (therapeutic index). For this purpose, drug-induced "minimal neurological deficits" (e.g., motor dysfunctions) are commonly quantified by simple tests, such as the rotarod test, in normal, i.e., nonepileptic animals. However, increasing evidence shows that chronic brain dysfunction associated with epilepsy may increase susceptibility to the AE of certain AEDs, e.g., N-methyl-D-aspartate (NMDA) receptor antagonists. The increased AE potential of such investigational drugs can be predicted by using kindled rats instead of normal rodents in preclinical drug evaluation studies. In the present experiments, we wished to determine whether kindled rats also exhibit an altered susceptibility to neurological adverse effects of standard AEDs, i.e., carbamazepine (CBZ), phenobarbital (PB), valproate (VPA), and diazepam (DZP). Abecarnil, a novel benzodiazepine (BZD) receptor agonist, was included in the study for comparison. All drugs were administered in diverse doses in kindled and nonkindled rats, and all behavioral alterations were scored in the cage and open field. Furthermore, the rotarod test was used to detect and quantify motor impairment induced by drug treatments. Kindled rats were more susceptible than nonkindled rats to motor impairment (ataxia and/or rotarod failures) induced by high doses of AEDs, although differences were noted between the drugs tested. VPA was the only drug that induced stereotyped behavior; it was much more potent in this respect in kindled than nonkindled rats. Abecarnil did not differ substantially in its AE in either subgroup of animals. Our data indicate that epileptogenesis induced by kindling renders the brain more susceptible to certain AE of AEDs.(ABSTRACT TRUNCATED AT 250 WORDS)

Transmitter amino acid levels in rat brain regions after amygdala-kindling or chronic electrode implantation without kindling: evidence for a pro-kindling effect of prolonged electrode implantation

Kindling is a chronic model of epilepsy characterized by a progressive increase in response to the same regularly applied stimulus. The biological basis of the kindling phenomenon requires to be determined, but several studies indicate that alterations in amino acidergic neurotransmission may be involved. In the present experiments, levels of glutamate, aspartate, GABA, glycine, and taurine were determined in 12 brain regions by HPLC in 3 groups of animals: (a) a group which was kindled via electrical stimulation of intraamygdala electrodes and was sacrificed 36 days after the last fully kindled seizure for neurochemical determinations; (b) a group of implanted but nonstimulated rats (surgical control group) in which neurochemical measurements were done at the same time after electrode implantation as the kindled group, and (c) a group of non-implanted, naive control rats. Compared to surgical controls, kindling induced a significant reduction of glutamate, GABA, and taurine in the brain stem (pons/medulla), whereas no differences between both groups were found in any of the other regions. However, both electrode-implanted groups differed significantly from non-implanted naive rats in several regions, indicating that electrode-implantation per se induced long-lasting alterations in transmitter amino acids. The most striking difference to naive controls was an increase of glycine levels in several regions in which this amino acid is known to potentiate glutamatergic transmission. In order to examine the functional consequences of prolonged electrode implantation, seizure thresholds were determined in groups of rats with short and prolonged electrode implantation.(ABSTRACT TRUNCATED AT 250 WORDS)

Kindling and electrode effects on the benzodiazepine receptors density of olfactory bulb and hippocampus after olfactory bulb kindling

The olfactory bulb (OB) kindling is a model of limbic secondary generalized epilepsy. Ten days after the completion of OB kindling, we have studied the long term effects of both electrode insertion and kindling on the binding of [3H]diazepam to crude mitochondrial fractions. On the one hand, we have shown that electrode implantation in sham-operated controls induced an obvious increase in benzodiazepine (BZD) receptor density (Bmax) only at the site of the electrode in comparison to sham-unoperated rats. These results might indicate an additional mechanism extending earlier observations reported by others, who have shown that prolonged electrode implantation induced changes in sham-operated and kindled rats. On the other hand, the long lasting effect of OB kindling on the binding parameters of [3H]diazepam was examined in the focus and in the hippocampus. The results indicate a bilateral increase of BZD receptors in the OB and an ipsilateral increase in the hippocampus. These changes might be a regulation phenomenon in response to a hyperexcitability state and to focal stimulations.

Facilitation of kindling by prior induction of long-term potentiation in the perforant path

Previous studies have revealed that a form of synaptic potentiation resembling long-term potentiation (LTP) occurs at various sites as a result of stimulation that leads to kindling. The present study evaluates what role this synaptic potentiation plays in the development of kindling following periodic stimulation of the entorhinal cortex of the rat. LTP was repetitively induced in the pathway from the entorhinal cortex (EC) to the dentate gyrus (DG) by daily stimulation with high frequency trains that led to LTP, but did not evoke afterdischarge (AD). Subsequently, animals received stimulation designed to induce kindling (that led to AD), and this stimulation was delivered once per day until kindled seizures were induced. While repetitive induction of LTP was not sufficient to produce kindling, prior induction of LTP significantly increased the rate of subsequent kindling as evidenced by a decrease in the number of kindling stimulations required to induce the kindled state. As a group, animals that had received stimulation designed to induce LTP developed kindled seizures after an average of 10 AD's, whereas a control group that had received non-potentiating stimulation required 25 AD's. These results indicate that LTP at EC-DG synapses cannot represent the mechanism of kindling following EC stimulation. However, synaptic potentiation at this site can facilitate the development of epileptogenesis in response to subsequent activation of the perforant path.

Down-regulation of alpha 2- and beta-adrenoceptor binding sites in rat cortex caused by amygdalar kindling

The role of central noradrenergic neurons in kindled seizures was assessed by comparison of alpha 2- and beta-adrenoceptor binding in the cerebral cortex from kindled and control rats. To minimize handling, which may modify kindling-induced changes in binding, the kindling protocol involved stimulation of the amygdala every hour for a maximum of 26 h. Twenty-four hours after kindling, down-regulation of beta-adrenoceptors was found in both olfactory cortex and the remaining neocortex, whereas alpha 2 down-regulation was confined to the olfactory cortex. At 21 days after kindling, the only change found was a down-regulation of beta-adrenoceptors in the neocortex. The results support the view that functional changes in central noradrenergic transmission are associated with the reduction in seizure threshold induced by kindling.

A study of the role of the cholinergic system in amygdaloid kindling in rats

The effect of atropine on kindling the amygdala of rats was tested by administering the drug in a dose of 25 mg/kg 1 h before each stimulus was applied. Rats tested with atropine kindled at the same rate as saline-treated controls. Cholinergic activity in the amygdala of rats was assessed, 4 weeks after the completion of kindling, by measuring both muscarinic receptor numbers and sodium-dependent high affinity choline uptake in tissue homogenates. There was no change in either of these parameters attributable to kindling. These results suggest that changes in the cholinergic system are not fundamental either to the development or the maintenance of kindling in the rat amygdala.