Genetically Epilepsy-prone Rats Research Articles

Divergent Effects of Systemic and Intracollicular CB Receptor Activation Against Forebrain and Hindbrain-Evoked Seizures in Rats.

Cannabinoid (CB) receptor agonists are of growing interest as targets for anti-seizure therapies. Here we examined the effect of systemic administration of the CB receptor agonist WIN 55,212-2 (WIN) against audiogenic seizures (AGSs) in the Genetically Epilepsy Prone Rat (GEPR)-3 strain, and against seizures evoked focally from the Area Tempestas (AT). We compared these results to the effect of focal administration of the CB1/2 receptor agonist CP 55940 into the deep layers of the superior colliculus (DLSC), a brain site expressing CB1 receptors. While systemic administration of WIN dose-dependently decreased AGS in GEPR-3s, it was without effect in the AT model. By contrast, intra-DLSC infusion of CP 55940 decreased seizures in both models. To determine if the effects of systemic WIN were dependent upon activation of CB1 receptors in the DSLC, we next microinjected the CB1 receptor antagonist SR141716, before WIN systemic treatment, and tested animals for AGS susceptibility. The pretreatment of the DLSC with SR141716 was without effect on its own and did not alter the anti-convulsant action of WIN systemic administration. Thus, while CB receptors in the DLSC are a potential site of anticonvulsant action, they are not necessary for the effects of systemically administered CB agonists.

The Acoustic Range Electric-magnetic Field Depresses Behavioral Seizure Activity

In the present study, we examined the effect of acoustic range of electric-magnetic field (EMF) on the behavioral manifestation of seizure in genetically epilepsy-prone rats (GEPRs) of Krushinsky-Molodkina strain. A 5-days exposure to EMF (10000 -15000 Hz frequency, 1,5 m/Tesla, during 20 min) resulted in partial or complete suppression of behavior seizure activity in GEPRs. Besides, on the background of EMF the latency period of first wild run was increased. On the same conditions, duration of wild run decreased. Audiogenic stimuli obtained in GEPRs changed the ECoG activity of sensomotoral cortex only in audiogenally-kindled animals. Findings of this study suggest that the regulation of the behavioral manifestation of seizure in GEPRs does not involve the cortex; it is mainly regulated by brainstem structures. We concluded that acoustic range of EMF could apply for suppression of behavioral manifestation of seizure. Audiogenic stimuli obtained in GEPRs changed the ECoG activity of sensomotoral cortex only in audiogenally-kindled animals. Findings of this study suggest that the regulation of the behavioral manifestation of seizure in GEPRs does not involve the cortex; it is mainly regulated by brainstem structures. Rostral part of cortex has significant role in manifestation of seizure activity in audiogenly kindling GEPRs. We concluded that acoustic range of EMF could apply for suppression of behavioral manifestation of seizure.

The Electromagnetic Field Changes on Emotional-motivated Behavior in Genetically Epilepsy-prone Rats

In the present study, we examined the effects of acoustic range electromagnetic field (EMF) on emotional-motivated behavior in genetically epilepsy-prone rats (GEPRs) of Krushinsky-Molodkina strain in Open field test. A 5-days exposure by EMF (10 -15 kHz, 1,5 mlTl, duration 20 min) resulted in partial or complete suppression of behavior seizure activity in GEPRs. The anxiety degree in irradiated GEPRs versus the controls (GEPRs without EMF exposure) was decreased, whereas the locomotion/exploratory activity was increased. A 5-days exposure by EMF in inbred white rats also changed the locomotion/exploratory activity in Open field test. We conclude that acoustic range of EMF can apply for suppression of behavioral manifestation of the seizure. These data support a potential role of EMF exposure in changing emotion regulation of anxiety, but the mechanisms of such influence are not known. Proposed biological mechanisms in anti-seizure and anxiolytic effects of EMF exposure in GEPRs might include normalization of neuroendocrine, neurotransmitter, and/or neurotrophic factors.

Genetically epilepsy-prone rats (GEPRs) and DBA/2 mice: Two animal models of audiogenic reflex epilepsy for the evaluation of new generation AEDs.

This review summarizes the current knowledge about DBA/2 mice and genetically epilepsy-prone rats (GEPRs) and discusses the contribution of such animal models on the investigation of possible new therapeutic targets and new anticonvulsant compounds for the treatment of epilepsy. Also, possible chemical or physical agents acting as proconvulsant agents are described. Abnormal activities of enzymes involved in catecholamine and serotonin synthesis and metabolism were reported in these models, and as a result of all these abnormalities, seizure susceptibility in both animals is greatly affected by pharmacological manipulations of the brain levels of monoamines and, prevalently, serotonin. In addition, both genetic epileptic models permit the evaluation of pharmacodynamic and pharmacokinetic interactions among several drugs measuring plasma and/or brain level of each compound. Audiogenic models of epilepsy have been used not only for reflex epilepsy studies, but also as animal models of epileptogenesis. The seizure predisposition (epileptiform response to sound stimulation) and substantial characterization of behavioral, cellular, and molecular alterations in both acute and chronic (kindling) protocols potentiate the usefulness of these models in elucidating ictogenesis, epileptogenesis, and their mechanisms. This article is part of a Special Issue entitled "Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic".

An abnormal GABAergic system in the inferior colliculus provides a basis for audiogenic seizures in genetically epilepsy-prone rats.

In this review of neuroanatomical studies of the genetically epilepsy-prone rat (GEPR), three main topics will be covered. First, the number of GABAergic neurons and total neurons in the inferior colliculus of GEPRs will be compared to those of the nonepileptic Sprague-Dawley rat. Next, the number of small neurons in the inferior colliculus will be described in both developmental and genetic analyses of GEPRs and their backcrosses. Last, results from two types of studies on the propagation pathways for audiogenic seizures in GEPRs will be shown. Together, these studies demonstrate a unique GABAergic, small neuron defect in the inferior colliculus of GEPRs that may play a vital role in the initiation and spread of seizure activity during audiogenic seizures. This article is part of a Special Issue entitled "Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic".

Rhythm and blues: Animal models of epilepsy and depression comorbidity

Clinical evidence shows a strong, bidirectional comorbidity between depression and epilepsy that is associated with decreased quality of life and responsivity to pharmacotherapies. At present, the neurobiological underpinnings of this comorbidity remain hazy. To complicate matters, anticonvulsant drugs can cause mood disturbances, while antidepressant drugs can lower seizure threshold, making it difficult to treat patients suffering from both depression and epilepsy. Animal models have been created to untangle the mechanisms behind the relationship between these disorders and to serve as screening tools for new therapies targeted to treat both simultaneously. These animal models are based on chemical interventions (e.g. pentylenetetrazol, kainic acid, pilocarpine), electrical stimulations (e.g. kindling, electroshock), and genetic/selective breeding paradigms (e.g. genetically epilepsy-prone rats (GEPRs), genetic absence epilepsy rat from Strasbourg (GAERS), WAG/Rij rats, swim lo-active rats (SwLo)). Studies on these animal models point to some potential mechanisms that could explain epilepsy and depression comorbidity, such as various components of the dopaminergic, noradrenergic, serotonergic, and GABAergic systems, as well as key brain regions, like the amygdala and hippocampus. These models have also been used to screen possible therapies. The purpose of the present review is to highlight the importance of animal models in research on comorbid epilepsy and depression and to explore the contributions of these models to our understanding of the mechanisms and potential treatments for these disorders.

BAD-Dependent Regulation of Fuel Metabolism and KATP Channel Activity Confers Resistance to Epileptic Seizures

Neuronal excitation can be substantially modulated by alterations in metabolism, as evident from the anticonvulsant effect of diets that reduce glucose utilization and promote ketone body metabolism. We provide genetic evidence that BAD, a protein with dual functions in apoptosis and glucose metabolism, imparts reciprocal effects on metabolism of glucose and ketone bodies in brain cells. These effects involve phosphoregulation of BAD and are independent of its apoptotic function. BAD modifications that reduce glucose metabolism produce a marked increase in the activity of metabolically sensitive K(ATP) channels in neurons, as well as resistance to behavioral and electrographic seizures in vivo. Seizure resistance is reversed by genetic ablation of the K(ATP) channel, implicating the BAD-K(ATP) axis in metabolic control of neuronal excitation and seizure responses.

Post-ictal analgesia in genetically epilepsy-prone rats is induced by audiogenic seizures and involves cannabinoid receptors in the periaqueductal gray

Post-ictal depression of consciousness occurs after generalized convulsive seizures, and includes analgesia, lasting for hours after electrically or chemically induced seizures in animals. The brain sites and mechanisms, mediating post-ictal analgesia, are unclear. The ventrolateral periaqueductal gray (PAG) is an important neuronal network site for mediating analgesia and also in generalized seizures, particularly in genetically epilepsy-prone rats (GEPRs). Endocannabinoids are implicated in mediating analgesia in several brain sites, including the PAG, and generalized seizures result in endocannabinoid release. This study evaluated if post-ictal analgesia occurs in GEPRs, following audiogenic seizures (AGS), and whether this analgesia involves endocannabinoid actions in PAG. Analgesia was evaluated, using thermal stimulation to evoke nociception, measuring changes in paw withdrawal latencies (PWLs) induced by AGS. Endocannabinoid involvement in post-ictal analgesia in GEPRs was evaluated, using focal bilateral microinjection of a cannabinoid (CB1) receptor antagonist (AM251) into PAG. AGS induced a significant increase in PWLs, lasting for ≥ 120 min. Microinjection of AM251 (100 and 200, but not 50 pmol/side) into PAG significantly decreased post-ictal analgesia in GEPRs. Endocannabinoids are also known to activate transient receptor potential vanilloid (TRPV1) receptors, but PAG microinjection of a TRPV1 receptor antagonist (capsazepine) did not affect post-ictal analgesia in GEPRs. These results indicate that AGS in GEPRs induce post-ictal analgesia, which is the first observation of this phenomenon in a genetic epilepsy model. These findings suggest an important role of PAG in post-ictal analgesia. The results also suggest that CB1 receptors in PAG are critical for mediating post-ictal analgesia in GEPRs.

Effect of Hypothyroidism on Postnatal Conjunctival Development in Rats

Aim: To study the conjunctival development in hypothyroid genetically epilepsy-prone rats (GEPRs) with serum T3 and T4 significantly lower than in normal rats. Methods: A structural, ultrastructural and histochemical study on the conjunctival epithelium of GEPRs and of control Sprague-Dawley (SD) rats before and after eyelid opening, with particular regard to goblet cell differentiation. Results: From birth to day 12, no goblet cells were demonstrated on the conjunctival surface of both strains, so that the epithelium was formed only by a cuboidal basal layer and by a superficial layer of roundish or flattened cells. On day 16, after the eyelid opening, Alcian blue (AB)-positive goblet cells filled with homogeneous granules were demonstrated isolated, in GEPRs, or clustered, in SD rats, in both the fornices and palpebral conjunctiva. The epithelium showed a basal layer and many layers of flattened cells and was taller in SD rats (8–10 layers) than in GEPRs (6–7 layers). At 3 months, the epithelium in SD rats was higher with generally clustered goblet cells, whilst in GEPRs goblet cells were both isolated or clustered. In both strains, the goblet cells showed a marked AB/periodic acid-Schiff positivity all over the conjunctival surface and were filled with granules of different density. In both strains, goblet cells were absent at birth and their appearance, as AB-positive cells, was concomitant with eyelid opening. Conclusions: Hypothyroid rats showed a conjunctival development different than that of normothyroid rats for both epithelial and goblet cells. It appears that thyroid hormone imbalance may influence conjunctival development.

Anticonvulsant and convulsant effects of cocaine in genetically epilepsy‐prone rats

Cocaine produces dose‐dependent increases in locomotor activity which can manifest as seizures at high doses. Cocaine is a local anesthetic that also blocks the uptake of norepinephrine (NE), dopamine (DA) and serotonin (5‐HT). Antidepressants that block the uptake of NE and/or 5‐HT produce anticonvulsant effects in genetically epilepsy‐prone rats (GEPRs) at doses below doses producing seizures. The purpose of this study was to determine if cocaine was capable of producing anticonvulsant effects against sound‐induced seizures in the GEPR. Procaine (local anesthetic) imipramine (NE and 5‐HT uptake blockade) and bupropion (DA uptake blockade) effects were also characterized. Anticonvulsant and convulsant dose response curves were determined in moderate seizure (GEPR‐3s) and severe seizure (GEPR‐9s) GEPRs. Effective dose‐50s for anticonvulsant effects and convulsant dose‐50s were determined for all 4 drugs. Cocaine, procaine, imipramine and bupropion all produced anticonvulsant effects in GEPR‐3s and GEPR‐9s at doses lower than doses producing convulsions. These results suggest that cocaine produces anticonvulsant effects at low doses and convulsant effects at high doses through its local anesthetic and NE, DA and 5‐HT uptake blockade mechanisms.

Seizure susceptibility is associated with altered protein expression of voltage-gated calcium channel subunits in inferior colliculus neurons of the genetically epilepsy-prone rat

The inferior colliculus (IC) is the consensus site for seizure initiation in the genetically epilepsy-prone rat (GEPR). We have previously reported that the current density of high threshold voltage-activated (HVA) calcium (Ca2+) channels was markedly enhanced in IC neurons of the GEPR-3 (moderate seizure severity substrain of the GEPR). The present study examines whether subunit protein levels of HVA Ca2+ channels are altered in IC neurons that exhibit enhanced Ca2+ current density. Quantification shows that the levels of protein expression of the Ca2+ channel pore-forming α1D (L-type) and α1E subunits (R-type) were significantly increased in IC neurons of seizure-naive GEPR-3s (SN-GEPR-3s) compared to control Sprague–Dawley (SD) rats. Significant increases and decreases in the levels of protein expression of Ca2+ channel regulatory β3 and α2δ subunits occurred in IC neurons of SN-GEPR-3s compared to control SD rats, respectively. No changes occurred in the protein expression of Ca2+ channel pore-forming α1A (P/Q-type), α1B (N-type) and α1C (L-type) subunits in IC neurons of SN-GEPR-3s compared to control SD rats. A single seizure selectively enhanced protein expression of Ca2+ channel α1A subunits in IC neurons of GEPR-3s. Thus, up-regulation of Ca2+ channel α1D and α1E subunits may represent the molecular mechanisms for the enhanced current density of L- and R-type of HVA Ca2+ channels in IC neurons of the GEPR, and may contribute to the genetic basis of their enhanced seizure susceptibility. The up-regulation of Ca2+ channel α1A subunits induced by seizures may contribute to the increasing IC neuronal excitability that results from repetitive seizures in the GEPR.

Protein expression of small conductance calcium-activated potassium channels is altered in inferior colliculus neurons of the genetically epilepsy-prone rat

The genetically epilepsy-prone rat (GEPR) exhibits inherited predisposition to sound stimuli-induced generalized tonic-clonic seizures (audiogenic reflex seizures) and is a valid model to study the physiopathology of epilepsy. In this model, the inferior colliculus (IC) exhibits enhanced neuronal firing that is critical in the initiation of reflex audiogenic seizures. The mechanisms underlying IC neuronal hyperexcitability that leads to seizure susceptibility are not as yet fully understood. The present report shows that the levels of protein expression of SK1 and SK3 subtypes of the small conductance Ca 2+-activated K + channels were significantly decreased, while SK2 channel proteins were increased in IC neurons of seizure-naive GEPR-3s (SN-GEPR-3), as compared to control Sprague-Dawley rats. No significant change was found in the expression of BK channel proteins in IC neurons of SN-GEPR-3s. Single episode of reflex audiogenic seizures in the GEPR-3s did not significantly alter the protein expression of SK1–3 and BK channels in IC neurons compared to SN-GEPR-3s. Thus, downregulation of SK1 and SK3 channels and upregulation of SK2 channels provide direct evidence that these Ca 2+-activated K + channels play important roles in IC neuronal hyperexcitability that leads to inherited seizure susceptibility in the GEPR.

Effects of high frequency repetitive transcranial magnetic stimulation (rTMS) on ictogenesis in EL mouse

This review summarizes the current knowledge about DBA/2 mice and genetically epilepsy-prone rats (GEPRs) and discusses the contribution of such animal models on the investigation of possible new therapeutic targets and new anticonvulsant compounds for the treatment of epilepsy. Also, possible chemical or physical agents acting as proconvulsant agents are described. Abnormal activities of enzymes involved in catecholamine and serotonin synthesis and metabolism were reported in these models, and as a result of all these abnormalities, seizure susceptibility in both animals is greatly affected by pharmacological manipulations of the brain levels of monoamines and, prevalently, serotonin. In addition, both genetic epileptic models permit the evaluation of pharmacodynamic and pharmacokinetic interactions among several drugs measuring plasma and/or brain level of each compound. Audiogenic models of epilepsy have been used not only for reflex epilepsy studies, but also as animal models of epileptogenesis. The seizure predisposition (epileptiform response to sound stimulation) and substantial characterization of behavioral, cellular, and molecular alterations in both acute and chronic (kindling) protocols potentiate the usefulness of these models in elucidating ictogenesis, epileptogenesis, and their mechanisms.This article is part of a Special Issue entitled "Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic".

Localization of the serotonergic terminal fields modulating seizures in the genetically epilepsy-prone rat

Serotonin (5-HT) has been shown to exert antiepileptic effects in a variety of generalized convulsive seizure models, particularly the genetically epilepsy-prone rat (GEPR). The present study was designed to identify the region/site(s) where 5-HT exerts anticonvulsant effects in the GEPR-9, a model in which sound-evoked generalized tonic-clonic seizures (GTCS) are highly sensitive to manipulations in 5-HT concentration. Because the 5-HT reuptake inhibitor, fluoxetine, was known to exert anticonvulsant effects in GEPR-9s via a 5-HT-dependent mechanism, we utilized selective regional 5-HT depletion in combination with systemic fluoxetine administration to find the site where a 5-HT deficit would prevent the anticonvulsant action of fluoxetine. Widespread destruction of serotonergic terminal fields or regionally specific terminal field destruction was achieved using intracerebroventricular and more target specific infusions of 5,7-dihydroxytryptamine. The capacity of fluoxetine to suppress seizures in GEPR-9s following a loss of 5-HT was then examined. The present findings show the anticonvulsant action of fluoxetine is markedly attenuated following the loss of midbrain 5-HT, particularly in the region of the superior colliculus, while forebrain and spinal cord 5-HT do not appear to play a role in the action of fluoxetine. The importance of the deep layers of the SC was confirmed by demonstrating that direct microinfusion of fluoxetine into the SC can suppress seizures in rats pretreated with the 5-HT(1A) receptor antagonist pindolol.

Seizures Beget Seizures: A Lack of Experimental Evidence and Clinical Relevance Fails to Dampen Enthusiasm

Three Brief Epileptic Seizures Reduce Inhibitory Synaptic Currents, GABAACurrents, and GABAA-Receptor Subunits. Evans MS, Cady CJ, Disney KE, Yang L, LaGuardia JJ. Epilepsia 2006;4710):1655–1664. PURPOSE: Cellular mechanisms activated during seizures may exacerbate epilepsy. γ-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in brain, and we hypothesized that brief epileptic seizures may reduce GABA function. METHODS: We used audiogenic seizures (AGSs) in genetically epilepsy-prone rats (GEPRs) to investigate effects of seizures on GABA-mediated inhibition in the presence of epilepsy. GEPRs are uniformly susceptible to AGSs beginning at 21 postnatal days. AGSs are brief convulsions lasting 20 s, and they begin in inferior colliculus (IC). We evoked three seizures in GEPRs and compared the results with those in seizure-naive GEPRs and nonepileptic Sprague-Dawley (SD) rats, the GEPR parent strain. RESULTS: Whole-cell recording in IC slices showed that GABA-mediated monosynaptic inhibitory postsynaptic currents (IPSCs) were reduced 55% by three brief epileptic seizures. Whole-cell recording in IC neuronal cultures showed that currents elicited by GABA were reduced 67% by three seizures. Western blotting for the alpha1 and alpha4 subunits of the GABAA receptor showed no statistically significant effects. In contrast, three brief epileptic seizures reduced gamma2 subunit levels by 80%. CONCLUSIONS: The effects of the very first seizures, in animals known to be epileptic, in an area of brain known to be critical to the seizure network, were studied. The results indicate that even brief epileptic seizures can markedly reduce IPSCs and GABA currents and alter GABAA-receptor subunit protein levels. The cause of the reductions in IPSCs and GABA currents is likely to be altered receptor subunit composition, with reduced gamma2 levels causing reduced GABAA-receptor sensitivity to GABA. Seizure-induced reductions in GABA-mediated inhibition could exacerbate epilepsy.

Three Brief Epileptic Seizures Reduce Inhibitory Synaptic Currents, GABAA Currents, and GABAA‐Receptor Subunits

Cellular mechanisms activated during seizures may exacerbate epilepsy. gamma-Aminobutyric acid (GABA) is the major inhibitory neurotransmitter in brain, and we hypothesized that brief epileptic seizures may reduce GABA function. We used audiogenic seizures (AGSs) in genetically epilepsy-prone rats (GEPRs) to investigate effects of seizures on GABA-mediated inhibition in the presence of epilepsy. GEPRs are uniformly susceptible to AGSs beginning at 21 postnatal days. AGSs are brief convulsions lasting approximately 20 s, and they begin in inferior colliculus (IC). We evoked three seizures in GEPRs and compared the results with those in seizure-naive GEPRs and nonepileptic Sprague-Dawley (SD) rats, the GEPR parent strain. Whole-cell recording in IC slices showed that GABA-mediated monosynaptic inhibitory postsynaptic currents (IPSCs) were reduced 55% by three brief epileptic seizures. Whole-cell recording in IC neuronal cultures showed that currents elicited by GABA were reduced 67% by three seizures. Western blotting for the alpha1 and alpha4 subunits of the GABA(A) receptor showed no statistically significant effects. In contrast, three brief epileptic seizures reduced gamma2 subunit levels by 80%. The effects of the very first seizures, in animals known to be epileptic, in an area of brain known to be critical to the seizure network, were studied. The results indicate that even brief epileptic seizures can markedly reduce IPSCs and GABA currents and alter GABA(A)-receptor subunit protein levels. The cause of the reductions in IPSCs and GABA currents is likely to be altered receptor subunit composition, with reduced gamma2 levels causing reduced GABA(A)-receptor sensitivity to GABA. Seizure-induced reductions in GABA-mediated inhibition could exacerbate epilepsy.

Brainstem Seizure Severity Regulates Forebrain Seizure Expression in the Audiogenic Kindling Model

Although sound-induced (audiogenic) seizures in the genetically epilepsy-prone rat (GEPR) initially occur independent of the forebrain, repeated audiogenic seizures recruit forebrain seizure circuits in a process referred to as audiogenic kindling. In GEPR-3s, audiogenic kindling results in facial and forelimb (F&F) clonic seizures that are typical of forebrain seizures. However, in GEPR-9s, audiogenic kindling produces posttonic all-limb clonus not usually observed during forebrain seizures. We hypothesized that the more severe brainstem seizures of the GEPR-9 prevent the expression of F&F clonic seizures during audiogenic kindling. Therefore attenuation of audiogenic seizures during audiogenic kindling in GEPR-9s should allow F&F clonic seizures to be expressed. Likewise, intensifying audiogenic seizure severity in GEPR-3s should inhibit audiogenically kindled F&F clonic seizures. We have tested this hypothesis in the present study. Lesions of the superior colliculus or treatment with low-dose phenytoin were used to suppress audiogenic seizure severity in GEPR-9s. Depletion of brain serotonin was used to increase the seizure severity in GEPR-3s. All GEPRs were then subjected to audiogenic kindling. Behavioral and electrographic seizures were assessed. Suppression of audiogenic seizure severity during audiogenic kindling in GEPR-9s increased the incidence forebrain seizure behavior. Kindled GEPR-9s that continued to display full tonic seizures did not exhibit forebrain convulsions, but did show posttonic clonus and forebrain seizure activity in the EEG. GEPR-3s chronically depleted of brain serotonin, along with displaying tonic brainstem seizures, tended to display less severe forebrain seizures during audiogenic kindling. These findings support the concept that severe brainstem seizures prevent the behavioral expression of forebrain seizures in audiogenically kindled GEPR-9s. It appears that the severe brainstem seizure of the GEPR-9 does not allow the forebrain seizure to manifest its typical behavioral concomitants despite electrographic evidence that spike-wave discharge is occurring in the forebrain.

Anticonvulsant effects of carbenoxolone in genetically epilepsy prone rats (GEPRs)

Carbenoxolone (CBX), the succinyl ester of glycyrrhetinic acid, is an inhibitor of gap junctional intercellular communication. Systemic administration of CBX was able to decrease the seizure severity score and to increase the latency time of seizure onset in genetically epilepsy prone rats (GEPRs). In particular, intravenous or intraperitoneal administration of carbenoxolone (5–30 mg/kg) produced a dose-dependent and significant reduction in the clonic and tonic phases of the audiogenic seizures in GEPRs. The anticonvulsant doses were not associated with an impairment of motor coordination. The bilateral microinjection of CBX (0.001–0.50 μg/0.5 μl) into the inferior colliculi, the substantia nigra (pars reticulata or compacta) and the inferior olivary complex was able to reduce the seizure severity score in a dose-dependent manner. The anticonvulsant effects were longer lasting after focal microinjection than after systemic administration. No anticonvulsant effects were observed following focal bilateral microinjections of glycyrrhizin into the same brain areas where CBX was shown to be effective.

Comparative fos immunoreactivity in the brain after forebrain, brainstem, or combined seizures induced by electroshock, pentylenetetrazol, focally induced and audiogenic seizures in rats

To help discern sites of focal activation during seizures of different phenotype, the numbers of Fos immunoreactive (FI) neurons in specific brain regions were analyzed following “brainstem-evoked,” “forebrain-evoked” and forebrain/brainstem combination seizures induced by a variety of methods. First, pentylenetetrazol (PTZ, 50 mg/kg) induced forebrain-type seizures in some rats, or forebrain seizures that progressed to tonic/clonic brainstem-type seizures in other rats. Second, minimal electroshock induced forebrain seizures whereas maximal electroshock (MES) induced tonic brainstem-type seizures in rats. Third, forebrain seizures were induced in genetically epilepsy-prone rats (GEPRs) by microinfusion of bicuculline into the area tempestas (AT), while brainstem seizures in GEPRs were induced by audiogenic stimulation. A final set was included in which AT bicuculline-induced forebrain seizures in GEPRs were transiently interrupted by audiogenic seizures (AGS) in the same animals. These animals exhibited a sequence combination of forebrain clonic seizure, brainstem tonic seizure and back to forebrain clonic seizures. Irrespective of the methods of induction, clonic forebrain- and tonic/clonic brainstem-type seizures were associated with considerable Fos immunoreactivity in several forebrain structures. Tonic/clonic brainstem seizures, irrespective of the methods of induction, were also associated with FI in consistent brainstem regions. Thus, based on Fos numerical densities (FND, numbers of Fos-stained profiles), forebrain structures appear to be highly activated during both forebrain and brainstem seizures; however, facial and forelimb clonus characteristic of forebrain seizures are not observable during a brainstem seizure. This observation suggests that forebrain-seizure behaviors may be behaviorally masked during the more severe tonic brainstem seizures induced either by MES, PTZ or AGS in GEPRs. This suggestion was corroborated using the sequential seizure paradigm. Similar to findings using MES and PTZ, forebrain regions activated by AT bicuculline were similar to those activated by AGS in the GEPR. However, in the combination seizure group, those areas that showed increased FND in the forebrain showed even greater FND in the combination trial. Likewise, those areas of the brainstem showing FI in the AGS model, showed an even greater effect in the combination paradigm. Finally, the medial amygdala, ventral hypothalamus and cortices of the inferior colliculi showed markedly increased FND that appeared dependent upon activation of both forebrain and brainstem seizure activity in the same animal. These findings suggest these latter areas may be transitional areas between forebrain and brainstem seizure interactions. Collectively, these data illustrate a generally consistent pattern of forebrain Fos staining associated with forebrain-type seizures and a consistent pattern of brainstem Fos staining associated with brainstem-type seizures. Additionally, these data are consistent with a notion that separate seizure circuitries in the forebrain and brainstem mutually interact to facilitate one another, possibly through involvement of specific “transition mediating” nuclei.

Glutamatergic activation of the amygdala differentially mimics the effects of audiogenic seizure kindling in two substrains of genetically epilepsy-prone rats

Comparisons of neuronal network mechanisms in closely related inherited seizure models are providing novel insights into epileptogenic pathophysiology. Genetically epilepsy-prone rats (GEPRs) exist in two substrains that inherit long-term susceptibility to behaviorally distinct audiogenic seizures (AGS). GEPR-3s exhibit generalized clonic AGS, while GEPR-9s exhibit generalized tonic AGS. After AGS kindling the tonic AGS of GEPR-9s is followed by generalized posttonic clonus (PTC), while the generalized clonic AGS is followed by facial and forelimb (F&F) clonus in GEPR-3s. PTC and F&F clonus are very rare in GEPRs before AGS kindling. The neuronal network subserving AGS in GEPR-9s lies exclusively in brainstem sites, but amygdala (AMG) and other sites are recruited into the network after AGS kindling. The present study attempted to mimic the effects of AGS kindling by bilaterally microinjecting subconvulsive doses of N-methyl- d-aspartate (NMDA) into the AMG of nonkindled GEPRs. NMDA (10 nmol/side) microinjected into AMG reversibly induced susceptibility to F&F clonus immediately following generalized clonic AGS in most nonkindled GEPR-3s. NMDA (7.5 nmol/side), microinjected into AMG temporarily induced susceptibility to generalized PTC immediately following tonic AGS in most nonkindled GEPR-9s. No seizures were induced in normal rats by these treatments, and no seizures were seen in GEPRs with these NMDA doses except those induced by acoustic stimuli. These findings support a critical role in AGS kindling for the AMG in the neuronal networks for both forms of AGS. However, the behavioral effect of the treatment was different in the two AGS substrains, suggesting interrelated but not identical pathophysiological mechanisms in these closely related epilepsy models.