Glutamate in the inferior colliculus plays a critical role in audiogenic seizure initiation

Delving into the significance of the His289Tyr single-nucleotide polymorphism in the glutamate ionotropic receptor kainate-1 (Grik1) gene of a genetically audiogenic seizure model

Genetic abnormalities affecting glutamate receptors are central to excitatory overload-driven neuronal mechanisms that culminate in seizures, making them pivotal targets in epilepsy research. Increasingly used to advance this field, the genetically audiogenic seizure hamster from Salamanca (GASH/Sal) exhibits generalized seizures triggered by high-intensity acoustic stimulation and harbors significant genetic variants recently identified through whole-exome sequencing. Here, we addressed the influence of the missense single-nucleotide polymorphism (C9586732T, p.His289Tyr) in the glutamate receptor ionotropic kainate-1 (Grik1) gene and its implications for the GASH/Sal seizure susceptibility. Using a protein 3D structure prediction, we showed a potential effect of this sequence variation, located in the amino-terminal domain, on the stability and/or conformation of the kainate receptor subunit-1 protein (GluK1). We further employed a multi-technique approach, encompassing gene expression analysis (RT-qPCR), Western blotting, and immunohistochemistry in bright-field and confocal fluorescence microscopy, to investigate critical seizure-associated brain regions in GASH/Sal animals under seizure-free conditions compared to matched wild-type controls. We detected disruptions in the transcriptional profile of the Grik1 gene within the audiogenic seizure-associated neuronal network. Alterations in GluK1 protein levels were also observed in various brain structures, accompanied by an unexpected lower molecular weight band in the inferior and superior colliculi. This correlated with substantial disparities in GluK1-immunolabeling distribution across multiple brain regions, including the cerebellum, hippocampus, subdivisions of the inferior and superior colliculi, and the prefrontal cortex. Notably, the diffuse immunolabeling accumulated within perikarya, axonal fibers and terminals, exhibiting a prominent concentration in proximity to the cell nucleus. This suggests potential disturbances in the GluK1-trafficking mechanism, which could subsequently affect glutamate synaptic transmission. Overall, our study sheds light on the genetic underpinnings of seizures and underscores the importance of investigating the molecular mechanisms behind synaptic dysfunction in epileptic neural networks, laying a crucial foundation for future research and therapeutic strategies targeting GluK1-containing kainate receptors.

Transcriptome of the Krushinsky-Molodkina Audiogenic Rat Strain and Identification of Possible Audiogenic Epilepsy-Associated Genes

Audiogenic epilepsy (AE), inherent to several rodent strains is widely studied as a model of generalized convulsive epilepsy. The molecular mechanisms that determine the manifestation of AE are not well understood. In the present work, we compared transcriptomes from the corpora quadrigemina in the midbrain zone, which are crucial for AE development, to identify genes associated with the AE phenotype. Three rat strains without sound exposure were compared: Krushinsky-Molodkina (KM) strain (100% AE-prone); Wistar outbred rat strain (non-AE prone) and “0” strain (partially AE-prone), selected from F2 KM × Wistar hybrids for their lack of AE. The findings showed that the KM strain gene expression profile exhibited a number of characteristics that differed from those of the Wistar and “0” strain profiles. In particular, the KM rats showed increased expression of a number of genes involved in the positive regulation of the MAPK signaling cascade and genes involved in the positive regulation of apoptotic processes. Another characteristic of the KM strain which differed from that of the Wistar and “0” rats was a multi-fold increase in the expression level of the Ttr gene and a significant decrease in the expression of the Msh3 gene. Decreased expression of a number of oxidative phosphorylation-related genes and a few other genes was also identified in the KM strain. Our data confirm the complex multigenic nature of AE inheritance in rodents. A comparison with data obtained from other independently selected AE-prone rodent strains suggests some common causes for the formation of the audiogenic phenotype.

Microglial MERTK eliminates phosphatidylserine-displaying inhibitory post-synapses

Glia contribute to synapse elimination through phagocytosis in the central nervous system. Despite the important roles of this process in development and neurological disorders, the identity and regulation of the "eat-me" signal that initiates glia-mediated phagocytosis of synapses has remained incompletely understood. Here, we generated conditional knockout mice with neuronal-specific deletion of the flippase chaperone Cdc50a, to induce stable exposure of phosphatidylserine, a well-known "eat-me" signal for apoptotic cells, on the neuronal outer membrane. Surprisingly, acute Cdc50a deletion in mature neurons causes preferential phosphatidylserine exposure in neuronal somas and specific loss of inhibitory post-synapses without effects on other synapses, resulting in abnormal excitability and seizures. Ablation of microglia or the deletion of microglial phagocytic receptor Mertk prevents the loss of inhibitory post-synapses and the seizure phenotype, indicating that microglial phagocytosis is responsible for inhibitory post-synapse elimination. Moreover, we found that phosphatidylserine is used for microglia-mediated pruning of inhibitory post-synapses in normal brains, suggesting that phosphatidylserine serves as a general "eat-me" signal for inhibitory post-synapse elimination.

Adrenergic Mechanisms of Audiogenic Seizure-Induced Death in a Mouse Model of SCN8A Encephalopathy

Sudden unexpected death in epilepsy (SUDEP) is the leading cause of death amongst patients whose seizures are not adequately controlled by current therapies. Patients with SCN8A encephalopathy have an elevated risk for SUDEP. While transgenic mouse models have provided insight into the molecular mechanisms of SCN8A encephalopathy etiology, our understanding of seizure-induced death has been hampered by the inability to reliably trigger both seizures and seizure-induced death in these mice. Here, we demonstrate that mice harboring an Scn8a allele with the patient-derived mutation N1768D (D/+) are susceptible to audiogenic seizures and seizure-induced death. In adult D/+ mice, audiogenic seizures are non-fatal and have nearly identical behavioral, electrographical, and cardiorespiratory characteristics as spontaneous seizures. In contrast, at postnatal days 20–21, D/+ mice exhibit the same seizure behavior, but have a significantly higher incidence of seizure-induced death following an audiogenic seizure. Seizure-induced death was prevented by either stimulating breathing via mechanical ventilation or by acute activation of adrenergic receptors. Conversely, in adult D/+ mice inhibition of adrenergic receptors converted normally non-fatal audiogenic seizures into fatal seizures. Taken together, our studies show that in our novel audiogenic seizure-induced death model adrenergic receptor activation is necessary and sufficient for recovery of breathing and prevention of seizure-induced death.

The Inferior Colliculus in Alcoholism and Beyond

Post-mortem neuropathological and in vivo neuroimaging methods have demonstrated the vulnerability of the inferior colliculus to the sequelae of thiamine deficiency as occurs in Wernicke-Korsakoff Syndrome (WKS). A rich literature in animal models ranging from mice to monkeys-including our neuroimaging studies in rats-has shown involvement of the inferior colliculi in the neural response to thiamine depletion, frequently accomplished with pyrithiamine, an inhibitor of thiamine metabolism. In uncomplicated alcoholism (i.e., absent diagnosable neurological concomitants), the literature citing involvement of the inferior colliculus is scarce, has nearly all been accomplished in preclinical models, and is predominately discussed in the context of ethanol withdrawal. Our recent work using novel, voxel-based analysis of structural Magnetic Resonance Imaging (MRI) has demonstrated significant, persistent shrinkage of the inferior colliculus using acute and chronic ethanol exposure paradigms in two strains of rats. We speculate that these consistent findings should be considered from the perspective of the inferior colliculi having a relatively high CNS metabolic rate. As such, they are especially vulnerable to hypoxic injury and may be provide a common anatomical link among a variety of disparate insults. An argument will be made that the inferior colliculi have functions, possibly related to auditory gating, necessary for awareness of the external environment. Multimodal imaging including diffusion methods to provide more accurate in vivo visualization and quantification of the inferior colliculi may clarify the roles of brain stem nuclei such as the inferior colliculi in alcoholism and other neuropathologies marked by altered metabolism.

Morphological and molecular correlates of altered hearing sensitivity in the genetically audiogenic seizure-prone hamster GASH/Sal

Rodent models of audiogenic seizures, in which seizures are precipitated by an abnormal response of the brain to auditory stimuli, are crucial to investigate the neural bases underlying ictogenesis. Despite significant advances in understanding seizure generation in the inferior colliculus, namely the epileptogenic nucleus, little is known about the contribution of lower auditory stations to the seizure-prone network. Here, we examined the cochlea and cochlear nucleus of the genetic audiogenic seizure hamster from Salamanca (GASH/Sal), a model of reflex epilepsy that exhibits generalized tonic-clonic seizures in response to loud sound. GASH/Sal animals under seizure-free conditions were compared with matched control hamsters in a multi-technical approach that includes auditory brainstem responses (ABR) testing, histology, scanning electron microscopy analysis, immunohistochemistry, quantitative morphometry and gene expression analysis (RT-qPCR). The cochlear histopathology of the GASH/Sal showed preservation of the sensory hair cells, but a significant loss of spiral ganglion neurons and mild atrophy of the stria vascularis. At the electron microscopy level, the reticular lamina exhibited disarray of stereociliary tufts with blebs, loss or elongated stereocilia as well as non-parallel rows of outer hair cells due to protrusions of Deiters' cells. At the molecular level, the abnormal gene expression patterns of prestin, cadherin 23, protocadherin 15, vesicular glutamate transporters 1 (Vglut1) and -2 (Vglut2) indicated that the hair-cell mechanotransduction and cochlear amplification were markedly altered. These were manifestations of a cochlear neuropathy that correlated to ABR waveform I alterations and elevated auditory thresholds. In the cochlear nucleus, the distribution of VGLUT2-immunolabeled puncta was differently affected in each subdivision, showing significant increases in magnocellular regions of the ventral cochlear nucleus and drastic reductions in the granule cell domain. This modified inputs lead to disruption of Vglut1 and Vglut2 gene expression in the cochlear nucleus. In sum, our study provides insight into the morphological and molecular traits associated with audiogenic seizure susceptibility in the GASH/Sal, suggesting an upward spread of abnormal glutamatergic transmission throughout the primary acoustic pathway to the epileptogenic region.

Inhibition of transient potential receptor vanilloid type 1 suppresses seizure susceptibility in the genetically epilepsy-prone rat

AIMS

Intracellular calcium plays an important role in neuronal hyperexcitability that leads to seizures. One calcium influx route of interest is the transient receptor potential vanilloid type 1 (TRPV1) channel. Here, we evaluated the effects of capsazepine (CPZ), a potent blocker of TRPV1 channels on acoustically evoked seizures (audiogenic seizures, AGS) in the genetically epilepsy-prone rat (GEPR-3), a model of inherited epilepsy.

METHODS

Male and female GEPR-3s were used. For the acute CPZ treatment study, GEPR-3s were tested for AGS susceptibility before and after treatment with various doses of CPZ (0, 1, 3, and 10 mg/kg; ip). For semichronic CPZ treatment study, GEPR-3s were tested for AGS susceptibility before and after 5-day CPZ treatment at the dose of 1 mg/kg (ip). The prevalence, latency, and severity of AGS were recorded and analyzed.

RESULTS

We found that acute CPZ pretreatment reduced the seizure severity in male GEPR-3s; the effect was dose-dependent. In female GEPR-3s, however, CPZ treatment completely suppressed the seizure susceptibility. Furthermore, semichronic CPZ treatment suppressed seizure susceptibility in female GEPR-3s, but only reduced the seizure severity in male GEPR-3s.

CONCLUSIONS

These findings suggest that the TRPV1 channel is a promising molecular target for seizure suppression, with female GEPR-3s exhibiting higher sensitivity than male GEPR-3s.

Identification of a Circadian Clock in the Inferior Colliculus and Its Dysregulation by Noise Exposure

Circadian rhythms regulate bodily functions within 24 h and long-term disruptions in these rhythms can cause various diseases. Recently, the peripheral auditory organ, the cochlea, has been shown to contain a self-sustained circadian clock that regulates differential sensitivity to noise exposure throughout the day. Animals exposed to noise during the night are more vulnerable than when exposed during the day. However, whether other structures throughout the auditory pathway also possess a circadian clock remains unknown. Here, we focus on the inferior colliculus (IC), which plays an important role in noise-induced pathologies such as tinnitus, hyperacusis, and audiogenic seizures. Using PER2::LUC transgenic mice and real-time bioluminescence recordings, we revealed circadian oscillations of Period 2 protein in IC explants for up to 1 week. Clock genes (Cry1, Bmal1, Per1, Per2, Rev-erbα, and Dbp) displayed circadian molecular oscillations in the IC. Averaged expression levels of early-induced genes and clock genes during 24 h revealed differential responses to day or night noise exposure. Rev-erbα and Dbp genes were affected only by day noise exposure, whereas Per1 and Per2 were affected only by night noise exposure. However, the expression of Bdnf was affected by both day and night noise exposure, suggesting that plastic changes are unlikely to be involved in the differences in day or night noise sensitivity in the IC. These novel findings highlight the importance of circadian responses in the IC and emphasize the importance of circadian mechanisms for understanding central auditory function and disorders.

SIGNIFICANCE STATEMENT

Recent findings identified the presence of a circadian clock in the inner ear. Here, we present novel findings that neurons in the inferior colliculus (IC), a central auditory relay structure involved in sound processing, express a circadian clock as evidenced at both the mRNA and protein levels. Using a reporter mouse that expresses a luciferase protein coupled to the core clock protein PERIOD2 (PER2::LUC), we could observe spontaneous circadian oscillations in culture. Furthermore, we reveal that the mRNA profile of clock-related genes in the IC is altered differentially by day or night noise exposure. The identification of a clock in the IC is relevant for understanding the mechanisms underlying dysfunctions of the IC such as tinnitus, hyperacusis, or audiogenic seizures.

Anticonvulsant effects of structurally diverse GABA(B) positive allosteric modulators in the DBA/2J audiogenic seizure test: Comparison to baclofen and utility as a pharmacodynamic screening model

The GABA(B) receptor has been indicated as a promising target for multiple CNS-related disorders. Baclofen, a prototypical orthosteric agonist, is used clinically for the treatment of spastic movement disorders, but is associated with unwanted side-effects, such as sedation and motor impairment. Positive allosteric modulators (PAM), which bind to a topographically-distinct site apart from the orthosteric binding pocket, may provide an improved side-effect profile while maintaining baclofen-like efficacy. GABA, the major inhibitory neurotransmitter in the CNS, plays an important role in the etiology and treatment of seizure disorders. Baclofen is known to produce anticonvulsant effects in the DBA/2J mouse audiogenic seizure test (AGS), suggesting it may be a suitable assay for assessing pharmacodynamic effects. Little is known about the effects of GABA(B) PAMs, however. The studies presented here sought to investigate the AGS test as a pharmacodynamic (PD) screening model for GABA(B) PAMs by comparing the profile of structurally diverse PAMs to baclofen. GS39783, rac-BHFF, CMPPE, A-1295120 (N-(3-(4-(4-chloro-3-fluorobenzyl)-6-methoxy-3,5-dioxo-4,5-dihydro-1,2,4-triazin-2(3H)-yl)phenyl)acetamide), and A-1474713 (N-(3-(4-(4-chlorobenzyl)-3,5-dioxo-4,5-dihydro-1,2,4-triazin-2(3H)-yl)phenyl)acetamide) all produced robust, dose-dependent anticonvulsant effects; a similar profile was observed with baclofen. Pre-treatment with the GABA(B) antagonist SCH50911 completely blocked the anticonvulsant effects of baclofen and CMPPE in the AGS test, indicating such effects are likely mediated by the GABA(B) receptor. In addition to the standard anticonvulsant endpoint of the AGS test, video tracking software was employed to assess potential drug-induced motor side-effects during the acclimation period of the test. This analysis was sensitive to detecting drug-induced changes in total distance traveled, which was used to establish a therapeutic index (TI = hypoactivity/anticonvulsant effects). Calculated TIs for A-1295120, CMPPE, rac-BHFF, GS39783, and A-1474713 were 5.31x, 5.00x, 4.74x, 3.41x, and 1.83x, respectively, whereas baclofen was <1. The results presented here suggest the DBA/2J mouse AGS test is a potentially useful screening model for detecting PD effects of GABA(B) PAMs and can provide an initial read-out on target-related motor side-effects. Furthermore, an improved TI was observed for PAMs compared to baclofen, indicating the PAM approach may be a viable therapeutic alternative to baclofen.

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.

Endogenous opioid peptide-mediated neurotransmission in central and pericentral nuclei of the inferior colliculus recruits μ1-opioid receptor to modulate post-ictal antinociception

BACKGROUND

The aim of the present work was to investigate the involvement of the μ1-endogenous opioid peptide receptor-mediated system in post-ictal antinociception.

METHODS

Antinociceptive responses were determined by the tail-flick test after pre-treatment with the selective μ1-opioid receptor antagonist naloxonazine, peripherally or centrally administered at different doses.

RESULTS

Peripheral subchronic (24 h) pre-treatment with naloxonazine antagonised the antinociception elicited by tonic-clonic seizures. Acute (10 min) pre-treatment, however, did not have the same effect. In addition, microinjections of naloxonazine into the central, dorsal cortical and external cortical nuclei of the inferior colliculus antagonised tonic-clonic seizure-induced antinociception. Neither acute (10-min) peripheral pre-treatment with naloxonazine nor subchronic intramesencephalic blockade of μ1-opioid receptors resulted in consistent statistically significant differences in the severity of tonic-clonic seizures shown by Racine's index (1972), although the intracollicular specific antagonism of μ1-opioid receptor decreased the duration of seizures.

CONCLUSION

μ1-Opioid receptors and the inferior colliculus have been implicated in several endogenous opioid peptide-mediated responses such as antinociception and convulsion. The present findings suggest the involvement of μ1-opiate receptors of central and pericentral nuclei of the inferior colliculus in the modulation of tonic-clonic seizures and in the organisation of post-ictal antinociception.

Differential susceptibility of astrocytic and neuronal function to 3-chloropropanediol in the rat inferior colliculus

We have previously shown that systemic administration of S(+)3-chloropropanediol (3-CPD) produces a morphological loss of astrocytes in specific nuclei of the rodent brain that precedes loss of both neurones and endothelial tight junctions. Here, we have evaluated the differential susceptibility of neuronal and astrocytic function to 3-CPD, in order to see if this parallels the morphological selectivity. To do this, we have developed an in vivo method for monitoring astrocyte function over time by giving hourly 20-min bolus challenge exposures to ammonia via an implanted microdialysis probe and measuring the resulting transient increases in the extracellular glutamine : glutamate ratio. These challenge ammonia exposures evoked a stable response for at least 5 h when the probe was implanted in the rat inferior colliculus, but caused no behavioural response or morphological damage. Although 3-CPD produced a rapid and sustained abolition of the ammonia response within 2 h, the field potential response of inferior collicular neurones to sound fell significantly to 75.0 ± 3.9% pre-dose at up to 8 h but then fell markedly, reaching 20.5 ± 3.7% at 2 days. Blood flow in the inferior colliculus also showed only late changes, increasing substantially at 2 days. Astrocyte damage at the EM level was seen from 3 h, followed by loss of astrocytes from 18 h to a minimum of 7 ± 10% control at 3 days. The rapid abolition of the ammonia response suggests that in addition to selective astrocyte death, 3-CPD also produces an earlier impairment of astrocyte function that precedes loss of neuronal function. This initial functional selectivity of 3-CPD provides a potential investigative tool in neurochemical studies of astrocyte-neuronal interactions.

Pontine reticular formation neurons are implicated in the neuronal network for generalized clonic seizures which is intensified by audiogenic kindling

The caudal pontine reticular formation nucleus (cPRF) is implicated in seizure propagation to the spinal cord in several forms of generalized convulsive seizures, including audiogenic seizures (AGS). Focal microinjection studies implicate cPRF as a requisite neuronal network site subserving generalized AGS in the moderate severity substrain of genetically epilepsy-prone rats (GEPR-3s). AGS in GEPR-3s culminate in generalized clonus, but daily repetition of AGS (AGS kindling) results in an additional seizure behavior, facial and forelimb (F and F) clonus, not seen prior to kindling. This study examined cPRF neuronal firing changes and seizure behaviors during AGS in GEPR-3s. We examined extracellular cPRF neuronal responses to acoustic stimuli (12 kHz) and observed neuronal firing during AGS. cPRF neurons exhibited onset responses to acoustic stimuli before and after AGS kindling. After AGS kindling, increased neuronal firing occurred, and response latencies were prolonged. Tonic neuronal firing occurred during generalized clonus, which changed to burst firing after AGS kindling. Burst firing also occurred during F and F clonus. Increased neuronal firing and the change from tonic to burst firing suggest that AGS kindling involves increased cPRF excitability. These data support an important role for cPRF neurons in generation of generalized clonus in unkindled GEPR-3s, which is increased by AGS kindling. The increased cPRF response latency might reflect a greater role of rostral components of the AGS neuronal network in transmission of acoustic responses to cPRF. This study also suggests that cPRF neurons may be involved in F and F clonus, which was unexpected since F and F clonus is thought to originate primarily in forebrain structures.

Involvement of Scn1b and Kcna1 ion channels in audiogenic seizures and PTZ-induced epilepsy

We have undertaken chemical genetic approach using Qingyangshenylycosides (QYS), a natural product compound, to explore the molecular mechanisms underlying different types of epilepsy models. Two animal models were used for these studies, i.e., audiogenic seizure (AGS) and pentylenetetrazol (PTZ)-induced generalized epilepsy in DBA/2J mice. We show that the latency of AGS is prolonged and the severity of seizures (the percentages of the tonus, Tonus_%) is reduced in the QYS-treated animals. These results indicate that QYS has anticonvulsant effect on the AGS model. However, we find that administration of QYS has an opposite effects on PTZ-induced generalized epilepsy. Both the latency of the generalized epilepsy and the latency of death are decreased after QYS treatment in PTZ-induced epilepsy. We examine the molecular basis of the distinct roles of QYS in these two epilepsy models by using gene expression data. Our results show that a voltage-gated sodium channel (Scn1b) and a voltage-gated potassium channel (Kcna1) are differentially expressed in AGS and PTZ-induced epilepsy models as well as in QYS-treated animals. Our results demonstrate that a chemical genetic approach may help to reveal both the molecular mechanisms of different epilepsies and the mechanism of action of the antiepileptic drugs.

Gene expression profiling reveals the mechanism of action of anticonvulsant drug QYS

We have examined the gene expression profiling of inferior colliculus from DBA/2J mice with high-intensity noise induced audiogenic seizure (AGS). We have also tested the effects of Qingyangshenylycosides (QYS), a traditional Chinese medicine, on the audiogenic seizure, and examined how the drug affected the gene expressions in inferior colliculus. Our results demonstrated that the latency was increased and the Tonus% of AGS was decreased in the animals treated with QYS, indicating that the drug effectively prevented audiogenic seizure. Gene expression analysis using Agilent oligo microarray showed that total of 134 genes were either up- or down-regulated during AGS. QYS prevented many of the AGS induced gene expression changes. Nevertheless, some of the AGS induced genes were further enhanced or reversed by QYS treatment. Our gene expression profiling data provided important information regarding the molecular mechanisms of AGS and the mechanism of action of QYS. Further analysis of the function of these genes may help to identify therapeutic targets for epilepsy.

Mixed forms of epilepsy in a subpopulation of WAG/Rij rats

Mixed forms of epilepsy in patients are often refractory. Therefore, animal models of comorbid convulsive and nonconvulsive seizure are needed for experimental research. Susceptibility to audiogenic convulsions was studied in a large group of young and adult WAG/Rij rats with inherited absence epilepsy. In 30% of adult rats, sound stimulation provoked audiogenic seizures of moderate intensity. The seizures had two excitation periods separated by a remarkably stable "arrest" of paroxysmal movements. Up to 20% of young WAG/Rij rats were also susceptible to audiogenic seizures, with a longer latency, lower intensity, and more simple seizure patterns. No difference in manifestations of spike-wave discharges was observed between the WAG/Rij rats with and without audiogenic seizures. This subpopulation of WAG/Rij rats genetically predisposed to absence and audiogenic seizures is proposed as an animal model suitable for investigation of basal mechanisms and pharmacological profiles of this mixed form of epilepsy.

Convulsive seizures induced by α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid microinjection into the mesencephalic reticular formation in rats

Effects of microinjections of a single 2 or 10 nmol dose of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) into the unilateral mesencephalic reticular formation (MRF) on behavior and on the electroencephalogram were examined in rats (n=30) over a 15-min period (Exp. 1); subsequent effects of sound stimulation with key jingling applied at 15, 30, and 45 min after the injection were observed (Exp. 2). The microinjections of a 2 nmol dose of AMPA (n=15) induced hyperactivity (15 of 15 rats) and running/circling (10 of 15 rats) in Exp. 1, and hyperactivity (5 of 15 rats) in Exp. 2. Moreover, the microinjections of a 10 nmol dose of AMPA (n=15) induced hyperactivity (15 of 15 rats), running/circling (13 of 15 rats), generalized tonic-clonic seizures (GTCS) (4 of 15 rats), and amygdala kindling-like seizures (AMKS) (8 of 15 rats) in Exp. 1; electroencephalographic seizure discharges were predominantly observed in the MRF during hyperactivity, running/circling and GTCS, while those predominantly observed in the amygdala were during AMKS. In Exp. 2, hyperactivity (15 of 15 rats), running/circling (14 of 15 rats) and GTCS (6 of 15 rats) were elicited by sound stimulation, although AMKS were not. The control group of rats (n=15) which received a single dose of saline microinjection into the unilateral MRF showed no behavioral or electroencephalographic changes in both Exp. 1 and 2. These findings suggest that potentiation of excitatory amino acid neurotransmission induced by AMPA injection into the MRF plays an important role not only in the development of hyperactivity, running/circling, GTCS and AMKS, but also in the development of audiogenic seizures.

Emergent properties of CNS neuronal networks as targets for pharmacology: application to anticonvulsant drug action

CNS drugs may act by modifying the emergent properties of complex CNS neuronal networks. Emergent properties are network characteristics that are not predictably based on properties of individual member neurons. Neuronal membership within networks is controlled by several mechanisms, including burst firing, gap junctions, endogenous and exogenous neuroactive substances, extracellular ions, temperature, interneuron activity, astrocytic integration and external stimuli. The effects of many CNS drugs in vivo may critically involve actions on specific brain loci, but this selectivity may be absent when the same neurons are isolated from the network in vitro where emergent properties are lost. Audiogenic seizures (AGS) qualify as an emergent CNS property, since in AGS the acoustic stimulus evokes a non-linear output (motor convulsion), but the identical stimulus evokes minimal behavioral changes normally. The hierarchical neuronal network, subserving AGS in rodents is initiated in inferior colliculus (IC) and progresses to deep layers of superior colliculus (DLSC), pontine reticular formation (PRF) and periaqueductal gray (PAG) in genetic and ethanol withdrawal-induced AGS. In blocking AGS, certain anticonvulsants reduce IC neuronal firing, while other agents act primarily on neurons in other AGS network sites. However, the NMDA receptor channel blocker, MK-801, does not depress neuronal firing in any network site despite potently blocking AGS. Recent findings indicate that MK-801 actually enhances firing in substantia nigra reticulata (SNR) neurons in vivo but not in vitro. Thus, the MK-801-induced firing increases in SNR neurons observed in vivo may involve an indirect effect via disinhibition, involving an action on the emergent properties of this seizure network.

Shared mechanisms of antidepressant and antiepileptic treatments: drugs and devices

Many treatments for the epilepsies and affective disorder share the properties of seizure suppression and mood stabilization. Moreover, affective disorders and the epilepsies appear to share partially similar pathogenic mechanisms. A component of the shared predisposition appears to arise from noradrenergic and serotonergic deficits. Increasing evidence supports the hypothesis that noradrenergic and/or serotonergic elevation is a mechanism of therapeutic benefit shared by most antidepressants and many antiepileptic medications. Medication induced alterations in GABAergic, glutamatergic, and CRH (corticotropin releasing hormone) containing neurons may also contribute to the shared therapeutic properties of antidepressant and antiepileptic medications.

AGS-induced expression of Narp is concomitant with expression of AMPA receptor subunits GluR1 and GluR2 in hippocampus but not inferior colliculus of P77PMC rats

To explore mechanisms of epileptogenesis in audiogenic seizures (AGS), we examined the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazoleopropionic acid (AMPA) receptor subunits GluR1 and GluR2 and of the GluR-associated protein Narp in the hippocampus and the inferior colliculus (IC) from AGS-susceptible P77PMC rats after a single AGS and audiogenic kindling. Western blotting and immunohistochemistry showed that Narp was rapidly induced in both the hippocampus and the IC by AGS. In the hippocampus, up-regulation of Narp was concomitant with GluR1 and GluR2 under both conditions of a single AGS and AGS kindling. In the IC, however, Narp was up-regulated, GluR2 down-regulated, and GluR1 unchanged after kindling. In comparison with kindling, neither GluR1 nor GluR2 was changed, while Narp significantly increased in the IC following a single AGS. These findings suggest that down-regulation of AMPA receptor GluR2 subunit in the IC may contribute to AGS-mediated epileptogenesis, and up-regulation of Narp in the IC may be involved in audiogenic seizures.

A static magnetic field modulates severity of audiogenic seizures and anticonvulsant effects of phenytoin in DBA/2 mice

RATIONALE

In a search for potential supplements or alternatives to the pharmacological treatment of epilepsy, we examined the effects of static magnetic fields on audiogenic seizures of DBA/2 mice.

METHODS

Two strains of DBA/2 mice were subjected to auditory stimulation that resulted sequentially in wild running, loss of righting, clonus, tonic hindlimb extension, and death in 80-95% of animals in different experiments. The incidence of seizure stages in groups of animals pretreated with a static magnetic field, phenytoin (PHT) or both was compared to the incidence in sham-exposed control mice.

RESULTS

Depending on magnetic flux density and duration of exposure to the field, seizure severity decreased significantly, but not completely, in both strains. However, incidence of five seizure stages was reduced in one strain, with about half of the mice seizure free. Two seizure stages (tonic hindlimb extension and death) were reduced significantly in the other. Magnetic field pretreatment potentiated the effect of PHT. Clonic seizures refractory to PHT or magnetic field pretreatment in DBA/2J mice responded to pretreatment with a combination of PHT and the magnetic field.

CONCLUSIONS

A static magnetic field had some anticonvulsant effects when employed alone. More robust effects were seen in combination with PHT. Further testing of magnetic fields for anticonvulsant effects and elucidation of mechanisms of action seem to be warranted.

Fos expression in GABAergic cells and cells immunopositive for NMDA receptors in the inferior and superior colliculi following audiogenic seizures in rats

Given the evidence that the inferior colliculus (IC) and superior colliculus (SC) seem to play key roles in connecting auditory pathways and seizure output pathways in the neuronal network for audiogenic seizures (AS) in rats, we examined Fos activation in GABAergic cells and cells immunopositive for glutamate N-methyl-D-aspartate (NMDA) receptors in the IC and SC following AS using the double-labeling procedure. Generalized tonic-clonic seizures (GTCS), which developed as an advanced form of AS in some of the susceptible rats, induced an increase in Fos expression in three IC substructures-the dorsal cortex of IC (DCIC), central nucleus of IC (CIC), and external cortex of IC (ECIC)-and in one SC substructure, the deep gray layer of SC (DpG). Compared with the rats showing GTCS, rats exhibiting wild running (WR) without proceeding to GTCS showed a different pattern of AS-induced Fos expression. The DpG in the WR animals showed no significant increase in the levels of Fos-like immunoreactivity. The degrees of Fos activation that occurred in GABAergic cells and cells immunopositive for NMDA receptors were similar in the DCIC, CIC, ECIC, and DpG following AS. These results suggest that Fos activation in the DpG is involved in the development from WR to GTCS in AS-susceptible rats. They also provide some evidence that some GABAergic neurons in the IC and SC and glutamatergic afferents (via NMDA receptors) to these structures are activated by AS.

Mutation of NMDA receptor subunit epsilon 1: effects on audiogenic-like seizures induced by electrical stimulation of the inferior colliculus in mice

It has been shown that the N-methyl-D-asparate (NMDA) receptor in the inferior colliculus is involved in the induction of audiogenic seizures (AGS). In the present study we examined audiogenic-like seizure susceptibility in GluR epsilon 1 null KO adult mice (n=32) and wild-type adult mice (n=28) by electrically stimulating the inferior colliculus (IC). Threshold current intensities of the GluR epsilon 1 KO mice for wild running, clonic and tonic seizures were higher than those of wild-type mice. In addition, the incidence rates of each seizure syndrome in GluR epsilon 1 KO mice were lower than in wild-type mice at each current intensity. These results show that GluR epsilon 1 KO mice were more resistant to audiogenic-like seizures induced by stimulating the IC. Thus, our findings suggest that the GluR epsilon 1 subunit plays an important role in regulating AGS.

Role of GABA abnormalities in the inferior colliculus pathophysiology - audiogenic seizures

gamma-Aminobutyric acid (GABA), acting at GABA(A) receptors, mediates inhibition in inferior colliculus (IC) central nucleus (ICc) neurons and plays a prominent role in mediating acoustically evoked non-monotonicity, offset inhibition, and binaural inhibition, and is also important in tonic inhibition. The IC plays an important role in a number of pathophysiological conditions that involve hearing, including tinnitus, age-related hearing loss, and audiogenic seizures (AGS). AGS are a major form of rodent neurological disorder that can be genetically mediated and can also be readily induced in both young and mature animals. A deficit in GABA-mediated inhibition in IC neurons has been shown to be a critical mechanism in genetic and induced forms of AGS. Thus, both endogenously evoked GABA-mediated inhibition and exogenously applied GABA are reduced in efficacy in IC neurons of rats that are susceptible to AGS. GABA-mediated inhibition in IC neurons is significantly more easily blocked by a GABA(A) antagonist in genetic and induced forms of AGS in vivo and in vitro. AGS can be induced in normal animals by treatments that reduce the effectiveness of GABA in the IC. Glutamate-mediated excitation is a critical element of neurotransmission in IC neurons, and excessive activation of glutamate receptors in the IC is also strongly implicated as the other major mechanism in the pathophysiology of AGS. These neurotransmitter abnormalities result in excessive firing of ICc neurons that acts as the critical initiation mechanism for triggering seizures in response to intense acoustic stimuli.

Audiogenic seizure susceptibility is induced by termination of continuous infusion of gamma-aminobutyric acid or an N-methyl-D-aspartic acid Antagonist into the inferior colliculus

The inferior colliculus (IC) is strongly implicated in seizure initiation in a genetic form of audiogenic seizures (AGS) and in AGS observed during ethanol withdrawal (ETX). Ethanol is known to block the actions of excitatory amino acids (EAA) and enhance the actions of gamma-aminobutyric acid (GABA) in several brain areas, including the IC. The present study investigated the effects on susceptibility to AGS following withdrawal from continuous blockade of N-methyl-D-aspartic acid (NMDA) receptors or continuous activation of GABA receptors in the IC. This involved infusion of GABA (1 M) or a competitive NMDA antagonist, DL-2-amino-7-phosphonoheptanoic acid (AP7, 1 mM), at 0.25 microl/h for 7 days using an Alzet osmotic minipump. Following abrupt termination of the infusion, AGS susceptibility began at 30 min. The incidence of AGS was 38.9 and 56.3% following GABA and AP7 withdrawal, respectively. The AGS behaviors observed during withdrawal, which included wild running and bouncing clonus, were very similar to those evoked by acoustic stimuli during ETX. AGS susceptibility lasted for several hours and in 13% of animals persisted for up to 6 months. The current results support diminished GABAergic and elevated glutamatergic function in the IC as the critical mechanisms and sites for AGS initiation. The present study, coupled with previous evidence that chronic ethanol exposure reduced GABA-mediated inhibition and enhanced EAA-mediated excitation, suggests that these amino acid receptor-mediated alterations in the IC are key elements in initiating AGS during ethanol withdrawal.

Differential effects of NMDA antagonists microinjections into the nucleus reticularis pontis caudalis on seizures induced by pentylenetetrazol in the rat

It has been shown that NMDA antagonists block the tonic but not the clonic component of seizures when they are injected in the oral region of the rat pontine reticular formation (PRF). The participation of the caudal PRF in the effects of NMDA antagonists upon the tonic and the clonic components of generalized seizures induced by pentylenetetrazol (PTZ) is unknown. The aim of the present study was to evaluate the effects of unilateral microinjections of competitive and non-competitive NMDA antagonists, 2-amino-7-phosphonoheptanoic acid (AP-7) and dizocilpine (MK-801), respectively, into the nucleus reticularis pontis caudalis of the rat PRF upon seizures induced by PTZ (70 mg/kg i.p.). MK-801 induced a dose-related decrease both in the incidence of generalized tonic-clonic seizures (GTCS) and in the presence of spikes in the EEG. MK-801 also increased GTCS latency. On the contrary, AP-7 did not have effects on GTCS. Interestingly, it induced ipsilateral circling behavior. These results suggest that in the caudal region of the rat PRF only non-competitive NMDA antagonists should block the generation of tonic and clonic components of generalized seizures.

Convulsive seizures induced by N-methyl-D-aspartate microinjection into the mesencephalic reticular formation in rats

Effects of microinjections of a single 2 or 10 nmol dose of N-methyl-D-aspartate (NMDA) into the unilateral mesencephalic reticular formation (MRF) on behavior and electroencephalogram were examined in rats (n=18) during a 15 min period (Exp. 1), and subsequent effects of sound stimulation with key jingling applied at 15, 30, and 45 min after the injections were observed (Exp. 2). The microinjections of 2 nmol dose of NMDA (n=10) induced hyperactivity (9 of 10 rats) and running/circling (8 of 10 rats) in Exp. 1, and hyperactivity (3 of 10 rats) in Exp. 2. Moreover, the microinjections of 10 nmol dose of NMDA (n=8) induced not only hyperactivity (8 of 8 rats) and running/circling (7 of 8 rats) but also generalized tonic-clonic seizures (GTCS) (5 of 8 rats) in Exp. 1; these seizure patterns were also elicited by sound stimulation in Exp. 2. The seizure patterns were accompanied by electroencephalographic seizure discharges in the MRF and the motor cortex. In contrast, the control group rats (n=10) which received a single dose of saline microinjection into the unilateral MRF showed no behavioral or electroencephalographic changes in both Exp. 1 and 2. These findings suggest that the MRF has an important role in the development of GTCS, which follows hyperactivity and running/circling, and that potentiation of excitatory neurotransmission in the MRF participates in the development of audiogenic seizures as well as GTCS.

Characterization of audiogenic-like seizures in naive rats evoked by activation of AMPA and NMDA receptors in the inferior colliculus

The role of glutamate receptors in the inferior colliculus (IC) in audiogenic and audiogenic-like seizures was investigated in adult rats with transient neonatal hypothyroidism by 0.02% propylthiouracil (PTU) treatment through mother's milk (PTU rats) and in naive rats treated intracisternally with N-methyl-d-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionic acid (AMPA), or cyclothiazide, an inhibitor of rapid AMPA receptor desensitization. All rats showed audiogenic or audiogenic-like seizures characterized by running fit (RF) and generalized tonic-clonic seizures (GTCS). While systemically administered MK-801 inhibited GTCS, intracisternally administered NBQX inhibited RF and GTCS in both audiogenic and audiogenic-like seizures. Auditory stimulation shortened the latency to GTCS induced by AMPA, but not NMDA, at a subclinical dose and further elongated the shortened duration of RF, but not GTCS, induced by MK-801 pretreatment. Furthermore, Northern blot analysis was used to evaluate the expression of the immediate-early gene c-fos in the IC following induction of audiogenic or audiogenic-like seizures. The significant induction of c-fos mRNA by audiogenic seizures in PTU rats or by AMPA- or cyclothiazide-induced seizures in naive rats was prominent in the IC. MK-801 suppressed c-fos mRNA expression in the IC induced by audiogenic seizures in PTU rats or by AMPA-induced seizures in naive rats. NBQX suppressed the expression of c-fos mRNA in the IC induced by AMPA-induced seizures but did not suppress c-fos mRNA in PTU rats or rats with cyclothiazide-induced seizures. Auditory stimuli failed to affect c-fos mRNA induction by AMPA. The present study suggests that audiogenic-like seizures can be reproduced by glutamate receptor agonists in which AMPA receptors are primarily linked to the initiation of audiogenic seizures (RF) while NMDA receptors presumably located within the IC are involved in the propagation of GTCS in audiogenic seizures.

Modulation of audiogenic seizures by histamine and adenosine receptors in the inferior colliculus

Susceptibility to behaviorally similar audiogenic seizures (AGS) occurs genetically and is inducible during ethanol withdrawal (ETX). Comparisons between AGS mechanisms of genetically epilepsy-prone rats (GEPR-9s) and ethanol-withdrawn rats (ETX-Rs) are yielding information about general pathophysiological mechanisms of epileptogenesis. The inferior colliculus (IC) is the AGS initiation site. Excitatory amino acid (EAA) abnormalities in the IC are implicated in AGS, and histamine and adenosine receptor activation each reduce EAA release and inhibit several seizure types. Previous studies indicate that focal infusion of an adenosine receptor agonist into the IC blocked AGS in GEPR-9s, but the effects of adenosine receptor activation in the IC on AGS in ETX-Rs are unknown. The effects of histamine receptor activation on either form of AGS are also unexamined. The present study evaluated effects of histamine or a nonselective adenosine A(1) agonist, 2-chloroadenosine, on AGS by focal microinjection into the IC. Ethanol dependence and AGS susceptibility were induced in normal rats by intragastric ethanol. Histamine (40 or 60 nmol/side) significantly reduced AGS in GEPR-9s, but histamine in doses up to 120 nmol/side did not affect AGS in ETX-Rs. 2-Chloroadenosine (5 or 10 nmol/side) did not affect AGS in ETX-Rs, despite the effectiveness of lower doses of this agent in GEPR-9s reported previously. Thus, histamine and adenosine receptors in the IC modulate AGS of GEPR-9s, but do not modulate ETX-induced AGS. The reasons for this difference may involve the chronicity of AGS susceptibility in GEPR-9s, which may lead to more extensive neuromodulation as compensatory mechanisms to limit the seizures compared to the acute AGS of ETX-Rs.

Phenytoin administration reveals a differential role of pontine reticular formation and periaqueductal gray neurons in generation of the convulsive behaviors of audiogenic seizures

The ventrolateral periaqueductal gray (PAG) and pontine reticular formation (PRF) are implicated in the neuronal network for audiogenic seizures (AGS). The AGS of genetically epilepsy-prone rats (GEPR-9s) culminate in tonic hindlimb extension (TE), and elevated acoustically evoked neuronal firing and burst firing, immediately preceding TE, have been observed in PAG and PRF. This study examined changes in PAG and PRF neuronal firing and behavior in GEPR-9s, following phenytoin administration. Recordings involved 16 PAG and nine PRF neurons in GEPR-9s. Phenytoin in doses (mean, 6. 3 mg/kg) that suppressed TE selectively did not consistently alter PAG neuronal firing. However, these doses of phenytoin resulted in significant (51.6% of control) suppression of PRF neuronal firing. Doses of phenytoin (mean, 8.3 mg/kg), which completely blocked AGS, significantly reduced PAG neuronal firing (64.6% of control), and more greatly suppressed PRF firing (25.8% of control). These results are consistent with a critical role for PRF neurons in generation of TE not evident for PAG. The suppression of PAG and PRF neuronal firing induced by phenytoin with complete seizure blockade is consistent with vital roles for both structures in the seizure network. The differential effects of phenytoin on structures requisite to the seizure network indicate that this experimental approach may be able to identify the most sensitive therapeutic target for anticonvulsant drugs, which could be critical to pharmacological suppression of specific seizure behaviors manifest in various types of convulsions, potentially including human epilepsy.

Differential roles in the neuronal network for audiogenic seizures are observed among the inferior colliculus subnuclei and the amygdala

The inferior colliculus (IC) is established as the initiation site within the neuronal network for audiogenic seizures (AGS), but the relative importance of the IC subnuclei in AGS is controversial. The lateral and basolateral subdivisions of the amygdala are implicated in the expansion of the AGS network that occurs during AGS kindling. However, the role of the amygdala in the AGS network in nonkindled AGS is unknown. NMDA receptors are implicated in modulation of AGS and in neurotransmission in both the IC and amygdala. Therefore, changes in AGS severity in genetically epilepsy-prone rats (GEPR-9s) were examined after bilateral focal microinjection into IC subnuclei or lateral/basolateral subdivisions of the amygdala of a competitive NMDA receptor antagonist, 3-((+)-2-carboxypiperazine-4-yl)propyl-1-phosphonic acid (CPP). Blockade of AGS in IC central nucleus (ICc) and external cortex (ICx) was observed at identical doses of CPP, but these doses were ineffective in IC dorsal cortex (ICd). Microinjection of CPP into the amygdala did not produce significant changes in AGS severity except at doses 20 times those effective in IC. The latter data contrast with the anticonvulsant effects of amygdala microinjections on seizure severity in kindled AGS reported previously. The present data in concord with neuronal recording studies of these nuclei suggest that the ICc is the most critical site in AGS initiation, the ICx in propagation, and that the ICd plays a lesser role in the AGS network. The amygdala does not appear to play a requisite role in the neuronal network for AGS in animals that have not been subjected to AGS kindling.

The periaqueductal grey is a critical site in the neuronal network for audiogenic seizures: modulation by GABA(A), NMDA and opioid receptors

The nuclei comprising the neuronal network for audiogenic seizures (AGS) are located primarily in the brainstem. Previous studies suggested a role for the periaqueductal grey (PAG) in the AGS network. The present study evaluated this possibility in genetically-epilepsy prone rats (GEPR-9s) by examining the effects of bilateral focal microinjection of a competitive NMDA receptor antagonist (DL-2-amino-7-phosphonoheptanoic acid (AP7), 1 and 5 nmol/side), a GABA(A) agonist (gaboxedol (THIP), 10 and 15 nmol) or an opioid peptide receptor antagonist (naloxone, 5 nmol) into PAG, based on the proposed role of these receptors in PAG neurotransmission. Blockade of NMDA receptors by AP7 (both doses) or activation of GABA(A) receptors with THIP (15 nmol/side) in the PAG suppressed AGS susceptibility. Naloxone displayed a seizure-suppressant effect that was delayed and incomplete. The seizure suppressant effect of AP7 or naloxone, unlike THIP, was observed at doses that did not produce motor quiescence. These data suggest that the PAG is a requisite nucleus in the neuronal network for AGS in GEPR-9s and that GABA(A), opioid peptide and NMDA receptors in the PAG modulate AGS propagation.

Modulation of the audiogenic seizure network by noradrenergic and glutamatergic receptors of the deep layers of superior colliculus

Recent studies suggest that the deep layers of superior colliculus (DLSC) play a role in the network for audiogenic seizures (AGS) in genetically epilepsy-prone rats (GEPR-9s). The present study examined the role of glutamatergic and noradrenergic receptors in DLSC in modulation of AGS susceptibility. The study examined effects of a competitive NMDA receptor antagonist [dl-2-amino-7-phosphonoheptanoic acid (AP7)] or an alpha1 noradrenergic agonist (phenylephrine) focally microinjected into DLSC as compared to effects in the inferior colliculus (IC) and pontine reticular formation (PRF), which are major established components of the AGS network. The results demonstrated that blockade of NMDA receptors in DLSC suppressed AGS susceptibility. AP7 microinjection was effective at relatively low doses in IC, but required higher doses in DLSC and PRF. The DLSC was relatively more sensitive to seizure reduction by the alpha1 noradrenergic agonist as compared to the IC and PRF. The anticonvulsant effect of AP7 was longer-lasting than phenylephrine in the DLSC and IC but not in the PRF. These data suggest that neurons in the DLSC are a requisite component for the neuronal network for AGS in GEPR-9s and that NMDA and alpha1 adrenoreceptors in this site may play important roles in the modulation of AGS propagation. The relatively greater sensitivity of DLSC to phenylephrine as compared to IC and PRF indicates that norepinephrine may be more important in the modulation of AGS in DLSC, which contrasts to the role of glutamate modulation. These data support recent neuronal recording data, which indicate that DLSC neurons play a critical role in AGS.

Neurons in the deep layers of superior colliculus play a critical role in the neuronal network for audiogenic seizures: mechanisms for production of wild running behavior

Recent investigations suggest that the deep layers of superior colliculus (DLSC) play a role in the neuronal network for audiogenic seizures (AGS). The present study examined DLSC neuronal firing and convulsive behavior simultaneously in freely-moving genetically epilepsy-prone rats (GEPR-9s) using chronically implanted microwire electrodes. An abrupt onset of acoustically-evoked firing at approximately 80-90 dB was observed in DLSC neurons of GEPR-9s, which was significantly above the normal threshold. DLSC neurons began to exhibit rapid tonic burst firing 1-2 s prior to the onset of the wild running behavior at the beginning of AGS. As the tonic phase of the seizure began, DLSC firing ceased, and only returned towards normal following post-ictal depression. These neuronal mechanisms may be relevant to other seizure models in which the DLSC is implicated. The temporal pattern of neuronal firing during AGS is specific to DLSC and differs markedly from those observed elsewhere in the AGS neuronal network. The temporal firing pattern suggests that the DLSC plays a primary role in the generation of the wild running phase of AGS. Previous studies indicate that the inferior colliculus is dominant during AGS initiation, and the pontine reticular formation is dominant during the tonic extension phase of AGS. Taken together these data suggest that the neurons in the neuronal network undergo a dominance shift as each specific convulsive behavior of AGS is elaborated.

NMDA receptors in the inferior colliculus are critically involved in audiogenic seizures in the adult rats with neonatal hypothyroidism

The effects of N-methyl-d-aspartate (NMDA) and non-NMDA receptor antagonists were compared on audiogenic seizures in the rats neonatally exposed to propylthiouracil (PTU). The rats treated with 0.02% PTU through mother's milk during days 0-19 after delivery showed a high incidence of audiogenic seizures consisting of running fit (RF) followed by generalized tonic-clonic seizure (GTCS) after matured. The systemic administration with MK-801, a NMDA receptor antagonist dose-dependently inhibited both RF and GTCS. NBQX (6-nitro-7-sulfamoylbenzo[f]quinoxaline-2,3-dione), a non-NMDA receptor antagonist, when systemically administered, failed to block audiogenic seizures. Audiogenic seizures caused a marked induction of c-fos messenger RNA (mRNA) in septal nucleus, bed nucleus of stria terminalis, amygdaloid nuclei, peripeduncular nucleus, and inferior colliculus, which was almost completely blocked by the pretreatment with MK-801. Bilateral microinjection of MK-801 into the inferior colliculus showed a tendency for inhibiting GTCS, but not RF, whereas CPP (3-(R)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid), a competitive NMDA receptor antagonist produced a significant inhibition against both RF and GTCS. These NMDA receptor antagonists administered into cisterna ambience, the floor of which is composed of inferior colliculus and neighboring structures, have shown potent blocking effects on both RF and GTCS. The present results suggest that NMDA receptors in the inferior colliculus, presumably in the subnucleus of external cortex may play the critical role in the initiation of audiogenic seizures in PTU-treated rats.

Ethanol and neurotransmitter interactions--from molecular to integrative effects

There is extensive evidence that ethanol interacts with a variety of neurotransmitters. Considerable research indicates that the major actions of ethanol involve enhancement of the effects of gamma-aminobutyric acid (GABA) at GABAA receptors and blockade of the NMDA subtype of excitatory amino acid (EAA) receptor. Ethanol increases GABAA receptor-mediated inhibition, but this does not occur in all brain regions, all cell types in the same region, nor at all GABAA receptor sites on the same neuron, nor across species in the same brain region. The molecular basis for the selectivity of the action of ethanol on GaBAA receptors has been proposed to involve a combination of benzodiazepine subtype, beta 2 subunit, and a splice variant of the gamma 2 subunit, but substantial controversy on this issue currently remains. Chronic ethanol administration results in tolerance, dependence, and an ethanol withdrawal (ETX) syndrome, which are mediated, in part, by desensitization and/or down-regulation of GABAA receptors. This decrease in ethanol action may involve changes in subunit expression in selected brain areas, but these data are complex and somewhat contradictory at present. The sensitivity of NMDA receptors to ethanol block is proposed to involve the NMDAR2B subunit in certain brain regions, but this subunit does not appear to be the sole determinant of this interaction. Tolerance to ethanol results in enhanced EAA neurotransmission and NMDA receptor upregulation, which appears to involve selective increases in NMDAR2B subunit levels and other molecular changes in specific brain loci. During ETX a variety of symptoms are seen, including susceptibility to seizures. In rodents these seizures are readily triggered by sound (audiogenic seizures). The neuronal network required for these seizures is contained primarily in certain brain stem structures. Specific nuclei appear to play a hierarchical role in generating each stereotypical behavioral phases of the convulsion. Thus, the inferior colliculus acts to initiate these seizures, and a decrease in effectiveness of GABA-mediated inhibition in these neurons is a major initiation mechanism. The deep layers of superior colliculus are implicated in generation of the wild running behavior. The pontine reticular formation, substantia nigra and periaqueductal gray are implicated in generation of the tonic-clonic seizure behavior. The mechanisms involved in the recruitment of neurons within each network nucleus into the seizure circuit have been proposed to require activation of a critical mass of neurons. Achievement of critical mass may involve excess EAA-mediated synaptic neurotransmission due, in part, to upregulation as well as other phenomena, including volume (non-synaptic diffusion) neurotransmission. Effects of ETX on receptors observed in vitro may undergo amplification in vivo to allow the excess EAA action to be magnified sufficiently to produce synchronization of neuronal firing, allowing participation of the nucleus in seizure generation. GABA-mediated inhibition, which normally acts to limit excitation, is diminished in effectiveness during ETX, and further intensifies this excitation.

Reduction of GABA and glutamate transporter messenger RNAs in the severe-seizure genetically epilepsy-prone rat

The genetically epilepsy-prone rat is an animal model of inherited generalised tonic-clonic epilepsy that shows abnormal susceptibility to audiogenic seizures and a lowered threshold to a variety of seizure-inducing stimuli. Recent studies suggest a crucial role for glutamate and GABA transporters in epileptogenesis and seizure propagation. The present study examines the levels of expression of the messenger RNAs encoding the glial and neuronal glutamate transporters, GLT-1 and EAAC-1, and the neuronal GABA transporter, GAT-1, in paired male genetically epileptic-prone rats and Sprague Dawley control rats using the technique of in situ hybridization. In a parallel study, semiquantitative immunoblotting was used to assess GLT-1 and EAAC-1 protein levels in similarly paired animals. Animals were assessed for susceptibility to audiogenic seizures on six occasions, and killed seven days following the last audiogenic stimulus exposure. Rat brains were processed for in situ hybridization with radioactive 35S-labelled oligonucleotide probes (EAAC-1 and GAT-1), 35S-labelled riboprobes (GLT-1), and Fluorescein-labelled riboprobes (GLT-1 and GAT-1) or processed for immunoblotting using subtype-specific antibodies for GLT-1 and EAAC-1. Semiquantitative analyses were carried out on X-ray film autoradiograms in several brain regions for both in situ hybridization and immunoblotting studies. Reductions in GAT-1 messenger RNA were found in genetically epileptic-prone rats in all brain regions examined (-8 to -24% compared to control). Similar reductions in GLT-1 messenger RNA expression levels were seen in cortex, striatum, and CA1 (-8 to -12%) of genetically epileptic-prone rats; the largest reduction observed was in the inferior colliculus (-20%). There was a tendency for a reduced expression of EAAC-1 messenger RNA in most regions of the genetically epileptic-prone rat brain although this reached statistical significance only in the striatum (-12%). In contrast, no significant differences in GLT-1 and EAAC-1 protein between genetically epileptic-prone rats and control animals were observed in any region examined, although there was a tendency to follow the changes seen with the corresponding messenger RNAs. These results show differences in the messenger RNA expression levels of three crucial amino acid transporters. For the two glutamate transporters, GLT-1 and EAAC-1, differences in messenger RNA levels are not reflected or are only partially reflected in the expression of the corresponding proteins.

Magnesium deficiency-dependent audiogenic seizures (MDDASs) in adult mice: a nutritional model for discriminatory screening of anticonvulsant drugs and original assessment of neuroprotection properties

A great many animal models for audiogenic seizures have been described. The extent to which these models may provide insight into neuroscience fields such as abnormal locomotor behavior (wild running), seizures and anticonvulsants, and neuroinsults and neuroprotectors is examined here by our study of magnesium deficiency-dependent audiogenic seizures (MDDASs) in adult mice. MDDASs were induced in all of the eight tested adult murine strains and are presented as a sequence of four successive components (latency, wild running, convulsion, and recovery phase periods). Compared with several classic seizure tests, the nutritional MDDAS model responded to low doses of prototype antiepileptic drugs (AEDs), including phenytoin (PHT), carbamazepine (CBZ), phenobarbital (PB), valproic acid (VPA), ethosuximide (ESM), and diazepam (DZP). Modulation by AEDs of the four components of MDDAS indicated that this seizure test was discriminatory, distinguishing between phenytoinergic (PHT, CBZ), GABAergic (PB, VPA, DZP), and ethosuximide (ESM) compounds. Suitability of the MDDAS test for evaluation of neuroprotective compounds was also examined: it showed partial (melatonin) and complete (WEB2170, an anti-PAF agent) reduction of recovery phase by non-anticonvulsant doses of test compounds. These neuroprotective responses were compared with neuroprotective potentials determined in a model of neonatal cerebral injury induced by focal injection of ibotenate (a glutamate analog). WEB2170 and melatonin reduced the size of lesions in white matter, but only WEB2170 protected cortical plate against ibotenate-induced lesions. In addition to the original neuroprotective behavior of WEB2170, studies on the neuroprotectors also supported GABAergic anticonvulsant activity of melatonin in the MDDAS test.

Removal of the inferior olive abolishes myoclonic seizures associated with a loss of olivary serotonin

Several lines of clinical evidence suggest that myoclonus is caused by a reduction of serotonin in the brain and hyperactivity of the inferior olive. We determined whether a change in serotonin content within the olivocerebellar system accompanied a predisposition to myoclonus and investigated the necessity of the inferior olive for a myoclonic seizure. The experiments employed the genetically epilepsy-prone rat that exhibits a profound myoclonic seizure in response to an auditory stimulus. We found that these animals demonstrated a significant reduction in the serotonergic innervation of the inferior olive without a significant change in the serotonergic innervation at any other level of the olivocerebellar circuit. The deficit in olivary serotonin was verified physiologically and pharmacologically by a reduced sensitivity of the genetically epilepsy-prone rat to the tremorogenic effect of harmaline, which is known to produce tremor through a mechanism that requires serotonergic innervation of the inferior olive. We quantified the timing of the myoclonic seizure of the genetically epilepsy-prone rat and found that its large amplitude 2-6 Hz clonus was always preceded by 9-10 Hz tremor that was synchronized among limbs. Ablation of the inferior olive by 3-acetylpyridine abolished the myoclonic seizure. The specificity of the deficit in olivary serotonin, the timing of the seizure, and the demonstration of the necessity of the inferior olive for myoclonus suggest that pathological inferior olivary activity contributes to the genesis of a myoclonic seizure.

Immunocytochemical localization of AMPA receptors in the rat inferior colliculus

Immunocytochemistry was used to study the distribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subtypes in the inferior colliculus (IC) of genetically epilepsy-prone rats (GEPR-9s) and normal Sprague-Dawley (SD) rats. The analysis was conducted using 3 antibodies specific for glutamate receptor subtypes, GluR 1, GluR 2/3, and GluR 4. Light microscopy showed that immunostaining of the IC was most dense with the GluR 2/3 antibody for both strains of animals. The amount of GluR 2/3 immunolabeling was similar for sound-stimulated GEPR-9s, seizure-naive GEPR-9s, and SD rats. The electron microscopy of GluR 2/3 in the IC revealed immunoreaction products associated with the postsynaptic densities of asymmetric synapses. The thin sections had comparable amounts of reaction product in dendrites or dendritic spines for both strains. Since the distribution and quantity of AMPA receptors in the IC of GEPR-9s and SD rats are similar, our results indicate that altered AMPA receptors are probably not the primary cause of seizure initiation in GEPR-9s.

Aberrant neuronal responsiveness in the genetically epilepsy-prone rat: acoustic responses and influences of the central nucleus upon the external nucleus of inferior colliculus

The inferior colliculus (IC) central nucleus (ICc), is critical for audiogenic seizure (AGS) initiation in the genetically epilepsy-prone rat (GEPR). The ICc lacks direct motor outputs but sends a major projection to the external nucleus of IC (ICx), which does project to the sensorimotor integration nuclei within the AGS neuronal network. The present study compared acoustic responses of ICx neurons in the GEPR and normal anesthetized rat and evaluated whether the GEPR exhibits functional abnormalities in the pathway from ICc to ICx. There is a significantly greater incidence of sustained repetitive response patterns to the acoustic stimulus in GEPR ICx neurons (75%) than in normal ICx neurons (24%). Following unilateral microinjection of N-methyl-D-aspartate (NMDA) into the contralateral ICc, acoustically-evoked ICx excitation and inhibition were each increased in normal animals, which is consistent with the mixed projections previously reported in this pathway and observed with electrical stimulation in the present study. The NMDA-induced ICx firing increase may be relevant to AGS, since, in previous studies, bilateral focal microinjection of NMDA into the ICc induced AGS susceptibility in normal rats [23]. However, the incidence and degree of the ICx neuronal response changes after NMDA microinjection was not abnormal in the GEPR. These data suggest that the hyperresponsiveness of ICx neurons may not involve abnormal transmission between the ICc and ICx, despite the elevated ICx neuronal responses to acoustic stimuli. However, the ICx hyperresponsivess of the GEPR, which is likely due to the known decrease in effectiveness of GABA-mediated inhibition in GEPR neurons, may be a major mechanism subserving the critical role that this structure plays in the AGS network.

Repetitive audiogenic seizures cause an increased acoustic response in inferior colliculus neurons and additional convulsive behaviors in the genetically-epilepsy prone rat

Previous studies indicate that daily repetition of audiogenic seizures (AGS) leads to audiogenic 'kindling' with increased seizure duration and additional seizural behaviors. The present study examined the neuronal correlates of this phenomenon. Extracellular single neuron firing and concomitant convulsive behaviors associated with 14 repetitive AGS were evaluated in the genetically epilepsy-prone rat severe seizure strain (GEPR-9). An increase in the number of acoustically-evoked action potentials in neurons of the central nucleus of inferior colliculus (ICc) was observed by the second day of AGS repetition, and peaked at day four. The ICc responses remained at similar enhanced level through day 14. ICc neuronal responses were completely absent for approximately two min post-ictally after a single AGS in all animals, but 80% of the animals undergoing repetitive AGS consistently exhibited neuronal firing in this post-ictal period. Post-tonic clonus and an increased duration of post-ictal behavioral depression were also observed with repetitive AGS. The increased ICc neuronal firing was observed prior to the appearance of the post-tonic clonus component of repetitive AGS. This suggests that the ICc neuronal firing increase may subserve, at least, the initial increase in AGS severity. However, changes in neuronal firing in nuclei of the neuronal network for AGS efferent to the ICc may be responsible for the increased AGS severity that occurs after the fourth day of AGS repetition.

Abnormalities in 5-HT1A and 5-HT1B receptor binding in severe-seizure genetically epilepsy-prone rats (GEPR-9s)

The present study was designed to determine whether abnormalities in serotonin receptor binding co-exist with the presynaptic serotonergic deficits that have previously been identified in the genetically epilepsy-prone rat (GEPR) brain. In vitro binding of [3H]8-OH-DPAT (0.16-10.3 nM) to 5-HT1A receptor sites was found to be decreased in the hippocampus of severe seizure GEPRs (GEPR-9s) when compared to nonepileptic control rats, while no difference in [3H]8-OH-DPAT binding was observed in the GEPR-9 corpora quadrigemina or midbrain tegmentum. The decreased binding of [3H]8-OH-DPAT to hippocampal membranes was due to a decrease in Bmax (P < 0.001), rather than to a change in the Kd. Conversely, in vitro binding of [125I]cyanopindolol (2-400 pM) to 5-HT1B receptor sites was increased in the GEPR-9 hippocampus, corpora quadrigemina and midbrain tegmentum when compared to nonepileptic control rats. The increased binding of [125I]cyanopindolol in all three regions resulted from an increase in the Bmax (P < 0.05), rather than a change in the Kd. These finding suggest that in addition to the innate reduction in 5-HT presynaptic markers, GEPR-9s also exhibit abnormalities in the density of 5-HT1A and 5-HT1B receptors in some regions of the brain. Inasmuch as serotonin acts to attenuate audiogenic seizures in GEPRs, these abnormalities in 5-HT receptor binding may contribute to the seizure susceptibility exhibited by these animals.

Pontine reticular formation neurons exhibit a premature and precipitous increase in acoustic responses prior to audiogenic seizures in genetically epilepsy-prone rats

The genetically epilepsy-prone rat (GEPR-9) exhibits elevated seizure sensitivity and audiogenic seizures (AGS). The pontine reticular formation (PRF) is implicated in the neuronal network for AGS in the GEPR-9. The present study examined PRF neuronal firing and convulsive behavior simultaneously in the GEPR-9. Chronically implanted microwire electrodes in PRF allowed single neuronal responses and behavior to be examined in freely-moving rats. PRF neurons in the GEPR-9 exhibit precipitous intensity-evoked increases at a significantly lower (approx. 15 dB SPL) intensity than normal Sprague-Dawley rats. PRF neurons in the GEPR-9 also exhibit increased auditory response latencies. At the onset of AGS (wild running) the firing rate of PRF neurons increased, and the rate of PRF firing increased dramatically as the tonic phase of the seizure began. During post-ictal depression the rate of PRF neuronal firing slowed, gradually returning to normal. This pattern of PRF periseizural neuronal firing changes differ dramatically in pattern and temporal characteristics from those previously observed in inferior colliculus (IC). The IC serves as the AGS initiation site. IC neurons show extensive firing increases prior to and during the initial wild running, silence during the tonic and post-ictal phases, and gradual recovery of responses thereafter. The changes in PRF neuronal firing pattern suggest that the PRF may play a major role in the generation of the tonic phase of AGS. The premature onset of the precipitous rise in PRF neuronal firing suggests that the influence of the IC on PRF neurons may be magnified in association with AGS susceptibility. The PRF neuronal firing increases observed in the present study coupled with previous observation of AGS blockade by PRF microinjections in the GEPR-9 further support an important role of the PRF in the propagation of AGS in the GEPR-9. The mechanisms of PRF firing elevation may also be relevant in other seizure models in which the brain-stem reticular formation is implicated.

N-Methyl-D-aspartate receptor subunits NR1 and NR2C are overexpressed in the inferior colliculus of audiogenic mice

Some non-DBA2 Albino Swiss mice exhibit noise induced epileptic seizures during a short period of postnatal development. Because N-methyl-D-aspartate (NMDA) glutamate ionotropic receptors are involved in the occurrence of audiogenic seizures, we investigated by in situ hybridization methods, the expression of the different subunits (NR1, NR2A, NR2B, NR2C) of this receptor in the central nucleus of the inferior colliculus (IC), a main relay of the auditory pathways. At postnatal day 20, the NR2C subunit is highly expressed in the IC of convulsive mice, while in non-convulsive mice a slight signal is only found for NR1, NR2A, and NR2B. In adult mice, the NR1 and NR2A signals are observed while the NR2B signal is almost undetectable. The audiogenic susceptibility may be related to the transient expression of the NR2C subunit during a brief neonatal period during which synaptic reorganization happens.

The role of the inferior colliculus in a genetic model of audiogenic seizures

Previous studies have shown the functional importance of the inferior colliculus (IC) for the propagation and initiation of audiogenic seizures in several models of epilepsy in rats. A review of the cell types and cytoarchitecture of the IC, including its three major subdivisions, is presented. Significant increases in GABA levels and the number of GABAergic neurons are found in the central nucleus of the IC (ICCN) of genetically epilepsy-prone rats (GEPR-9s) as compared to Sprague-Dawley rats that do not display audiogenic seizures. Two independent anatomical methods were used to determine the number of GABAergic neurons, immunocytochemistry and in situ hybridization. In both types of preparation, the labeled cells in the ICCN appeared to be of different sizes but the number of small cells with diameters less than 15 microns showed the greatest increase. Nissl-stained sections showed that the total number of neurons in the ICCN was increased in GEPR-9s and indicated that the increase in GABAergic neurons was not due to a change in the phenotype of collicular neurons from non-GABAergic to GABAergic. The number of small neurons in Nissl-stained sections of the ICCN was shown to correlate with seizure severity in the offspring of crosses made between Sprague-Dawley rats and GEPR-9s. Furthermore, the GEPR-3s that display moderate seizures showed a significant increase in the number of small neurons in the ICCN, and the magnitude of this increase was predicted from this correlation. Finally, the use of knife cuts through the midbrain indicated that the ICCN sends an important projection to the external nucleus and that this projection plays a vital role in the propagation of seizure activity from the site of seizure initiation in the ICCN. It remains to be resolved how the increase in small GABAergic neurons in the ICCN is responsible for the known pharmacological defects observed at GABAergic synapses.

Ethanol withdrawal induces increased firing in inferior colliculus neurons associated with audiogenic seizure susceptibility

Ethanol withdrawal (ETX) in ethanol-dependent rats results in susceptibility to seizures, including generalized tonic-clonic audiogenic seizures (AGS). The inferior colliculus (IC) is strongly implicated in AGS initiation during ETX, but IC neuronal mechanisms subserving AGS are unclear. The present study examined IC (central nucleus) single neuronal firing during repeated (4 day) intragastric ethanol administration and during ETX. This involved microwire electrodes implanted chronically into freely moving rats and acoustic stimulation in intensities up to 105 dB SPL. During initial ethanol administration the animals were stuporous, and IC spontaneous neuronal firing and acoustically evoked firing at high stimulus intensities were significantly reduced. This firing reduction is consistent with the action of ethanol to enhance gamma-aminobutyric acid (GABA)-mediated inhibition, which is prominent in IC neurons at high stimulus intensities. During ETX the animals were agitated, and spontaneous IC neuronal firing and acoustically evoked firing at all stimulus intensities were significantly increased during the period of AGS susceptibility. Previous studies indicate that IC neuronal responses are tightly regulated by GABA and glutamate. The IC firing increases during ETX in the present study may involve the down-regulation of GABAA receptors and supersensitivity of glutamate receptors reported to occur during ETX. Previous studies also indicate that focal blockade of GABAA receptors or activation of glutamate receptors produces AGS susceptibility in normal rats. Therefore, the IC neuronal firing increases observed in the present study may play a critical role in initiation of AGS during ethanol withdrawal.

Effects of glutamate, N-methyl-D-aspartate, high potassium, and hypoxia on unit discharges in CA1 area of hippocampal slices of DBA and C57 mice

We studied effects of L-glutamate, N-methyl-D-aspartate (NMDA), high K+, and hypoxia on spontaneous unit discharges in stratum pyramidale of CA1 region of hippocampal slices in DBA and C57 mice aged 3-4 and 5-6 weeks. Application of L-glutamate (0.5-2.0 mM), NMDA (5-20 microM), high K+ (8.5 mM), and a brief period of hypoxia (1 min) to the perfused artificial cerebrospinal fluid (ACSF) all produced different degrees of spontaneous high-frequency discharges from CA1 area of hippocampal slices of both DBA and C57 mice. Two types of responses recorded extracellularly occurred after these manipulations: high-frequency repetitive single spikes and bursts of multiple population spikes. The rate and type of responses from CA1 region of hippocampal slices after these manipulations were different and depended on the strain and age of mice and the nature of manipulations. In general, hippocampal slices from audiogenic seizure-susceptible DBA mice were more sensitive than those from audiogenic seizure-resistant C57 mice, and hippocampal slices from younger animals were more susceptible than those from older ones. Thus, DBA mice aged 3-4 weeks of age were most susceptible and C57 mice aged 5-6 weeks were least susceptible to all these pharmacological, ionic, and hypoxic manipulations. Bursts of multiple population spikes were the most common responses in DBA mice and in younger animals, and repetitive single spikes were the predominant responses in C57 mice and in older animals. In all groups of animals, the average spontaneous discharge rate was highest after L-glutamate perfusion, next highest after NMDA, and lowest after high K+ and hypoxia. The latency of the appearance of spontaneous epileptiform activity from CA1 region of hippocampal slices was long (> 2 min) after NMDA perfusion and short (< 1 min) after L-glutamate, high K+ and hypoxia. The duration of the increased spontaneous discharges was short (-1 min) after L-glutamate perfusion, long (> 3 min) after high K+ and hypoxia, and between short and long after NMDA perfusion. These results suggest that age and strain of animal and nature of stimulus precipitate different patterns of epileptiform activity in CNS.