Differences in Evolution of Epileptic Seizures and Topographical Distribution of Tissue Damage in Selected Limbic Structures Between Male and Female Rats Submitted to the Pilocarpine Model

Age differences in organophosphorus nerve agent-induced seizure, blood brain barrier integrity, and neurodegeneration in midazolam-treated rats

Exposure to organophosphorus nerve agents irreversibly inhibits acetylcholinesterase and may lead to cholinergic crisis and seizures. Although benzodiazepines are the standard of care after nerve agent-induced status epilepticus, when treatment is delayed for up to 30 min or more, refractory status epilepticus can develop. Adult male rodents are often utilized for evaluation of therapeutic efficacy against nerve agent exposure. However, there may be age and sex differences in toxicity and in therapeutic response. We previously reported that juvenile rats are less susceptible to the lethal effects of soman compared to adults, while pups are the most susceptible. Here, we report on age and sex differences in delayed midazolam treatment efficacy on survival, seizures and brain pathology. Male and female pups, juvenile and adult rats were exposed to an equitoxic dose of soman and treated with atropine sulfate and the oxime asoxime chloride (HI-6 dimethanesulphonate) 1 min after exposure and with midazolam 40 min after seizure onset, determined by EEG in juvenile and adult rats, and by behavior in pups. Survival, seizure data, and spontaneous recurrent seizures were evaluated. Brains were processed to assess neurodegeneration, neuroinflammation, and blood brain barrier (BBB) integrity. Juvenile and adult rats exposed to soman and treated with midazolam had BBB disruption, epileptogenesis, neurodegeneration, microglial activation, and astrogliosis; adult rats had poorer outcomes. Pups and juvenile rats exposed to soman had poor survival prior to midazolam treatment but most survived once treated; overall, neurodegeneration or disrupted BBB integrity was not detected in midazolam-treated pups. We found that age is a determinant factor in soman-induced toxicity and response to standard medical countermeasures. In addition, we observed sex differences in response to soman in juveniles and males with respect to body weight growth curves and in neuronal loss in juveniles and adults. Adjunct therapies to midazolam are warranted and it is important to evaluate both age and sex as factors in therapeutic response.

Sex influences on hippocampal kindling-induced seizures in middle-aged mice

Epilepsy is a chronic neurological disorder, and its prevalence presents a bimodal distribution with high incidences in children and older adults. The incidence of epilepsy does not generally differ between men and women; however, whether this holds true for new-onset epilepsy in older adults is unclear. While studies in animal models of epilepsy may help explore the biological mechanisms relevant to the influences of sex on epileptogenesis, relatively little information is available regarding sex differences in the genesis of epileptic seizures in middle-aged animals. In this study, we addressed this knowledge gap using a mouse model of extended hippocampal kindling. C57 black mice aged between the ages of 12 and 13 months underwent hippocampal kindling as this age roughly corresponds to middle age in humans (∼50 years). Relative to male mice, female mice showed faster-progressing and more severe evoked seizures, a higher tendency to experience spontaneous seizures in the early stage of extended kindling, less frequent expression of hippocampal interictal spikes, and insignificant decreases in hippocampal theta rhythm. Collectively, these results demonstrated the existence of sex-specific differences in hippocampal kindling-induced seizures and suggested that middle-aged female mice have greater but variable susceptibility to hippocampal kindling-induced epileptogenesis compared with male mice of similar age.

PPARβ/δ Agonist GW0742 Modulates Microglial and Astroglial Gene Expression in a Rat Model of Temporal Lobe Epilepsy

The role of astroglial and microglial cells in the pathogenesis of epilepsy is currently under active investigation. It has been proposed that the activity of these cells may be regulated by the agonists of peroxisome proliferator-activated nuclear receptors (PPARs). This study investigated the effects of a seven-day treatment with the PPAR β/δ agonist GW0742 (Fitorine, 5 mg/kg/day) on the behavior and gene expression of the astroglial and microglial proteins involved in the regulation of epileptogenesis in the rat brain within a lithium-pilocarpine model of temporal lobe epilepsy (TLE). TLE resulted in decreased social and increased locomotor activity in the rats, increased expression of astro- and microglial activation marker genes (Gfap, Aif1), pro- and anti-inflammatory cytokine genes (Tnfa, Il1b, Il1rn), and altered expression of other microglial (Nlrp3, Arg1) and astroglial (Lcn2, S100a10) genes in the dorsal hippocampus and cerebral cortex. GW0742 attenuated, but did not completely block, some of these impairments. Specifically, the treatment affected Gfap gene expression in the dorsal hippocampus and Aif1 gene expression in the cortex. The GW0742 injections attenuated the TLE-specific enhancement of Nlrp3 and Il1rn gene expression in the cortex. These results suggest that GW0742 may affect the expression of some genes involved in the regulation of epileptogenesis.

A Novel Rat Infant Model of Medial Temporal Lobe Epilepsy Reveals New Insight into the Molecular Biology and Epileptogenesis in the Developing Brain

Although several adult rat models of medial temporal lobe epilepsy (mTLE) have been described in detail, our knowledge of mTLE epileptogenesis in infant rats is limited. Here, we present a novel infant rat model of mTLE (InfRPil-mTLE) based on a repetitive, triphasic injection regimen consisting of low-dose pilocarpine administrations (180 mg/kg. i.p.) on days 9, 11, and 15 post partum (pp). The model had a survival rate of >80% and exhibited characteristic spontaneous recurrent electrographic seizures (SRES) in both the hippocampus and cortex that persisted into adulthood. Using implantable video-EEG radiotelemetry, we quantified a complex set of seizure parameters that demonstrated the induction of chronic electroencephalographic seizure activity in our InfRPil-mTLE model, which predominated during the dark cycle. We further analyzed selected candidate genes potentially relevant to epileptogenesis using a RT-qPCR approach. Several candidates, such as the low-voltage-activated Ca2+ channel Cav3.2 and the auxiliary subunits β 1 and β 2, which were previously reported to be upregulated in the hippocampus of the adult pilocarpine mTLE model, were found to be downregulated (together with Cav2.1, Cav2.3, M1, and M3) in the hippocampus and cortex of our InfRPil-mTLE model. From a translational point of view, our model could serve as a blueprint for childhood epileptic disorders and further contribute to antiepileptic drug research and development in the future.

RNA Sequencing Demonstrates Ex Vivo Neocortical Transcriptomic Changes Induced by Epileptiform Activity in Male and Female Mice

Seizures are generally associated with epilepsy but may also be a symptom of many other neurological conditions. A hallmark of a seizure is the intensity of the local neuronal activation, which can drive large-scale gene transcription changes. Such changes in the transcriptional profile likely alter neuronal function, thereby contributing to the pathological process. Therefore, there is a strong clinical imperative to characterize how gene expression is changed by seizure activity. To this end, we developed a simplified ex vivo technique for studying seizure-induced transcriptional changes. We compared the RNA sequencing profile in mouse neocortical tissue with up to 3 h of epileptiform activity induced by 4-aminopyridine (4AP) relative to control brain slices not exposed to the drug. We identified over 100 genes with significantly altered expression after 4AP treatment, including multiple genes involved in MAPK, TNF, and neuroinflammatory signaling pathways, all of which have been linked to epilepsy previously. Notably, the patterns in male and female brain slices were almost identical. Various immediate early genes were among those showing the largest upregulation. The set of down-regulated genes included ones that might be expected either to increase or to decrease neuronal excitability. In summary, we found the seizure-induced transcriptional profile complex, but the changes aligned well with an analysis of published epilepsy-associated genes. We discuss how simple models may provide new angles for investigating seizure-induced transcriptional changes.

Wireless EEG Recording of Audiogenic Seizure Activity in Freely Moving Krushinsky-Molodkina Rats

This study investigates audiogenic epilepsy in Krushinsky-Molodkina (KM) rats, questioning the efficacy of conventional EEG techniques in capturing seizures during animal restraint. Using a wireless EEG system that allows unrestricted movement, our aim was to gather ecologically valid data. Nine male KM rats, prone to audiogenic seizures, received implants of wireless EEG transmitters that target specific seizure-related brain regions. These regions included the inferior colliculus (IC), pontine reticular nucleus, oral part (PnO), ventrolateral periaqueductal gray (VLPAG), dorsal area of the secondary auditory cortex (AuD), and motor cortex (M1), facilitating seizure observation without movement constraints. Our findings indicate that targeted neural intervention via electrode implantation significantly reduced convulsive seizures in approximately half of the subjects, suggesting therapeutic potential. Furthermore, the amplitude of brain activity in the IC, PnO, and AuD upon audiogenic stimulus onset significantly influenced seizure severity and nature, highlighting these areas as pivotal for epileptic propagation. Severe cases exhibited dual waves of seizure generalization, indicative of intricate neural network interactions. Distinctive interplay between specific brain regions, disrupted during convulsive activity, suggests neural circuit reconfiguration in response to escalating seizure intensity. These discoveries challenge conventional methodologies, opening avenues for novel approaches in epilepsy research and therapeutic interventions.

Long-term ANT-DBS effects in pilocarpine-induced epileptic rats: A combined 9.4T MRI and histological study

Deep brain stimulation (DBS) of the anterior nucleus of the thalamus (ANT) appears to be effective against seizures in animals and humans however, its therapeutic mechanisms remain elusive. This study aimed to combine 9.4T multimodal magnetic resonance imaging (MRI) with histology to investigate the longitudinal effects of long-term ANT-DBS in pilocarpine-induced epileptic rats. Status epilepsy (SE) was induced by LiCl-pilocarpine injection in 11 adult male Sprague-Dawley rats. Four weeks after SE, chronic epileptic rats underwent either ANT-DBS (n = 6) or sham-DBS (n = 5) surgery. Electroencephalography (EEG) and spontaneous recurrent seizures (SRS) were recorded for 1 week. The T2-weighted image and images from resting-state functional MRI (rs-fMRI) were acquired at three states: before SE, at 4 weeks post-SE, and at 5 weeks post-DBS. Volumes of the hippocampal subregions and hippocampal-related functional connectivity (FC) were compared longitudinally. Finally, antibodies against neuronal nuclei (NeuN) and glial fibrillary acidic proteins were used to evaluate neuronal loss and astrogliosis in the hippocampus. Long-term ANT-DBS significantly reduced seizure generalization in pilocarpine-induced epileptic rats. By analyzing the gray matter volume using T2-weighted images, long-term ANT-DBS displayed morphometric restoration of the hippocampal subregions. Neuronal protection of the hippocampal subregions and inhibition of astrogliosis in the hippocampal subregions were observed in the ANT-DBS group. ANT-DBS caused reversible regulation of FC in the insula-hippocampus and subthalamic nucleus-hippocampus. Long-term ANT-DBS provides comprehensive protection of hippocampal histology, hippocampal morphometrics, and hippocampal-related functional networks.

Sex and estrous cycle stage shape left-right asymmetry in chronic hippocampal seizures in mice

Lateralization of hippocampal function is indicated by varied outcomes of patients with neurological disorders that selectively affect one hemisphere of this structure, such as temporal lobe epilepsy (TLE). The intrahippocampal kainic acid (IHKA) injection model of TLE allows for targeted damage to the left or right hippocampus, enabling systematic comparison of effects of left-right asymmetry on seizure and non-seizure outcomes. Although varying non-seizure phenotypic outcomes based on injection side in dorsal hippocampus were recently evaluated in this model, differences in chronic seizure patterns in left- (IHKA-L) vs. right-injected (IHKA-R) IHKA animals have yet to be evaluated. Here, we evaluated hippocampal seizure incidence in male and female IHKA-L and IHKA-R mice. Females displayed increased electrographic seizure activity compared to males at both 2 months and 4 months post-injection (mpi). In addition, IHKA-L females showed higher seizure frequency than IHKA-R on diestrus and estrus at 2 mpi, but seizure duration and time in seizures were only higher in IHKA-L females on diestrus. These cycle stage-associated changes, however, did not persist to 4 mpi. Furthermore, this lateralized difference in seizure burden was not observed in males. These results indicate for the first time that the side of IHKA injection can shape chronic electrographic seizure burden. Overall, these results demonstrate a female-specific left-right asymmetry in hippocampal function can interact with estrous cycle stage to shape chronic seizures in mice with epilepsy, with implications for neural activity and behavior in both normal and disease states.

Selective activation of the hypothalamic orexinergic but not melanin-concentrating hormone neurons following pilocarpine-induced seizures in rats

Introduction

Sleep disorders are common comorbidities in patients with temporal lobe epilepsy (TLE), but the underlying mechanisms remain poorly understood. Since the lateral hypothalamic (LH) and the perifornical orexinergic (ORX) and melanin-concentrating hormone (MCH) neurons are known to play opposing roles in the regulation of sleep and arousal, dysregulation of ORX and MCH neurons might contribute to the disturbance of sleep-wakefulness following epileptic seizures.

Methods

To test this hypothesis, rats were treated with lithium chloride and pilocarpine to induce status epilepticus (SE). Electroencephalogram (EEG) and electromyograph (EMG) were recorded for analysis of sleep-wake states before and 24 h after SE. Double-labeling immunohistochemistry of c-Fos and ORX or MCH was performed on brain sections from the epileptic and control rats. In addition, anterograde and retrograde tracers in combination with c-Fos immunohistochemistry were used to analyze the possible activation of the amygdala to ORX neural pathways following seizures.

Results

It was found that epileptic rats displayed prolonged wake phase and decreased non-rapid eye movement (NREM) and rapid eye movement (REM) phase compared to the control rats. Prominent neuronal activation was observed in the amygdala and the hypothalamus following seizures. Interestingly, in the LH and the perifornical nucleus, ORX but not MCH neurons were significantly activated (c-Fos+). Neural tracing showed that seizure-activated (c-Fos+) ORX neurons were closely contacted by axon terminals originating from neurons in the medial amygdala.

Discussion

These findings suggest that the spread of epileptic activity from amygdala to the hypothalamus causes selective activation of the wake-promoting ORX neurons but not sleep-promoting MCH neurons, which might contribute to the disturbance of sleep-wakefulness in TLE.

Respiratory dysfunction in two rodent models of chronic epilepsy and acute seizures and its link with the brainstem serotonin system

Patients with drug-resistant epilepsy can experience respiratory alterations, notably during seizures. The mechanisms underlying long-term alterations in respiratory function remain unclear. As the brainstem 5-HT system is a prominent modulator of respiratory function, this study aimed at determining whether epilepsy is associated with alterations in both the respiratory function and brainstem serotonin (5-HT) system in rats. Epilepsy was triggered by pilocarpine-induced status epilepticus in rats. Our results showed that 30–50% of epileptic (EPI) rats exhibited a sharp decrease in oxygen consumption (SDOC), low metabolic rate of oxygen, and slow regular ventilation (EPI/SDOC + rats). These alterations were detected only in rats with chronic epilepsy, independent of behavioral seizures, were persistent over time, and not associated with death. In these rats, 5-HT fiber density in the nucleus tractus solitarius was lower than that in the control and EPI/SDOC− rats. Both EPI/SDOC + rats and DBA/2 mice that present with audiogenic-induced seizure followed by fatal respiratory arrest—a model of sudden and expected death in epilepsy—had increased transcript levels of tryptophan hydroxylase 2 and 5-HT presynaptic transporter. Thus, our data support that 5-HT alterations are associated with chronic and acute epilepsy-related respiratory dysfunction.