Localized cortical injections of ethosuximide suppress spike-and-wave activity and reduce the resistance to kindling in genetic absence epilepsy rats (GAERS)

Alpha-2a adrenergic receptor activation in genetic absence epilepsy: An absence status model?

OBJECTIVE

The objective of the study was to propose a candidate animal model of absence status epilepticus induced by specific alpha-2a adrenergic receptor (α2AR) activation. We also aim to investigate the responsiveness of this model to classical anti-status or anti-absence medications.

METHODS

An α2AR agonist, dexmedetomidine (DEX), was injected intracerebroventricularly into adult rats with genetic absence epilepsy, and their electroencephalography (EEG) was recorded. The total duration, number, and mean duration of each spike-and-wave discharges (SWDs) were evaluated. The blocks of absence status events were classified as the initial and second sets of absence statuses. Ethosuximide (ETX) was administered as a pretreatment to another group of rats and later injected with 2.5 μg DEX. In addition, ETX, valproic acid (VPA), diazepam (DIAZ), and atipamezole (ATI) were administered after induced status-like events following DEX administration. Power spectral characteristics and coherence analysis were performed on the EEG to assess the absence status events and sleep.

RESULTS

The 2.5 μg dose of DEX increased the total SWD duration and induced continuous SWDs up to 26 min. Following the initial absence status event, sleep was induced; then, the second period of absence status-like activities were initiated. ETX pretreatment blocked the occurrence of absence status-like activities. Power spectral density analyses revealed that DEX-induced post-sleep activities had higher power in delta frequency band (1-4 Hz) and attenuated power of 7 Hz harmonics (14 and 21 Hz) than the pre-injection seizure. The mean duration of SWDs were decreased in all the groups, but occasional prolonged activities were seen in ETX or VPA-injected rats but not with DIAZ or ATI.

SIGNIFICANCE

This study presents an absence status epilepticus animal model that is activated by α2AR activation to investigate the pathophysiological role of absence status. Unlike other agents ATI switched off the second set of absence statuses to normal SWDs, without sedation or lethargy, can show it may preferentially block absence status-like activity.

THE PLAIN LANGUAGE SUMMARY

This study proposes a rat model for prolonged seizures, resembling absence status epilepticus. Activating the brain's alpha-2a adrenergic receptor with dexmedetomidine induced seizures lasting up to 26 minutes. Ethosuximide pretreatment and post-treatment with valproic acid, diazepam, and atipamezole decreased induced seizures. The findings suggest this model is valuable for studying absence status epilepticus. In addition, atipamezole normalized abnormal seizures without sedation, hinting at its potential for targeted treatment and further research.

Fabrication of ethosuximide loaded alginate/polyethylene oxide scaffolds for epilepsy research using 3D-printing method

Epilepsy is a medical condition that causes seizures and impairs the mental and physical activities of patients. Unfortunately, over one-third of patients do not receive adequate relief from oral Antiepileptic Drugs (AEDs) and continue to experience seizures. In addition to that, long term usage of Antiepileptic Drugs can cause a range of side effects. To overcome this problem, the precision of 3D printing technology is combined with the controlled release capabilities of biodegradable polymers, allowing for tailored and localized AED delivery to specific seizure sites. As a result of this novel technique, therapeutic outcomes can be enhanced, side effects of AEDs are minimized, and patient-specific dosage forms can be created. This study focused on the use of ethosuximide, an antiepileptic drug, at different concentrations (10, 13, and 15 mg) loaded into 3D-printed sodium alginate and polyethylene oxide scaffolds. The scaffolds contained varying concentrations (0.25%, 0.50%, and 0.75% w/v) and had varying pores created by 3D patterning sizes from 159.86 ± 19.9 µm to 240.29 ± 10.7 µm to optimize the releasing system for an intracranial administration. The addition of PEO changed the Tg and Tm temperatures from 65°C to 69°C and from 262°C to 267°C, respectively. Cytotoxicity assays using the human neuroblastoma cell line (SH-SY5Y) showed that cell metabolic activity reached 130% after 168 h, allowing the cells to develop into mature neural cells. In vitro testing demonstrated sustained ethosuximide release lasting 2 hours despite crosslinking with 3% CaCl2. The workpaves the way for the use of ethosuximide -loaded scaffolds for treating epilepsy.

Tapping the Brakes: Cellular and Synaptic Mechanisms that Regulate Thalamic Oscillations

Thalamic oscillators contribute to both normal rhythms associated with sleep and anesthesia and abnormal, hypersynchronous oscillations that manifest behaviorally as absence seizures. In this review, we highlight new findings that refine thalamic contributions to cortical rhythms and suggest that thalamic oscillators may be subject to both local and global control. We describe endogenous thalamic mechanisms that limit network synchrony and discuss how these protective brakes might be restored to prevent absence seizures. Finally, we describe how intrinsic and circuit-level specializations among thalamocortical loops may determine their involvement in widespread oscillations and render subsets of thalamic nuclei especially vulnerable to pathological synchrony.

Increased inhibitory synaptic activity in the hippocampus (CA1) of genetic absence epilepsy rats: Relevance of kindling resistance

PURPOSE

Genetic absence epilepsy rats from Strasbourg (GAERS), a well-validated genetic rat model for typical absence epilepsy, are known to manifest a resistance to secondary generalization of abnormal focal electrical activity evoked by kindling. The mechanism of this resistance is still unclear. In order to understand the possible mechanism of kindling resistance, we investigated for the first time, the differences of short-term synaptic plasticity by using a paired-pulse paradigm as an indicator of GABAergic activity in CA1 region of hippocampus in GAERS and non-epileptic Wistar rats in-vivo.

METHODS

Rats were subjected to kindling process, basolateral amygdala was stimulated twice a day, with a supra-threshold current, until they displayed limbic or convulsive seizures. One hour after the last kindling stimulation, evoked field potentials from CA1 pyramidal layer of hippocampus were recorded in-vivo under urethane anesthesia.

RESULTS

In response to supra-threshold kindling stimulations GAERS showed a significantly delayed kindling progression and displayed a significant increase in hippocampal excitability at early stages of kindling that is the critical for the development of convulsive seizures. In control rats that were not received kindling stimulation, paired-pulse depression (PPD) was significantly pronounced in GAERS with respect to the Wistar group. During the kindling course, PPD was gradually reduced in the Wistar rats as kindling progression was advanced. However in GAERS, PPD ratios were not significantly changed at early stages of kindling. When GAERS reached convulsive stage, their PPD ratios became similar to that of Wistar rats.

DISCUSSION

The increased inhibition in paired-pulse responses at early stages of kindling in GAERS suggests the role of augmented GABAergic activity as one of the underlying mechanisms of kindling resistance observed in genetic rat models of absence epilepsy.

Ethosuximide Affects Paired-Pulse Facilitation in Somatosensory Cortex of WAG\Rij Rats as a Model of Absence Seizure

PURPOSE

The interaction between somatosensory cortex and thalamus via a thalamocortical loop is a theory behind induction of absence epilepsy. Inside peri-oral somatosensory (S1po) and primary somatosensory forelimb (S1fl) regions, excitatory and inhibitory systems are not balanced and GABAergic inhibitory synapses seem to play a fundamental role in short-term plasticity alterations.

METHODS

We investigated the effects of Ethosuximide on presynaptic changes by utilizing paired-pulse stimulation that was recorded from somatosensory cortex in 18 WAG\Rij rats during epileptic activity. A twisted tripolar electrode including two stimulating electrodes and one recording electrode was implanted into the S1po and S1FL according to stereotaxic landmarks. Paired-pulses (200 µs, 100-1000 µA, 0.1 Hz) were applied to somatosensory cortex at 50, 100, 400, 500 ms inter-pulse intervals for 50 min period.

RESULTS

The results showed that paired-pulse facilitation was significantly reduced at all intervals in all times, but compared to the control group of epileptic WAG/Rij rats (p<0.05), it was exceptional about the first 10 minutes after the injection. At the intervals of 50 and 100 ms, a remarkable PPD was found in second, third, fourth and fifth 10-min post injection.

CONCLUSION

These experiments indicate that Ethosuximide has effects on presynaptic facilitation in somatosensory cortex inhibitory loops by alteration in GABA levels that leads to a markedly diminished PPF in paired-pulse stimulation.

Transplantation of GABAergic Interneurons into the Neonatal Primary Visual Cortex Reduces Absence Seizures in Stargazer Mice

Epilepsies are debilitating neurological disorders characterized by repeated episodes of pathological seizure activity. Absence epilepsy (AE) is a poorly understood type of seizure with an estimated 30% of affected patients failing to respond to antiepileptic drugs. Thus, novel therapies are needed for the treatment of AE. A promising cell-based therapeutic strategy is centered on transplantation of embryonic neural stem cells from the medial ganglionic eminence (MGE), which give rise to gamma-aminobutyric acidergic (GABAergic) interneurons during embyronic development. Here, we used the Stargazer (Stg) mouse model of AE to map affected loci using c-Fos immunohistochemistry, which revealed intense seizure-induce activity in visual and somatosensory cortices. We report that transplantation of MGE cells into the primary visual cortex (V1) of Stg mice significantly reduces AE episodes and lowers mortality. Electrophysiological analysis in acute cortical slices of visual cortex demonstrated that Stg V1 neurons exhibit more pronounced increases in activity in response to a potassium-mediated excitability challenge than wildtypes (WT). The defective network activity in V1 was significantly altered following WT MGE transplantation, associating it with behavioral rescue of seizures in Stgs. Taken together, these findings present MGE grafting in the V1 as a possible clinical approach in the treatment of AE.

Antiepileptic action of N-palmitoylethanolamine through CB1 and PPAR-α receptor activation in a genetic model of absence epilepsy

N-palmitoylethanolamine (PEA), an endogenous fatty acid ethanolamide, plays a key role in the regulation of the inflammatory response and pain through, among others, activation of nuclear peroxisome proliferator-activated receptors (PPAR-α). Endogenous cannabinoids play a protective role in several central nervous system (CNS) disorders, particularly those associated with neuronal hyperexcitability. We investigated the effects of PEA and the role of PPAR-α in absence epilepsy using the WAG/Rij rat model. PEA, anandamide (AEA), a PPAR-α antagonist (GW6471) and a synthetic CB1 receptor antagonist/inverse agonist (SR141716) were administered to WAG/Rij rats in order to evaluate the effects on epileptic spike-wave discharges (SWDs) on EEG recordings. We studied also the effects of PEA co-administration with SR141716 and GW6471 and compared these effects with those of AEA to evaluate PEA mechanism of action and focusing on CB1 receptors and PPAR-α. Both PEA and AEA administration significantly decreased SWDs parameters (absence seizures). In contrast, GW6471 was devoid of effects while SR141716 had pro-absence effects. The co-administration of SR141716 with PEA or AEA completely blocked the anti-absence effects of these compounds. GW6471 antagonized PEA's effects whereas it did not modify AEA's effects. Furthermore, we have also measured PEA, AEA and 2-AG (2-arachidonoylglycerol) brain levels identifying significant differences between epileptic and control rats such as decreased PEA levels in both thalamus and cortex that might contribute to absence epilepsy. Our data demonstrate that PEA has anti-absence properties in the WAG/Rij rat model and that such properties depend on PPAR-α and indirect activation of CB1 receptors. This article is part of the Special Issue entitled 'New Targets and Approaches to the Treatment of Epilepsy'.

T-type calcium channels in burst-firing, network synchrony, and epilepsy

Low voltage-activated (LVA) T-type calcium channels are well regarded as a key mechanism underlying the generation of neuronal burst-firing. Their low threshold for activation combined with a rapid and transient calcium conductance generates low-threshold calcium potentials (LTCPs), upon the crest of which high frequency action potentials fire for a brief period. Experiments using simultaneous electroencephalography (EEG) and intracellular recordings demonstrate that neuronal burst-firing is a likely causative component in the generation of normal sleep patterns as well as some pathophysiological conditions, such as epileptic seizures. However, less is known as to how these neuronal bursts impact brain behavior, in particular network synchronization. In this review we summarize recent findings concerning the role of T-type calcium channels in burst-firing and discuss how they likely contribute to the generation of network synchrony. We further outline the function of burst-firing and network synchrony in terms of epileptic seizures. This article is part of a Special Issue entitled: Calcium channels.

Focal administration of neuropeptide Y into the S2 somatosensory cortex maximally suppresses absence seizures in a genetic rat model

PURPOSE

Neuropeptide Y (NPY) is an inhibitory neurotransmitter that suppresses focal and generalized seizures in animal models. In this study, we investigated the sites within the thalamocortical circuit that NPY acts to suppress seizures in genetic absence epilepsy rats from Strasbourg (GAERS).

METHODS

In conscious freely moving GAERS, NPY was administered via intracerebral microcannulae implanted bilaterally into one of the following regions: primary somatosensory cortex (S1), secondary somatosensory cortex (S2), the primary motor cortex (M1), caudal nucleus reticular thalamus (nRT), or ventrobasal thalamus (VB). Animals received vehicle and up to three doses of NPY, in a randomized order. Electroencephalography (EEG) recordings were carried out for 30 min prior to injection and 90 min after the injection of NPY or vehicle.

KEY FINDINGS

Focal microinjections of NPY into the S2 cortex suppressed seizures in a dose-dependent manner, with the response being significantly different at the highest dose (1.5 mm) compared to vehicle for total time in seizures postinjection (7.2 ± 3.0% of saline, p < 0.01) and average number of seizures (9.4 ± 4.9% of saline, p < 0.05). In contrast NPY microinjections into the VB resulted in an aggravation of seizures.

SIGNIFICANCE

NPY produces contrasting effects on absence-like seizures in GAERS depending on the site of injection within the thalamocortical circuit. The S2 is the site at which NPY most potently acts to suppress absence-like seizures in GAERS, whereas seizure-aggravating effects are seen in the VB. These results provide further evidence to support the proposition that these electroclinically "generalized" seizures are being driven by a topographically restricted region within the somatosensory cortex.

Ethosuximide modifies network excitability in the rat entorhinal cortex via an increase in GABA release

Ethosuximide is the drug of choice for treating generalized absence seizures, but its mechanism of action is still a matter of debate. It has long been thought to act by disrupting a thalamic focus via blockade of T-type channels and, thus, generation of spike-wave activity in thalamocortical pathways. However, there is now good evidence that generalized absence seizures may be initiated at a cortical focus and that ethosuximide may target this focus. In the present study we have looked at the effect ethosuximide on glutamate and GABA release at synapses in the rat entorhinal cortex in vitro, using two experimental approaches. Whole-cell patch-clamp studies revealed an increase in spontaneous GABA release by ethosuximide concurrent with no change in glutamate release. This was reflected in studies that estimated global background inhibition and excitation from intracellularly recorded membrane potential fluctuations, where there was a substantial rise in the ratio of network inhibition to excitation, and a concurrent decrease in excitability of neurones embedded in this network. These studies suggest that, in addition to well-characterised effects on ion channels, ethosuximide may directly elevate synaptic inhibition in the cortex and that this could contribute to its anti-absence effects. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Seizure-like thalamocortical rhythms initiate in the deep layers of the cortex in a co-culture model

The oscillatory rhythms underlying many physiological and pathological states, including absence seizures, require both the thalamus and cortices for full expression. A co-culture preparation combining cortical and thalamic explants provides a unique model for investigating how such oscillations initiate and spread. Here we investigated the dynamics of synchronized thalamocortical activity by simultaneous measurement of field-potential recordings and rapid imaging of Ca(2+) transients by fluorescence methods. Spontaneous sustained hypersynchronized "seizure-like" oscillations required reciprocal cortico-thalamocortical connections. Isolated cortical explants can independently develop brief discharges, while thalamic explants alone were unable to do so. Rapid imaging of Ca(2+) transients demonstrated deep-layer cortical initiation of oscillatory network activity in both connected and isolated explants. Further, cortical explants derived from a rat model of genetic absence epilepsy showed increased bursting duration consistent with an excitable cortex. We propose that thalamocortical oscillatory network activity initiates in deep layers of the cortex with reciprocal thalamic interconnections enabling sustained hyper-synchronization.