Reduced expression of perineuronal nets in the normotopic somatosensory cortex of the tish rat.

The tish (telencephalic internal structural heterotopia) rat is a naturally occurring and unique model of a malformation of cortical development (MCD) arising from a sponeantous mutation in the Eml1 gene. Tish rats are characterized by a macroscopic bilateral heterotopic dysplastic cortex (HDCx) and an overlaying, intact normotopic neocortex (NNCx). These two cortices are functional and have been reported to innervate and establish connections with subcortical regions including the thalamus, resulting in a dual-cortical representation. Additionally, impaired GABAergic neurotransmission and early-onset spike wave discharge bursts have been reported in developing tish rats. Perineuronal nets (PNNs) are specialized extraceullar matrix structures that predominately surround and stabilize parvalbumin-positive (PV+) GABAergic interneurons and are essential components of the neural landscape. Here, we report a significant reduction in the average number of WFA+-PNNs in the normotopic somatosensory cortex (NSSCx) of the tish rat at two developmental time points, P16 and P35, corresponding to a decrease in the number of PV+ interneurons ensheathed by a PNN in the NSSCx. Compared with control animals, PNN expression was partially, but significantly restored following treatment with insulin-like growth factor 1 (IGF-1). These data suggest that the 'dual cortical representation' in the setting of an MCD reduces the cortical activation necessary for proper PNN expression likely contributing to the impairments in GABAergic neurotransmission and network excitability previously identified in the tish rat.

A deletion in Eml1 leads to bilateral subcortical heterotopia in the tish rat.

Children with malformations of cortical development (MCD) are at risk for epilepsy, developmental delays, behavioral disorders, and intellectual disabilities. For a subset of these children, antiseizure medications or epilepsy surgery may result in seizure freedom. However, there are limited options for treating or curing the other conditions, and epilepsy surgery is not an option in all cases of pharmacoresistant epilepsy. Understanding the genetic and neurobiological mechanisms underlying MCD is a necessary step in elucidating novel therapeutic targets. The tish (telencephalic internal structural heterotopia) rat is a unique model of MCD with spontaneous seizures, but the underlying genetic mutation(s) have remained unknown. DNA and RNA-sequencing revealed that a deletion encompassing a previously unannotated first exon markedly diminished Eml1 transcript and protein abundance in the tish brain. Developmental electrographic characterization of the tish rat revealed early-onset of spontaneous spike-wave discharge (SWD) bursts beginning at postnatal day (P) 17. A dihybrid cross demonstrated that the mutant Eml1 allele segregates with the observed dysplastic cortex and the early-onset SWD bursts in monogenic autosomal recessive frequencies. Our data link the development of the bilateral, heterotopic dysplastic cortex of the tish rat to a deletion in Eml1.

The Role of Kainate Receptors in the Pathophysiology of Hypoxia-Induced Seizures in the Neonatal Mouse.

Kainate receptors (KARs) are glutamate receptors with peak expression during late embryonic and early postnatal periods. Altered KAR-mediated neurotransmission and subunit expression are observed in several brain disorders, including epilepsy. Here, we examined the role of KARs in regulating seizures in neonatal C57BL/6 mice exposed to a hypoxic insult. We found that knockout of the GluK2 subunit, or blockade of KARs by UBP310 reduced seizure susceptibility during the period of reoxygenation. Following the hypoxic insult, we observed an increase in excitatory neurotransmission in hippocampal CA3 pyramidal cells, which was blocked by treatment with UBP310 prior to hypoxia. Similarly, we observed increased excitatory neurotransmission in CA3 pyramidal cells in an in vitro hippocampal slice model of hypoxic-ischemia. This increase was absent in slices from GluK2-/- mice and in slices treated with UBP310, suggesting that KARs regulate, at least in part, excitatory synaptic neurotransmission following in vivo hypoxia in neonatal mice. Data from these hypoxia models demonstrate that KARs, specifically those containing the GluK2 subunit, contribute to alterations in excitatory neurotransmission and seizure susceptibility, particularly during the reoxygenation period, in neonatal mice. Therapies targeting KARs may prove successful in treatment of neonates affected by hypoxic seizures.

Stiripentol in refractory status epilepticus.

Benzodiazepines (BZDs), which enhance γ-aminobutyric acid (GABAA ) receptor-mediated inhibition, are the first-line therapy for treatment of status epilepticus (SE). However, pharmacoresistance to BZDs develops rapidly after SE initiation. This is due to an activity-dependent internalization of BZD-sensitive GABAA receptors during SE. Stiripentol (STP) is a positive allosteric modulator of GABAA receptors with a unique subunit selectivity profile. We report that in a rodent model of SE, STP terminates behavioral seizures and remains effective in established SE when seizures have become BZD resistant. The anticonvulsant effects of STP are age dependent, with greater potency in juvenile animals. Whole cell recordings from dentate granule cells in hippocampal slices reveal that STP potentiates GABAergic inhibitory postsynaptic currents (IPSCs) and tonic GABAergic currents by acting at a site on the GABAA receptor that is separate from the benzodiazepine binding site. This potentiation persists in established SE, whereas potentiation of GABAergic inhibition by BZDs is lost. STP potentiates IPSCs in juvenile animals with greater potency than in adult animals. We suggest that STP, either alone or as add-on therapy, may prove useful in treating established and BZD-resistant status epilepticus. Furthermore, STP may be particularly effective in terminating SE in children when SE is most prevalent.

Stiripentol is anticonvulsant by potentiating GABAergic transmission in a model of benzodiazepine-refractory status epilepticus.

Benzodiazepines (BZDs) are first-line therapy for treatment of status epilepticus (SE). However, BZD treatment is negatively affected by seizure duration due to decreases in BZD-sensitive GABA(A) receptors during prolonged SE. Stiripentol (STP) is an anticonvulsant that is used as add-on treatment for Dravet Syndrome. Recent studies have shown that STP is a positive allosteric modulator of the GABA(A) receptor. The subunit selectivity of STP at this receptor suggests that it would be anticonvulsant in both brief as well as prolonged SE. We tested this possibility by comparing the ability of STP and diazepam (DZP), a commonly used BZD, to terminate behavioral convulsions in a rodent model of pharmacoresistant SE. We found that STP was anticonvulsant in this model and remained effective during prolonged SE, unlike DZP which exhibited a 14 fold increase in its ED(50). Whole cell recording from hippocampal slices from these animals revealed that STP potentiated GABAergic IPSCs, as well as tonic GABAergic current by acting at a site on the GABA(A) receptor separate from the BDZ binding site. Potentiation of GABAergic currents by STP remained intact during prolonged SE, while potentiation by DZP was lost. Both IPSC potentiation and anticonvulsant activity of STP were greater in younger animals than in adults. These findings suggest that at doses that yield therapeutically relevant concentrations, STP is anticonvulsant by potentiating GABAergic inhibition and that the subunit selectivity profile of STP enables it to remain effective despite GABA(A) receptor subunit changes during prolonged SE.

The natural products magnolol and honokiol are positive allosteric modulators of both synaptic and extra-synaptic GABA(A) receptors.

The National Center for Complementary and Alternative Medicine (NCCAM) estimates that nearly 40% of adults in the United States use alternative medicines, often in the form of an herbal supplement. Extracts from the tree bark of magnolia species have been used for centuries in traditional Chinese and Japanese medicines to treat a variety of neurological diseases, including anxiety, depression, and seizures. The active ingredients in the extracts have been identified as the bi-phenolic isomers magnolol and honokiol. These compounds were shown to enhance the activity of GABA(A) receptors, consistent with their biological effects. The GABA(A) receptors exhibit substantial subunit heterogeneity, which influences both their functional and pharmacological properties. We examined the activity of magnolol and honokiol at different populations of both neuronal and recombinant GABA(A) receptors to characterize their mechanism of action and to determine whether sensitivity to modulation was dependent upon the receptor's subunit composition. We found that magnolol and honokiol enhanced both phasic and tonic GABAergic neurotransmission in hippocampal dentate granule neurons. In addition, all recombinant receptors examined were sensitive to modulation, regardless of the identity of the α, ß, or γ subunit subtype, although the compounds showed particularly high efficacy at δ-containing receptors. This direct positive modulation of both synaptic and extra-synaptic populations of GABA(A) receptors suggests that supplements containing magnolol and/or honokiol would be effective anxiolytics, sedatives, and anti-convulsants. However, significant side-effects and risk of drug interactions would also be expected.