Neurosteroid-sensitive δ-GABAA receptors: A role in epileptogenesis?

Identification and verification of key molecules in the epileptogenic process of focal cortical dysplasia

Focal cortical dysplasia (FCD) represents a common developmental malformation associated with drug-resistant epilepsy (DRE) among children. However, the exact molecular mechanisms behind this condition are still unclear. In our study, FCD-associated microarray data from the Gene Expression Omnibus (GEO) database were analyzed. A comprehensive series of bioinformatics analyses were conducted, including screening for differentially expressed genes (DEGs), functional enrichment analysis, weighted gene co-expression network analysis (WGCNA), and protein-protein interaction (PPI) analysis. Subsequently, a freezing lesion (FL) rat model was developed to validate expression levels of hub genes along with the molecular pathways behind FCD epileptogenicity. 320 DEGs were identified, and functional enrichment analysis revealed significant enrichment of these DEGs in "Neuroinflammatory response", "Cytokine production involved in immune response", and "Macrophage activation". Ultimately, 5 potential hub genes (CYBB, ITGAM, FCG3A, LY86, and CD86) were pinpointed. Notably, 4 hub genes (CYBB, ITGAM, FCG3A, and CD86) were validated in in vivo experiments, suggesting possible associations with neuroinflammation triggered by microglia. This underscores the tight relationship between microglia-induced neuroinflammation and the pathological progression of epileptic seizures in FCD. ITGAM, FCG3A, CD86, CYBB, and LY86 may emerge as promising candidate biomarkers, influencing diagnostic and therapeutic strategies.

Why ketamine

We present the rationale for testing ketamine as an add-on therapy for treating benzodiazepine refractory (established) status epilepticus. In animal studies, ketamine terminates benzodiazepine refractory status epilepticus by interfering with the pathophysiological mechanisms and is a neuroprotectant. Ketamine does not suppress respiration when used for sedation and anesthesia. A Series of reports suggest that ketamine can help terminate refractory and super refractory status epilepticus. We propose to use 1 or 3 mg/Kg ketamine intravenously based on animal-to-human conversion and pharmacokinetic studies. This paper was presented at the 8th London-Innsbruck Colloquium on Status Epilepticus and Acute Seizures held in September 2022.

Antiepileptogenic effects of trilostane in the kainic acid model of temporal lobe epilepsy

OBJECTIVE

Epileptogenesis after status epilepticus (SE) has a faster onset in rats treated to reduce brain levels of the anticonvulsant neurosteroid allopregnanolone with the 5α-reductase inhibitor finasteride; however, it still has to be evaluated whether treatments aimed at increasing allopregnanolone levels could result in the opposite effect of delaying epileptogenesis. This possibility could be tested using the peripherally active inhibitor of 3β-hydroxysteroid dehydrogenase/Δ^5-4 isomerase trilostane, which has been shown repeatedly to increase allopregnanolone levels in the brain.

METHODS

Trilostane (50 mg/kg) was administered subcutaneously once daily for up to six consecutive days, starting 10 min after intraperitoneal administration of kainic acid (15 mg/kg). Seizures were evaluated by video-electrocorticographic recordings for 70 days maximum, and endogenous neurosteroid levels were assessed by liquid chromatography-electrospray tandem mass spectrometry. Immunohistochemical staining was performed to evaluate the presence of brain lesions.

RESULTS

Trilostane did not alter the latency of kainic acid-induced SE onset or its overall duration. When compared to the vehicle-treated group, rats receiving six daily trilostane injections presented a remarkable delay of the first spontaneous electrocorticographic seizure and subsequent tonic-clonic spontaneous recurrent seizures (SRSs). Conversely, rats treated with only the first trilostane injection during SE did not differ from vehicle-treated rats in developing the SRSs. Notably, trilostane did not modify neuronal cell densities or the overall damage in the hippocampus. In comparison to the vehicle group, repeated administration of trilostane significantly decreased the activated microglia morphology in the subiculum. As expected, allopregnanolone and other neurosteroid levels were remarkably increased in the hippocampus and neocortex of rats treated for 6 days with trilostane, but pregnanolone was barely detectable. Neurosteroids returned to basal levels after a week of trilostane washout.

SIGNIFICANCE

Overall, these results suggest that trilostane led to a remarkable increase in allopregnanolone brain levels, which was associated with protracted effects on epileptogenesis.

In the fast lane: Receptor trafficking during status epilepticus

Status epilepticus (SE) remains a significant cause of morbidity and mortality and often is refractory to standard first-line treatments. A rapid loss of synaptic inhibition and development of pharmacoresistance to benzodiazepines (BZDs) occurs early during SE, while NMDA and AMPA receptor antagonists remain effective treatments after BZDs have failed. Multimodal and subunit-selective receptor trafficking within minutes to an hour of SE involves GABA-A, NMDA, and AMPA receptors and contributes to shifts in the number and subunit composition of surface receptors with differential impacts on the physiology, pharmacology, and strength of GABAergic and glutamatergic currents at synaptic and extrasynaptic sites. During the first hour of SE, synaptic GABA-A receptors containing γ2 subunits move to the cell interior while extrasynaptic GABA-A receptors with δ subunits are preserved. Conversely, NMDA receptors containing N2B subunits are increased at synaptic and extrasynaptic sites, and homomeric GluA1 ("GluA2-lacking") calcium permeant AMPA receptor surface expression also is increased. Molecular mechanisms, largely driven by NMDA receptor or calcium permeant AMPA receptor activation early during circuit hyperactivity, regulate subunit-specific interactions with proteins involved with synaptic scaffolding, adaptin-AP2/clathrin-dependent endocytosis, endoplasmic reticulum (ER) retention, and endosomal recycling. Reviewed here is how SE-induced shifts in receptor subunit composition and surface representation increase the excitatory to inhibitory imbalance that sustains seizures and fuels excitotoxicity contributing to chronic sequela such as "spontaneous recurrent seizures" (SRS). A role for early multimodal therapy is suggested both for treatment of SE and for prevention of long-term comorbidities.

Virally-induced expression of GABAA receptor δ subunits following their pathological loss reveals their role in regulating GABAA receptor assembly

Decreased expression of the δ subunit of the GABA_A receptor (GABA_AR) has been found in the dentate gyrus in several animal models of epilepsy and other disorders with increased excitability and is associated with altered modulation of tonic inhibition in dentate granule cells (GCs). In contrast, other GABA_AR subunits, including α4 and γ2 subunits, are increased, but the relationship between these changes is unclear. The goals of this study were to determine if viral transfection of δ subunits in dentate GCs could increase δ subunit expression, alter expression of potentially-related GABA_AR subunits, and restore more normal network excitability in the dentate gyrus in a mouse model of epilepsy. Pilocarpine-induced seizures were elicited in DOCK10-Cre mice that express Cre selectively in dentate GCs, and two weeks later the mice were injected unilaterally with a Cre-dependent δ-GABA_AR viral vector. At 4-6 weeks following transfection, δ subunit immunolabeling was substantially increased in dentate GCs on the transfected side compared to the nontransfected side. Importantly, α4 and γ2 subunit labeling was downregulated on the transfected side. Electrophysiological studies revealed enhanced tonic inhibition, decreased network excitability, and increased neurosteroid sensitivity in slices from the δ subunit-transfected side compared to those from the nontransfected side of the same pilocarpine-treated animal, consistent with the formation of δ subunit-containing GABA_ARs. No differences were observed between sides of eYFP-transfected animals. These findings are consistent with the idea that altering expression of key subunits, such as the δ subunit, regulates GABA_AR subunit assemblies, resulting in substantial effects on network excitability.

Neurosteroid replacement therapy for catamenial epilepsy, postpartum depression and neuroendocrine disorders in women

Neurosteroids are involved in the pathophysiology of many neuroendocrine disorders in women. This review describes recent advancements in pharmacology of neurosteroids and emphasizes the benefits of neurosteroid replacement therapy for the management of neuroendocrine disorders such as catamenial epilepsy (CE), postpartum depression (PPD) and premenstrual brain conditions. Neurosteroids are endogenous modulators of neuronal excitability. A variety of neurosteroids are present in the brain including allopregnanolone (AP), allotetrahydro-deoxycorticosterone and androstanediol. Neurosteroids interact with synaptic and extrasynaptic GABA_A receptors in the brain. AP and related neurosteroids, which are positive allosteric modulators of GABA_A receptors, are powerful anticonvulsants, anxiolytic, antistress and neuroprotectant agents. In CE, seizures are most often clustered around a specific menstrual period in women. Neurosteroid withdrawal-linked plasticity in extrasynaptic receptors has been shown to play a key role in catamenial seizures, anxiety and other mood disorders. Based on our extensive research spanning two decades, we have proposed and championed neurosteroid replacement therapy as a rational strategy for treating disorders marked by neurosteroid-deficiency, such as CE and other related ovarian or menstrual disorders. In 2019, AP (renamed as brexanolone) was approved for treating PPD. A variety of synthetic neurosteroids are in clinical trials for epilepsy, depression and other brain disorders. Recent advancements in our understanding of neurosteroids have entered a new era of drug discovery and one that offers a high therapeutic potential for treating complex brain disorders.

Molecular mechanisms of sex differences in epilepsy and seizure susceptibility in chemical, genetic and acquired epileptogenesis

This article provides a succinct overview of sex differences in epilepsy and putative molecular mechanisms underlying sex differences in seizure susceptibility in chemical, genetic, and acquired epileptogenesis. The susceptibility to excitability episodes and occurrence of epileptic seizures are generally higher in men than women. The precise molecular mechanisms remain unclear, but differences in regional morphology and neural circuits in men and women may explain differential vulnerability to seizures and epileptogenic cascades. Changes in seizure sensitivity can be attributed to steroid hormones, including fluctuations in neurosteroids as well as neuroplasticity in their receptor signaling systems. Other potential neurobiological bases for sex differences in epilepsies include differences in brain development, neurogenesis, neuronal chloride homeostasis, and neurotrophic and glial responses. In catamenial epilepsy, a gender-specific neuroendocrine condition, epileptic seizures are most often clustered around a specific menstrual period in adult women. A deeper understanding of the molecular and neural network basis of sex differences in seizures and response to antiepileptic drugs is highly warranted for designing effective, sex-specific therapies for epilepsy, epileptogenesis, and seizure disorders.

Progesterone modulates neuronal excitability bidirectionally

Progesterone acts on neurons directly by activating its receptor and through metabolic conversion to neurosteroids. There is emerging evidence that progesterone exerts excitatory effects by activating its cognate receptors (progesterone receptors, PRs) through enhanced expression of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs). Progesterone metabolite 5α,3α-tetrahydro-progesterone (allopregnanolone, THP) mediates its anxiolytic and sedative actions through the potentiation of synaptic and extrasynaptic γ-aminobutyric acid type-A receptors (GABAARs). Here, we review progesterone's neuromodulatory actions exerted through PRs and THP and their opposing role in regulating seizures, catamenial epilepsy, and seizure exacerbation associated with progesterone withdrawal.

Allopregnanolone and Pregnanolone Are Reduced in the Hippocampus of Epileptic Rats, but Only Allopregnanolone Correlates with Seizure Frequency

BACKGROUND

Neurosteroids modulate epileptic activity by interacting with the γ-aminobutyric acid type A receptor, but their brain levels are still undetermined.

OBJECTIVES

We aimed to establish neurosteroid levels in the neocortex and hippocampus by liquid chromatography/mass spectrometry in epileptic rats.

METHODS

Kainic acid-treated rats were continuously monitored up to 9 weeks to determine seizure frequency by video electrocorticography (n = 23) and compared to age-matched controls monitored in the same manner (n = 11).

RESULTS

Decreased allopregnanolone (-50%; p < 0.05, Mann-Whitney test) and pregnanolone levels (-64%; p < 0.01) were found in the hippocampus, whereas pregnenolone sulfate, pregnenolone, progesterone, and 5α-dihydroprogesterone were nonsignificantly reduced. No changes were found in the neocortex. Moreover, allopregnanolone (but not pregnanolone) levels were positively correlated with seizure frequency (r2 = 0.4606, p < 0.01).

CONCLUSION

These findings indicate a selective reduction in hippocampal levels of 3α-reduced neurosteroids. This reduction was partially mitigated by seizures in the case of allopregnanolone.

Neurosteroids and Focal Epileptic Disorders

Neurosteroids are a family of compounds that are synthesized in principal excitatory neurons and glial cells, and derive from the transformation of cholesterol into pregnenolone. The most studied neurosteroids—allopregnanolone and allotetrahydrodeoxycorticosterone (THDOC)—are known to modulate GABA_A receptor-mediated transmission, thus playing a role in controlling neuronal network excitability. Given the role of GABA_A signaling in epileptic disorders, neurosteroids have profound effects on seizure generation and play a role in the development of chronic epileptic conditions (i.e., epileptogenesis). We review here studies showing the effects induced by neurosteroids on epileptiform synchronization in in vitro brain slices, on epileptic activity in in vivo models, i.e., in animals that were made epileptic with chemoconvulsant treatment, and in epileptic patients. These studies reveal that neurosteroids can modulate ictogenesis and the occurrence of pathological network activity such as interictal spikes and high-frequency oscillations (80–500 Hz). Moreover, they can delay the onset of spontaneous seizures in animal models of mesial temporal lobe epilepsy. Overall, this evidence suggests that neurosteroids represent a new target for the treatment of focal epileptic disorders.

The Need to Intervene Before Time Point 2: Evidence From Clinical and Animal Data That Status Epilepticus Damages the Brain

Status epilepticus, a condition characterized by abnormally prolonged seizures, has the potential to cause irreversible, structural or functional, injury to the brain. Unfavorable consequences of these seizures include mortality, the risk of developing epilepsy, and cognitive impairment. We highlight key findings of clinical and laboratory studies that have provided insights into aspects of cell death, and anatomical and functional alterations triggered by status epilepticus that support the need to intervene before time point 2, the time after which the risk of these long-term consequences increases.

Reduced neurosteroid potentiation of GABAA receptors in epilepsy and depolarized hippocampal neurons

OBJECTIVE

Neurosteroids regulate neuronal excitability by potentiating γ-aminobutyric acid type-A receptors (GABARs). In animal models of temporal lobe epilepsy, the neurosteroid sensitivity of GABARs is diminished and GABAR subunit composition is altered. We tested whether similar changes occur in patients with epilepsy and if depolarization-induced increases in neuronal activity can replicate this effect.

METHODS

We determined GABAR α4 subunit expression in cortical tissue resected from pediatric epilepsy patients. Modulation of human GABARs by allopregnanolone and Ro15-4513 was measured in Xenopus oocytes using whole-cell patch clamp. To extend the findings obtained using tissue from epilepsy patients, we evaluated GABAR expression and modulation by allopregnanolone and Ro15-4513 in cultured rat hippocampal neurons exposed to high extracellular potassium (HK) to increase neuronal activity.

RESULTS

Expression of α4 subunits was increased in pediatric cortical epilepsy specimens encompassing multiple pathologies. The potentiation of GABA-evoked currents by the neurosteroid allopregnanolone was decreased in Xenopus oocytes expressing GABARs isolated from epilepsy patients. Furthermore, receptors isolated from epilepsy but not control tissue were sensitive to potentiation by Ro15-4513, indicating higher expression of α4 βx γ2 subunit-containing receptors. Correspondingly, increasing the activity of cultured rat hippocampal neurons reduced allopregnanolone potentiation of miniature inhibitory postsynaptic currents (mIPSCs), increased modulation of tonic GABAR current by Ro15-4513, upregulated the surface expression of α4 and γ2 subunits, and increased the colocalization of α4 and γ2 subunit immunoreactivity.

INTERPRETATION

These findings suggest that seizure activity-induced upregulation of α4 βx γ2 subunit-containing GABARs could affect the anticonvulsant actions of neurosteroids.

Neurosteroid regulation of GABAA receptors: A role in catamenial epilepsy

The female reproductive hormones progesterone and estrogen regulate network excitability. Fluctuations in the circulating levels of these hormones during the menstrual cycle cause frequent seizures during certain phases of the cycle in women with epilepsy. This seizure exacerbation, called catamenial epilepsy, is a dominant form of drug-refractory epilepsy in women of reproductive age. Progesterone, through its neurosteroid derivative allopregnanolone, increases γ-aminobutyric acid type-A receptor (GABAR)-mediated inhibition in the brain and keeps seizures under control. Catamenial seizures are believed to be a neurosteroid withdrawal symptom, and it was hypothesized that exogenous administration of progesterone to maintain its levels high during luteal phase will treat catamenial seizures. However, in a multicenter, double-blind, phase III clinical trial, progesterone treatment did not suppress catamenial seizures. The expression of GABARs with reduced neurosteroid sensitivity in epileptic animals may explain the failure of the progesterone clinical trial. The expression of neurosteroid-sensitive δ subunit-containing GABARs is reduced, and the expression of α4γ2 subunit-containing GABARs is upregulated, which alters the inhibition of dentate granule cells in epilepsy. These changes reduce the endogenous neurosteroid control of seizures and contribute to catamenial seizures.

Role of NMDA receptors in the pathophysiology and treatment of status epilepticus

This review considers the role of N-methyl-d-aspartate receptors in the pathophysiology and treatment of status epilepticus (SE). NMDA receptors play a critical role in sustaining SE by mediating the plasticity of γ-aminobutyric acid (GABA)-_A and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, neuronal loss, and epileptogenesis. In parallel, there is growing interest in using the NMDA receptor antagonist ketamine in the treatment of refractory SE. Ketamine has proved to be safe for use in refractory and super-refractory SE in patients. The pilot studies also suggest that ketamine may be efficacious for termination of refractory SE.

GABA type a receptor trafficking and the architecture of synaptic inhibition

Ubiquitous expression of GABA type A receptors (GABAA R) in the central nervous system establishes their central role in coordinating most aspects of neural function and development. Dysregulation of GABAergic neurotransmission manifests in a number of human health disorders and conditions that in certain cases can be alleviated by drugs targeting these receptors. Precise changes in the quantity or activity of GABAA Rs localized at the cell surface and at GABAergic postsynaptic sites directly impact the strength of inhibition. The molecular mechanisms constituting receptor trafficking to and from these compartments therefore dictate the efficacy of GABAA R function. Here we review the current understanding of how GABAA Rs traffic through biogenesis, plasma membrane transport, and degradation. Emphasis is placed on discussing novel GABAergic synaptic proteins, receptor and scaffolding post-translational modifications, activity-dependent changes in GABAA R confinement, and neuropeptide and neurosteroid mediated changes. We further highlight modern techniques currently advancing the knowledge of GABAA R trafficking and clinically relevant neurodevelopmental diseases connected to GABAergic dysfunction. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 238-270, 2018.

Genetic and Molecular Regulation of Extrasynaptic GABA-A Receptors in the Brain: Therapeutic Insights for Epilepsy

GABA-A receptors play a pivotal role in many brain diseases. Epilepsy is caused by acquired conditions and genetic defects in GABA receptor channels regulating neuronal excitability in the brain. The latter is referred to as GABA channelopathies. In the last two decades, major advances have been made in the genetics of epilepsy. The presence of specific GABAergic genetic abnormalities leading to some of the classic epileptic syndromes has been identified. Advances in molecular cloning and recombinant systems have helped characterize mutations in GABA-A receptor subunit genes in clinical neurology. GABA-A receptors are the prime targets for neurosteroids (NSs). However, GABA-A receptors are not static but undergo rapid changes in their number or composition in response to the neuroendocrine milieu. This review describes the recent advances in the genetic and neuroendocrine control of extrasynaptic and synaptic GABA-A receptors in epilepsy and its impact on neurologic conditions. It highlights the current knowledge of GABA genetics in epilepsy, with an emphasis on the neuroendocrine regulation of extrasynaptic GABA-A receptors in network excitability and seizure susceptibility. Recent advances in molecular regulation of extrasynaptic GABA-A receptor-mediated tonic inhibition are providing unique new therapeutic approaches for epilepsy, status epilepticus, and certain brain disorders. The discovery of an extrasynaptic molecular mechanism represents a milestone for developing novel therapies such as NS replacement therapy for catamenial epilepsy.

Decreased surface expression of the δ subunit of the GABAA receptor contributes to reduced tonic inhibition in dentate granule cells in a mouse model of fragile X syndrome

While numerous changes in the GABA system have been identified in models of Fragile X Syndrome (FXS), alterations in subunits of the GABA_A receptors (GABA_ARs) that mediate tonic inhibition are particularly intriguing. Considering the key role of tonic inhibition in controlling neuronal excitability, reduced tonic inhibition could contribute to FXS-associated disorders such as hyperactivity, hypersensitivity, and increased seizure susceptibility. The current study has focused on the expression and function of the δ subunit of the GABA_AR, a major subunit involved in tonic inhibition, in granule cells of the dentate gyrus in the Fmr1 knockout (KO) mouse model of FXS. Electrophysiological studies of dentate granule cells revealed a marked, nearly four-fold, decrease in tonic inhibition in the Fmr1 KO mice, as well as reduced effects of two δ subunit-preferring pharmacological agents, THIP and DS2, supporting the suggestion that δ subunit-containing GABA_ARs are compromised in the Fmr1 KO mice. Immunohistochemistry demonstrated a small but statistically significant decrease in δ subunit labeling in the molecular layer of the dentate gyrus in Fmr1 KO mice compared to wildtype (WT) littermates. The discrepancy between the large deficits in GABA-mediated tonic inhibition in granule cells in the Fmr1 KO mice and only modest reductions in immunolabeling of the δ subunit led to studies of surface expression of the δ subunit. Cross-linking experiments followed by Western blot analysis demonstrated a small, non-significant decrease in total δ subunit protein in the hippocampus of Fmr1 KO mice, but a four-fold decrease in surface expression of the δ subunit in these mice. No significant changes were observed in total or surface expression of the α4 subunit protein, a major partner of the δ subunit in the forebrain. Postembedding immunogold labeling for the δ subunit demonstrated a large, three-fold, decrease in the number of symmetric synapses with immunolabeling at perisynaptic locations in Fmr1 KO mice. While α4 immunogold particles were also reduced at perisynaptic locations in the Fmr1 KO mice, the labeling was increased at synaptic sites. Together these findings suggest that, in the dentate gyrus, altered surface expression of the δ subunit, rather than a decrease in δ subunit expression alone, could be limiting δ subunit-mediated tonic inhibition in this model of FXS. Finding ways to increase surface expression of the δ subunit of the GABA_AR could be a novel approach to treatment of hyperexcitability-related alterations in FXS.