Anticonvulsant action of allopregnanolone in immature rats

Epilepsy Research in the Institute of Physiology of the Czech Academy of Sciences in Prague

Starting from simple clinical statistics, the spectrum of methods used in epilepsy research in the Institute of Physiology of the Czechoslovak (now Czech) Academy of Sciences progressively increased. Professor Servít used electrophysiological methods for study of brain activity in lower vertebrates, neuropathology was focused on electronmicroscopic study of cortical epileptic focus and ion-sensitive microelectrodes were used for studies of cortical direct current potentials. Developmental studies used electrophysiological methods (activity and projection of cortical epileptic foci, EEG under the influence of convulsant drugs, hippocampal, thalamic and cortical electrical stimulation for induction of epileptic afterdischarges and postictal period). Extensive pharmacological studies used seizures elicited by convulsant drugs (at first pentylenetetrazol but also other GABA antagonists as well as agonists of glutamate receptors). Motor performance and behavior were also studied during brain maturation. The last but not least molecular biology was included into the spectrum of methods. Many original data were published making a background of position of our laboratory in the first line of laboratories interested in brain development.

Neurosteroids and Seizure Activity

Still circa 25% to 30% of patients with epilepsy cannot be efficiently controlled with available antiepileptic drugs so newer pharmacological treatment options have been continuously searched for. In this context, a group of endogenous or exogenous neurosteroids allosterically positively modulating GABA-A receptors may offer a promising approach. Among endogenous neurosteroids synthesized in the brain, allopregnanolone or allotetrahydrodeoxycorticosterone have been documented to exert anticonvulsant activity in a number of experimental models of seizures-pentylenetetrazol-, bicuculline- pilocarpine-, or 6 Hz-induced convulsions in rodents. Neurosteroids can also inhibit fully kindled seizures and some of them have been reported to counteract maximal electroshock-induced convulsions. An exogenous neurosteroid, alphaxalone, significantly elevated the threshold for maximal electroconvulsions in mice but it did not potentiate the anticonvulsive action of a number of conventional antiepileptic drugs against maximal electroshock-induced seizures. Androsterone not only elevated the threshold but significantly enhanced the protective action of carbamazepine, gabapentin and phenobarbital against maximal electroshock in mice, as well. Ganaxolone (a 3beta-methylated analog of allopregnanolone) needs special consideration for two reasons. First, it performed better than conventional antiepileptic drugs, diazepam or valproate, in suppressing convulsive and lethal effects of pentylenetetrazol in pentylenetetrazol-kindled mice. Second, ganaxolone has been evaluated in the randomized, double-blind, placebo-controlled phase 2 trial in patients with intractable partial seizures, taking maximally 3 antiepileptic drugs. The initial results indicate that add-on therapy with ganaxolone resulted in reduced seizure frequency with adverse effect being mainly mild to moderate. Possibly, ganaxolone may be also considered against catamenial seizures. Some positive effects of ganaxolone as an adjuvant were also observed in children with refractory seizures and its use may also prove efficient for the management of neonatal seizures associated with hypoxic injury. Neurosteroids positively modulating GABA-A receptor complex exert anticonvulsive activity in many experimental models of seizures. Their interactions with antiepileptic drugs seem ambiguous in mice. Initial clinical data indicate that ganaxolone may provide a better seizure control in patients with drug-resistant epilepsy.

Allopregnanolone augments epileptiform activity of an in-vitro mouse hippocampal preparation in the first postnatal week

In the immature brain the neurotransmitter γ-amino butyric acid (GABA) mediates a membrane depolarization and can contribute to both, inhibition and excitation. Therefore the consequences of a positive modulation of GABA(A) receptors by neurosteroids on epileptiform activity are hard to predict. In order to analyze whether neurosteroids attenuate or exaggerate epileptiform activity in the immature brain, we investigated the effect of the neurosteroid allopregnanolone on epileptiform activity in an in-toto hippocampus preparation of early postnatal mice (postnatal days 4-7) using field potential recordings. These in-vitro experiments revealed that 0.5 μmol/L allopregnanolone had no effect on ictal-like epileptiform activity, but increased the occurrence of interictal epileptiform events. The allopregnanolone-induced enhancement of interictal epileptiform activity could be blocked by a selective inhibition of synaptic GABA_A receptors. In contrast, allopregnanolone had no effect on interictal epileptiform activity upon enhanced extrasynaptic GABAergic activity. Patch-clamp experiments demonstrated that allopregnanolone prolonged the decay of GABAergic postsynaptic currents, but had no effect on tonic GABAergic currents. We conclude from these results that allopregnanolone can enhance excitability in the immature hippocampus viaprolonged synaptic GABAergic currents. This potential effect of neurosteroids on brain excitability should be considered if they are applied as anticonvulsants to premature or early postnatal babies.

Allopregnanolone and neuroinflammation: a focus on multiple sclerosis

The progesterone derivative allopregnanolone (ALLO) is one of the most widely studied compounds among neurosteroids. Through interactions with GABA-A receptors expressed by neurons and glial cells, ALLO has been shown to affect diverse aspects of neural cell physiology, including cell proliferation and survival, migration, and gene expression. Recent data point to important roles for ALLO in different neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, and multiple sclerosis (MS). Dysregulation in ALLO biosynthesis pathways has been reported in brain tissue from MS patients as well as in the central nervous system (CNS) tissue derived from MS animal models. Administration of ALLO has been shown to ameliorate neurobehavioral deficits together with neuropathology and inflammation in the CNS of animals with autoimmune demyelination. These findings are in line with previous reports indicating growth- and differentiation-promoting actions of ALLO on neurons and glial cells as well as its neuroprotective effects in the context of other CNS diseases. Nonetheless, these findings have also raised the possibility that ALLO might influence leukocyte biology and associated neuroinflammatory mechanisms independent of its neuroregenerative properties. Herein, we review the current knowledge regarding the role of ALLO in the pathogenesis of MS, and discuss the potential cellular and molecular pathways that might be influenced by ALLO in the context of disease.

Neuroactive steroids for the treatment of status epilepticus

Benzodiazepines are the current first-line standard-of-care treatment for status epilepticus but fail to terminate seizures in about one third of cases. Synaptic GABAA receptors, which mediate phasic inhibition in central circuits, are the molecular target of benzodiazepines. As status epilepticus progresses, these receptors are internalized and become functionally inactivated, conferring benzodiazepine resistance, which is believed to be a major cause of treatment failure. GABAA receptor positive allosteric modulator neuroactive steroids, such as allopregnanolone, also potentiate synaptic GABAA receptors, but in addition they enhance extrasynaptic GABAA receptors that mediate tonic inhibition. Extrasynaptic GABAA receptors are not internalized, and desensitization of these receptors does not occur during continuous seizures in status epilepticus models. Here we review the broad-spectrum antiseizure activity of allopregnanolone in animal seizure models and the evidence for its activity in models of status epilepticus. We also demonstrate that allopregnanolone inhibits ongoing behavioral and electrographic seizures in a model of status epilepticus, even when there is benzodiazepine resistance. Parenteral allopregnanolone may provide an improved treatment for refractory status epilepticus.

Up-regulation of neurosteroid biosynthesis as a pharmacological strategy to improve behavioural deficits in a putative mouse model of post-traumatic stress disorder

Benzodiazepines remain the most frequently used psychotropic drugs for the treatment of anxiety spectrum disorders; however, their use is associated with the development of tolerance and dependence. Another major hindrance is represented by their lack of efficacy in many patients, including patients with post-traumatic stress disorder (PTSD). For these nonresponders, the use of selective serotonin reuptake inhibitors (SSRIs) has been the therapy of choice. In the past decade, clinical studies have suggested that the pharmacological action of SSRIs may include the ability of these drugs to normalise decreased brain levels of neurosteroids in patients with depression and PTSD; in particular, the progesterone derivative allopregnanolone, which potently, positively and allosterically modulates the action of GABA at GABA(A) receptors. Preclinical studies using the socially-isolated mouse as an animal model of PTSD have demonstrated that fluoxetine and congeners ameliorate anxiety-like behaviour, fear responses and aggressive behaviour expressed by such mice by increasing corticolimbic levels of allopregnanolone. This is a novel and more selective mechanism than serotonin reuptake inhibition, which, for half a century, has been considered to be the main molecular mechanism for the therapeutic action of SSRIs. Importantly, this finding may shed light on the high rates of SSRI resistance among patients with PTSD and depression, comprising disorders in which there appears to be a block in allopregnanolone synthesis. There are several different mechanisms by which such a block may occur, and SSRIs may only be corrective under some conditions. Thus, the up-regulation of allopregnanolone biosynthesis in corticolimbic neurones may offer a novel nontraditional pharmacological target for a new generation of potent nonsedating, anxiolytic medications for the treatment of anxiety, depression, and PTSD: selective brain steroidogenic stimulants.

Allopregnanolone potentiates the glutamate-mediated seizures induced by 4-aminopyridine in rat hippocampus in vivo

Excitatory and inhibitory neurotransmission in the central nervous system can be modulated by neurosteroids. We previously found that in rat hippocampal slices allopregnanolone (3α-hydroxy-5α-pregnan-20-one), a positive GABA(A) receptor modulator, suppresses the epileptic discharges induced by 4-aminopyridine (4-AP), a convulsant K(+) channel blocker that stimulates glutamate release. Here, we tested the action of allopregnanolone on the epileptogenic and excitotoxic effects of the intrahippocampal administration of 4-AP in vivo. Drugs were perfused by a microdialysis cannula-electrode in the dorsal hippocampus and the EEG was recorded. Extracellular levels of aspartate, glutamate and GABA were analyzed by HPLC in the microdialysis fractions, and 24 h after the experiment the hippocampus was studied histologically. 4-AP induced intense epileptic discharges, increased the extracellular levels of aspartate, glutamate, and GABA by 383, 420, and 245%, respectively, and produced a notable neurodegeneration in CA1 and CA3 areas. Allopregnanolone administration alone did not affect the electrical activity, amino acids levels or cellular morphology, but when co-infused with 4-AP incremented 55-77% the duration of the epileptic discharges, and potentiated 32-49% the release of glutamate in comparison with 4-AP alone. The 4-AP-induced neurodegeneration was not modified by allopregnanolone. The NMDA receptor antagonist MK-801 protected against the epilepsy and neurodegeneration produced by 4-AP, and allopregnanolone did not affect this protection. We conclude that, differently from the observations in vitro, allopregnanolone potentiated the stimulatory effect of 4-AP on glutamate release and that this may explain the potentiation of the epileptogenic effect of 4-AP in vivo.

In a mouse model relevant for post-traumatic stress disorder, selective brain steroidogenic stimulants (SBSS) improve behavioral deficits by normalizing allopregnanolone biosynthesis

The pathophysiological role of the neurosteroid 3alpha-hydroxy-5alpha-pregnan-20-one (allopregnanolone) in neuropsychiatric disorders has been highlighted in several recent investigations. For instance, allopregnanolone levels are decreased in the CSF of patients with post-traumatic stress disorder (PTSD) and major unipolar depression. Neurosteroidogenic antidepressants, including fluoxetine and analogs, correct this decrease in a manner that correlates with improved depressive symptoms. PTSD-like behavioral dysfunctions, including heightened aggression, exaggerated fear, and anxiety-like behavior associated with a decrease in corticolimbic allopregnanolone content are modeled in mice by protracted social isolation stress. Allopregnanolone is not only synthesized by principal glutamatergic and gamma-aminobutyric acid (GABA)ergic neurons, but also locally, potently, positively, and allosterically modulates GABA action at postsynaptic and extrasynaptic GABAA receptors. Hence, this paper will review preclinical studies, which show that in socially isolated mice, rather than selective serotonin reuptake inhibitor mechanisms, allopregnanolone biosynthesis in glutamatergic corticolimbic neurons offers a nontraditional target for fluoxetine to decrease signs of aggression, normalize fear responses, and decrease anxiety-like behavior. At low selective serotonin reuptake inhibitor-inactive doses, fluoxetine and related congeners potently increase allopregnanolone levels by acting as potent selective brain steroidogenic stimulants (SBSS), thereby facilitating GABAA receptor neurotransmission and improving behavioral dysfunctions. Although the precise molecular mechanisms that underlie the action of these drugs are not fully understood, findings from socially isolated mice may ultimately generate insights into novel drug targets for the treatment of psychiatric disorders, such as anxiety and panic disorders, depression, and PTSD.

New therapeutic approaches for epilepsies, focusing on reorganization of the GABAA receptor subunits by neurosteroids

Neurosteroids such as allopregnanolone (THP) act as positive allosteric modulators of gamma-aminobutyric acid (GABA)A receptors and have exerted anticonvulsant properties. However, their role in the regulation of epileptogenesis is unclear. It has been shown that circulating levels of THP fluctuate during development and seizure episodes. Furthermore, both chronic administration of THP and its withdrawal transiently increase expression of the alpha4 subunit of the GABAA receptor in the brain. The steroidogenic enzymes, 5-alpha-reductase (5aR) and 3-alpha-hydroxysteroid dehydrogenase (3aHSD) have been identified as well, indicating that various cell types are involved in the biosynthesis of neuroactive steroids in the brain. The purpose of the present study is to examine how GABAA receptor-modulating neurosteroids contribute to the epileptogenesis by using the epileptic mutant EL mouse. Male EL mice and control animals, DDY mice, were used. EL mice show secondary generalized seizures, which initiate primarily at the parietal cortex and generalize through the hippocampus. In the interictal period during development, changes of THP, 5aR, 3aSDH, and GABAA receptor alpha4, gamma2, and delta subunits were investigated by western blotting in the hippocampus. In EL mice, levels of the neurosteroid THP and the steroidogenic enzymes 5aR and 3aSDH significantly increased at 3 weeks of age, and rapidly decreased thereafter (5-10 weeks). The sharp withdrawal was observed before mice experienced frequent seizures. In contrast, GABAA alpha4, gamma2, and delta expressions were upregulated (3-8 weeks). In the brain of EL mice, positive neurosteroids such as THP were withdrawn before mice experienced repetitive seizures, which may likely be a trigger for ictogenesis and epileptogenesis. Furthermore, reorganization of the GABAA receptor subunits may lead to a hypersensitivity of the receptor to neurosteroids. Therefore, GABAA receptor-regulating neurosteroids may be a promising target for the development of novel antiepileptic agents.

The role of neurosteroids in the pathophysiology and treatment of catamenial epilepsy

Catamenial epilepsy is a multifaceted neuroendocrine condition in which seizures are clustered around specific points in the menstrual cycle, most often around perimenstrual or periovulatory period. Generally, a twofold or greater increase in seizure frequency during a particular phase of the menstrual cycle could be considered as catamenial epilepsy. Based on this criteria, recent clinical studies indicate that catamenial epilepsy affects 31-60% of the women with epilepsy. Three types of catamenial seizures (perimenstrual, periovulatory and inadequate luteal) have been identified. However, there is no specific drug available today for catamenial epilepsy, which has not been successfully treated with conventional antiepileptic drugs. Elucidation of the pathophysiology of catamenial epilepsy is a prerequisite to develop specific targeted approaches for treatment or prevention of the disorder. Cyclical changes in the circulating levels of estrogens and progesterone play a central role in the development of catamenial epilepsy. There is emerging evidence that endogenous neurosteroids with anticonvulsant or proconvulsant effects could play a critical role in catamenial epilepsy. It is thought that perimenstrual catamenial epilepsy is associated with the withdrawal of anticonvulsant neurosteroids. Progesterone and other hormonal agents have been shown in limited trials to be moderately effective in catamenial epilepsy, but may cause endocrine side effects. Synthetic neurosteroids, which enhance the tonic GABA-A receptor function, might provide an effective approach for the catamenial epilepsy therapy without producing hormonal side effects.