Relationships between benzodiazepine receptors, impairment of GABAergic transmission and convulsant activity of beta-CCM: a PET study in the baboon Papio papio

Seizing the moment: Zebrafish epilepsy models

Zebrafish are now widely accepted as a valuable animal model for a number of different central nervous system (CNS) diseases. They are suitable both for elucidating the origin of these disorders and the sequence of events culminating in their onset, and for use as a high-throughput in vivo drug screening platform. The availability of powerful and effective techniques for genome manipulation allows the rapid modelling of different genetic epilepsies and of conditions with seizures as a core symptom. With this review, we seek to summarize the current knowledge about existing epilepsy/seizures models in zebrafish (both pharmacological and genetic) and compare them with equivalent rodent and human studies. New findings obtained from the zebrafish models are highlighted. We believe that this comprehensive review will highlight the value of zebrafish as a model for investigating different aspects of epilepsy and will help researchers to use these models to their full extent.

Differential sensitivity to inverse agonists of GABA(A)/benzodiazepine receptors in rats with genetic absence-epilepsy

A strain of Wistar rats, genetic absence epilepsy rats from Strasbourg (GAERS), was selected and inbred over 40 generations for occurrence of spontaneous spike-wave discharges characteristic of absence seizures, simultaneously with a strain of non-epileptic rats (NER). GAERS demonstrate an excessive sensitivity to antagonists of the GABA(A) receptor. The sensitivity to convulsions induced by various inverse agonists of the GABA(A)/benzodiazepine receptor was compared in GAERS and NERs. The beta-carbolines FG 7142 and DMCM, and the imidazobenzodiazepines RO 19-4603 and the alpha 5-selective RY 024 were several times more convulsant in GAERS than in NERs. The largest differences were found with the non-selective RO 19-4603- and FG 7142. The proconvulsant imidazobenzodiazepine RO 15-4513, binding also to diazepam-insensitive receptors, had low efficacy. The high affinity binding of GABA(A)/BZD receptors with (3H) RO 15-1788 in the brain of naive rats and after administration of FG 7142 did not differ in GAERS and NERs. The data indicate that the hypersensitivity of GAERS to various inverse agonists of the GABA(A)/benzodiazepine receptor involves cortical GABA(A) receptors and is not related to differential activity of a subunit-selective receptor.

Management issues for women with epilepsy: a review of the literature

OBJECTIVE

A review of literature referable to management issues for women with epilepsy (WWE) was undertaken for the development of a practice parameter.

BACKGROUND

Epilepsy is a common neurologic condition with gender-related management implications. Although reviews of this topic often focus on pregnancy-related issues for WWE, specific health concerns for WWE are present throughout all phases of reproductive life.

METHODS

An OVID MEDLINE literature search was conducted for 1965 to 1997 using the following key words/phrases and cross referencing: epilepsy/ seizures and pregnancy, anticonvulsants, antiepileptic drugs (AEDs), teratogenesis, oral contraceptives, birth defects, folate/folic acid, vitamin K, metabolic bone disease, and breast-feeding.

RESULTS

Pregnancy outcome literature for WWE spans several decades. Methodology varies and interpretation is complicated by modern management strategies. Contributions of socioeconomic factors, AEDs, maternal epilepsy, and seizures during pregnancy to adverse pregnancy outcomes have not been clearly delineated. There is a biologic basis for recommendations concerning contraception, folate supplementation, vitamin K use in pregnancy, breast-feeding, metabolic bone disease, catamenial epilepsy, and reproductive endocrine disorders, but no outcome studies afford a strong evidence base for practice recommendation.

CONCLUSIONS

WWE face health issues for which there is no available outcome literature to guide decision making. The urgent need for studies in many of these areas is highlighted by expanded treatment options with new AEDs and epilepsy surgery.

In vivo modulation of benzodiazepine receptor function after inhibition of endogenous gamma-aminobutyyric acid synthesis

The influence of decreased endogenous gamma-aminobutyric acid (GABA) concentration on benzodiazepine receptor function was studied in the brain of living baboons. Positron emission tomography and the radiotracer [11C]flumazenil combined with electroencephalography were used to determine the pharmacological properties of two bezodiazepine receptors agonists, diazepam and bretazenil, in baboons pre-treated or not with DL-allylglycine (an inhibitor of GABA synthesis). Our results show that, in vivo, DL-allylglycine reduces the affinity of benzodiazepine receptors for their agonists without altering the intrinsic capability of agonists to allosterically modulate GABAergic transmission.

Histamine H1 receptor occupancy in human brains after single oral doses of histamine H1 antagonists measured by positron emission tomography

1. Histamine H1 receptor occupancy in the human brain was measured in 20 healthy young men by positron emission tomography (PET) using [11C]-doxepin. 2. (+)-Chlorpheniramine, a selective and classical antihistamine, occupied 76.8 +/- 4.2% of the averaged values of available histamine H1 receptors in the frontal cortex after its administration in a single oral dose of 2 mg. Intravenous administration of 5 mg (+)-chlorpheniramine almost completely abolished the binding of [11C]-doxepin to H1 receptors (H1 receptor occupancy: 98.2 +/- 1.2%). 3. Terfenadine, a nonsedative antihistamine, occupied 17.2 +/- 14.2% of the available H1 receptors in the human frontal cortex after its administration in a single oral dose of 60 mg. 4. There was no correlation between H1 receptor occupancy by terfenadine and the plasma concentration of the active acid metabolite of terfenadine in each subject. 5. PET data on human brain were essentially compatible with those on H1 receptor occupancy in guinea-pig brain determined by in vivo binding techniques, although for the same H1 receptor occupancy the dose was less in human subjects than in guinea-pigs. 6. The PET studies demonstrated the usefulness of measuring H1 receptor occupancy with classical and second-generation antihistamines in human brain to estimate their unwanted side effects such as sedation and drowsiness quantitatively.

GABA and epilepsy: their complex relationship and the evolution of our understanding

Because of its abundance in the brain, its ability to produce hyperpolarizing inhibition of almost all neurons, its association with benzodiazepines, and the discovery that many convulsants inhibited its synthesis, gamma-aminobutyric acid (GABA) has often appeared to be the key to epilepsy. Many assumed that "primary" or "genetic" epilepsy must be a disorder of GABA synapses and that GABA agonists would be universal anticonvulsants if permeability and drug metabolism were controlled. The GABA synthetic gene was a logical "candidate gene" for epilepsy. However, the GABA-deficiency theory of epilepsy is less convincing today. GABA agonists were found to intensify seizures in some rodent and human cases. Absence and other generalized seizures in humans often worsened when treated with GABA transaminase inhibitors such as gamma-vinyl-GABA. Surprisingly, the GABA transaminase inhibitors appear to be more useful in partial than in generalized epilepsies. Neuronal GABA uptake blockers are proconvulsant. GABA agonists aggravate seizures in several mutants, ranging from the photosensitive baboon to the genetically epilepsy-prone rat. How can this be understood? Muscimol injections into the pedunculopontine nucleus increase seizures due to systematically administered convulsants, while the receptor blocker bicuculline suppresses seizures after injection into several brain regions, including the striatum. The result of inhibiting inhibitory circuits is excitation. Studies with GABA uptake blockers and the GABAB agonist baclofen are presented in which their combined administration provoked seizures in rats. Baclofen was shown also to increase the incidence of seizures evoked by pentylenetetrazole without increasing seizures due to local injections of excitatory amino acids. Baclofen antagonized the myoclonic effect of 5-hydroxytryptophan in rats with serotonin lesions. Baclofen augments some seizures and inhibits others. Selective inhibition of a particular tract, whether GABAergic or not, may have convulsant or anticonvulsant effects, depending on its connections and the state of the organism. GABAA receptor stimulation is usually but not always anticonvulsant. GABAB receptor stimulation may facilitate absence seizures and related primary generalized seizures. GABAB receptors may be abnormal in some forms of nonfocal epilepsy seen in childhood. It is likely that mutations of GABA transporter and GABAA receptor genes will be found in humans but they will probably not be patients with "pure epilepsy."

In vivo bidirectional modulatory effect of benzodiazepine receptor ligands on GABAergic transmission evaluated by positron emission tomography in non-human primates

The central type benzodiazepine receptor (BDZr), an allosteric modulatory site of the GABAA receptor-anion channel, has been shown in vitro to respond to drugs with positive efficacy (agonists), zero efficacy (competitive antagonists) and drugs with negative efficacy (inverse agonists). However, this general concept of the function of BDZr drugs has rarely been assessed in intact living brain. We report here in on a non-invasive in vivo assessment of the intrinsic efficacies of BDZr drugs in the brain of non-human primates. We have performed an in vivo simultaneous determination of fractional BDZr occupancy and the resulting pharmacological efficacies of the full agonist diazepam, the partial agonist bretazenil, the antagonist flumazenil (Ro15-1788), the partial inverse agonist Ro15-4513 and the full inverse agonist methyl beta-carboline-3-carboxylate (beta-CCM). Positron emission tomography (PET) was used to estimate fractional BDZr occupancy measured as the in vivo displacement in the brain of the positron emitter radioligand, [11C]flumazenil. Simultaneously, the proconvulsant or anticonvulsant efficacies of the BDZr drugs were measured as their abilities to facilitate or counteract the central effects of an infusion of pentylenetetrazol, a non-competitive GABA antagonist acting on the picrotoxin site of the receptor complex. This was measured using electroencephalographic recording (EEG). Our results show that, in vivo, the fractional receptor occupancy by a given drug is perfectly correlated with its resulting graded pharmacological effects, as predicted from the competitive drug receptor interaction theory. Furthermore, the slope of the relationship between fractional receptor occupancies and the resulting pharmacological effects (an index of intrinsic efficacy) strictly depends on the BDZr ligand considered. Diazepam displayed a strong positive intrinsic efficacy, and, in contrast, beta-CCM a marked negative one. Between these two extremes, the partially active drugs bretazenil and Ro15-4513, which required a large fractional receptor occupancy to produce significant anti- or proconvulsant effects, respectively, displayed only a weak intrinsic efficacy. Flumazenil did not produce any significant pharmacological effect. We observed that the in vivo intrinsic efficacies of diazepam, flumazenil and beta-CCM correlate with their intrinsic efficacies as measured by their modulatory effects on the GABA-dependent membrane chloride conductance in vitro. Thus, the intrinsic efficacies measured using PET and EEG are likely to reflect the different in vivo abilities of BDZr drugs to induce or stabilize the GABAA-benzodiazepine chloride channel in a given conformation.(ABSTRACT TRUNCATED AT 400 WORDS)