Alkyl-substituted gamma-butyrolactones as potential tools in the study and treatment of epilepsy

Alteration of GLUR2 expression in the rat brain following absence seizures induced by gamma-hydroxybutyric acid

We explored the involvement of the glutamate receptor subunit B (GluR2) in the mechanism of absence seizures induced by gamma-hydroxybutyric acid (GHB). The expression and distribution of GluR2 protein in rat brain were examined during and after GHB-induced absence seizures. The data indicate that GluR2 protein expression significantly decreases following the onset of absence seizures. The suppression of GluR2 expression was prolonged and it outlasted the duration of the continuous absence seizure activity. The alteration of GluR2 protein levels was accompanied by a re-distribution of GluR2 expression from laminae V to IV in cerebral cortex. We also analyzed the duration and latency of absence seizures induced by GHB 72 h following an initial GHB-induced absence seizure, a time when suppression of GluR2 protein was maximal. The second absence seizure was significantly more prolonged than the first. These data may indicate that the putative down-regulation of GluR2 following GHB-induced absence seizure could have contributed to the potentiation of subsequent seizures in animals. A related hypothesis posed by the data is that down-regulation of GluR2 is involved in the mechanisms of the maintenance of recurrent absence seizure activity once it is initiated and therefore, may contribute to the chronicity of seizures in absence epilepsy.

Effects of anticonvulsant lactams on in vitro seizures in the hippocampal slice preparation

Some 3,3-disubstituted 2-pyrrolidinones and 2-piperidinones (five- and six-membered ring lactams, respectively) possess potent in vivo anticonvulsant activity. In vitro these lactams potentiate GABA(A) receptor-mediated chloride currents, which is thought to be the mechanism by which they exert their therapeutic effects. However, the apparent affinity for these GABA(A) interactions is low: EC50s range from hundreds of micromolar to low millimolar values. In order to more completely characterize the activities of these compounds, it was necessary to know the concentrations required to curtail epileptiform activity in an intact neural network, and the mechanism by which this occurs. To address these questions, we used two methods of inducing ictal activity in hippocampal-entorhinal cortical slices: 4-aminopyridine (4-AP) and low Mg2+. We found that 3,3-diethyl-2-pyrrolidinone (diethyl-lactam) prevents seizure-like discharges with IC50s of 1.1 and 2.1 mM in the two models, respectively. These values are nearly identical to the EC50 value obtained in whole-cell studies of diethyl-lactam's GABA(A) receptor modulation. The addition of the GABA(A) antagonist picrotoxin to the low Mg2+ ACSF produced seizures which persisted during diethyl-lactam application. Neither 3-benzyl-3-ethyl-2-piperidinone (3-BEP) nor alpha-ethyl-alpha-methyl-gamma-thiobutyrolactone (alpha-EMTBL), two compounds which are similar to diethyl-lactam, but demonstrate picrotoxin-insensitive inhibition of voltage-dependent currents, diminished low Mg2+/picrotoxin seizure activity. Our results support the hypothesis that diethyllactam and related compounds exert their anticonvulsant activity primarily, if not exclusively, by modulating the GABA(A) receptor.

Structure-activity studies of fluoroalkyl-substituted gamma-butyrolactone and gamma-thiobutyrolactone modulators of GABA(A) receptor function

Dihydro-2(3H)-furanones (gamma-butyrolactones) and dihydro-2(3H)-thiophenones (gamma-thiobutyrolactones) containing fluoroalkyl groups at positions C-3, C-4, and C-5 of the heterocyclic rings were prepared. The anticonvulsant/convulsant activities of the compounds were evaluated in mice. Brain concentrations of the compounds were determined and the effects of the compounds on [35S]-tert-butylbicyclophosphorothionate ([35S]TBPS) binding to the picrotoxin site on GABAA receptors were investigated. The effects of the compounds on GABAA receptor function were studied using electrophysiological methods and cultured rat hippocampal neurons. Fluorination at C-3 results in either subtle or pronounced effects on the pharmacological activity of the compounds. When hydrogens are replaced with fluorines at the methylene carbon of an ethyl group, as in 3-(1,1-difluoroethyl)dihydro-3-methyl-2(3H)-furanone (1), the anticonvulsant actions of the compound are not much changed from those found for the corresponding alkyl-substituted analogue. In marked contrast, fluorination at the methyl carbon of the ethyl group, as in dihydro-3-methyl-3-(2,2,2-trifluoroethyl)-2(3H)-furanone (3), produces a compound having convulsant activity. This convulsant activity seems to be due to an increased affinity of the compound for the picrotoxin site on GABAA receptors caused by an interaction that involves the trifluoromethyl group. Results obtained with gamma-butyrolactones containing either a 3-(1-trifluoromethyl)ethyl or a 3-(1-methyl-1-trifluoromethyl)ethyl substituent indicate that the interactions of the trifluoromethyl group with the picrotoxin binding site are subject to both stereochemical and steric constraints. Sulfur for oxygen heteroatom substitution, as in the corresponding gamma-thiobutyrolactones, affects the type (competitive, non-competitive, etc.) of binding interactions that these compounds have with the picrotoxin site in a complex manner. Fluorination of alkyl groups at the C-4 and C-5 positions of gamma-butyrolactones having convulsant activity increases convulsant potency.

Common modes of action of gamma-butyrolactones and pentylenetetrazol on the GABAA receptor-ionophore complex

The effects of pentylenetetrazol and bicyclic gamma-butyrolactones of similar stereostructures were studied on the convulsant and benzodiazepine binding sites and chloride ionophore activity of the gamma-aminobutyric acid (GABAA) receptor-complex. Bicyclic gamma-butyrolactones displayed millimolar IC50 values and low stereoselectivities on [35S]t-butylbicyclophosphorothionate (TBPS) binding to the convulsant sites in synaptosomal membranes of rat forebrains. Ring saturation of bicyclic gamma-butyrolactones decreased their IC50 values by one order of magnitude. The IC50 values of saturated bicyclic gamma-butyrolactones and pentylenetetrazol were increased by GABA versus its antagonist R 5135 (3 alpha-hydroxy-16-imino-5 beta,17-aza-androstan-11-one). A bicyclic gamma-butyrolactone and pentylenetetrazol accelerated the dissociation of [35S]TBPS, displaced [3H]flumazenil binding in two phases and blocked the muscimol-elicited chloride currents in patch-clamped cortical neurones in culture in a similar manner. These similar effects on binding and ionophore function support their common modes of action on the GABAA receptor-ionophore complex.

Effect of gamma-crotonolactone in experimental models of generalized absence seizures

The effect of the unsaturated lactone, gamma-crotonolactone (GCL), against spike wave discharges (SWD) in two pharmacological models of generalized absence seizures in rat was investigated. The models used were the gamma-hydroxybutyrate (GHB) model and the low dose pentylenetetrazole model. GCL pretreatment resulted in a significant decrease in duration of SWD in both models. In addition, this compound reduced the ability of the agonist, muscimol, to enhance SWD duration in the GHB model and was effective in developing animals as well as in adult animals with GHB-induced SWD. These data suggest that GCL may be a useful tool in the study of basic mechanisms of SWD generation in experimental absence seizures.