Changes in the gene expression of GABAA receptor subunit mRNAs in the septum of rats subjected to pentylenetetrazol-induced kindling

Chemical kindling was induced in rats by long-term administration of pentylenetetrazol (PTZ) (30 mg/kg three times a week for 9 weeks). The effects of such kindling on the abundance of transcripts encoding subunits of the gamma-aminobutyric acid type A (GABAA) receptor in the brain were measured by RNase protection assay. Kindled rats were examined either 3 or 30 days after discontinuation of PTZ treatment. The amounts of gamma2L and gamma2S subunit mRNAs were significantly increased in the hippocampus and cerebral cortex of kindled rats 3 and 30 days after treatment discontinuation, compared with those observed in control rats, and these effects were prevented by the concomitant administration of the anticonvulsant abecarnil. In contrast, the amounts of alpha1 and beta2 subunit mRNAs in these two brain regions did not differ significantly between kindled and control rats. The abundance of alpha1, beta2, gamma2L and gamma2S subunit mRNAs was decreased in the septum of rats 3 or 30 days after discontinuation of treatment with PTZ either alone or in combination with abecarnil. The amounts of none of the four subunit mRNAs measured differed significantly between the striatum or frontal cortex of kindled rats and control rats 3 days after drug discontinuation. Immunohistochemical analysis with antibodies to choline acetyltransferase revealed a marked decrease in the number of cholinergic neurons in the septum of kindled rats 30 days after discontinuation of PTZ treatment; this effect was not prevented by the administration of abecarnil. These results suggest that long-term treatment with PTZ induces a loss of GABAA receptors in the septum.

Molecular and functional adaptation of the GABA(A) receptor complex during pregnancy and after delivery in the rat brain

The abundance of gamma-aminobutyric acid receptor type A (GABAA receptor) subunit mRNAs and polypeptides as well as muscimol-stimulated 36Cl- uptake were measured in rat cerebral cortex or hippocampus at various times during pregnancy and after delivery. RNase protection assays revealed that the amount of the gamma2L subunit mRNA decreased progressively during pregnancy, in the cerebral cortex and hippocampus, and then returned to control values around the time of delivery. A similar pattern was observed for the alpha5 subunit mRNA in the cerebral cortex, whereas no significant changes were apparent for alpha1, alpha2, alpha3, alpha4, beta1, beta2, beta3 and gamma2S subunit mRNAs. The amounts of gamma2 and alpha1 proteins in the cerebral cortex were measured by immunoblot analysis; whereas the abundance of gamma2 protein decreased during pregnancy, no change was detected in the amount of alpha1 protein. Evaluation for functional significance of the down-regulated gamma2 and alpha5 subunit was made by determining the GABAA receptor function assessed by measurement of muscimol-stimulated 36Cl- uptake in cerebral cortical membrane vesicles. Muscimol-induced 36Cl- uptake was markedly reduced during of pregnancy compared with rats in oestrus. At this same time, the potentiating effects of diazepam and allopregnanolone on muscimol stimulation of 36Cl- uptake also were reduced. In contrast, the effects of muscimol, allopregnanolone and diazepam were significantly increased, relative to animals in oestrus, after delivery.

High affinity central benzodiazepine receptor ligands: synthesis and structure-activity relationship studies of a new series of pyrazolo[4,3-c]quinolin-3-ones

A large series of 2-aryl(heteroaryl)-2,5-dihydropyrazolo[4,3-c]quinolin- 3-(3H)-ones, carrying appropriate substituents at the quinoline and N2-phenyl rings, were prepared and tested as central benzodiazepine receptor ligands. Results from structure-affinity relationship studies were in full agreement with previously proposed pharmacophore models and, in addition, quantitative structure-activity analysis gave further significant insight into the main molecular determinants of high benzodiazepine receptor affinity. The intrinsic activity of some active ligands was also determined and preliminary discussed.

Synthesis and anticonvulsant activity of some 1,2,3,3a-tetrahydropyrrolo[2,1-b]-benzothiazol-, -thiazol- or -oxazol-1-ones in rodents

To identify more potent anticonvulsant agents and to gain insights into the structural properties determining the potency of a new class of anticonvulsants, some 3a-substituted tetrahydropyrrolo[2,1-b]benzothiazol-1-ones (1a-d) and the thiazole and oxazole analogues (2a-c and 3a-c, respectively) have been synthesized and tested for anticonvulsant activity against isoniazid-induced seizures in rodents. The most active compound, 2a, with a median effective dose (ED50, i.p.) of 24.3 mg kg-1 and 15.9 mg kg-1 in mice and in rats, respectively, was more extensively investigated and found to strengthen the effects of diazepam. No clear correlation was observed between the anticonvulsant activity and molecular lipophilicity descriptors of compounds 1-3. Structural similarity between the antiepileptic drug phenobarbital and compounds 1-3 was evidenced by molecular modelling studies and used to derive preliminary structure-activity relationships. The results demonstrate that 2a is an attractive candidate as an anticonvulsant agent worthy of further study and may help the design of other anticonvulsant drugs.

Biochemical evaluations of the effects of loreclezole and propofol on the GABAA receptor in rat brain

The effects of loreclezole on the function of the gamma-aminobutyric acid type A (GABAA) receptor complex in rat cerebral cortical membrane preparations were compared with those of propofol and diazepam. Loreclezole and propofol modulated [3H]muscimol binding and t-[35S]butylbicyclophosphorothionate ([35S]TBPS) binding to washed and unwashed membranes with potencies and efficacies greater than those of diazepam. Loreclezole and propofol enhanced [3H]flunitrazepam binding to washed membranes with efficacies lower than those of GABA and muscimol. Both loreclezole and propofol showed biphasic effects on [35S]TBPS binding to washed membranes: at low concentrations (5 to 10 microM), both drugs, with different efficacies, enhanced [35S]TBPS binding whereas, at higher concentrations (30 to 100 microM), they inhibited this biochemical parameter. In contrast, diazepam enhanced [35S]TBPS binding to washed membranes at all concentrations tested. The combination of loreclezole with GABA, at a concentration (0.3 microM) that only slightly increased [35S]TBPS binding to washed membranes, reversed the increase in binding elicited by loreclezole (5 to 10 microM) and significantly potentiated the inhibitory effect exerted by higher concentrations (30 to 100 microM) of this drug. Similar effects were observed with the combination of GABA and propofol. However, GABA had no effect on the enhancement of [35S]TBPS binding induced by diazepam. The ability of GABA to reverse and potentiate the effects of loreclezole and propofol on [35S]TBPS binding to washed membranes was shared by pentobarbital (200 microM) and alphaxalone (3 microM). These anesthetics showed greater efficacies in combination with pentobarbital (200 microM) and alphaxalone (3 microM). These anesthetics showed greater efficacies in combination with propofol than with loreclezole. These results suggest that, unlike diazepam, loreclezole and propofol may activate the receptor-associated Cl- channel in the absence of GABA. Furthermore, the difference in the pharmacological profiles of loreclezole and propofol may result from their different effectiveness in activating the receptor Cl- channel directly.

Failure of chronic treatment with abecarnil to induce contigent and noncontingent tolerance in pentylenetetrazol-kindled rats

We examined the effect of chronic treatment with abecarnil, a selective agonist at gamma-aminobutyric acid(A) (GABA(A)) receptors, on the development of tolerance to its anticonvulsant effect in pentylenetetrazole (PTZ)-kindled rats. We used two different experimental protocols to differentiate between pharmacological (noncontingent) and contingent tolerance. In one group of animals, kindling was suspended and abercarnil (1mg/kg intraperitoneally, i.p.) was administered three times daily for 15 days. In a second group of rats, PTZ-kindling was continued during chronic treatment with abecarnil. Tolerance to the anticonvulsant effect of a subsequent challenge dose of abecarnil (0.5 mg/kg i.p.) did not develop in either experimental group.

Direct activation of GABAA receptors by loreclezole, an anticonvulsant drug with selectivity for the beta-subunit

Loreclezole, an anticonvulsant and antiepileptic compound, potentiates gamma-aminobutyric acid (GABA) type A receptor function, by interacting with a specific allosteric modulatory site on receptor beta-subunits. A similar selectivity for GABAA receptor beta-subunits is apparent for the direct activation of receptor-operated Cl- channels, by the general anesthetics propofol and pentobarbital. The ability of loreclezole to activate GABAA receptors directly has now been compared, biochemically and electrophysiologically, with that of propofol. In well-washed rat cortical membranes (devoid of endogenous GABA), loreclezole and propofol increased t-[35S]butylbicyclophosphorothionate ([35S]TBPS) binding by up to 28% (at 5 microM) and 80% (at 10 microM), respectively. Higher concentrations (50-100 microM) of both compounds inhibited [35S]TBPS binding with great efficacy, an effect mimicked by GABA. In contrast, the benzodiazepine diazepam increased [35S]TBPS binding, but failed to inhibit this parameter, even at high concentrations. At concentrations of 50-100 microM, loreclezole induced inward Cl- currents in the absence of GABA, in Xenopus oocytes expressing human recombinant GABAA receptors, comprised of alpha 1-, beta 2- and gamma 2S-subunits. At 100 microM, the current evoked by loreclezole was 26% of that induced by 5 microM GABA. The current evoked by 100 microM propofol was 98% of that induced by 5 microM GABA. Currents induced by loreclezole, like those evoked by propofol, were potentiated by diazepam in a flumazenil-sensitive manner and blocked by either bicuculline or picrotoxin. These data suggest that loreclezole shares, with propofol, an agonistic action at GABAA receptors containing the beta 2-subunit and that the different efficacies of the two compounds in this regard, may underlie the difference in their pharmacological profiles. The failure of loreclezole to activate GABAA receptors containing the beta 1-subunit may be responsible for its lack of hypnotic effect.