Chronic treatment with flumazenil enhances binding sites for convulsants at recombinant alpha(1)beta(2)gamma(2S) GABA(A) receptors

Anticonvulsive effects of protodioscin against pilocarpine-induced epilepsy

Epilepsy is associated with increased morbidity and mortality together and places a large financial burden on individuals and society. To evaluate the anticonvulsant action of protodioscin (PDSN) in experiments with animals with pilocarpine-induced convulsions. We assessed the activity of PDSN in pilocarpine induced seizures in combination with different agents which are acting via diverse receptors, such as atropine, memantine, nimodipine, diazepam, and flumazenil, to determine the exact receptors responsible for the action of PDSN. Furthermore, the level of antioxidant markers was investigated in the cerebellum and cerebral cortex in mice to define the antioxidant action of PDSN. The effects of PDSN on proapoptotic markers (i.e., Bcl-2, Bax, and caspase-3) was investigated via western blot analysis. PDSN significantly enhanced latency to the first convulsion and survival compared to the group treated with pilocarpine alone. Moreover, PDSN improved animal survival, and subjects experiencing no convulsions. Striatal glutamate and aspartate levels were not modified, and gamma amino butyric acid (GABA) levels increased, as a result of treatment with PDSN. The results suggest that the anticonvulsive action of PDSN is dependent on inhibitory amino acids. PDSN treatment also significantly decreased nitrite levels in the blood and brain cortex compared to the normal control. In the western blot analysis, PDSN exerted its neuroprotective effect via the upregulation of Bcl-2 and downregulation of Bax and caspase-3. The results of this study suggest that PDSN exerts neuroprotective effects via multiple mechanisms.

The role of transcriptional and translational mechanisms in flumazenil-induced up-regulation of recombinant GABA(A) receptors

The aim of this study was to further elucidate the mechanisms involved in adaptive changes of GABA(A) receptors following prolonged exposure to flumazenil, the antagonist of benzodiazepine binding sites on GABA(A) receptors. The effects of prolonged flumazenil treatment were studied on recombinant alpha(1)beta(2)gamma(2S) GABA(A) receptors stably expressed in human embryonic kidney (HEK 293) cells. Using radioligand binding experiments we found enhancement in the maximum number of [(3)H]muscimol labeled binding sites in different preparations of HEK 293 cells. The parallel increase of [(3)H]flunitrazepam binding sites in the membranes was reduced in the presence of actinomycin D and cycloheximide, inhibitors of RNA and protein synthesis, respectively. Chronic flumazenil also raised the steady-state level of mRNA encoding alpha(1) receptor subunit. The results suggest that the up-regulation of GABA(A) receptors, observed after prolonged flumazenil treatment is at least partly due to increased de novo synthesis of receptor proteins at both transcriptional and translational level.

Allosteric uncoupling and up-regulation of benzodiazepine and GABA recognition sites following chronic diazepam treatment of HEK 293 cells stably transfected with alpha1beta2gamma2S subunits of GABA (A) receptors

Benzodiazepines are drugs known to produce tolerance and dependence and also to be abused and co-abused. The aim of this study was to further explore the mechanisms that underlie adaptive changes in GABA(A) receptors following prolonged exposure to these drugs. Human embryonic kidney (HEK 293) cells stably expressing recombinant alpha1beta2gamma2s GABA(A) receptors were exposed for 72 h to a high concentration of diazepam (50 microM) in the absence or presence of other drugs. Radioligand binding studies were used to determine the parameters of [(3)H]flunitrazepam and [(3)H]muscimol binding sites and allosteric interactions between these sites. Prolonged treatment with diazepam increased the maximum number (B (max)) of [(3)H]flunitrazepam and [(3)H]muscimol binding sites in the membranes, and of [(3)H]muscimol binding sites on the surface of HEK 293 cells. There was no change in the affinity (K (d)) of binding sites. The diazepam-induced increase in the B (max) value of [(3)H]flunitrazepam binding sites was reduced by two GABA(A) receptor antagonists, gabazine (1 and 10 microM) and picrotoxin (100 microM). In addition, it was reduced by cycloheximide (5 microg/ml), a protein synthesis inhibitor, and actinomycin D (7.5 microg/ml), an RNA synthesis inhibitor. Flumazenil (5 microM), the antagonist of benzodiazepine binding sites, also up-regulated [(3)H]flunitrazepam recognition sites. Simultaneous treatment with diazepam and flumazenil failed to produce an additive up-regulation. GABA (1 nM - 1 mM)-induced potentiation of [(3)H]flunitrazepam binding to membranes obtained from diazepam (50 microM)-pretreated cells was markedly reduced, suggesting functional uncoupling between GABA and benzodiazepine binding sites. The results suggest that diazepam up-regulated benzodiazepine binding sites on stably expressed GABA(A) receptors by stimulating their synthesis at both the transcriptional and translational levels. A comparable increase of [(3)H]muscimol binding sites expressed on the surface of intact HEK 293 cells suggests that internalisation of surface receptors presumably can not explain the uncoupling.