The present study investigated the role of NMDA (N-methyl-D-aspartate) receptors in the hypersusceptibility to seizures induced by the benzodiazepine inverse agonist DMCM (methyl-6,7-dimethoxy-4-ethyl-β-carboline-3-carboxylate) during diazepam withdrawal in mice, using behavioral and biochemical approaches. The seizure threshold of DMCM was markedly decreased during diazepam withdrawal, reflecting withdrawal hyperexcitability in response to physical dependence. The decrease in the seizure threshold of DMCM in diazepam-withdrawn mice was inhibited by the non-competitive NMDA receptor antagonists MK-801 ((+)-5-methyl-10,11- dihydro-5H-dibenzo(a,d)cycloheptan-5,10-imine maleate; 50 μg/kg, s.c.) and ifenprodil (20 mg/kg, i.p.). The effective doses of these compounds were lower than those required to prevent DMCM-induced seizures in chronically vehicle-treated mice. Since MK-801 and ifenprodil do not only bind to NMDA receptors but also to σ receptors, the present study also investigated the effects of σ receptor ligands. The decrease in the seizure threshold of DMCM in diazepam-withdrawn mice was not modified by the σ receptor agonist, (+)-pentazocine (5 mg/kg, s.c.), or the σ receptor antagonist, NE-100 (N,N-dipropyl-2-[4-methoxy-3-(2-phenylethoxy)phenyl]-ethylamine monohydrochloride; 5 mg/kg, i.p.). Furthermore, the latency to the expression of wild running induced by intracerebroventricular administration of NMDA (60 ng/mouse) was also significantly lower in diazepam-withdrawn mice than in vehicle-treated control mice. On the other hand, there was no difference in the spermidine concentration between vehicle-treated control and diazepam-withdrawn mice. In a receptor binding experiment, the B(max) value for [3H]-MK-801 binding was significantly increased in cerebrocortical tissues from diazepam-withdrawn mice, while the K(d) value did not change in either group. However, the acute addition of a high concentration of diazepam (10 and 100 μM) in vitro did not alter [3H]-MK-801 binding in cerebrocortical membrane preparations. The behavioral experiments suggest that NMDA receptor antagonists may suppress benzodiazepine withdrawal responses, while the biochemical study reveals upregulation of the NMDA receptor, which may play an important role in the hypersusceptibility to DMCM-induced seizure in diazepam-withdrawn mice.
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