Anticonvulsant actions of MK-801 on the lithium-pilocarpine model of status epilepticus in rats.

MK-801, a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, was tested for anticonvulsant effects in rats using two seizure models, coadministration of lithium and pilocarpine and administration of a high dose of pilocarpine alone. Three major results are reported. First, pretreatment with MK-801 produced an effective and dose-dependent anticonvulsant action with the lithium-pilocarpine model but not with rats treated with pilocarpine alone, suggesting that different biochemical mechanisms control seizures in these two models. Second, the anticonvulsant effect of MK-801 in the lithium-pilocarpine model only occurred after initial periods of seizure activity. This observation is suggested to be an in vivo demonstration of the conclusion derived from in vitro experiments that MK-801 binding requires agonist-induced opening of the channel sites of the NMDA receptor. Third, although it is relatively easy to block seizures induced by lithium and pilocarpine by administration of anticonvulsants prior to pilocarpine, it is more difficult to terminate ongoing status epilepticus and block the lethality of the seizures. Administration of MK-801 30 or 60 min after pilocarpine, i.e., during status epilepticus, gradually reduced electrical and behavioral seizure activity and greatly enhanced the survival rate. These results suggest that activation of NMDA receptors plays an important role in status epilepticus and brain damage in the lithium-pilocarpine model. This was further supported by results showing that nonconvulsive doses of NMDA and pilocarpine were synergistic, resulting in status epilepticus and subsequent mortality.