Characterisation of N-methyl-D-aspartate (NMDA)-induced kindling.

The term kindling refers to the gradual development of epileptiform activity as a result of repeated, spaced stimulations of specific brain regions e.g. amygdala, hippocampus. cerebral cortex. Electrically kindled seizures are a valuable model of human complex partial epilepsy and evidence is accumulating for an involvement of excitatory amino acids (EAAs), and in particular glutamate. in this form of epileptogenesis [l-61. We have recently demonstrated that repeated, focal administration of low (subconvulsant) doses of the selective EAA receptor agonist NMDA also results in the progressive development of full limbic seizure activity (71. We now describe in detail the properties of these chemically-induced seizures. Details of the experimental procedures are described elsewhm [2]. Briefly. male Sprague-Dawley rats (280-3208) were implanted with a guide cannulahipolar electrode assembly into the right basolateral amygdala under halothanehitrous oxide anaesthesia. The assembly was secured to the skull with two stainless steel anchor screws using cyanoacrylate cement and zinc powder. The chemical kindling procedure (daily focal microinjection of NMDA in 0.5~1 50mM phosphate buffer, pH 7.4) was begun at least one week after surgery. Motor seizure activity was rated on a scale of 6 5 based on that of Racine [8]. Repeated daily microinjection of NMDA, Znmol, into the basolateral amygdala evoked seizures of increasingly progmssive severity. Animals initially displayed jaw and facial myoclonus followed by unilateral, then bilateral forelimb jerking and myoclonus and ultimately rearing and falling. All animals progressed to general id seizure activity following 17.7f1.5 (mean fS.E.M.; n=6) injections. The developing motor and electroencephalographic seizure activity closely resembled that seen during electrical kindling. In contrast to electrical kindling, however, NMDA-treated animals showed repeated, brief seizure episodes occasionally continuing for up to 10-15 min. Control. vehicle-injected, animals behaved normally, only rarely showing signs of facial muscle twitching. Daily co-administration of the competitive NMDA receptor antagonist AP7. 70nmo1, with NMDA, Znmol, markedly suppressed the development of the seizure response, with most animals showing only occasional facial myoclonus or transient chewing with jaw myoclonus. After 20 daily intra-amygdaloid injections, by which time most NMDA-treated animals had reached their maximal seizure stage. the NMDNAP7-treated p u p showed a mean seizure stage of only 0.4fo.1 (n=5) i.e. no significant kindling had occurred (b 0.05 compared with control group; Willcoxon's two-sample rank test). With subsequent daily NMDA administration alone, kindling developed at a rate similar to that seen in the NMDA-kindled group. The NMDNAP7-tnated group showed no other clear behavioural changes. One month after NMDA-induced kindling all animals responded to a single injection of NMDA, Znmol, with generalised seizure activity, demonstrating the durability of the kindled state. The mean seizure response of the group after 1 month did not differ significantly from that observed immediately following kindling (3.7fo.5 compared with 4.3f0.2, respectively: PM.05). In fully NMDA kindled animals, co-administration of the potent NMDA receptor antagonist CGP 37849, lnmol, with NMDA Znmol, inhibited all generalised seizure activity. Only brief periods of jaw myoclonus with occasional head bobbing