Interstrain differences in cognitive functions in rats in relation to status epilepticus

Cognitive functions of Long Evans (N = 30) and Wistar. rats (N = 32) were c ompared using a Morris water maze. Under control conditions the Long Evans rats were more efficient in this test, their average escape latency after 5 days of training (6.4 +/- 0.1 s, mean +/- S.E.M.) was significantly shorte r than that of the Wistar rats (11.0 +/- 0.1 s). When the training was comp leted seizures were induced by an intraperitoneal injection of pilocarpine (330 mg/kg in the Long Evans strain and 350 mg/kg in the Wistar rats) 30 mi n after pretreatment with N-methylscopolamine (1 mg/kg i.p.). Clonazepam (1 mg/kg i.p.) was used to interrupt clonic seizures after 2 hours of continu ous activity. Approximately one quarter of rats in both strains did not dev elop seizures. Severe convulsive status epilepticus was common in Long Evan s rats (23 out of 30). In contrast, only 12 Wistar rats generated convulsiv e status epilepticus and the same number of animals exhibited only bursts o f motor seizures separated by periods without convulsions (temporary seizur es). Mortality after pilocarpine-induced status epilepticus was considerabl y higher in the Long Evans rats than in the Wistar rats. After a latency of 2-3 weeks spontaneous recurrent seizures appeared in all animals surviving status. Cognitive memory was tested during the 'silent period' between sta tus and recurrent seizures. The Long Evans rats were unable to find the pla tform at the 3rd and 6th day after status but then their performance rapidl y improved. The performance of the Wistar rats undergoing status epilepticu s was seriously deteriorated and it never normalized, whereas the animals w ith temporary seizures exhibited only a transitory marginal prolongation of latencies. The hippocampal formation was damaged by status epilepticus in rats of both strains - the Long Evans rats exhibited more extensive damage of subfields CA1 and CA3, whereas in the Wistar rats a complete destruction of hilar neurons was observed in addition to partial CA1 and CA3 damage. ( C) 2000 Elsevier Science B.V. All rights reserved.