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Differences in the behaviour of Sprague--Dawley and Lewis rats during repeated passive avoidance procedure: effect of amphetamine.

The present paper investigated the differences in passive avoidance learning between Sprague--Dawley and Lewis rats. After initial habituation (experimental Part 1), measured as latencies to enter the dark, preferable compartment, the effect of treatment with amphetamine (8 mg kg(-1)b.w.), the retention performance compared with controls (saline) was tested in both rat strains in Parts 2--4. The intervals between Parts 2--4 were 24 or 49 days. Each experimental part consisted of testing lasting 6 days. On the 7th day the rats received drug treatment 1 h before the application of foot shock. The differences between rat strains were already detectable at the beginning of the study. During the repeated exposures of rats in Part 1, only Lewis rats, in contrast to Sprague--Dawley rats, exhibited the habituation. The repeated testing of rats in Parts 2--4, due to previous experience with an aversive stimulus, was considered as the retention test. In Parts 2--3 we observed only minor differences in the responses of both rat strains tested. Also no significant differences were observed between rat strains after amphetamine treatment that induced an amnesia-like effect in all retention trials. However, data shown in Part 4 revealed the largest differences between both strains. Control Lewis rats exhibited significantly higher retention responses than Sprague--Dawley rats. In the latter strain we observed no differences in avoidance latencies between controls and amphetamine treated rats. In Lewis rats the difference in avoidance performance between controls and amphetamine treated animals was highly significant due to their enhanced retention performance. In conclusion, the results presented in this study extend the known behavioural differences in tested rat strains to the passive avoidance procedure that, in addition, was performed for a total period of 4 months. Due to a known deficiency of hypothalamo-pituitary-adrenal axis activity in Lewis rats it can be hypothesized that the behavioural dissociation of this strain from Sprague--Dawley rats could be related to the different activity of this regulatory axis in the rat strains tested.