Repeated exposure to rats has persistent genotype-dependent effects on learning and locomotor activity of apolipoprotein E knockout and C57B1/6 mice

Recently we have shown that an experimentally controlled encounter of mice with rats ('rat stress') some time before actual behavioural testing either abolished or induced behavioural deficits in the Morris water maze. depend ing on the genotype of the mice: apolipoprotein E knockout mice (apoE0/0) a nd wild type mice. Here we report that previous rat stress: (i) facilitated learning of a circular hole board task in apoE0/0 mice and impaired learni ng in wild type mice, thereby abolishing genotype-dependent differences (ii ) although both genotypes preferred the dark compartment when tested in a l ight ida rk-preference task 3 months after rat stress. locomotor activity w as reduced in apoE0/0 and increased in wild type mice, thus genotype differ ences were amplified, (iii) both genotypes responded with a differential re gulation of bodyweight during exposure to rats, which persisted for 3 month s apoE0/0 mice decreased while wild type mice increased their body weight; (iv) the high emotional reactivity (defecation boli) measured during behavi oural tasks was not affected in apoE0/0 mice, whereas a decrease was observ ed in wild type mice. Thus. pre-experimental confrontation of mice with rat s shifts behaviour and physiological responses and eliminates some of the g enotype-dependent differences. (C) 2001 Elsevier Science B.V. All rights re served.