Figure 1.

Impaired extinction of fear memory in E2/E2 mice. (a) Experimental animals. Mice expressing human apolipoprotein E (apoE) were crossed to generate the six APOE genotypes. Male mice from the F2 generation (box) were used in this study. (b) All groups demonstrate robust acquisition of conditioned fear, as shown by an increase in freezing across the fear conditioning session. (c) There was no difference in time spent freezing by the conclusion of the fear conditioning session (average freezing during the final minute). (d) No differences in sensitivity to the aversive stimulus were observed, as noted by the activity response during the mild foot shocks. (e and f) E2/E2 mice show impaired extinction of the conditioned fear memory, as evidenced by a slower decline in freezing (slope of extinction curve) in response to the context; n=9–12. Groups labeled with dissimilar letters are significantly different (p<0.05, analysis of variance (ANOVA) followed by t-test).