Figure 2.

Effects of extending context pre-exposure time on retention of contextual fear memory. (A) The level of percent time freezing did not differ between WT and SK2+/T mice whether the mice received either one 5-min context pre-exposure or three 5-min context pre-exposure sessions on Day 1. The data for the mice that received one 5-min context pre-exposure session are plotted in S1 together with the data from the first context pre-exposure session of those mice that received three 5-min context pre-exposure sessions (S1, S2, and S3). Note the break in the ordinate axis. A repeated measures ANOVA on total percent time freezing for the 3 × 5-min pre-exposure mice yielded a significant effect of session, F_(2,64) = 8.22; P < 0.002, a nonsignificant interaction of genotype X session, F_(2,64) = 2.24; n.s., and a nonsignificant main effect of genotype, F_(1,32) = 1.83; n.s. These data indicate that while freezing increased over the course of the three 5-min exposures to the conditioning chambers, there was no genotypic difference in freezing behavior prior to conditioning. The increase in amount of time the mice spent immobile would be expected to increase as the animals habituate to the locomotor activational effects of exposure to a novel environment. Genotypic differences in freezing behavior during the second and third 5-min pre-exposure sessions may reflect delayed habituation by the SK2+/T mice. (B) The freezing data from the conditioning trials presented on Day 2 are plotted separately for the 1 × 5-min group (left) and 3 × 5-min group (right) for clarity. Both groups of mice exhibited similar levels of freezing during the conditioning session. The gray vertical bars indicate the 30-sec periods during the presentation of each auditory tone CS. (C) Retention of contextual fear memory was tested at 24 h after conditioning. (C-1) SK2+/T mice that received one 5-min context pre-exposure session exhibited significantly less freezing during the context test than did comparably trained WT mice. This result is consistent with the data of Figure 1C. In contrast, SK2+/T mice that received three 5-min context pre-exposure sessions exhibited freezing that was comparable to the WT mice. (C-2) The freezing measures for the two groups of mice that received three 5-min pre-exposure sessions were calculated in 30-sec bins and plotted accordingly to examine the dynamic of freezing over the course of the 5-min context test. These data suggest that the rate and degree of extinction of context fear memory was equivalent across the two groups of mice. (D) Retention of cued fear memory was tested ∼1 h after the context test on Day 3. There were no differences in percent freezing between genotypes before the tone was presented (Pre-tone), and SK2+/T mice exhibited significantly less freezing in response to the tone compared with WT mice, regardless of context pre-exposure condition. All error bars, SEM. Significant differences are indicated by an asterisk (* P < 0.05; see text for details).