Fig. 1

Facilitated extinction of contextual fear memory in Trpm2^−/−mice. (a) Time line of the contextual fear conditioning procedure. The freezing re-sponse during habituation (BL) and acquisition was analyzed for 1 min per trial. A foot shock (2-s, 0.7 mA) was given at the end of habituation and the first three conditioning trials. (b) Trpm2^−/− mice displayed reduced contextual fear acquisition (two-way repeated measures ANOVA, genotype: F(1,44) = 23.53, p < 0.0001; shock: F(3,132) = 248.3, p < 0.0001; genotype × shock interaction: Interaction, F(3,132) = 12.26, p < 0.0001; Bonferroni post hoc, 1st, p < 0.0001; 2nd, p < 0.0001; 3rd, p = 0.0276; WT, n = 23; Trpm2^−/−, n = 23). (c) Similar levels of freezing during fear retrieval 24 h after CFC and the first 5 min of extinction training session E1 (unpaired two-tailed t test, genotype: t(46) = 0.1876, p = 0.852; WT, n = 24; Trpm2^−/−, n = 24). (d) Time line of the contextual fear extinction procedure. During the extinction phase, the mice were placed in the chamber for 5 min without reinforcing shock. 24 h later, consolidated extinction memory was recalled by monitoring freezing behavior for 2 min in the original chamber. (e) Trpm2^−/− mice showed a faster rate of contextual fear extinction over the 7-day course of extinction training (two-way repeated measures ANOVA, genotype: F(1,46) = 6.369, p = 0.0151; day: F(6,276) = 65.95, p < 0.0001; genotype × day interaction: Interaction F(6,276) = 3.067, p = 0.0064; Bonferroni post hoc, E2, p > 0.999; E3, p > 0.999; E4, p = 0.1266; E5, p = 0.0556; E6, p = 0.0085; E7, p = 0.0278; WT, n = 24; Trpm2^−/−, n = 24). Extinction retrieval tests 24 h (at 8 d: retrieval 1) and 21 d (at 28 d: retrieval 2) after extinction training showed that Trpm2^−/− mice had less context-dependent freezing behavior to the conditioning context 24 h and 21 d after extinction training than WT mice (unpaired two-tailed t test, 24 h, p = 0.0065, WT, n = 8; Trpm2^−/−, n = 8; 28 d, genotype: t(23) = 3.535, p = 0.0018, WT, n = 12; Trpm2^−/−, n = 13). (f) Remote memory. Conditioned mice without extinction training were returned to the context 28 d later for the remote memory test. There were no significant differences in the percentage durations of freezing between WT and Trpm2^−/− mice at day 28 (p = 0.2405). Animal freezing is measured as percent time spent freezing over a given test period. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001 compared with WT littermates. Numbers in parentheses denote the number of mice in each group used for the experiment. All data are mean ± SEM. Detailed statistics in Supplementary Information