Defective fear extinction in low shock-conditioned VPAC2-KO mice. On Day 1, mice were trained with 3 tone-shock pairings (A). Then, mice were subjected to the contextual fear memory test on Day 2 (B), cued fear memory test on Day 3 (C), and cued fear extinction paradigm on Days 4 and 5 (D). Results are expressed as the mean ± S.E.M. of 12 mice per group. BL: Baseline. ††P < 0.01, compared with the baseline. *P < 0.05, compared with wild-type.