Figure 4.

Within-session extinction of conditioned fear in NPY, Y₁, Y₂ and Y_1/Y₂ receptor double KO mice. (A–C) Histograms (right panels) show impaired fear extinction of NPY KO mice demonstrated by equal % of freezing to the first three and last three CS of extinction sessions on day 2 and day 3, (D–F) compared to WT, Y₁ receptor KO mice show significantly higher % of freezing to the last three CS on extinction day 2 but not on extinction day 3, indicating a delayed extinction process, (G–I) Y₂ receptor KO mice demonstrate equal % of freezing as WT controls on both extinction days, (J–L) Y_1/Y₂ receptor double KO mice display increased freezing to the first and last three CS on both extinction days. Data shown are means ± SEM, repeated two-way anova with Bonferroni post hoc test for expression of conditioned fear, *P < 0.05, **P < 0.01, ***P < 0.001.