(A) Experimental design. Mice received stereotaxic injection of AAV2.1:hSyn_DIO_hM4Di_mCherry (hM4Di) or control virus (AAV2.1:hSyn_mCherry) into nucleus reuniens (RE), and CAV2:Cre_GFP into dCA1. (B) Representative microphotographs of hM4Di expression in RE and the extent of viral expression (blue, minimal; dark blue, maximal). (C) Experimental timeline. (D) Summary of data for freezing levels of the hM4Di group during the training (Training: RM ANOVA with Fisher’s LSD post hoc test, effect of time: F(1, 21)=446, p<0.001; effect of CNO: F(1, 21)=0.165, p=0.689; time × CNO interaction: F(1, 21)=6.64, p=0.018), fear extinction session (Extinction: RM ANOVA with Šidak’s post hoc test, effect of time: F(1, 21)=95.55, p<0.001; effect of CNO: F(1, 21)=0.259, p=0.616; time × CNO interaction: F(1, 21)=0.1740, p=0.681), and fear extinction test (Test: unpaired t-test: t(20) = 3.26, p=0.002; effect size: 25.442 [95CI 9.492; 41.392]). (E) Summary of data for freezing levels of the control group during the training (Training: RM ANOVA with Fisher’s LSD post hoc test, effect of time: F(1, 10)=119 p<,001; effect of CNO: F(1, 10)=2,86 p=,122; time × CNO interaction: F(1, 10)=3,13 p=,107), fear extinction session (Extinction: RM ANOVA with Šidak’s post hoc test, effect of time: F(2,897, 31,86)=21,32, p<0.001; effect of CNO: F(1, 11)=2,320,, p=0.156; time × CNO interaction: F(5, 55)=1,472, p=0.240) and fear extinction test (Test: unpaired t-test: t(10) = 0.149, p=0.884; effect size: 1.362 [95CI –19.016; 21.749]). (F) Experimental timeline and summary of data for freezing levels during the training. After stereotactic injection of DIO_hM4Di into RE and CAV:Cre_GFP into dCA1, mice underwent CFC (Training: RM ANOVA with Šidak’s post hoc test, effect of time: F(1, 14)=218, p<0.001; effect of CNO: F(1, 14)=0.001, p=0.974; time × CNO interaction: F(1, 14)=0.010, p=0.919), CFE session (Extinction 1: RM ANOVA with Šidak’s post hoc test, effect of time: F(1, 14)=45.5, p<0.001; effect of CNO: F(1, 14)=0.121, p=0.73; time × CNO interaction: F(1, 14)=0.020, p=0.89), short-term CFE memory test (Test 1: t-test: t(14) = 2.45, p=0.028; effect size: 23.935 [95CI 8.310; 39.561]), long-term CFE memory test (Extinction 2: RM ANOVA with Šidak’s post hoc test, effect of time: F(1, 14)=23.1, p<0.001; effect of CNO: F(1, 14)=10.5, p=0.006; time × CNO interaction: F(1, 14)=0.028, p=0.87) and exposure to the context B (CtxB: unpaired t-test: t(14) = 0.057, p=0.955; effect size: –3.306 [95CI –22.850; 16.239]). Due to technical problems, only 6 mice out of 12 from the CNO group were trained on day 4 (Test 2 and CtxB). (G) Experimental timeline and summary of data for freezing levels during the training. Mice were stereotaxically injected into RE with AAV2.1 encoding hM4Di and cannulae placed into dCA1. Next, mice underwent CFC (Training: RM ANOVA with Šidak’s post hoc tests, effect of time: F(1, 12)=146, p<0.001; effect of CNO: F(1, 12)=1.60. p=0.23; time × CNO interaction: F(1, 12)=0.12, p=0.72), CFE session (Extinction: RM ANOVA with Šidak’s post hoc test, effect of time: F(1, 15)=36, p<0.001; effect of CNO: F(1, 15)=2.14, p=0.16; time × CNO interaction: F(1, 15)=4.59, p=0.49) and long-term fear extinction memory test (unpaired t-test, t(15) = 2.30. p=0.036; effect size: 22.803 [95CI 2.383; 43.223]). (H) Experimental timeline and summary of data for freezing levels during the training. Mice were stereotaxically injected into RE with AAV2.1 encoding mCherry and cannulae were placed into dCA1. After surgery, they underwent CFC (Training: RM ANOVA with Šidak’s post hoc tests, effect of time: F(1, 10)=84.3, p<0.001; effect of CNO: F(1, 10)=2.18, p=0.17; time × CNO interaction: F(1, 10)=1.61, p=0.23), CFE session (RM ANOVA with Šidak’s post hoc test, effect of time: F(1, 11)=25.8, p<0.001; effect of CNO: F(1, 11)=0.006, p=0.93; time × CNO interaction: F(1, 11)=0.12, p=0.72) and CFE memory test (unpaired t-test, t(11) = 1.12, p=0.28; effect size: 13.718 [95CI 13.256; 40.696]). (I) Microphotography and schematic representations of cannulae placement. Means +/- SEM are shown.