Figure 2. Hippocampal miR-124 mediates behavioral responses to chronic stress. A, Schematics of AAV engineered to overexpress GFP together with either miR-124 or mock under the control of the Camk2a promoter. B, C, Successful transduction of miR-124 using AAV-mediated gene transfer. B, Northern blotting shows overexpression of miR-124 in the hippocampus. C, Left, Coronal brain sections with distance from bregma, adapted from Franklin and Paxinos (2008). Right, Corresponding images showing transgene expression (GFP, green) after vector microinjection (counterstained with 4′,6-diamidino-2-phenylindole, blue). D, Schematic of the experimental design used to demonstrate reversal of CUMS-induced depression-like behaviors by miR-124 overexpression in the hippocampus. Mice were injected with the AAV–miR-124 overexpression vector or AAV–GFP control vector in the bilateral hippocampus. After 3 weeks, they were subjected to CUMS for 6 weeks and then tested in the behavioral assays. E–H, Behavioral results. E, SI time in the SI. F, Immobility time in the FST. G, Latency to feeding in the NSF. H, Sucrose preference in the SPT. miR-124 overexpression blocked the CUMS-induced decrease in SI (E), increased latency to feeding (G), and reduction in sucrose preference (H). n = 10–13 mice per group. *p < 0.05. I, Schematic of the experimental design for testing the effects of miR-124 downregulation on susceptibility to stress-induced depression-like behaviors. Mice were bilaterally implanted with cannulae directed to the hippocampus. One week after surgery, mice were subjected to mild RRS (1 h/d for 14 d). The miR-124 inhibitor or negative control miRNA was infused every 3 d during the mild RRS session. After the mild RRS session, behavioral tests were performed. J, Location of the cannula tips. K, Northern blotting showing reduced mature miR-124 expression in mice injected with miR-124 inhibitor. L–O, Results of behavioral tests. L, SI time in the SI. M, Immobility time in the FST. N, Latency to feeding in the NSF. O, Sucrose preference in the SPT. This mild RRS protocol did not induce depression-like behaviors compared with the NS condition and the miR-124 inhibitor did not affect baseline responses (NS groups). However, the miR-124 inhibitor reduced interaction time (L), prolonged latency to feeding (N), and decreased sucrose preference (O) in mild RRS-exposed mice, indicative of enhanced susceptibility to depression-like behavior effects. n = 10–13 mice per group. *p < 0.05. All data are presented as mean ± SEM.