Real-time place preference (RTPP)

The implanted optic fiber of the tested mouse was connected to a blue light laser via patch cords (Fiblaser, Shanghai, China) and a fiber-optic rotary joint (Doric Lenses, Quebec, Canada), and placed in a behavioral arena containing two equivalent chambers. Each chamber is 50(H) x 25(L) x 25(W) cm. The two chambers are connected by a 12.5 cm wide gate. Mice can travel freely between two chambers through the gate. Before test, for both ChR2 and control group, one of the equivalent chambers was randomly assigned as the light-paired chamber (stimulation side). The first mouse was placed in the non-stimulation side at the onset of the experiment. The laser (473 nm laser, 5 ms pulse duration, 10–15 mW intensity, 20 Hz frequency) was turned on once the mouse traveled to the light-paired chamber and was kept on when it stayed in the chamber. The laser was turned off when the mouse traveled to the non-stimulation chamber. The test was finished after 20 min. Then we moved on to test the next mouse. At this time, the former non-stimulation chamber was assigned as light-paired chamber, while the previous light-paired chamber was reversed in the similar way. The assigned stimulation and non-stimulation chambers were always switched between two tested mice within the same group for counterbalancing the natural preference of the behavioral chambers. All behavioral tests were recorded using an HD digital video camera (Sony, Shanghai, China) and the offline analyses were performed by a video tracking software (ANY-maze; Stoelting Co, IL) to automatically separate the whole 20 min tests into 4 sections (5 min for each section).