Activity chamber

Animals were placed in activity chambers (43 × 43 cm, Med Associates, Inc.) for 3-min sessions. Total distance traveled (centimeter) and spatial location (time in ‘center’ or ‘margin’) were tracked with an overhead camera and Ethovision XT 7.0 software (Noldus Information Technology Inc., Wageningen, The Netherlands).

Drug-induced locomotion were measured using Med Associates activity chambers as previously described [31 (link)]. The open field chamber (27 × 27 × 20.5) are placed within light- and air-controlled sound-attenuating boxes (64 cm × 45 cm × 42 cm). Locomotion was detected by interruption of infrared beams by the body of the mouse (16 photocells in each horizontal axis located 1 cm above the activity chamber floor, as well as 16 photocells elevated 4 cm above the chamber floor to detect rearing behaviors). Data were collected and analyzed by Med Associates Activity Monitor 7 software. Activity testing was performed with prior habituation wherein, on day 1, animals were placed in the chamber for 30 min to acclimate to the testing environment. 2 days later, animals were again habituated for 30 min, injected with sterile saline (0.9% NaCl) and activity was recorded for 60 min post-injection. On the final day of testing, mice received drug injections following an initial 30 min habitation and activity was again monitored for 60 min post-injection. Apomorphine HCl hemihydrate (Sigma, 5 mg/kg, St. Louis, MO, USA) was dissolved in sterile saline and administered subcutaneously (s.c.). Quinpirole hydrochloride (Sigma, 1 mg/kg) was dissolved in sterile saline and administered via intraperitoneal (i.p.) injection.

Rat exploratory behavior was tested prior to fear conditioning and extinction in a Med Associates activity chamber (43.2 × 43.2 × 30.5 cm) consisting of clear plastic sides and a white Plexiglas floor. This apparatus detects horizontal and vertical motion via two parallel grids of intersecting infrared beams (one at floor level and one at a height that the rat can only reach by jumping or rearing up on its hind legs). A linked computer records the rat’s X,Y,Z coordinates every 50 ms. Rats were tested in two different setups: (1) an open-field setup, consisting of direct placement in the bare apparatus as described above and (2) a light–dark setup, in which a dark Plexiglas box with a rat-sized doorway covers exactly half of the chamber. Rats were tested for 10 min each day for 2 days for each setup (for a total of 40 min over 4 days), and the order of the setups was counterbalanced. Days 1 and 2 of the open field were conducted identically: the rat was placed in a corner of the chamber and allowed to move freely. Days 1 and 2 of the light–dark test were conducted differently: on Day 1, the rat was restricted to exploring the dark box; on Day 2, the rat was placed in the dark box, but allowed to move freely between dark and light compartments. Luminance (outside of the dark box) was provided by overhead incandescent lights adjusted to provide 100 lux at the level of the chamber.