All auditory fear conditioning and extinction procedures were performed using the Ugo Basile Fear Conditioning System (Ugo Basile Srl). Briefly, mice were first handled and habituated to the conditioning chamber for five successive days. The conditioning chambers (17 cm by 17 cm by 25 cm) equipped with stainless steel shocking grids were connected to a precision feedback current-regulated shocker (Ugo Basile Srl). During fear conditioning, the chamber walls were covered with black and white checkered wallpapers, and the chambers were cleaned with 75% ethanol (context A). On day 1, mice were conditioned individually in context A with five pure tones (CS; 4 kHz, 76 dB, 30 s each) delivered at variable intervals (20 to 180 s), and each tone was co-terminated with a foot shock (US; 0.5 mA, 2 s each). ANY-maze software (Stoelting Co.) was used to automatically control the delivery of tones and foot shocks. Conditioned mice were returned to their home cages 30 s after the end of the last tone, and the floor and walls of the cage were cleaned with 75% ethanol for each mouse. For retrieval, 24 hours after conditioning, animals received two CS-alone presentations in a test chamber, which had gray nonshocking plexiglass floor and dark gray wallpapers and was washed with 4% acetic acid solution between the tests for individual mice (context B). In extinction experiments, animal trained in context A with five CS-US pairings on day 1 was presented with 20 CS without foot shock in context B on day 2. On day 3, mice received eight CS-alone presentations in the extinction context. During the behavioral test, the chamber was placed in a sound-attenuating enclosure with ventilation fan and a single house light (Ugo Basile Srl). The movement of the mouse in the conditioning or test chamber was recorded using a near-infrared camera and analyzed in real time with ANY-maze software. Fear response was operationally defined as measurable behavioral freezing (more than 1-s cessation of movement), which was automatically scored and analyzed by ANY-maze software. The time spent on freezing during the tone (cue) was measured for each tone presentation.

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