Fear Conditioning Test

For the Fear Conditioning experiment, the procedure was performed as described by Vidal et al. (2018) (Figure 2A). Contextual and tone-cued fear conditioning tests were performed using the Fear Conditioning apparatus (Stoelting) and the AnyMaze Video Tracking System. The mice underwent three days of testing: a training day, a tone-cued-in-a-novel-context testing day, and a contextual testing day. In the training session, each mouse received five tone-shock pairings. The shock (0.5 mA, 50 Hz, 2 s) was delivered 18 s after the end of the tone (70 dB, 2 kHz, 20 s). In the following session (the tone-cued testing day), each mouse was placed in a novel context for 3 min and they were exposed to three tones identical to the ones of the training day, but they did not receive any shock. In the last session, each mouse was placed in a context identical to the one used in the training day for 5 min, but they were not exposed to any tone or shock. The time that the animals spent freezing in the testing sessions was used as a measure of the memory of the association between the tones and shocks, and the tone and the environment, respectively.

Schematic protocol of the Fear Conditioning Test (A). Mean ± S.E.M of the time that TS and CO mice under bexarotene or vehicle treatment spent freezing during the training session, and in the tone-cued and the contextual memory tests sessions in the Fear Conditioning test (B). **: p < 0.01; ***: p < 0.001 TS Vh vs. CO Vh, #: p < 0.05 TS Bx vs. TS Vh; Fisher’s LSD post-hoc tests.