TDK conference 2017

Fear conditioning is a well-known memory test, where the unconditioned stimulus (most commonly foot-shocking) is linked to the context or a conditioned stimulus. The fear memory is established during acquisition session. This is followed by retention sessions, where shocks are not delivered and the previously formed fear memory is recalled. The intensity of fear was determined by the overall time animals spent with freezing (complete motionlessness) behavior. In our experiments we investigated the effects of 3 treatments on fear expression and extinction:1) different cues as conditional stimuli, 2) changing of the inner walls of the shocking chamber prior to the first retention session, 3) engagement in rewarded cognitive tasks between retention sessions.

In experiment 1, rats were tested in 6 situations. In the “illuminated” setting the shocking chamber was constantly illuminated, no conditional cues were presented. Similarly to this, in the “illuminated+sound” setup, the chamber was illuminated, but this time audible sound cue was applied as a conditional stimuli. In the “dark” setting the chamber was neither illuminated nor paired with a conditional cue. During the “dark+sound”, the “dark+light” and the “dark+sound+light” paradigms an auditory, a visual cue or a combination of both were presented, respectively. The experiment started with an acquisition session (six foot shocks were delivered, preceded by 10 s long conditional cues), followed by 5 retention sessions, and ended with another shocking session.

With the exception of the “illuminated” paradigm, which induced a very low level of freezing, groups did not differ significantly. Intensity of freezing was the highest on the first retention day and fear response got gradually extinct during the consecutive retentions, and significantly increased back again on the next shocking session. By this, we conclude that the main conditional stimulus was the procedure itself.

In the experiment 2, the acquisition session was followed by two retention sessions at 24 hours and day 28. A 2 × 2 design was applied with the following treatments: 1.) the chamber walls were the same or “different in the retention sessions than in acquisition, 2.) animals were handled and engaged in cognitive tasks between the retention sessions, while the other group remained undisturbed. Changing the walls had only marginally significant effect on fear response whereas engagement between the retention days decreased freezing.

Study was funded by: Hungarian National Brain Research Program (NAP), contract # KTIA_NAP_13-2014-0015.