Conditioned Lick Suppression: Assessing Contextual, Cued, and Context-cue Compound Fear Responses Independently of Locomotor Activity in Mice.

Pavlovian fear conditioning is a widely used procedure to assess learning and memory processes that has also been extensively used as a model of post-traumatic stress disorder (PTSD). Freezing, the absence of movement except for respiratory-related movements, is commonly used as a measure of fear response in non-human animals. However, this measure of fear responses can be affected by a different baseline of locomotor activity between groups and/or conditions. Moreover, fear conditioning procedures are usually restricted to a single conditioned stimulus (e.g., a tone cue, the context, etc.) and thus do not depict the complexity of real-life situations where traumatic memories are composed of a complex set of stimuli associated with the same aversive event. To overcome this issue, we use a conditioned lick suppression paradigm where water-deprived mice are presented with a single conditioned stimulus (CS, a tone cue or the context) previously paired with an unconditioned stimulus (US, a foot shock) while consuming water. We use the ratio of number of licks before and during the CS presentation as a fear measure, thereby neutralizing the potential effect of locomotor activity in fear responses. We further implemented the conditioned lick suppression ratio to assess the effect of cue competition using a compound of contextual and tone cue conditioned stimuli that were extinguished separately. This paradigm should prove useful in assessing potential therapeutics and/or behavioral therapies in PTSD, while neutralizing potential confounding effects between locomotor activity and fear responses on one side, and by considering potential cue-competition effects on the other side.