Murine GRPR and Stathmin Control in Opposite Directions both Cued Fear Extinction and Neural Activities of the Amygdala and Prefrontal Cortex

Extinction is an integral part of normal healthy fear responses, while it is compromised in several fear-related mental conditions in humans, such as post-traumatic stress disorder (PTSD). Although much research has recently been focused on fear extinction, its molecular and cellular underpinnings are still unclear. The development of animal models for extinction will greatly enhance our approaches to studying its neural circuits and the mechanisms involved. Here, we describe two gene-knockout mouse lines, one with impaired and another with enhanced extinction of learned fear. These mutant mice are based on fear memory-related genes, stathmin and gastrin-releasing peptide receptor (GRPR). Remarkably, both mutant lines showed changes in fear extinction to the cue but not to the context. We performed indirect imaging of neuronal activity on the second day of cued extinction, using immediate-early gene c-Fos. GRPR knockout mice extinguished slower (impaired extinction) than wildtype mice, which was accompanied by an increase in c-Fos activity in the basolateral amygdala and a decrease in the prefrontal cortex. By contrast, stathmin knockout mice extinguished faster (enhanced extinction) and showed a decrease in c-Fos activity in the basolateral amygdala and an increase in the prefrontal cortex. At the same time, c-Fos activity in the dentate gyrus was increased in both mutant lines. These experiments provide genetic evidence that the balance between neuronal activities of the amygdala and prefrontal cortex defines an impairment or facilitation of extinction to the cue while the hippocampus is involved in the context-specificity of extinction

Schematic representation of the connectivity of brain areas involved in fear extinction.

<p>There is a balance between the amygdala, hippocampus and prefrontal cortex during normal fear reaction in wildtype mice (<b>A</b>). In GRPR KO mice there is a shift of the balance between the basolateral amygdala and prefrontal cortex towards stronger activation of the basolateral amygdala leading to higher freezing (<b>B</b>). Stronger neural activity in the prefrontal cortex leads and lesser in the basolateral amygdala leads to less freezing in stathmin KO mice (<b>C</b>).</p

Cued fear extinction is controlled by stathmin and GRPR in opposite directions.

<p>(<b>A</b>) Protocol used for acquisition (left) and extinction (right) of cued fear conditioning. (<b>B</b>) Acquisition (left) and extinction (right) performances of GRPR WT and KO mice (11 WT and 12 KO). (<b>C</b>) Acquisition (left) and extinction (right) performances of stathmin WT and KO mice (16 WT and 11 KO). Acquisition performance is expressed as percentage of freezing during tone-shock pairings and extinction performance is expressed as percentage of freezing during 5 blocks (4 tones) for 4 days of extinction. Results are presented as mean ± SEM. R, renewal. *represents significant difference between groups during one block of a daily session; # represents significant difference between groups during the whole extinction phase.</p

Stathmin and GRPR are not involved in contextual fear extinction.

<p>(<b>A</b>) Representation of the protocol used for acquisition (tone-shock explicitly unpaired, left) and extinction (right) of contextual fear conditioning. (<b>B</b>) Acquisition (left) and extinction (right) performances of GRPR WT and KO mice (12 WT and 12 KO). (<b>C</b>) Acquisition (left) and extinction (right) performances of stathmin WT and KO mice (11 WT and 11 KO). Acquisition performance is expressed as percentage of freezing minute by minute and extinction performance is expressed as percentage of freezing during 10 minutes of the session during 4 days of extinction. Results are presented as mean ± SEM. # represents significant difference between groups during the whole extinction phase.</p