Identification of neuronal ensembles involved in remote fear memory extinction impairments

Abstract

Remote memory consolidation, extinction, and its impairments have been of interest to researchers for years, especially due to its clinical relevance in patients with emotional disorders, like PTSD (Post Traumatic Stress Disorder), anxiety, and phobias. Its neuronal substrates are key elements to understand the persistent nature of remote memory and open a new perspective to novel therapeutic approaches for human fear-related disorders. While the majority of reports investigate the mechanisms and ensembles of recent fear memory extinction (hours to days following conditioning), only a few refer to the remote time point (i.e. weeks after conditioning), usually describing successful memory extinction. The neuronal correlates of impaired remote fear memory extinction were yet beyond the scope. Here we present selective impairment of contextual remote fear memory extinction in alpha calcium/calmodulin-dependent protein kinase II (CaMKII) autophosphorylation-limited mice (T286A+/-). To map brain regions involved in this phenomenon, we applied screening of c-Fos expression, a neuroplasticity marker, across 23 brain areas following contextual fear conditioning and extinction of recent (1-day old) and remote (30-days old) fear memory in WT and T286A+/- mice. Following impaired remote fear memory extinction in T286A+/- mice, we found upregulated c-Fos expression in the entorhinal cortex (ENT), nucleus reuniens (RE), centromedial (CM), mediodorsal (MD), anterodorsal (AD) thalamic nuclei, and medial septum (MS), compared to WT animals performing normal remote fear memory extinction. Thus our data suggest that CaMKII-autophosphorylation-dependent c-Fos expression in these areas controls distant contextual fear extinction and may shed light on these brain regions as potential targets for therapeutic strategies against emotional disorders such as PTSD.