Age-dependent changes in autophosphorylation of alpha calcium/calmodulin dependent kinase II in hippocampus and amygdala after contextual fear conditioning

The hippocampus and amygdala are essential brain regions responsible for contextual fear conditioning (CFC). The autophosphorylation of alpha calciumcalmodulin kinase II (aCaMKII) at threonine-286 (T286) is a critical step 3 implicated in long-term potentiation (LTP), learning and memory. However, the changes in aCaMKII levels with aging and training in associated brain regions are not fully understood. Here, we studied how aging and training affect the levels of phosphorylated (T286) and proportion of phosphorylated:total aCaMKII in the hippocampus and amygdala. Young and aged mice, naïve (untrained) and trained in CFC, were analysed by immunohistochemistry for the levels of total and phosphorylated aCaMKII in the hippocampus and amygdala. We found that two hours after CFC training, young mice exhibited a higher level of phosphorylated and increased ratio of phosphorylated:total aCaMKII in hippocampal CA3 stratum radiatum. Furthermore, aged untrained mice showed a higher ratio of phosphorylated:total aCaMKII in the CA3 region of the hippocampus when compared to the young untrained group. No effect of training or aging were seen in the central, lateral and basolateral amygdala regions, for both phosphorylated and ratio of phosphorylated:total aCaMKII. These results show that aging impairs the training-induced upregulation of autophosphorylated (T286) aCaMKII in the CA3 stratum radiatum of the hippocampus. This indicates that distinct age-related mechanisms underlie CFC that may rely more heavily on NMDA receptor-dependent plasticity in young age.