Layer-specific analysis of cortical fear engram in mice

Associative learning is a fundamental mechanism for acquiring new experience by the brain. Though molecular, structural and synaptic aspects of this process have been thoroughly studied, less is known about how it happens at the level of cortical circuitries. According to the theory of predictive coding, associative learning leads to the formation of cortical representations or engrams that affect the supragranular (L2/3) and infragranular (L5/6) cortical layers in different ways. We utilized the cued (auditory) fear conditioning paradigm and layer-specific c-Fos neuroimaging of associative (prelimbic and cingulate) and auditory cortical areas after memory retrieval in mice to elucidate this issue. Retrieval of associative memory about auditory conditioned stimulus (CS) resulted in the preferential activation of cingulate and infralimbic cortices in conditioned mice compared to unpaired, CS-only controls and home cage mice. This activation was specific in all layers of cingulate cortex as well as in infragranular layers of prelimbic cortex. In auditory cortex in all mice, presented with auditory stimuli during retrieval (conditioned, unpaired and sound control groups) there was strong a c-fos activation. Such activation was found in all layers. Finally, we used correlation analysis and showed the c-fos expression in prelimbic and primary auditory but not cingulate cortex was highly-correlated across layers. The present findings indicate that associative memory cortical engram is unequal across layers and preferentially includes infragranular neurons.