Second-order threat conditioning in the amygdala-posterior piriform cortex network

Fear, while crucial for survival, is a component of a myriad of psychiatric illnesses in its extreme. Persistent fear memories can form through processes such as second-order conditioning (SOC), during which a second-order conditioned stimulus (CS2) acquires significance by associating with a first-order conditioned stimulus (CS1). The neural circuitry underlying SOC, particularly the roles of sensory cortices, remains poorly understood. Here we explore the mechanisms of olfactory SOC in rats, focusing on the basolateral amygdala (BLA) and posterior piriform cortex (pPC). Our results demonstrate that NMDAR-dependent plasticity in both regions is essential for SOC. The BLA mediates the CS2-CS1 association, while the pPC, receiving inputs from the locus coeruleus and BLA, is critical for memory acquisition and retrieval. Single-nucleus multiomics analysis of Fos⁺ ensembles in both regions reveals distinct yet overlapping gene activation profiles in excitatory neurons, accompanied by global chromatin remodeling. These findings highlight the specific yet coordinated roles of these structures in supporting learning and memory.