Role of the basolateral amygdala in learning and relearning context conditioned fear and its extinction.

The basolateral complex of the amygdala (BLA) is a key component of the neuronal circuitry underlying the acquisition and the extinction of Pavlovian conditioned fear. The present series of experiments examined the role of neuronal activity and NMDA receptors (NMDAr) activation in the BLA on learning and relearning context conditioned fear and its extinction. Disruption of neuronal activity in the BLA prevented the acquisition of fear responses to a novel, a moderately familiar or a highly familiar context. It also prevented the reacquisition of fear responses to a conditioned or an extinguished context. Local blockade of NMDAr containing the NR2B subunit prior to training extinction or re-extinction impaired the short- and long-term loss of fear responses. In contrast, a similar blockade subsequent to training extinction or re-extinction left the long-term loss of fear responses unaffected. Disruption of neuronal activity in the BLA prior to training extinction and re-extinction depressed fear responses. It impaired the long-term loss of fear produced by extinction training but spared and even facilitated the long-loss of fear produced by re-extinction training when extinction had already been learned. The exact same outcome was observed when neuronal activity in the BLA was disrupted subsequent to training extinction and re-extinction. These findings suggest that the BLA is critical for both learning and relearning context conditioned fear. In contrast, the BLA is necessary for learning but not relearning extinction of conditioned fear. This implies that once extinction has been learned, others structures support the retrieval and the expression of extinction memory. This is consistent with current neural model of extinction that involves interactions between several neural substrates including the BLA and the medial prefrontal cortex