The role of the amygdala in emotional memories : a multidisciplinary approach

This thesis investigates the role of the amygdala for the establishment of fear memories with a multidisciplinary approach, including behavioural, psychopharmacological, genetic, molecular, and electrophysiological techniques in rats or mice, under healthy or pathological conditions. This research program aims to shed light on the acquisition and storage of emotional memories in the amygdala and closely interconnected brain areas. In one line of experiments, the molecular mechanisms leading to the establishment of fear memory traces in the amygdala were investigated. For this purpose, the functional role of the polysialylated neural cell adhesion molecule PSA-NCAM, expressed in the synaptic junction, was assessed in the amygdala – and also prefrontal cortex and hippocampus – with psychopharmacological and genetic approaches and tasks that strongly rely on these brain areas. Two lines of studies were followed: 1) amygdala-targeted cleavage and enhancement of PSA-NCAM in rats and 2) general cleavage of PSA-NCAM throughout the brain using genetically modified mice. Taken together, both approaches show that amygdaloid PSA-NCAM plays no role in the acquisition and storage of fear memories, but is rather involved in their extinction. Furthermore, the results confirm the importance of PSA-NCAM in hippocampus mediated learning and for the first time show that prefrontal cortex mediated learning depends on PSA-NCAM. These results suggest that PSA-NCAM is selectively involved in some, but not all, synaptic plasticity processes in the brain. In another line of experiments, the valproic acid (VPA) animal model of autism was used to investigate a possible contribution of the amygdala towards the autistic pathology. VPA was injected once at a specific time point during gestation, the time of neural tube closure. The offspring of such treated rats were first characterized in a broad set of behavioural tasks. It was found that VPA-treated offspring exhibited very specific behavioural anomalies closely resembling autistic symptomotology, such as impaired social interaction, exploration and recognition, enhanced repetitive behaviours, impaired sensorimotor gating and increased anxiety, while other behavioural parameters were left unharmed. Once the validity of the model was established, amygdala functionality was assessed. The results demonstrated that VPA-treated offspring exhibited highly enhanced conditioned fear memories, which generalized to other stimuli and were resistant to extinction. Electrophysiological in vitro recordings in the amygdala revealed hyper-reactivity towards stimulation and enhanced activity-induced synaptic plasticity. These results imply that enhanced activity and plasticity in the amygdala may underlie the exaggerated fear memories. Furthermore it is suggested in this thesis that a hyper-reactive amygdala may underlie some of the most basic symptoms observed in autism: reduced social interactions and resistance to rehabilitation.