Initial conditioning and re-conditioning recruit different populations of 'fear neurons' in the basal amygdala of rats.

'Fear neurons' in the basal amygdala (Ba) acquire excitatory responsiveness to conditioned stimuli (CS) after fear conditioning and are believed to encode aversive valence of conditioned fear. However, it is unclear whether identical fear conditioning sessions given at different times engage the same population of 'fear neurons'. Here, we recorded electrical activity from single neurons in the Ba while the same fear conditioning paradigm was performed at two different times. Conditioned fear was monitored during CS presentation after each conditioning session in order to identify 'fear neurons'. Surprisingly, we found that initial conditioning and re-conditioning recruited different populations of 'fear neurons' in the Ba. We performed a control experiment in which conditioned fear was monitored twice after a single fear conditioning session. The majority of the 'fear neurons', which were activated during the first retrieval, were re-activated during the second retrieval, suggesting that conditioning-induced 'fear neurons' are stable. Our findings, therefore, suggest that 'fear neurons' in the Ba encode specific learned events as well as their aversive valence.