Longitudinal recordings of single units in the basal amygdala during fear conditioning and extinction.

The balance between activities of fear neurons and extinction neurons in the basolateral nucleus of the basal amygdala (BA _L ) has been hypothesized to encode fear states after extinction. However, it remains unclear whether these neurons are solely responsible for encoding fear states. In this study, we stably recorded single-unit activities in the BA _L during fear conditioning and extinction for 3 days, providing a comprehensive view on how different BA _L neurons respond during fear learning. We found BA _L neurons that showed excitatory responses to the conditioned stimulus (CS) after fear conditioning ('conditioning-potentiated neurons') and another population that showed excitatory responses to the CS after extinction ('extinction-potentiated neurons'). Interestingly, we also found BA _L neurons that developed inhibitory responses to the CS after fear conditioning ('conditioning-inhibited neurons') or after extinction ('extinction-inhibited neurons'). BA _L neurons that showed excitatory responses to the CS displayed various functional connectivity with each other, whereas less connectivity was observed among neurons with inhibitory responses to the CS. Intriguingly, we found correlative neuronal activities between conditioning-potentiated neurons and neurons with inhibitory responses to the CS. Our findings suggest that distinct BA _L neurons, which are responsive to the CS with excitation or inhibition, encode various facets of fear conditioning and extinction.