Substantia nigra, nucleus basalis magnocellularis and basolateral amygdala roles in extinction of contextual fear conditioning in the rat

Fear conditioning is accepted as a useful experimental paradigm to investigate anxious disorders following stress. In this field it is important to understand the mechanisms underlying the extinction of conditioned fear. In the rat it has been shown that the amygdalar basolateral nucleus plays a crucial role in all memorization phases of this type of memory (acquisition, consolidation, retrieval, and also reconsolidation and extinction). Recent results show that both the substantia nigra and nucleus basalis magnocellularis, two sites strongly connected with the basolateral amygdala are also involved in the consolidation of contextual fear conditioning. The aim of the present work is to investigate if latter two sites, besides the basolateral amygdala, are also involved in the extinction of the conditioned fear response. The results show that tetrodotoxin-induced inactivation of post-extinction training of either site does not impair the extinction process, which instead is impaired by inactivation of the basolateral amygdala. Thus, the present results confirm previous ones which show that diverse memorization phases (post-acquisition consolidation, extinction, reconsolidation) may be sustained by different neural sites and circuits.