Lesions of the Basal Amygdala Block Expression of Conditioned Fear But Not Extinction

Although the role of the amygdala in acquisition of conditioned fear is well established, there is debate concerning the intra-amygdala circuits involved. The lateral nucleus of the amygdala (LA) is thought to be an essential site of plasticity in fear conditioning. The LA has both direct and indirect [via the basal nuclei; basal amygdala (BA)] projections to the central nucleus (Ce) of the amygdala, an essential output for fear behaviors. Lesions of the LA or Ce prevent acquisition of conditioned freezing to a conditioned stimulus, but BA lesions do not, suggesting that the BA is not normally involved in fear conditioning. If true, posttraining BA lesions should also have no effect. Replicating previous studies, we found that rats given electrolytic BA lesions before training acquired conditioned fear normally. They also showed normal long-term retention and extinction of conditioned fear. Unexpectedly, BA lesions made after training completely blocked expression of conditioned fear. Despite this deficit, lesioned rats were able to learn a new tone-shock association. Thus, although the LA-Ce system is sufficient for fear acquisition in the absence of the BA, it is not sufficient when the BA is present, suggesting that the BA is an important site of plasticity in fear conditioning. The pattern of lesion deficits we observed (after but not before training) might be explained by homeostatic mechanisms that balance plasticity over multiple inputs, regulating the influence of the BA and LA onto Ce output neurons.



Figure 5. Model to account for deficits with posttraining, but not pretraining, lesions. We suggest that homeostatic mechanisms that balance plasticity over multiple inputs could regulate the influence of the BA, LA, and MGm onto CeM neurons. 1, The CeM receives inputs from the MGm, BA, and LA via ITC cells (dashed line). 2, After fear conditioning, BA inputs gain a large proportion of the total plasticity, inhibiting the development of plasticity in neighboring inputs. 3, Posttraining lesions of the BA remove this plasticity, leaving the system subthreshold for producing a fear response. 4, With pretraining removal of the BA, however, increased plasticity in the LA and MGm supports fear learning. For simplicity, projections from the MGm to the LA and from the LA to the BA are not shown.

David Anglada-Figueroa, and Gregory J. Quirk
Lesions of the Basal Amygdala Block Expression of Conditioned Fear But Not Extinction
J. Neurosci. 25: 9680-9685; doi:10.1523/JNEUROSCI.2600-05.2005

http://www.jneurosci.org/cgi/content/full/25/42/9680