Neurotoxic lesions of the lateral nucleus of the amygdala decrease conditioned fear but not unconditioned fear of a predator odor: comparison with electrolytic lesions

Abstract

Considerable evidence suggests that the lateral (LA) and basal (BA) nuclei of the amygdala are sites of plasticity and storage of emotional memory. Recent arguments, however, have seriously challenged this view, suggesting that the effects of amygdala lesions are attributable to interference with performance of fear behavior and not learning and memory. One way to address this controversy is to measure the same behavioral response during both conditioned and unconditioned fear. This is done in the present study by measuring fear-related freezing behavior after electrolytic and neurotoxic lesions of the LA or LA/BA nuclei in rats in a contextual fear conditioning paradigm and unconditioned fear to a predator odor. Electrolytic LA lesions attenuated post-shock freezing, retention test freezing, and freezing to the predator odor trimethylthiazoline (TMT). In contrast, excitotoxic NMDA lesions of the LA had no effect on post-shock freezing but significantly attenuated retention test freezing. Furthermore, excitotoxic LA lesions did not diminish freezing to TMT. Larger excitotoxic lesions that included the BA significantly reduced freezing in both the post-shock and retention tests but did not appreciably decrease freezing to TMT. The results suggest that the LA is important for memory of learned fear but not for generation of freezing behavior. In addition, the BA plays a role in freezing in conditioned fear situations but not in unconditioned fear. The studies suggest that the LA and BA play different roles in fear conditioning, but neither of them has a significant role in unconditioned freezing to a predator odor.

Fig. 1.

Coronal brain sections of electrolytic lesions from the LA group (A) and LA/ASTR group (B). The lesions are outlined inblack. In A, the LA lesions had only slight damage of the ASTR, basal nucleus of the amygdala, and central nucleus of the amygdala. The lesions of the rat shown inB were primarily located in the ASTR, sparing much of the LA. This rat was unique because other rats in the LA/ASTR group had lesions of both the LA and ASTR. Behaviorally, attentuation of freezing in this rat was similar to the rest of the group.Ba, Basal nucleus of the amygdala;Ce, central nucleus of the amygdala; La, lateral nucleus of the amygdala.

Fig. 2.

Schematic representations of largest and smallest electrolytic lesions in the amygdala at four rostral to caudal coronal levels (numbers indicate millimeters posterior from bregma). The sections are of the left ventral quadrants of coronal drawings containing the amygdala from the atlas of Paxinos and Watson (1998). Relevant structures are labeled in the left row of drawings. Lesions of the LA and LA/ASTR are in themiddle and right rows, respectively. Thegray areas represent the animals with smallest lesion and the black areas represent the largest lesion of each group. DEn, Dorsal endopiriform nucleus.

Fig. 3.

Effects of electrolytic lesions on short-term and long-term memory of conditioned fear as measured by the percent of time spent freezing in the post-shock period and retention test, respectively. Freezing in the post-shock period was significantly more than during the retention test for all groups. Overall freezing (both post-shock and retention) in the LA and LA/ASTR groups was significantly less that the no-lesion group. There was no interaction effect.

Fig. 4.

Percentage of time rats with electrolytic lesions of the amygdala spent freezing to TMT. Freezing before TMT presentation (Pre TMT) did not differ between the groups. The three groups differed from each other on the percentage of freezing during TMT. Only the LA/ASTR lesions completely blocked freezing (there was no significant difference in freezing before and during TMT).

Fig. 5.

Representative NMDA lesions in the amygdala. On the left is a low-magnification cresyl violet-stained coronal section of the amygdala. On the right are magnified regions (50×) of the lateral and basal nuclei of a single coronal section from each of the three experimental groups. TheNo Lesion specimen demonstrates normal cell morphology in lateral and basal nuclei. In the LA Lesion samples, gliosis dominates the lateral nucleus, but normal neuronal cell morphology is found the basal nucleus. The LA+ Lesionsamples have only gliosis in both the lateral and basal nuclei.

Fig. 6.

Schematic representations of largest and smallest NMDA lesions in the amygdala at four rostral to caudal coronal levels (numbers indicate millimeters posterior from bregma). The sections are of the left ventral quadrants of coronal drawings containing the amygdala from the atlas of Paxinos and Watson (1998). Relevant structures are labeled in the left row. Lesions of the LA and LA+ groups are in the middle and right rows, respectively. The gray areas represent the animals with smallest lesion and the black areasrepresent the largest lesion of each group. DEn, Dorsal endopiriform nucleus.

Fig. 7.

NMDA lesion of the LA and LA+ on conditioned fear.A, Effects of NMDA lesions on short-term and long-term contextually conditioned fear as measured by the percentage of time spent freezing in the post-shock period and retention test, respectively. Freezing in the post-shock period was significantly more than during the retention test for all groups. The LA+ group had significantly less overall freezing than the no-lesion and LA groups. The post-shock freezing of the no-lesion and LA groups did not differ; however, there was significantly less freezing during the retention test in the LA group compared with the no-lesion group.B, The percentage of decrease in freezing in the retention test compared with the post-shock period. Both the LA and LA+ groups had a significantly greater percentage decrease of freezing in the retention test from the post-shock period than the no-lesion group.

Fig. 8.

Percentage of time rats with NMDA lesions of the amygdala spent freezing to TMT. Freezing before TMT presentation did not differ between the groups. There was a slight but statistically significant decrease in freezing in the LA+ group compared with the no-lesion group.