Abstract: |
Formation, storage and expression of
aversive Pavlovian CS-US associations is dependent on the amygdala, specifically
the lateral (LA) and central (CE) nuclei. Pavlovian fear conditioning is
an important early step in instrumental active avoidance (AA) learning.
Multiple electrophysiology, pharmacology and lesion studies suggest that
the amygdala is also necessary for the acquisition of AA, and plays a transient
role in maintaining these responses. However, these studies have generally
not distinguished between the roles of the different amygdalar nuclei. Previous
work in our laboratory suggests that different outputs of the LA mediate
passive versus active responses to aversive cues
(Amorapanth et al., 2000; Choi et al., submitted). The LA and CE mediate
passive defensive behaviors while the LA and the basal nucleus of the amygdala
(B) seem to mediate active defensive behaviors. In this set of experiments,
we examined the effects of bilateral electrolytic or sham lesions of the
LA, B or CE on the acquisition of Sidman AA. Following recovery from surgeries,
all rats were given 8 Sidman AA training sessions. Sidman AA was carried
out in a two-way avoidance chamber in which shuttling once every 30s prevented
the delivery of footshocks. A repeated measures ANOVA comparing the mean
number of AA responses per training block (two training sessions/block)
showed a significant effect of Group F(3, 246) = 20.87; p < 0.01. Bonferonni
post-tests analysis revealed that the LA and B groups performed significantly
less AA responses on blocks 2, 3, and 4 than the Sham group (ps < 0.05
for B; 0.01 for LA). Moreover, post-hoc analysis indicated that the CE group
showed a significantly higher mean number of AA responses than the Sham
group on block 1 (p < 0.05). Together, these results suggest that AA
responses were impaired by pre-training LA or B lesions, but facilitated
by pre-training CE lesions during early stages of training. These results
are consistent with the hypothesis that the LA and B mediate active defensive
behaviors while the CE mediates passive defensive behaviors that constrain
AA. |