PMC

PL is necessary for fear expression, but not extinction memory. (a) Cannula placements in PL. (b) Inactivation of PL (MUS: n=14; SAL: n=11) prior to extinction training (Day 2) reduced conditioned freezing during trial blocks 1–5. ^*p<0.05, Tukey's post hoc. During extinction retrieval (Day 3), however, MUS-infused rats displayed normal levels of freezing. (b, inset) Effects on suppression were similar to conditioned freezing. Inactivation reduced suppression on trial block 1 during extinction training (Day 2). ^*p<0.05, Student's t-test. During extinction retrieval (Day 3), however, MUS-infused rats displayed normal levels of suppression during the first trial block. Data are represented as group averages.

vHPC is necessary for both fear expression and extinction memory. (a) Cannula placements in vHPC. (b) Inactivation of vHPC (MUS: n=14; SAL: n=14) prior to extinction training (Day 2) reduced conditioned freezing during trial blocks 1–7 and 10. ^*p<0.05, Tukey's post hoc. During extinction retrieval (Day 3), MUS-infused rats displayed significantly higher levels of freezing during trial blocks 2–4. ^*p<0.05, Tukey's post hoc. (b, inset) Effects on suppression were similar to conditioned freezing. Inactivation reduced suppression on trial block 1 during extinction training (Day 2). ^*p<0.05, Student's t-test. During extinction retrieval (Day 3), MUS-infused rats displayed significantly higher levels of suppression during the first trial block. ^*p<0.05, Student's t-test. Data are represented as group averages.

BLA is necessary for both fear expression and extinction memory. (a) Cannula placements in BLA. (b) Inactivation of BLA (MUS: n=11; SAL: n=12) prior to extinction training (Day 2) reduced conditioned freezing during trial blocks 1–3. ^*p<0.05, Tukey's post hoc. During extinction retrieval (Day 3), MUS-infused rats displayed higher levels of freezing during trial blocks 1–3. ^*p<0.05, Tukey's post hoc. (b, inset) Effects on suppression were similar to conditioned freezing. Inactivation reduced suppression on trial block 1 during extinction training (Day 2). ^*p<0.05, Student's t-test. During extinction retrieval (Day 3), MUS-infused rats displayed significantly higher levels of suppression during the first trial block. ^*p<0.05, Student's t-test. Data are represented as group averages.

Circuits of fear expression and extinction memory. (a) Fear expression. Freezing during the first trial block of extinction training for each muscimol group is represented as percent of saline controls (100%). PL and vHPC could augment fear expression through their projections to BLA, either in parallel or in series. (b) Extinction memory. Freezing during trial blocks 1–2 of extinction retrieval (drug free) for each muscimol group is represented as percent of saline controls (100%). Extinction training results in plasticity (gray starburst) at vHPC and/or BLA inputs to IL. Extinction-related plasticity may also occur at IL inputs to intercalated (ITC) cell masses in the amygdala. After extinction, activation of ITCs would inhibit amygdala output via projections to CeA, resulting in low fear.

IL is necessary for extinction memory, but not fear expression. (a) Cannula placements in IL. (b) Inactivation of IL (MUS: n=12; SAL: n=12) prior to extinction training (Day 2) did not affect conditioned freezing, but impaired the acquisition of extinction memory, as noted by higher levels of freezing during trial blocks 4–8. ^*p<0.05, Tukey's post hoc. During extinction retrieval (Day 3), MUS-infused rats displayed impaired extinction memory, as noted by higher levels of freezing during trial blocks 1–2. ^*p<0.05, Tukey's post hoc. (b, inset) Effects on suppression were similar to freezing. Inactivation did not affect suppression on trial block 1 during extinction training (Day 2). During extinction retrieval (Day 3), however, MUS-infused rats displayed a trend toward higher suppression during the first trial block. p=0.08, Student's t-test. Data are represented as group averages, and bars indicate SEM.