Fig. 4.

Timeline (Panel A), fear extinction behavior (Panel B), and cholinesterase activity (CHE) in plasma and BLA (Panels C–F) for Experiment 2. Rats were implanted with a DSI telemetry device for cardiovascular recordings (data not reported). Rats then underwent auditory fear conditioning acquisition via three tone-shock pairings (CS+, N = 30) followed by contextual fear recall 24 h later (both in Context A, identical to Context C in Experiment 1). Extinction learning was then assessed 24 h later with 3 min in Context B (identical to Context D in Experiment 1) to assess unconditioned freezing followed by exposure to 20 tones. Extinction recall was assessed 48 h later followed immediately by euthanasia and collection of blood and brains for CHE analysis. Panel B shows group differences between extinction resistant (ER) and extinction competent (EC) rats emerged during cue-induced fear (first 5 tone presentations), extinction learning (last 10 tone presentations of extinction learning), and extinction recall (first 5 tones of extinction recall), but not acquisition (during three tone-shock pairings). Rats were divided into ER and EC groups based a median split of the average freezing during extinction learning (see Sharko et al., 2017). Panels C and D show that ER rats had lower levels of post-test plasma and BLA CHE activity compared EC rats. Panels E and F demonstrate that not only were BLA and plasma CHE positively correlated (r = 0.49, P < 0.01), but BLA CHE activity was negatively correlated with the average percent freezing during the last ten tones of extinction learning (r = −0.44, P = 0.02). * = P < 0.05 for ER versus EC. Data are represented as mean ± SEM. Closed squares = ER rats, open circles = EC rats.