Fig. 1.

Extinction learning of olfactory threat memory is impaired in aged rats. (A) Schematic of olfactory threat conditioning and extinction training. (B) Time course of % freezing during recall and extinction training (n = 13 adult, n = 12 aged). Adult rats showed significantly reduced freezing compared to aged rats following 6 d of extinction training. Another cohort of adult rats (n = 6) that did not undergo extinction training showed high freezing to the conditioned odor when tested 6 d following odor threat conditioning. (C) Measurement of percentage of freezing on recall day showed no difference in threat learning acquisition between adult and aged groups. (D) Measurement of percentage of extinction on the last day of extinction training (% recall freezing − % freezing on the last training day)/(% recall freezing) showed impaired extinction in the aged group. (E) Examples of cFos expression in the anterior piriform cortex of adult (n = 5 Odor; n = 4 Cond.; n = 5 Extinc.) and aged rats (n = 7 Cond. and n = 5 Extinc.). (F) Comparisons of cFos cell counts in various groups of adult and aged rats showed no difference among groups. (G) No correlation of cFos⁺ cell numbers in the anterior piriform cortex and % freezing in individual rats. Red and black circles indicate data from the adult and aged groups, respectively. (H) Examples of cFos expression in the posterior piriform cortex of adult (n = 5 each group) and aged rats (n = 7 Cond. and n = 5 Extinc.). (I) Comparisons of cFos cell counts in various groups of adult and aged rats showed increased cFos expression following odor threat conditioning and reduction to control levels following extinction training in adult rats only. In aged rats, no difference in cFos expression was observed between the conditioned rats and rats that underwent extinction training. (J) Positive correlation of cFos⁺ cell number in the posterior piriform cortex and % freezing in individual rats. Red and black circles indicate data from the adult and aged groups, respectively. Odor: odor exposure only; Cond.: olfactory threat-conditioned; Extinc.: extinction-trained (Scale bars, 50 µm). *P < 0.05, **P < 0.01.