Adolescent conditioning affects rate of adult fear, safety and reward learning during discriminative conditioning

Abstract

Fear and reward memories formed in adulthood are influenced by prior experiences. Experiences that occur during sensitive periods, such as adolescence, can have an especially high impact on later learning. Fear and reward memories form when aversive or appetitive events co-occur with initially neutral stimuli, that then gain negative or positive emotional load. Fear and reward seeking behaviours are influenced by safety cues, signalling the non-occurrence of a threat. It is unclear how adolescent fear or reward pre-conditioning influences later dynamics of these conditioned emotions, and conditioned safety. In this study, we presented male rats with adolescent fear or reward pre-conditioning, followed by discriminative conditioning in adulthood. In this discriminative task, rats are simultaneously conditioned to reward, fear and safety cues. We show that adolescent reward pre-conditioning did not affect the rate of adult reward conditioning, but instead accelerated adult safety conditioning. Adolescent fear pre-conditioning accelerated adult fear and reward seeking behaviours but delayed adult safety expression. Together, our results suggest that the dynamics of safety conditioning can be influenced by adolescent priming of different valences. Taking adolescent experiences into consideration can have implications on how we approach therapy options for later learned fear disorders where safety learning is compromised.

Figure 1

Paradigm outline. In adolescence (P30), rats were exposed to the future conditioning context alone, reward conditioning, to the safety cue unpaired to footshocks, or fear conditioning (cxt-ctr, ADSC-R, ADSC-U, ADSC-F). In adulthood (P75), rats received reward conditioning (r), followed by 1 habituation session to familiarize rats to the later fear and safety cues. Next, rats underwent 4 days of discriminative conditioning (DC) followed by extinction training and extinction recall.

Figure 2

Adolescent fear conditioning accelerated adult reward conditioning. Averaged % time spent in the port during each reward cue was used to assess reward seeking behaviour. Over the 5 days of reward conditioning (r1-5) ADSC-F rats presented the steepest learning curve, despite insignificant baseline differences at r1 (a). This effect in the ADSC-F group was not already apparent within the first reward conditioning session (b). r1-5: reward session 1-5. i: interval, 5 consecutive reward trials were averaged to form an interval; ^####p < 0.0001, ^###p < 0.001, ^##p < 0.01 and ^#p < 0.05 vs r1 (a) or i1 (b), ^p < 0.05 vs ADSC-U of the same interval. Data are mean + SEM.

Figure 3

Adolescent conditioning does not affect reward seeking during adult discriminative conditioning (DC) or reward extinction. Throughout DC all rats spent more time in the port upon reward cue presentation than upon presentation of any other cue (a–d). During extinction, all groups significantly and uniformly reduced their time in the port (e), which persisted to the extinction recall (f). i: interval, 5 consecutive reward trials were averaged to form an interval; ^####p < 0.0001 and ^#p < 0.05: reward vs all other cues, ⁺p < 0.05 vs fear cue of the same group. Data are mean + SEM.

Figure 4

Adolescent conditioning affects fear expression and rate of safety learning during adult discriminative conditioning. Averaged % time freezing during each cue was used to assess fear related behaviour. The ADSC-F group showed increased freezing to the fear and the fear + safety cue in DC1 and 2 (a,b). Safety learning is defined as a significant suppression of freezing to the fear + safety cue, compared to the fear cue alone. ADSC-R rats showed significant safety learning as early as DC2 (b–d), whereas the ADSC-F group did not show significance until DC4 (d). The cxt-ctr group showed safety learning from DC3 onwards (c,d) and the ADSC-U group only in DC3 (c,d). During extinction, all groups reduced their fear response, but at different rates (e) and in the extinction recall fear suppression was maintained in all groups except the ADSC-U group (f). i: interval, 5 consecutive fear trials were averaged to form an interval; ****p < 0.0001, **p < 0.01, *p < 0.05 group differences within cue, ^####p < 0.0001, ^###p < 0.001, ^##p < 0.01, ^#p < 0.05 fear + safety cue vs fear within group (a–d,f) or vs i1 within group (e), ⁺⁺⁺p < 0.001, ⁺p < 0.05 vs i2 within group (e) and, ^&&p < 0.01 vs i3 within group (e). Data are mean + SEM.

Figure 5

Adolescent fear conditioning facilitated re-learning of the fear cue-shock association, but did not influence freezing to the first fear cue presentation. Averaged % time freezing was analysed individually for each fear cue in DC1 (a) and DC2 (c). Freezing levels to the first fear cue, i.e. before any shock was presented are low in all groups, but noticeably accelerated after the first fear cue-shock pairing in the ADSC-F group (a). Fear + safety cues between two fear cue-shock presentations were averaged to form intervals (i) for DC1 (b) and DC2 (d). In DC1, ADSC-F rats showed high freezing levels in i1, after the first fear cue-shock presentation (b). At DC2, freezing levels did not change over the course of the session (c,d). Note that in DC2 fear + safety cues were presented before the first fear cue shock pairing, resulting in five intervals. f: fear cue; ****p < 0.0001, ***p < 0.001, **p < 0.01, *p < 0.05 between group difference vs ADSC-F within the same interval; ^p < 0.05 between group difference vs ADSC-U within the same interval; ^####p < 0.0001, ^###p < 0.001, ^##p < 0.01, ^#p < 0.05 within group difference vs f1 (a) or i1 (b); ⁺⁺⁺p < 0.001, ⁺⁺p < 0.01 within group difference vs f2 (a) or i2 (b), and ^&p < 0.05 within group difference vs i3 (b). Data are mean + SEM.

Figure 6

Adolescent conditioning had minimal effects on contextual freezing. Averaged % time freezing during the first (B) and last (E) two minutes of adolescent conditioning (a), R1 (b) and DC1-4 (c–f) was used as a measure for contextual fear. Immediately after adolescent conditioning, freezing was elevated in the ADSC-U group compared to its level in the beginning and compared to the other 3 groups (a). Freezing upon the first adult re-exposure to the chamber was absent in each group (b). Throughout discriminative conditioning, contextual freezing remained low but reached significance in the ADSC-F group in DC1 (c) and in the ADSC-U group in DC2-4 (d–f), when compared to the beginning of the session. ****p < 0.0001, **p < 0.01 between group difference vs ADSC-F within the same interval ^####p < 0.0001, ^##p < 0.01, ^#p < 0.05 within group difference vs B. Data are mean + SEM.