Appetitive behavioral traits and stimulus intensity influence maintenance of conditioned fear

Abstract

INDIVIDUAL DIFFERENCES IN APPETITIVE LEARNING HAVE LONG BEEN REPORTED, AND GENERALLY DIVIDE INTO TWO CLASSES OF RESPONSES: cue- vs. reward-directed. The influence of cue- vs. reward-directed phenotypes on aversive cue processing, is less well understood. In the current study, we first categorized rats based on their predominant cue-directed orienting responses during appetitive Pavlovian conditioning. Then, we investigated the effect of phenotype on the latency to exit a familiar dark environment and enter an unfamiliar illuminated open field. Next, we examined whether the two phenotypes responded differently to a reconsolidation updating manipulation (retrieval+extinction) after fear conditioning. We report that the rats with a cue-directed ("orienting") phenotype differentially respond to the open field, and also to fear conditioning, depending on US-intensity. In addition, our findings suggest that, regardless of appetitive phenotype or shock intensity, extinction within the reconsolidation window prevents spontaneous recovery of fear.

Keywords: extinction; fear conditioning; open field; orienting; reconsolidation.

Figure 1

Timeline of experimental design. Rats were first tested for their willingness to enter an illuminated open field. Rats then received appetitive conditioning (App. cond.) with 56 light-food pairings in Context A. On their last day of appetitive conditioning rats were classified as Orienters and Nonorienters. After 3–5 days, both groups were fear conditioned (Fear cond.) with 3 tone-shock pairings of either 0.7 or 1.0 mA in Context B (indicated by gray shading). 24 h after fear conditioning, rats were exposed to a single cue retrieval trial (Ret) or a typical extinction session (No ret). For rats in the Ret group that received a cue exposure and those in the No ret group that received a context exposure, the exposure occurred 10 min prior to beginning the extinction session. 24 h after extinction, rats were tested for long-term memory (LTM), and 3 weeks later tested for spontaneous recovery. Context change is indicated by shading.

Figure 2

(A,B) Conditioned orienting and food cup approach for the Orienters and Nonorienters. Mean ± s.e.m number of orienting bouts (^*p < 0.0001) (A) or food cup entries (p = 0.42) (B) averaged for last 8 trials of training. Orienters showed significantly more orienting than Nonorienters, but the food cup response was equivalent between groups. (C) Latency to exit the dark insert and enter the illuminated open field. Orienters exited significantly more quickly than the Nonorienters (p = 0.05). Activity, as measured by the total distance traveled within both fields (C), did not differ between Orienters and Nonorienters (p = 0.37).

Figure 3

(A,B) Freezing during fear conditioning with a 0.7 mA and 1.0 mA footshock. (A) Orienters and Non-orienters showed no differences in freezing during conditioning when the US was a 0.7 mA footshock (p = 0.49). (B) Non-orienters froze significantly more than Orienters during fear conditioning when the US was 1.0mA footshock (p = 0.04). Each conditioning session involved three CS-US pairings. (C,D) Contextual freezing 24 h after fear conditioning to either a 0.7 or 1.0 mA footshock. (C) There were no significant differences between Orienters and Non-orienters in freezing to the fear conditioning context when the US was 0.7 mA footshock (p = 0.09) and overall context freezing was extremely low. (D) Non-orienters froze significantly more than Orienters to the fear conditioning context when the US was a 1.0 mA footshock (^*p = 0.03).

Figure 4

Cue-induced freezing at the beginning of extinction, end of extinction, during LTM test, and spontaneous recovery. (A) For rats conditioned with a 0.7 mA shock, a retrieval trial prevented spontaneous recovery (i.e., there was no significant increase in freezing from the end of extinction to spontaneous recovery test; Orienters p = 0.206, Non-orienters p = 0.732). While neither group showed significant spontaneous recovery, Non-orienters froze significantly less than Orienters during the test for spontaneous recovery (^#p = 0.041). Rats receiving typical extinction treatment did show a significant increase in freezing (Orienters p = 0.014, Non-orienters p = 0.032). (B) Rats conditioned with a 1.0 mA shock showed the same pattern of results: a retrieval trial prior to extinction attenuated spontaneous recovery (Orienters p = 0.524, Nonorienters p = 0.235). Rats exposed to typical extinction showed a significant increase in freezing (Orienters p < 0.001, Non-orienters p = 0.032). While neither Orienters nor Non-orienters that received ret+ext showed significant increases in freezing from the end extinction to LTM or spontaneous recovery tests, Orienters showed significantly less freezing than Non-orienters at both time points (LTM ^#p = 0.011, spontaneous recovery ⁺p = 0.045).