Predictability and heritability of individual differences in fear learning

Abstract

Our objective was to characterize individual differences in fear conditioning and extinction in an outbred rat strain, to test behavioral predictors of these individual differences, and to assess their heritability. We fear-conditioned 100 Long-Evans rats, attempted to extinguish fear the next day, and tested extinction recall on the third day. The distribution of freezing scores after fear conditioning was skewed, with most rats showing substantial freezing; after fear extinction, the distribution was bimodal with most rats showing minimal freezing, but a substantial portion showing maximal freezing. Longer rearing episodes measured prior to conditioning predicted less freezing at the beginning of extinction, but differences in extinction learning were not predicted by any baseline exploratory behaviors. We tested the heritability of extinction differences by breeding rats from the top and bottom 20 % of freezing scores during extinction recall. We then ran the offspring through the same conditioning/extinction procedure, with the addition of recording ultrasonic vocalizations throughout training and testing. Only a minority of rats emitted distress vocalizations during fear acquisition, but the incidence was less frequent in the offspring of good extinguishers than in poor extinguishers or randomly bred controls. The occurrence of distress vocalizations during acquisition predicted higher levels of freezing during fear recall regardless of breeding line, but the relationship between vocalization and freezing was no longer evident following extinction training, at which point freezing levels were influenced only by breeding and not by vocalization. The heritability (h2) of extinction recall was estimated at 0.36, consistent with human estimates.Electronic supplementary materialThe online version of this article (doi:10.1007/s10071-014-0752-1) contains supplementary material, which is available to authorized users.

Activation of orexin/hypocretin neurons is associated with individual differences in cued fear extinction

“…As seen by others in rat strains [12, 13, 17], Long Evans rats showed significant individual differences in the extinction of cue-conditioned freezing. For analysis purposes, animals were divided into high and low freezing groups based on a median split of the average percent freezing observed during the last 10 minutes of the cue (within trial extinction) session.…”

“…Analysis of the behavioral data in the 23 animals during the extinction trial suggested that the data were not normally distributed (Shapiro-Wilk normality test, P<0.004). A frequency distribution analyzing the average percent freezing during the last 10 tone presentations of the first extinction trial suggested distinct groups of subjects within the population, similar to differences in extinction learning/recall seen by other groups [12, 13]. Therefore, for analysis purposes, animals were divided into high and low freezers based on a median split of the average percent freezing duration during the last 10 minutes of the cue trial (within trial extinction) on day 3 of the protocol.…”

Preventing the return of fear using reconsolidation updating and methylene blue is differentially dependent on extinction learning

“…11 for a detailed description of selection criteria and breeding procedures). Briefly, for four generations, Long-Evans rats were fear conditioned to a pure tone (20 s, 5 kHz, 80 dB) co-terminating with a footshock (0.7 mA for 0.5 s).…”

Rats with differential self-grooming expression in the elevated plus-maze do not differ in anxiety-related behaviors

“…In this direction, it has been shown that individual differences are important predictors for the reactivity to stressful stimulation [7][8][9]. The reasons and specific individual traits that can contribute to variations on the expression of defensive behaviors are still unclear, although being under intense investigation [10,11].…”