Experience modulates vicarious freezing in rats: a model for empathy

Abstract

The study of the neural basis of emotional empathy has received a surge of interest in recent years but mostly employing human neuroimaging. A simpler animal model would pave the way for systematic single cell recordings and invasive manipulations of the brain regions implicated in empathy. Recent evidence has been put forward for the existence of empathy in rodents. In this study, we describe a potential model of empathy in female rats, in which we studied interactions between two rats: a witness observes a demonstrator experiencing a series of footshocks. By comparing the reaction of witnesses with or without previous footshock experience, we examine the role of prior experience as a modulator of empathy. We show that witnesses having previously experienced footshocks, but not naïve ones, display vicarious freezing behavior upon witnessing a cage-mate experiencing footshocks. Strikingly, the demonstrator's behavior was in turn modulated by the behavior of the witness: demonstrators froze more following footshocks if their witness froze more. Previous experiments have shown that rats emit ultrasonic vocalizations (USVs) when receiving footshocks. Thus, the role of USV in triggering vicarious freezing in our paradigm is examined. We found that experienced witness-demonstrator pairs emitted more USVs than naïve witness-demonstrator pairs, but the number of USVs was correlated with freezing in demonstrators, not in witnesses. Furthermore, playing back the USVs, recorded from witness-demonstrator pairs during the empathy test, did not induce vicarious freezing behavior in experienced witnesses. Thus, our findings confirm that vicarious freezing can be triggered in rats, and moreover it can be modulated by prior experience. Additionally, our result suggests that vicarious freezing is not triggered by USVs per se and it influences back onto the behavior of the demonstrator that had elicited the vicarious freezing in witnesses, introducing a paradigm to study empathy as a social loop.

Figure 1. Illustration of experimental design of Experiment 1.

Pairs of rats were exposed to the Empathy Test context for 15 minutes (Habituation). Twenty-four hours later, witnesses were placed in the other context, and either received or not a number of footshocks (Pre-Exposure Training). Twenty four hours later, the witnesses were tested for long-term retention of this experience by replacing them in the pre-exposure context and measuring freezing (Pre-Exposure Test). Twenty-four hours later, demonstrator - witness pairs were placed again in the 2 compartments of the Empathy context. This time, the demonstrator (right) receives shocks through the floor grid while the witness (left) can hear, see and smell the demonstrator through a perforated Plexiglas dividing screen. The lowest panel schematizes the time course of the Empathy Test session.

Figure 2. Behavior of 4 witness groups in Empathy Test.

Naïve control witness (NcW), experienced control witness (EcW), naïve shock witness (NsW), experienced shock witness (EsW). (A) % Average freezing before shock and during shock period by witnesses. ***p<0.001 EsW compared to all the other witness groups. (B) % Change in locomotor activity before shock, shock and after shock periods. % Change in locomotion is relative to the first time bin that served as baseline and thus has a value of zero by definition. $p<0.05, $$p<0.01 EsW compared to EcW; ##p<0.01, ###p<0.01 EsW compared to NcW; ***p<0.001 EsW compared to NsW. (C) % Time spent in window zone by witnesses. $p<0.05, $$p<0.01 EsW compared to EcW; ##p<0.01 EsW compared to NcW; *p<0.05, **p<0.01 EsW compared to NsW. All data is presented as mean ± S.E.M (n = 11–15 per group).

Figure 3. Social modulation of freezing in witnesses and demonstrators.

(A) % Average freezing before shock and during shock period by demonstrators paired with naive (D(NsW) and experienced (D(EsW) witnesses. (B) Correlation between freezing levels of experienced shock witness (EsW) and their paired demonstrator (D(EsW)). (C) % Freezing levels of naïve (NsW) and experienced shock witness (EsW) before shock (BS) and during footshock trials (1^st to 5^th). (D) % Freezing of demonstrator group paired with naïve (D(NsW) and experienced (D(EsW)) witnesses before shock (BS) and during footshock trials (1^st to 5^th). **p<0.01, ***p<0.001 compared to respective groups. All data is presented as mean ± S.E.M (n  =  11–15 per group).

Figure 4. (A) Example sound spectrograms illustrating (1) a 40 min sound track containing USVs recorded in Experiment 1, (2) USVs in a 10 second time window detail, (3) the result of the automated detection of USVs in Matlab, with epochs containing a single 22 kHz-USV shown in yellow.

(B) % of naïve shock witness (NsW)-demonstrator (D(NsW) pairs and % experienced shock witness (EsW) and Demonstrator (D(EsW) pairs that emitted USVs. (C) Correlation between the number of emitted USVs and % average freezing response in shock period by both demonstrator groups (paired with naïve shock witness D(NsW) and paired with experienced shock witness D(EsW) together). (D) Correlation between the number of USVs and % average freezing behavior in shock period by naïve shock witness (NsW) and experienced shock witness (EsW) groups.

Figure 5. % Freezing behavior of Naïve (Naïve-Control, Naïve-USV) and Experienced groups (Experienced-Control and Experienced-USV) before and during control and USV sound stimulus in Experiment 2.

All data is presented as mean ± S.E.M (n  =  10–11 per group).