Fig 1.
Theoretical rationale behind the selection of BALB/cJ mice as study subjects.
Based on literature data indicating that BALB/cJ mice exhibit very low levels of social behaviour and on the hypothetical association between social behaviour and emotional contagion, we selected the upper and lower quartiles of BALB/cJ mice as our study population. The full line indicates the theoretical distribution of emotional contagion in a heterogeneous population of different mouse strains; the dashed line represents the theoretical distribution of BALB/cJ mice and the shaded rectangles represent our study population characterised by emotional contagion resistant (ECR; left rectangle) and prone (ECP; right rectangle) populations. Importantly whilst emotional contagion resistant theoretically represent an extreme population, emotional contagion prone subjects shall locate around the average of the general population.
Fig 2.
Timing, expressed in weeks, of the behavioural and physiological investigations in male BALB/cJ mice.
Fig 3.
(a) Schematic representation of the experimental apparatus. (left) The Demonstrator mouse (D) is injected with 1% formalin concentration whilst the 3 Observers (O) mice with 0.2% formalin concentration. O mice are then tested in visual and olfactory contact with a D mouse. This setting was used in EXPERIMENTS 1, 2, 3. (middle) Control individuals (CTRL) are injected with 0.2% formalin concentration and tested in the absence of a D mouse. This setting was used in EXPERIMENT 1. (right) The D mouse is injected with 1% formalin concentration whilst the 3 O mice with 0.2% formalin concentration. O mice are then tested in the presence of a D mouse providing olfactory but not visual cues. This setting was used in EXPERIMENT 3. (b) Paw-licking behaviour in mice injected with different formalin concentrations in EXPERIMENT 1. Controls (CTRL) curve: male mice tested individually for paw-licking response to injection in a rear paw with 0.2% formalin concentration. Observers (O) curve: mice tested socially for paw-licking response to injection with 0.2% formalin concentration (i.e. in the presence of a Demonstrator mouse injected with 1% formalin concentration). Demonstrators (D) curve: mice tested socially for paw-licking response upon 1% formalin concentration. ** p < 0.01, O vs CTRL during interval 0–5; * p < 0.05, O vs CTRL during interval 16–20. (inset) Integral response of CTRL, O and D mice, obtained as the average of the values constituting Fig 3B. ** p < 0.01, significantly different from CTRL; ## p < 0.01 significantly different from O.
Fig 4.
Behavioural results and plasma corticosterone profile following restraint stress.
(a) Identification of subgroups based on time spent in paw-licking behaviour. Paw-licking behaviour (s) scored in the emotional contagion test by mice belonging to the lower (25%), intermediate (50%) and upper quartiles (25%). The former and the latter constituted, respectively, the emotional contagion resistant (ECR) and the emotional contagion prone (ECP) subgroups. Mice whose values were comprised in the two intermediate quartiles (25% below and 25% above the median) were excluded from further analyses. (b) Social behaviour. Affiliative behaviours scored in the resident-intruder test by mice belonging to ECR and ECP subgroups. Data are expressed as seconds out of 10-min testing. * p < 0.05, ECR vs ECP. (c) Plasma corticosterone profile. Plasma corticosterone concentrations before a 25-min restraint stress (t0), immediately after (t25) and 35, 95 and 155 minutes later (t60, t120, t180). (inset) Comparison for the area under the curve (AUC) response, as a measure of corticosterone release in response to restraint stress, between mice belonging to ECR and ECP subgroups. * p < 0.05, ECR vs ECP. (d) Emotional memory in the cued fear-conditioning test. Comparison of time spent in freezing (in the cued fear-conditioning test) between subjects belonging to ECR and ECP subgroups. Data are expressed as seconds out of 10-min testing. * p < 0.05, ECR vs ECP.
Table 1.
Behavioural and steroid hormonal profiles of ECR and ECP subgroups.
Fig 5.
Evaluation of the neuropeptides BDNF, oxytocin and vasopressin.
(a) BDNF concentrations in selected brain areas. Levels of BDNF measured in prefrontal cortex, striatum, hypothalamus and hippocampus, in mice belonging to emotional contagion resistant (ECR) and emotional contagion prone (ECP) subgroups. (b) Oxytocin concentrations in selected brain areas. Levels of the neuropeptide oxytocin measured in prefrontal cortex, striatum, hypothalamus and hippocampus, in mice belonging to ECR and ECP subgroups. * p < 0.05, ECR vs ECP. (c) Vasopressin concentrations in selected brain areas. Levels of the neuropeptide vasopressin measured in prefrontal cortex, striatum and hippocampus, in mice belonging to ECR and ECP subgroups. * p < 0.05, ECR vs ECP.
Fig 6.
Representative western blots of BDNF, oxytocin and vasopressin receptors.
The figure shows representative western blots of TrkB (BDNF receptor), OT-R (oxytocin receptor), V1a, V3 (vasopressin receptors) and GAPDH (for normalization) with the corresponding molecular weight (expressed in kDa), in the prefrontal cortex, striatum, hypothalamus and hippocampus of mice belonging to emotional contagion resistant (ECR) and emotional contagion prone (ECP) subgroups.
Table 2.
Density of TrkB (BDNF receptors), OT-R (oxytocin receptors), V1a and V3 (vasopressin receptors) in mice belonging to ECR and ECP subgroups.