Anxiolytic Treatment Impairs Helping Behavior in Rats

Abstract

Despite decades of research with humans, the biological mechanisms that motivate an individual to help others remain poorly understood. In order to investigate the roots of pro-sociality in mammals, we established the helping behavior test, a paradigm in which rats are faced with a conspecific trapped in a restrainer that can only be opened from the outside. Over the course of repeated test sessions, rats exposed to a trapped cagemate learn to open the door to the restrainer, thereby helping the trapped rat to escape (Ben-Ami Bartal et al., 2011). The discovery of this natural behavior provides a unique opportunity to probe the motivation of rodent helping behavior, leading to a deeper understanding of biological influences on human pro-sociality. To determine if an affective response motivates door-opening, rats receiving midazolam, a benzodiazepine anxiolytic, were tested in the helping behavior test. Midazolam-treated rats showed less helping behavior than saline-treated rats or rats receiving no injection. Yet, midazolam-treated rats opened a restrainer containing chocolate, highlighting the socially specific effects of the anxiolytic. To determine if midazolam interferes with helping through a sympatholytic effect, the peripherally restricted beta-adrenergic receptor antagonist nadolol was administered; nadolol did not interfere with helping. The corticosterone response of rats exposed to a trapped cagemate was measured and compared to the rats' subsequent helping behavior. Rats with the greatest corticosterone responses showed the least helping behavior and those with the smallest responses showed the most consistent helping at the shortest latency. These results are discussed in terms of their implications for the interaction between stress and pro-social behavior. Finally, we observed that door-opening appeared to be reinforcing. A novel analytical tool was designed to interrogate the pattern of door-opening for signs that a rat's behavior on one session influenced his behavior on the next session. Results suggest that helping a trapped rat has a greater motivational value than does chocolate. In sum, this series of experiments clearly demonstrates the fundamental role of affect in motivating pro-social behavior in rodents and the need for a helper to resonate with the affect of a victim.

Keywords: altruism; emotional contagion; empathy; helping; midazolam; rodent.

FIGURE 1

(A) The mean (±SEM) latency to door-opening for all rats (n = 80) decreased across the 12 days of testing, suggestive of learning. (B) The decay in door-opening latency across testing sessions differed between the groups of rats tested (n = 16 per group). (C) The average opening latency during the final 3 days of testing, when latencies had plateaued, was significantly greater for rats treated with a high dose of MDZ than for rats that received no injection (^∗∗p = 0.01) or an injection of nadolol (^∗p = 0.04).

FIGURE 2

The variability of opening latency within groups is illustrated for rats tested with a trapped rat (A) or with chocolate chips (B) using box plots (40, 50, 60 percentile lines with the median marked in red, 10 and 90 percentile whiskers) showing latencies across the 12 days of testing. All individual latencies are ilustrated for days 1, 6, and 12 (hollow blue circles). Frequency histograms of latencies on those days are shown at the right for each group. In all groups except the high MDZ rats tested with a trapped rat and saline rats tested with chocolate, there was a shift from long to short latencies.

FIGURE 3

Day to day reinforcement occurred with a probability that was greater than chance for all groups in the trapped condition, except rats injected with the high dose of MDZ. A model that took into account the effects of learning and individual differences was constructed. The chance distribution of %SO (proportion of sequential openings) values from 10,000 matrices is shown in histogram form for each group tested with a trapped rat. The observed %SO is marked by the red dotted line in each panel and the two-tailed probability of the observed %SO occuring by chance listed. For uninjected rats (A) or rats injected with saline (B), nadolol (C) or the low dose of MDZ (D), the observed %SO was significantly greater than would be expected by chance. However, in the case of rats injected with the high dose of MDZ (E), the observed %SO was less than 85% of the chance %SO values.

FIGURE 4

(A) Using the same symbols as in Figure 1 (open black, uninjcted; open red, saline; pink, low MDZ; solid red, high MDZ), the frequency of opening streaks (consecutive day openings) of different lengths is illustrated for streaks of lengths from 2 to 12 days. At the left is the number of rats that opened at least once. (B) Again, using the same symbols as in Figure 1, the median length of the longest streak (±25 and 75 percentiles) is graphed as a function of the median testing day (±25 and 75 percentiles) on which the streak began. The gray line at the top shows the optimal possible performance (e.g., rats that began opening on day 1 could achieve a streak of 12 days). (C) The failure of a rat to open for one or more days is termed a “break.” An analysis of breaks for rats that opened on at least two consecutive days on days 9–12 (uninjected, n = 10; saline, n = 7; nadolol, n = 8; low MDZ, n = 6; high MDZ, n = 2) shows that rats treated with MDZ were more likely to take at least one break (filled red circles, right axis). Rats treated with MDZ also took longer breaks on average than did rats from the other groups (black columns, left axis). The individual points for all rats considered in this analysis are illustrated by the hollow blue circles.

FIGURE 5

(A) The mean (±SEM) latency to door-opening for all rats tested with a chocolate-containing restrainer (n = 24) decreased across the 12 days of testing, suggestive of learning. (B) The decay in door-opening latency across testing sessions differed between the groups of rats tested (n = 8 per group). (C) The average opening latency during the final three days of testing, at a time when latencies had stabilized, was significantly less for rats treated with a low dose of MDZ than for rats that received saline (^∗p = 0.04).

FIGURE 6

Data from rats tested with a trapped rat and injected with (A) saline, (B) low MDZ, or (C) high MDZ was resampled to match the statistical power of chocolate conditions. The distributions of p(%SO—Null)s from 100 bootstrapped samples from each trapped rat condition are shown in the histograms. The medians of these bootstrapped p(%SO—Null)s (dashed line) are greater than the orginial p(%SO—Null) values (crosses) from the complete data set of trapped rat conditions, showing reduction in statistical power. The p(%SO—Null) of rats tested with chocolate (red dots) were compared to that of rats tested with a trapped rat.

FIGURE 7

(A) The corticosterone response of each free rat evoked by vicarious distress is positively correlated with the average opening latency over the 12 days of testing. behavior. (B) There was no correlation between the the trapped rat’s corticosterone response and the ensuing opening behavior of the free rat.