Validation of a novel social investigation task that may dissociate social motivation from exploratory activity

Abstract

The present series of studies provide validation of a new paradigm that uniquely combines the assessment of the propensity to engage in social investigation with measures of (nonsocial) exploratory activity in rats. Assessment of this social investigation paradigm indicated that (a) rats showed a robust preference for social investigation over nonsocial exploratory activity, (b) female rats showed a greater preference for social investigation than male rats, (c) no signs of habituation in these responses were observed when rats were tested once daily for 4 consecutive days, (d) the preference for social investigation was stable and robust in both the dark and light periods of the daily light cycle for 5 consecutive days, and (e) testing under bright light conditions suppressed social investigation. In addition, acute administration of opiate drugs, low dose morphine (1.0 mg/kg) and naltrexone (1.0 mg/kg) produced a more robust attenuation of social investigation than nonsocial exploratory activity. Amphetamine increased both forms of investigation and haloperidol had the opposite effect, but the overall preference for social investigation over exploratory activity remained largely intact after both amphetamine and haloperidol injection. Together, these findings validate the use of this behavioral task to assess changes in social-motivation and general exploratory activity. Importantly, the task is bi-directionally sensitive to subject characteristics (i.e., sex), drug manipulations which modulate social motivation, and environmental manipulations.

Figure 1

Diagram of the apparatus employed for the assessment of social investigation and exploratory activity. Boxes are separated by 12 cm and illuminated by a dim light above the apparatus.

Figure 2

Baseline investigatory behavior in the apparatus depicted in Figure 1. The top panel of graphs shows that in a 30 min test session, the time spent investigating at the social hole increases (top left) while the number of head pokes decreases (top middle), yielding an overall increase in mean poke duration (top right). Moreover, no habituation was observed in behavioral responding across 4 successive days of testing, and a robust preference to explore the hole where social reinforcement is possible was observed on all test days. In the bottom panel of graphs, data are cumulated across the entire 30 min test session. Although a significant preference for social investigation was observed in both sexes, female rats spent significantly more time in social investigation than male rats. All data are expressed as means ± SEM; an asterisk (*) indicates a significant difference between male and female rats at the social hole (p<.05).

Figure 3

Modulation of investigatory behavior by several drugs. The top panel of graphs shows that peripheral administration of morphine (1 mg/kg i.p.) or naltrexone (1 mg/kg i.p.) alone or in combination reduced the amount of time spent in social investigation. Overall, the impact of opioid drugs was much greater on social investigation than on nonsocial exploratory activity. The bottom panel shows that peripheral administration of amphetamine (0.25 mg/kg i.p.) produced a robust facilitation of both social investigation and exploratory activity, but that the overall preference for the social hole was relatively intact. Haloperidol treatment (0.25 mg/kg i.p.) dramatically reduced activity at both holes irrespective of whether it was administered alone or in combination with amphetamine. All data are expressed as means ± SEM; an asterisk (*) indicates a significant difference between the groups indicated (p<.05).

Figure 4

The impact of light/dark cycle on investigatory behavior. During the 30 min test sessions, the time spent investigating (top panel), the number of head pokes (middle panel), and mean poke duration (bottom panel) at both the social and non-social holes were measured in both light and dark phases of daily light cycling for 5 successive days. No effects of light/dark cycling were found; Time and number of social hole were stably greater than those of non-social hole in both light and dark phases on all test sessions. All data are expressed as means ± SEM.

Figure 5

The effect of illumination on investigatory behavior. The time spent investigating (top panel), the number of head pokes (middle panel), and mean poke duration (bottom panel) at both the social and non-social holes were scored under dimly lit and bright lit conditions. The number of head pokes at social hole, but not non-social hole, was significantly lower under bright lit condition compared to dim lit condition. All data are expressed as means ± SEM; an asterisk (*) indicates a significant difference between the conditions indicated (p<.05).