Social interactions and 50 kHz ultrasonic vocalizations in adolescent and adult rats

Abstract

Human adolescents drink partly to facilitate their social interactions, a social facilitatory effect of ethanol also seen in adolescent rats tested under familiar test circumstances. To explore the role of hedonic affect in ethanol-induced social facilitation, this study assessed 50 kHz ultrasonic vocalizations (USVs) in pair-housed adolescent (P29-37) and adult (P71-79) Sprague-Dawley male rats during social interactions. On each of eight test days, animals were socially isolated for 3 h and then injected intraperitoneally with 0 (saline), 0.25 or 0.5 g/kg ethanol. Twenty-five minutes later they were placed alone in a familiar test chamber for a 5-min period, followed by a 10-min encounter with a similarly injected peer. USVs were recorded during this 10-min period, while social interactions were videotaped for later scoring. BECs were measured immediately post-test on day 8. Although the 0.25 g/kg dose of ethanol facilitated play fighting in adolescents but not adults, ethanol had little to no effect on 50 kHz USV production under these test circumstances. USV production was higher in adults than adolescents, despite adolescents consistently engaging in more social behavior. To the extent that 50 kHz USVs index the hedonic value of social interactions, these findings support the conclusion that elevations in social behavior normally evident in adolescents may not be related to increases in hedonic sensitivity for social stimuli.

Figure 1

Frequency of crossing through the center aperture between the two sides of the test chamber during the 5-min period prior to the social interaction test. Data collapsed across ethanol dose. Vertical lines in these and subsequent figures reflect SEM values.

Figure 2

Total number of 50 kHz USVs emitted by each pair of animals during the social interaction test across days. Data are collapsed across ethanol dose.

Figure 3

Frequency of play fighting in adolescent and adult animals across test days. Figure insert shows frequency of play fighting in adolescents collapsed across placement order. Asterisk (*) indicates significant difference from saline-injected group in post-hoc analyses on data collapsed across placement order.

Figure 4

Frequency of social contact behavior in adolescent and adult animals. Data are collapsed across ethanol dose.

Figure 5

Frequency of social investigation (left) and social preference/avoidance (right) in adolescent and adult animals Positive values of the coefficient indicate social preference, negative values indicate social avoidance. Data are collapsed across ethanol dose and test day.