Chronic tolerance to the social consequences of ethanol in adolescent and adult Sprague-Dawley rats

Adolescence is a time when experimentation with ethanol becomes normative, with high levels of use becoming apparent in some adolescents. Little is known, however, as to whether ethanol adaptations emerging in adolescents with repeated ethanol use are similar to those emerging in adults. The presents study used a rodent model to investigate the development of chronic tolerance to ethanol-induced alterations in social behavior. The study focused both on ethanol-induced social facilitations, typically evident in adolescents but not adult animals at low doses of ethanol, as well as the inhibition of social behavior occurring at higher doses in both adolescent and adult rats. Adolescent and adult male and female Sprague-Dawley rats were injected intraperitoneally with either isotonic saline or 1 g/kg ethanol for 7 consecutive days: postnatal day (P) 27–33 for adolescents and P62–68 for adults. Acute effects of ethanol (0, 0.25, 0.5, 0.75, and 1 g/kg) on social behavior, social motivation (measured in terms of social preference), and locomotor activity were assessed 48 h after the last chronic exposure using a modified social interaction test in a familiar environment. Adolescents chronically exposed to ethanol developed tolerance to ethanol-induced social facilitation. Animals of both ages likewise developed chronic tolerance to ethanol-induced social inhibition. Metabolic tolerance emerged in adults, as indexed by a decrease in blood ethanol concentrations after chronic ethanol at this age, whereas only functional tolerance was evident in adolescents. Unexpectedly, chronic ethanol diminished baseline levels of social preference in adolescents, but made them more responsive to ethanol-induced enhancement of social preference. Chronic ethanol exposure in adulthood, however, only induced tolerance to the suppressing effects of higher ethanol doses on social preference. Thus, whereas adolescents and adult both develop adaptations following repeated exposure to ethanol, adolescents are more vulnerable to the disruptive effects of chronic ethanol exposure on social preference than their more mature counterparts.