Age differences in the expression of acute and chronic tolerance to ethanol in male and female rats

Background: Ontogenetic differences in response to ethanol (EtOH) challenge have been observed under a variety of circumstances, including varying reports of developmental differences in the expression of tolerance to EtOH. The purpose of the present experiment was to further explore potential differences in acute (AT) and chronic (CT) tolerance expression between adolescent and adult, male and female Sprague-Dawley rats, using the social interaction test. Methods: AT and CT to the social suppressing effects of a moderate dose of EtOH was assessed in adolescent and adult rats following intraperitoneal injections of 2.0g/kg EtOH or saline daily for 10days. At test, adults and adolescents were challenged with 1.0 or 1.25g/kg EtOH, respectively, with AT and CT assessed at 5 and 25minutes postinjection using ratios of impairment to brain ethanol concentrations (BrECs) at each time period (CT) and within-session declines in impairment relative to BrECs (AT). Results: In adolescents, 10days of EtOH pre-exposure resulted in evidence of CT at 25minutes postinjection, perhaps associated with an enhanced expression of AT. Among adults, signs of CT were seen at 5minutes postinjection in adults, and may reflect neuroadaptations unassociated with AT, as with evidence of tolerance emerging only in adult control animals repeatedly exposed to saline injection prior to EtOH challenge on test day. Sex differences in tolerance expression were not observed at either age. Conclusions: Our results show ontogenetic differences between adolescents and adults in the short- and long-term neuroadaptations that they express in response to repeated perturbations with EtOH. Together these findings add age of exposure and time of testing within the intoxication period as critical variables to be considered when exploring the complex relationship between AT and CT.