Ontogeny of rapid tolerance to the hypnotic effects of ethanol.

BACKGROUND: Alcohol use typically begins during adolescence. Little is known regarding the antecedents and impact of this early alcohol use, although age-related changes in the neural systems modulating alcohol sensitivity could contribute to the propensity of adolescents to consume alcohol. We have previously observed a marked ontogenetic decline in acute tolerance to ethanol (EtOH), with preweanling pups exhibiting the most acute tolerance (Silveri and Spear, 1998). The objective of this study was to assess whether rapid tolerance shows a similar ontogenetic profile.
METHODS: In Experiment I of the present study, female and male Sprague-Dawley preweanling rats at postnatal day (P) 16 or young adults at P56 were examined. On Day 1, preweanlings were pretreated with saline, 1.5, 3.5, or 5.0 g/kg EtOH and adults were pretreated with saline or 3.5 g/kg EtOH. This was followed by challenge with 3.5 g/kg EtOH and assessment of ethanol-induced sleep time on Day 2. In Experiment II, adolescents at P36 were pretreated with saline, 3.5, or 5.0 g/kg EtOH on Day 1 before Day 2 assessment of sleep time after 3.5 g/kg EtOH. Upon awakening, animals were killed and whole brains removed and analyzed for brain alcohol levels (BrALs).
RESULTS: Rapid tolerance was observed in P36 and P56 animals, with EtOH-pretreated animals exhibiting significantly shorter sleep times than age-matched animals treated with saline. In contrast, no evidence for rapid tolerance was seen after any pretreatment dose at P16. Interestingly, although EtOH-pretreated adults woke at higher BrALs than saline-treated adults, this pattern was not evident in the adolescents, which suggests that at this age, metabolic tolerance may play a prominent role in the expression of rapid tolerance. Thus, it seems as though the ability to develop rapid tolerance to EtOH hypnosis emerges during adolescence and continues into adulthood.
CONCLUSIONS: Together with previous findings that adolescent animals also display substantial acute tolerance to alcohol (Silveri and Spear, 1998), the propensity of adolescents for developing these two forms of tolerance may have important implications for the increased voluntary consumption of alcohol at this age relative to other ages.