Rapid tolerance and locomotor sensitization in ethanol-naive adolescent rhesus macaques

Background: Acute and chronic tolerance, as well as locomotor sensitization, have been linked to ethanol intake. This study examined the change in response between 2 acutely administered doses of ethanol in adolescent rhesus macaques, with the objective of investigating rapid tolerance and locomotor sensitization to the behavioral effects of ethanol, and whether these phenomena are related to voluntary ethanol consumption in nonhuman primates. Methods: Rhesus macaques (n = 109, 42 males, 67 females) were administered 2 sequential intravenous doses of ethanol (2.2 g/kg for males, 2.0 g/kg for females) separated by a period of 5 to 30 days. Following each injection, subjects underwent a 30-minute behavior assessment. Behavioral data were summarized using factor analysis, and compared between the 2 doses using repeated measures ANOVA. The relationship between behavioral response measures and the number of days between doses was analyzed using regression analyses. Following the second ethanol dose, subjects were given free access to an aspartame-sweetened 8.4% ethanol solution for 1 hour a day for 4 weeks. Percent change in behavioral response measures from dose 1 to dose 2 was analyzed for associations with ethanol consumption using multiple regression analyses. Results: Factor analysis yielded 3 factors: ataxia, stimulation, and jumping. From dose 1 to dose 2 there was a significant decrease in ataxia and a significant increase in stimulation. Peak blood ethanol concentration did not differ between doses. There were no significant associations between the number of days between doses and the magnitude of change in response for any of the behavioral measures. Percent change in stimulation from dose 1 to dose 2 was positively associated with subsequent oral ethanol consumption only in females tested in a social setting. Conclusions: Adolescent rhesus macaques develop rapid tolerance to the motor-impairing effects of alcohol, while at the same time developing locomotor sensitization. These changes in response are not necessarily short lived, and may persist for some time following the first ethanol dose. Clear and consistent associations between rapid tolerance and locomotor sensitization and ethanol intake levels have yet to be demonstrated, however.