3H-amphetamine concentrations in the brains of young and aged rats: implications for assessment of drug effects in aged animals.

The distribution of amphetamine in the brain, liver, and fat of 3-month-old Wistar rats was compared to the distribution in 24-month-old rats. Animals were killed 20 or 65 min after IP administration of 2.5 mg/kg of 3H-amphetamine. Amphetamine levels in tissues were measured by homogenizing organs in acid and extracting radioactive amphetamine into toluene. Amounts were quantified by scintillation counting. Amphetamine concentrations were twice as high in the brains of the old rats as in young animals at both time points. The liver and fat of old rats also contained more amphetamine. These findings lead us to question the common practice in aging studies of administering compounds using total body weight as the reference point.