Caffeine sensitivity in the neonatal rat.
The motoric activity of infant rats at 1, 10 and 15 days of age was determined following treatment with caffeine, a central nervous system stimulant. At all three ages responses to increasing caffeine dosage described a curvilinear function in which activity first increased and then decreased. These results show that the basic adult response pattern to caffeine is exhibited by infants as young as 1 day of age. However, 1-day-old rats were less sensitive to caffeine than the 10- and 15-day-old pups. Their maximal activity was reached at the 80 mg/kg dose while 10- and 15-day-old rats were most active in the 20--40 mg/kg dose range. These results contribute to the growing body of evidence that the neonatal rat can be used to assess the behavioral effects of exposure to environmental agents during development.