Siegel, Lawrence Allen.

Infant rats display differences in duration of loss-of-righting (LOR) in response to an hypnotic dose of morphine sulfate. These differences in LOR duration are predictive of the rats' preference for drinking morphine solutions as adults. Infants tested at 16 days of age were designated Short-, Medium-, or Long-sleep based upon a 2.5 mg/kg dose of morphine sulfate administered intraperitoneally. Infants displaying long durations of LOR (long-sleep) subsequently display a marked preference for morphine solutions when tested as adults. Conversely, infants that displayed little or no LOR (short-sleep) did not consume as much of the morphine solutions in subsequent testing. This effect was consistent across the animals tested and appears to be independent of the screening dose. These findings demonstrate that adult differences in preference for drinking morphine solutions can be predicted in infancy.