Neonatal undernutrition alters responsiveness to morphine in mature rats: a possible source of epiphenomena in developmental drug studies.

We studied the effect of neonatal undernutrition and its attendant stresses on the behavior and thermoregulation of adult rats in the absence and presence of morphine. Undernutrition was accomplished by fostering half the pups in each litter to a nonpregnant, nonlactating female rat every other day for the first 6 days of life. As a control, the remaining pups were fostered to lactating rats. Significant alterations in body and brain weight and in brain DNA, RNA and protein were noted in undernourished rats at 10 and 21 days of age. At 6 months of age, no difference was observed in unconditioned exploratory behavior or in the acquisition and performance of a conditioned autoshaped lever touch response. Differential sensitivity to morphine was observed in nourished and undernourished rats performing the autoshaped response at asymptotic levels. At 1 year of age, undernourished rats displayed lower rectal temperatures than controls and showed an altered thermic response to morphine. These data demonstrate that poor nutritional status and other nondrug factors may be responsible for the altered thermoregulation and opiate sensitivity observed in rodents perinatally addicted to opioids, effects generally regarded as specific consequences of early opiate exposure.