Early undernutrition blocks the effect of naltrexone on rat exploratory behavior.

1. Young rats were undernourished by feeding their dams an 8% protein diet (w/w) from birth until weaning (21 days); the dams of control rats were fed a 20% protein diet (w/w) until weaning. All offspring were fed a 20% protein diet from 21 to 120-150 days of age, when they were tested. 2. The involvement of endogenous opioids in the exploratory activity (rearing response and crossing frequency in a 2-compartment open-field test) was evaluated by examining the effects of a single injection of naltrexone (0.75 mg/kg, ip) 6 min prior to testing. 3. Naltrexone decreased exploratory activity (rearing) by approximately 50% in normal rats but had no effect on undernourished rats. 4. These data provide additional evidence for an alteration in the opioid system of undernourished rats.