Development and adult pituitary-adrenal function in female rats injected with morphine during different postnatal periods

Developmental, neuroendocrine and behavioral effects of early morphine addiction in female rat pups were investigated. compared with controls, body weights of morphine-injected animals, regardless of treatment age, were depressed during treatment and remained depressed for periods up to 49 days; growth rates of these animals were decreased during drug treatment but temporarily surpassed those of controls following termination of morphine administration. Delayed eye opening and early vaginal opening occurred in some of the animals administered morphine before Day 14. Behaviorally, animals treated with the narcotic from Days 15–21 showed impaired activity in the open field test; all animals, in any early morphine treatment group, showed normal resting pituitary-adrenal activity at 30 days of age and showed no neuroendocrine signs of protracted dependence. However, rats neonatally narcotic-treated on Days 15–21 showed a decreased corticosterone rise in response to morphine challenge of 30 mg/kg for as late as Day 62. The persistence of the weight and endocrine effects suggest that exposure of the immature female rat to morphine results in prolonged morphine-specific alteration of brain mechanisms concerned with behavioral and neuroendocrine processes and supports the suitability of using a rat model to study long-term effects of neonatal narcotic addiction.