Chronic exposure to the opioid antagonist naltrexone during pregnancy: Maternal and offspring effects

The role of endogenous opioids in pregnancy and parturition, and the influence exerted on prenatal and postnatal features of the offspring, were studied in rats. Females received daily injections of 50 mg/kg naltrexone (NTX), a dosage found to block opioids from interacting with opioid receptors for 24 h, throughout pregnancy. No effects on the length of gestation, course of pregnancy, litter size, or the viability of the mother or offspring were noted. The body weights, crown-rump lengths, and wet and dry weights of the brain, heart, kidney, liver, and skeletal muscle (triceps surae) in neonates delivered by NTX-treated rats were substantially elevated compared to newborn animals of saline-injected mothers. Offspring exposed to NTX during prenatal life were larger in body weight and length, and organ wet and dry weights on postnatal days 10 and 21. By weaning (day 21), body weights of NTX-exposed rats were 36% greater than controls, and increases were observed in the wet weights of brain (18%), heart (42%), kidney (38%), lungs (22%), liver (44%), and triceps surae (246%). These data lead us to hypothesize that native opioids are important growth-inhibiting, tonically active regulators of prenatal ontogeny, and that events occurring in prenatal life are determinants to postnatal outcome insofar as somatic development.