Opioid receptor blockade during prenatal life modifies postnatal behavioral development

The ontogeny of physical characteristics, spontaneous motor, and sensorimotor behaviors of preweaning rats, as well as ambulation and emotionality at weaning (day 21) were studied in rats exposed to 50 mg/kg naltrexone (NTX) or saline (controls) daily throughout gestation by maternal administration; all animals were cross-fostered to untreated mothers at birth. Morphine challenge tests and nociceptive measures revealed that this dosage of opioid antagonist blocked opioid receptors for 24 h. At birth and weaning, animals in the NTX group weighed 12 and 20%, respectively, more than control offspring. The age at which a specific physical characteristic, spontaneous motor behavior, or reflex initially appeared and the age at which 100% of the animals demonstrated a particular characteristic/behavior often were accelerated in animals prenatally exposed to NTX. The frequency of ambulation was subnormal in the NTX group, and the frequency and/or incidence of rearing, grooming, wet-dog shakes, and defecation were reduced from normal levels in these opioid antagonist-exposed rats. These results imply that interactions of endogenous opioid systems during embryogenesis are determinants of somatic, physical, and behavioral development in postnatal life.