Opioid receptor blockade during prenatal life influences postnatal behavioral development

Opioid peptides regulate pre- and postnatal development. To examine the contribution of prenatal opioid interaction on postnatal behavioral development, the ontogeny of physical characteristics, spontaneous motor and sensorimotor behaviors, as well as ambulation and emotionality at weaning, were studied in rats exposed to 50 mg/kg naltrexone (NTX) or saline (controls) daily throughout gestation by maternal administration. All offspring were cross-fostered to untreated mothers at birth and received no further drug treatment. Morphine challenge of nociception revealed that this dosage of opioid antagonist blocked opioid receptors for 24 hr The age at which a specific physical characteristic (eg., hair covering, eye opening), spontaneous motor behavior (e.g., head turn with return, crawling), or reflex (e.g., bar grasping, auditory 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, as well as the frequency and/or incidence of rearing, grooming, wet-dog shakes, and defecation. These results imply that interactions of endogenous opioid systems during embryogenesis are determinants of somatic, physical and behavioral development in postnatal life.