Naltrexone's influence on neurobehavioral development

The ontogeny of spontaneous motor and sensorimotor behaviors of preweaning rats, as well as ambulation, emotionality, and nociception at weaning (day 21), were studied in rats given chronic administration of 1 or 50 mg/kg naltrexone from birth to day 21. The age at which a specific spontaneous motor behavior or performance initially appeared and the age at which 100% of the animals demonstrated a particular behavior were accelerated in animals given 50 mg/kg naltrexone, but delayed in rats injected with 1 mg/kg naltrexone. In general, ambulation, emotionality, and nociceptive responses were not affected by naltrexone treatment, although the frequency of face-washing in both naltrexone groups and activity cage performance in the 50 mg/kg naltrexone group deviated from control levels. Observations of head-shake and wet-dog shake behaviors in naltrexone-treated animals at 2 hr and 10 hr post-drug injection were similar to controls with the exception of an abnormal increase in the 1 mg/kg naltrexone group at 10 hr. Although these results may imply that endogenous opioid systems play a role in regulating neurobehavioral development, further study is needed to distinguish whether these changes are a consequence of the somatic and morphological alterations known to occur with naltrexone administration or if the timetable of behavioral ontogeny is governed by endorphin-opiate receptor interaction.