Effect of chronic morphine exposure during the neonatal period on the anxiety- like behavior of immature rats

Introduction: Several studies have reported anti-anxiety effects of morphine in adult rats. The present study examined the effect of chronic morphine injections in infancy and before puberty on anxiety-like behavior in immature rats. Methods: Neonate rats (n=35) were randomly chosen and divided into two groups. On postnatal days 8-14, one group received saline and the other one received morphine. On postnatal day 21, each group was divided into subgroups. These subgroups received either morphine or saline according to the type of group on postnatal days 22-28. Finally, on postnatal days 22 and 28, rate of anxiety was studied in a plus maze. Results: On day 24 after birth, morphine increased percentage of open arm time in all groups (P<0.001). This percentage on day 28 was highest for morphine groups compared with the control group (P<0.001). The number of open arm entry on day 24 after birth was significantly increased, for both groups treated with morphine (P<0.05). The greatest difference was observed on day 28 for re-treated rats with fixed dose of morphine compared to the control group (P<0.001). Locomotor activity on days 24 and 28 after birth for both groups treated with morphine was more than the other groups (P <0.05). Conclusion: Chronic morphine administration in the neonatal period caused reduced anxiety-like behavior in immature rats. Also, re-exposure to morphine at a fixed dose had an age related anti-anxiety effect that increased in older rats.