Modulatory Effects of the Glucocorticoid and Opioid Systems on Anxiety-Related Behavior in Young and Mature Rats

Considering that there is limited information on interaction between age and effects of the opioid system and glucocorticoids in determination of the anxiety level, we examined the influences of 1 mg/kg dexamethasone and 20 mg/kg RU486 (as an agonist and an antagonist of glucocorticoid receptors), and also of 5 mg/kg morphine and 20 mg/kg naloxone (as an agonist and an antagonist of the opioid system) on the above level in young and mature male Wistar rats. The percentage of time in the open arms in the plus-maze test was evaluated as an index of anxiety behavior, and the percentage of number of entries in the closed arms was measured as an index of locomotor activity. Morphine (5 mg/kg) and dexamethasone (1 mg/kg) exerted anxiolytic effects in both young and mature rats, while these agents reduced locomotor activity only in young animals. RU486 could prevent the anxiolytic effect of morphine, and the anxiolytic effect of dexamethasone was inhibited by naloxone in young animals; this, however, was not observed in mature rats. Thus, there is an interactive effect between glucocorticoids and the opioid system in mediation of anxiety, and the respective events are age-dependent.