interaction between glucocorticoids and opioids in nociception in young and adult rats

Background: The aim of this study was to examine the relationship between the glucocorticoid and opioid systems in the modulation of nociception in young and adult rats. Materials and methods: The experiments were done in young and adult Wistar rats, using morphine 5 mg/kg, and naloxone 2 mg/kg as a ^-opioid receptor agonist. Dexamethasone 1 mg/kg and mifepristone (RU486) 20 mg/kg were used as a glucocorticoid receptor agonist and antagonist, respectively. Hind paw licking latency was measured by hot plate after intrap-eritoneal administration of drugs. Results: The results showed that morphine and dexamethasone had significant analgesic effects (P < 0.001, P < 0.01, respectively) in both age groups. Coadministration of morphine and dexamethasone did not induce a greater analgesic effect in comparison with morphine alone in either age group. Mifepristone prevented the analgesic effect of morphine in the adult animals (P < 0.001), but had no effect in young animals. The analgesic effect of dexamethasone was inhibited by naloxone in both groups (P < 0.01). Conclusion: These results suggest that glucocorticoids regulate opioid-induced analgesia from the age of puberty, but opioids regulate glucocorticoid-induced analgesia in prepubescent animals. Thus, there is a clear overlapping effect between the two pain modulation systems.