ANTAGONISM OF SWIM-STRESS-INDUCED ANTINOCICEPTION BY THE DELTA-OPIOID RECEPTOR ANTAGONIST NALTRINDOLE IN ADULT AND YOUNG-RATS

The avialability of the non-peptide δ-opioid receptor antagonist naltrindole has provided the possibility for in vivo studies on the function of δ-opioid receptors. We have studied the effects of naltrindole on swim-stress-induced antinociception in adult and neonatal rats. Adult, 25 and 20 day old rats were stressed by warm water (20° C) swimming for 3 min periods and antinociception was assessed by the tail immersion test (50°C). Naltrindole (0.5 and 1 mg kg-1) antagonized swim-stress-induced antinociception in adult and 25 day old rats but in 20 day old rats naltrindole (1 mg kg-1) was without effect. Antinociception induced by the highly μ-opioid receptor selective agonist alfentanil was completely antagonized by naloxone (11 mg kg-1) but virtually unaffected by naltrindole (1 mg kg-1). Neither naloxone nor naltrindole (1 mg kg-1) antagonized swim-stress-induced rises in plasma corticosterone in adult rats at the time of peak antinociception. In conclusion, naltrindole shows in vivo antagonism of opioid-mediated responses. Swim-stress induced antinociception is mediated through the δ-opioid receptor in 25 day old and adult rats and through the μ-opioid site in 20 day old animals.