Antagonism of swim‐stress‐induced antinociception by the δ‐opioid receptor antagonist naltrindole in adult and young rats

Open Access
Publisher Website
Google Scholar
Abstract
1 The availability 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. 2 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). 3 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−l) was without effect. 4 Antinociception induced by the highly μ-opioid receptor selective agonist alfentanil was completely antagonized by naloxone (1 mg kg−l) but virtually unaffected by naltrindole (1 mg kg−1). 5 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. 6 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.