Mediation of swim-stress antinociception by the opioid delta 2 receptor in the mouse.

The present study has characterized the antinociceptive response to cold water swim-stress (CWSS) in mice using opioid-selective antagonists as well as tolerance and cross-tolerance approaches. Mice subjected to CWSS using water at 5 degrees C for 3 min showed a marked antinociceptive response in the tail-flick test, which reached approximately 90% after +10 min, and which persisted for 15 to 20 min. This antinociceptive response (at +10 min) was antagonized by naloxone or by the delta antagonist ICI 174,864. Additionally, the CWSS response was antagonized by the opioid delta 2 antagonist, naltrindole-5'-isothiocyanate, but not by the delta 1 antagonist, [D-Ala2,Leu5,Cys6]enkephalin, or by the mu antagonist, beta-funaltrexamine or by the kappa antagonist, norbinaltorphimine. Although the CWSS-induced antinociceptive effect was blocked by some delta antagonists and tolerance resulted from the CWSS-induced response, the decrease in body temperature after each CWSS exposure was not affected by the opioid antagonists and reliably occurred in CWSS-tolerant mice, suggesting that the observed antinociception was independent of changes in body temperature. In mice rendered tolerant to the antinociceptive actions of the mu agonist, [D-Ala2,NMPhe4,Gly-ol] enkephalin, or to [D-Pen2,D-Pen5]enkephalin (predominantly a delta 1 agonist), the CWSS-induced antinociceptive response was unaltered. In contrast, in mice tolerant to the delta 2 agonist, [D-Ala2,Glu4]deltorphin, the CWSS-induced antinociceptive response was markedly and significantly reduced.(ABSTRACT TRUNCATED AT 250 WORDS)