The effect of various doses of the selective delta agonist
BUBU (Tyr-D-Ser(O-t-butyl)-
Gly-Phe-Leu-Thr(O-t-butyl) on the vocalization threshold to paw pressure were compared in normal and arthritic rats, a suitable clinical model of
chronic pain. In both group of rats, the
intravenous administration of
BUBU (6, 9, 12 mg/kg in normal and 1.5, 3, 6 mg/kg in arthritic rats) led to significant antinociceptive effects. The same dose of
BUBU (6 mg/kg i.v.) produced a much more potent antinociceptive effect in arthritic than in normal rats, and a dose as low as 1.5 mg/kg produced a significant
analgesic effect in the arthritic animal, whereas at 3 mg/kg
BUBU was ineffective in normal rats. The
analgesic effects of
BUBU (9 mg/kg in normal and 3 mg/kg in arthritic rats) were completely prevented by the selective delta antagonist
naltrindole (1 mg/kg i.v. a dose devoid of
analgesic potency per se), while they were not affected by the selective mu antagonist
naloxone (0.05 mg/kg i.v.). In addition, 3 mg/kg i.v. of
BUBU remained effective in
morphine tolerant arthritic rats. These results suggest that
delta opioid receptor activation can modulate the transmission of cutaneous mechanical nociceptive information in rats, especially in inflammatory
pain conditions.