Highly delta selective antagonists in the RVM attenuate the antinociceptive effect of PAG DAMGO

THE present study tested the hypothesis that endogenous opioid peptides act ing at the delta-opioid receptor (DOR) in the rostral ventromedial medulla (RVM) contribute to the antinociception elicited by the mu-opioid receptor (MOR) agonist DAMGO in the midbrain periaqueductal gray (PAG). Following mi croinjection of DAMGO into the PAG, either the highly selective DOR antagon ist TIPP[Psi] or the DOR2 antagonist naltriben (NTB) Nas microinjected into the RVM. Both TIPP[Psi] (1.0 mu g) and NTB (5.0ng) significantly attenuate d the analgesic effect of PAG DAMGO but had no effect when given before PAG saline. These results confirm and extend previous studies suggesting that PAG mu-opioids activate a descending system with a DOR mediated endogenous opioid link in the RVM. (C) 1999 Lippincott Williams & Wilkins.