Abstract
Microinjection of [ d-Ala 2, MePhe 4, Gly(ol) 5]enkephalin (DAMGO) into either the periaqueductal gray (PAG) or the rostral ventral medulla (RVM) elicts analgesia in the tailflick assay in the rat. Co-administration of DAMGO into both regions together results in a profound synergistic interaction similar to that we previously reported with morphine. U50,488H and DPDPE are inactive when given into either region. [ d-Ala 2, Glu 4]Deltorphin (deltorphin), on the other hand, elicits an analgesic response, although the maximal response is less than than μ agonists. Co-administration of DAMGO into one region with deltorphin in the other also results in a significant synergy. However, co-administration of DAMGO and deltorphin together in the same region gives only additive effects. These results confirm the existence of μ/μ synergy between the PAG and RVM. κ1 and δ1 agents are inactive, but the δ 2 agonist deltorphin is active in both regions. Our results indicate the presence of μ/δ 2 synergy between the PAG and RVM which appears to involve interactions of pathways rather than receptori nteractions at the cellular level.
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