Synergy between mu/delta-opioid receptors mediates adenosine release from spinal cord synaptosomes

Morphine releases adenosine from the spinal cord and this contributes to spinal antinociception. The present study examined possible interactions between mu- and subclasses of delta-opioid receptors in the release of adenosine. Nanomolar (10(-8), 10(-9) M) concentrations of morphine release adenosine from spinal cord synaptosomes under conditions of partial depolarization with elevated K+, and this component of release is mediated by activation of mu-opioid receptors. Subnanomolar (10(-10), 10(-11) M) concentrations of the mu-opioid receptor agonists morphine, [N-MePhe3,D-Pro4]morphiceptin, and [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAMGO) have minimal effects on the release of adenosine from the spinal cord. However, [D-Pen2,D-Pen5]enkephalin (DPDPE), a delta 1-opioid receptor agonist, and [D-Ala2,Cys4]deltorphin, a delta 2-opioid receptor agonist, at doses which exhibit no intrinsic effects (10(-8) and 10(-7) M), shifted the dose-response curve for mu-opioid receptor-evoked adenosine release to the left in a dose-dependent manner. DPDPE was more potent than [D-Ala2,Cys4]deltorphin when combined with the highly selective mu-opioid receptor agonist [N-MePhe3,D-Pro4]morphiceptin, but these agents showed similar activity with the less selective agonists DAMGO and morphine. Simultaneous activation of mu- and delta-opioid receptors generates a synergistic release of adenosine from spinal cord synaptosomes. Although agonists for both delta 1- and delta 2-opioid receptor subtypes produce this response, the delta 1-opioid receptor agonist is more potent at eliciting this effect when the most selective mu-opioid receptor ligand is used.