Differential properties of mu and delta opiate binding sites studied with highly selective ligands

Differences in binding kinetics, ions and nucleotides interactions with rat brain opiate mu and delta receptor subtypes were investigated using respectively DAGO (Tyr-D-Ala-Gly-MePhe-Gly-ol) and DTLET (Tyr-D-Thr-Gly-Phe-Leu-Thr) as highly selective mu and delta ligands. On the basis of kinetic experiments the delta-agonist binding is an extremely slow process as compared to that of the mu-agonist. Displacement experiments by DTLET of [3H] DAGO binding and Scatchard analysis of the binding of [3H] DTLET in the absence and presence of DAGO demonstrated that an independent model of interaction to two independent sites best describes the observed data. Steady state binding of [3H] DAGO is decreased by GMP-P(NH)P while this nucleotide is ineffective in reducing the binding of DTLET. However in presence of NaCl, GMP-P(NH)P reduces the specific binding at both sites in a concentration dependent manner. The analysis of the nucleotide inhibition of delta-binding at various concentrations of NaCl suggests that the non-hydrolyzable nucleotide can exerts its effects only on a prebound Na+ receptor. These data suggest that mu and delta-receptors may assume different affinity states depending upon the presence of Na+ and nucleotide.