Opioid binding to rat and guinea-pig neural membranes in the presence of physiological cations at 37 degrees C.

We have identified mu, delta and kappa opioid binding sites in four types of neural membranes under conditions which include physiological concentrations of ions and an incubation temperature of 37 degrees C. We hypothesize that binding parameters determined under these conditions should be more directly comparable with physiological experiments than parameters obtained under conditions of low ionic concentration and at low temperature. By using either a radioligand which is selective for a single type of opioid binding site or a relatively nonselective radioligand in the presence of an unlabeled selective ligand, we have isolated binding to single populations of sites. Saturation and displacement data were analyzed with the aid of a computerized nonlinear curve fitting program. [3H]Tyr-D-Ala-Gly-(Me)Phe-Gly-ol bound to a single population of sites with the characteristics of mu receptors, as determined by saturation and displacement analysis. Binding to the mu site represented 70% of the total specific opioid binding in rat brain, but only 20 to 30% in guinea-pig tissues. [3H][D-Ala2-D-Leu5]enkephalin bound almost equally well to mu and delta sites, but the delta site could be examined by the inclusion of unlabeled Tyr-D-Ala-Gly-(Me)Phe-Gly-ol in the incubations. [3H]Ethylketocyclazocine bound mu and kappa sites, and Tyr-D-Ala-Gly-(Me)Phe-Gly-ol was also used to block the mu component in experiments in which we studied kappa binding. Binding to kappa sites represented 50 to 60% of the total in guinea-pig tissues, but less than 20% in rat brain.