[3H]Diprenorphine Binding to k-Sites in Guinea-Pig and Rat Brain: Evidence for Apparent Heterogeneity

Abstract

The binding of the unselective opioid antagonist [3H]diprenorphine to homogenates prepared from rat brain and from guinea-pig brain and cerebellum has been studied in HEPES buffer containing 10 mM Mg2+ ions. Sequential displacement of bound [3H]diprenorphine by ligands with selectivity for .mu., .delta.-, and .kappa.-opioid receptors uncovers the multiple components of binding. In the presence of cold ligands that occupy all .mu.-, .delta.-, and .kappa.-sites, opioid binding still remains. This binding represents 20% of total specific sites and is displaced by naloxone. The nature of these undefined opioid binding sites is discussed.