Characterization of [³H]morphine binding to interleukin-1-activated thymocytes

We have previously reported that interleukin-1-induced proliferation of thymocytes is accompanied by the appearance of [³H] morphine binding sites on these cells. In the present study, we have characterized these binding sites. They differ from classical opioid receptors in the brain in several ways, including: 1) lack of stereoselectivity; 2) relatively low affinity (K(d) = 50 nM) and high capacity (B(max) = 3 pmol/mg of protein); 3) binding is strongly inhibited by Ca⁺⁺, Mg⁺⁺, Mn⁺⁺ and Cl^- ions and 4) binding is inhibited by proteinase K or E and by phospholipase A₂ but not trypsin treatment of thymocyte membranes. The binding sites, which were found largely on the CD4⁺ subset of T-cells, also showed a preference for opioid alkaloids over peptides. These [³H]morphine binding sites may mediate a negative feedback effect on interleukin-1-induced proliferation of thymocytes in vivo.