Immunoregulation by opioid peptides: I. Absence of classical opioid receptor on human mononuclear cells

Potent effects of opiates and opioid peptides on the immune responsiveness of mononuclear cells have recently been described. The mechanism by which the effects are mediated was explored by examining the opioid receptor properties of these cells. Human mononuclear cells (B and T lymphocytes and monocytes) were examined for their ability to bind 3H-naloxone, a mu opioid ligand, and 3H-D-Ala2-D-Leu5-enkephalin, a delta opioid ligand. Both ligands bound to nonadherent cells as well as monocytes in a non-specific fashion and could not be competitively displaced by the addition of unlabeled compound. Cells which were stimulated in culture with mitogens did not develop receptors which bound either ligand in a specific fashion. Homogenization of human mononuclear cells prior to measuring binding significantly reduced non-specific binding of radioligands. Although under these conditions a limited amount of net specific binding was observed, this binding was not concentration dependent or saturable and varied considerably between individuals. These observations suggest that opioid effects on immune responsiveness are not mediated through a mononuclear opioid receptor.