Naloxone is an inappropriate antagonist of met-enkephalin-modulated superoxide anion release.

Met-enkephalin (MENK) increased superoxide anion release by human polymorphonuclear cells (PMNs) at a physiological (10(-10) M) concentration and decreased release at a relatively high (10(-8) M) concentration. Surprisingly, naloxone (NAL), used as a specific antagonist in these experiments, displayed immunomodulatory actions of its own, increasing superoxide anion release at 10(-10) and 10(-8) M concentrations. Although both were effective, the dose-response curves were different for NAL and MENK.

Interaction of Met-enkephalin and corticosteroids in immunomodulation.

The effect of Met-enkephalin (MENK) on several immune functions, corticosterone (CS) and adrenocorticotropin (ACTH) levels in the plasma was studied in adrenalectomized (ADX) and sham-adrenalectomized (SADX) mice. Multiple Met-enkephalin injections (10 mg/kg per day in two injections 12 h apart, for 4 days) increased the plaque-forming cell response to sheep erythrocytes in the spleen and enhanced the proliferation of spleen cells in vitro. These effects were comparable in sham-adrenalectomized and adrenalectomized mice.