In vivo effects of chronic treatment with [Met⁵]-enkephalin on hematological values and natural killer cell activity in athymic mice

The role of endogenous opioids in immunological mechanisms was examined by subjecting athymic (nu/nu) mice to chronic injections of the opioid agonist [Met⁵]enkephalin (MET) or continuous opioid receptor blockade with naltrexone (NTX). After 8 days of treatment, neither excess peptide nor deprivation of opioids from receptors had any effect on body weight, spleen index (spleen to body weight ratio), total and differential white blood cell counts, and natural killer (NK) cell activity in peripheral blood or splenic lymphocytes. At 28 days, chronic treatment with MET or NTX had no effect on any of these parameters with the exception of an elevation from controls in NK cell activity in peripheral blood in mice receiving NTX, and subnormal NK cell activity related to splenic lymphocytes in the MET group. These results suggest that chronic exposure to an opioid agonist, or persistent opioid receptor blockade, have little influence on a variety of immunological properties in athymic mice, suggesting that native opioids such as MET do not play a marked role in defense mechanisms in the athymic mouse.