Leu-enkephalin enhances interferon secretion in mice.

This work has explored the possibility that alterations of NK activity induced by the opioid pentapeptide leu-enkephalin (LENK) may reflect the alterations in secretion of IFN, an important regulator of NK activity. The NK activity in the spleen of mice was determined in parallel with the plasma IFN level 24 and 48h after an i. p. injection of LENK (10 mg/kg). A known inducer of IFN secretion, polyinosinic-polycytidylic acid (poly-IC), was also used. LENK injection significantly increased basal IFN secretion 24 and 48h later. The level was comparable to that induced by poly-IC. However, LENK was not able to augment the poly-IC-increased IFN level. The increase of IFN at 48 h coincided with a mild enhancement of NK activity in the spleens, but 24 h after LENK injection, the increased IFN level in plasma was associated with a significant drop of splenic NK activity. LENK did not affect the NK activity stimulated with poly-IC. Naloxone (20 mg/kg), an opioid receptor-blocking agent, only partly diminished the LENK-induced IFN secretion. However, naloxone itself increased the plasma IFN level. These data indicate that the immunomodulatory effects of the opioid peptide LENK in vivo are associated with alterations of IFN secretion.