Peripheral effects of methionine-enkephalin on inflammatory reactions and behavior in the rat

Methionine-enkephalin (Met-Enk) induces notable alterations in immune and c entral nervous system functions. The present study was conducted in order t o compare peripheral and central effects of Met-Enk on nonspecific immunity , open field behavior and pain perception in the rat. The results showed th at 0.2 mg/kg of Met-Enk given intraperitoneally (i.p.) increased concanaval in A (Con-A)-induced paw edema and enhanced basal and phorbol myristate ace tate (PMA)-stimulated H2O2 production of peritoneal macrophages. Met-Enk-in duced immunopotentiation was antagonized by anti-Met-Enk antibodies (anti-M et-Enk-Ig) and quaternary naltrexone (qNtx). Met-Enk injected i.p. produced an increase of horizontal and vertical locomotor activity in the open fiel d that was reversed by i.p. administration of anti-Met-Enk-Ig and qNtx. The dose of 0.2 mg/kg of Met-Enk applied i.p. did not affect the number of wri thes in the test of analgesia. Intracerebroventricular (i.c.v.) injection o f Met-Enk, given in a dose that was previously shown to be immunostimulator y, enhanced only basal H2O2 production of peritoneal macrophages, and anti- Met-Enk-Ig antagonized this effect. Besides, i.c.v. treatment with anti-Met -Enk-Ig increased and decreased H2O2 production of peritoneal macrophages u nder basal and stimulated conditions, respectively, Met-Enk and anti-Met-En k-Ig injected i.c.v. did not influence activity in the open field and pain sensitivity. Thus, the i.c.v. dose of Met-Enk that was sufficient to modula te immune functions did not influence behavior. It may be concluded that Me t-Enk modulated nonspecific immune responses and open field behavior by per ipheral mechanisms. Copyright (C) 2000 S. Karger AG, Basel.