Enkephalins and immunity. I: In vivo suppression and potentiation of humoral immune response.

BALB/c mice and Wistar rats immunized with sheep red blood cells and ovalbumin were treated intraperitoneally with different doses of methionine-enkephalin, leucine-enkephalin, and naloxone. Large doses of enkephalins (10-5 mg/kg b.w.) induced a significant decrease in hemolysin-forming cell response and production of hemagglutinating antibody. Immunosuppression induced by enkephalin was dose-dependent. In rats met-enkephalin was a more potent immunosuppressor than leu-enkephalin. Rats injected with 2.5 mg/kg b.w. of enkephalins into the lateral ventricle of the brain showed more pronounced immune suppression than did animals treated intraperitoneally with 5 mg/kg b.w. of enkephalins. These neuropeptides, and met-enkephalin in particular, exhibited a protective action against anaphylactic shock in rats sensitized to ovalbumin. In those animals, passive cutaneous anaphylaxis and elaboration of precipitating anti-ovalbumin antibody were considerably reduced. On the other hand, small doses of enkephalins stimulated humoral immune responses in the rat. Thus, it appears that enkephalins both suppress and potentiate immune responsiveness, depending on the dose used. As for naloxone, a large dose of this blocker of opioid receptors enhanced humoral immune reactions in the rat.