Enhancement of interleukin-1 production in mouse peritoneal macrophages by methionine-enkephalin.

Methionine-enkephalin, an endogenous opioid, has been reported to have some effects on immune responses. By thymocyte proliferation method, we herein report that methionine-enkephalin over a wide range of concentrations (1 pmol-0.1 mumol/L) significantly increases both extracellular interleukin-1 release and intracellular interleukin-1 production from peritoneal macrophages induced by lipopolysaccharide in mice. Naloxone, having no effect per se on interleukin-1 production, does not block the enhancing effect of the neuropeptide. Interleukin-1 production was also elevated following ip methionine-enkephalin into mice. The results suggest that methionine-enkephalin mediates the enhancement of interleukin-1 synthesis and release as well, and that the effect is not mediated through classical opioid receptors. The results also provide the further links between immune and nervous systems.