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Abstract
We investigated the effect of several opioid peptides on the activation of murine
peritoneal exudate macrophages (M phi) in vitro. M phi were treated with interferon
(IFN) as a priming agent and bacterial lipopolysaccharide (LPS) as a triggering agent
in the presence or absence of opioid peptides. M phi activation was assessed by their
tumoricidal activity. When treatment with IFN and LPS resulted in a high level activation
of M phi, dynorphin-A exerted no further enhancing effect. When treatment induced
only weak activation, however, dynorphin-A augmented the M phi activation. Leucine-enkephalin,
methionine-enkephalin, and also beta-endorphin had augmenting effects. An opioid receptor
antagonist, naloxone, reduced the effect of dynorphin-A and beta-endorphin. When M
phi were treated sequentially with IFN and LPS, beta-endorphin operated in combination
with LPS only. Moreover, beta-endorphin was effective for already activated M phi.
These results indicate that opioid peptides act on M phi via classical opioid receptors,
and that responsiveness to opioid peptides is induced in the triggering stage of M
phi activation.