The β-endorphin inhibition of mitogen-induced splenocytes proliferation is mediated by central and peripheral paracrine/autocrine effects of the opioid

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In this study we show that the opioid peptide β-endorphin exerts a tonic inhibitory effect on the proliferative response of splenocytes to the polyclonal mitogen phytohemoagglutinin throughout two separate sites of action: one central and one peripheral. The intracerebroventricular administration of β-endorphin, in fact, induces a significant inhibition of splenocyte proliferation. In contrast, both the intracerebroventricular and the peripheral administration of anti-β-endorphin γ globulins induce a significant increase in proliferation. Moreover, an increase of splenocyte proliferation was observed also after the intravenous administration of γ globulins and intraperitoneal naloxone, and this effect was still present in hypophysectomized rats. The data reported suggest that β-endorphin exerts a tonic inhibitory effect on proliferation, acting centrally, and peripherally throughout a paracrine/autocrine mechanism. FACS experiments show that the effect observed is not the consequence of an alteration of lymphocyte trafficking induced by the opioid.