SELECTIVE EFFECT OF METHOXYINDOLES ON THE LYMPHOCYTE-PROLIFERATION AND MELATONIN BINDING TO ACTIVATED HUMAN LYMPHOID-CELLS

Three pineal methoxyindoles (melatonin (Mel), 5-methoxytryptamine (5-M TA) and 5-methoxytryptophol (5-MTO)) were studied for their ability to influence the proliferative response of human peripheral blood lympho cytes (PBL) and tonsillar lymphocytes (TL) following activation with c oncanavalin A (ConA) in vitro. The ConA-stimulated DNA synthesis was a ffected in a different dose-dependent mode by the methoxyindoles teste d. Melatonin and 5-MTO inhibited and 5-MTA increased the ConA-induced [H-3]thymidine incorporation in PBL and TL. The initial screening for 2-[I-125]iodomelatonin binding using a single point assay revealed sig nificantly increased specific binding to PBL and TL after 72-h stimula tion with ConA as compared to the non-activated cell cultures. Coincub ation of separate lymphocyte cultures with ConA and Mel or 5-MTO resul ted in inhibition of the specific 2-[I-125]iodomelatonin binding (85% and 74%, respectively). The specific binding determined in the presenc e of 5-MTA did not differ from control values. Series of saturation an d competition experiments were performed to examine the binding charac teristics of ConA-stimulated lymphocytes for 2-[I-125]iodomelatonin. T he radioligand labelled binding sites of high affinity (K-d = 0.14 +/- 0.03 nM) and low capacity (B-max = 6.8 +/- 1 5 fM/mg protein). Compet itive studies with a variety of indoles determined the following order of relative potency for inhibition of 2-[I-125]iodomelatonin binding in TL: 2-iodomelatonin > melatonin > > 5-methoxytryptophol. 5-Methoxyt ryptamine did not show displacement potency for the labelled ligand. C ollectively, our data suggest that pineal hormones might be directly i nvolved in the regulation of the T-lymphoproliferative response of hum an lymphoid cells. We show the availability of melatonin receptors, wh ich seem to be an intrinsic characteristic of activated human lymphocy te populations. While the effects of Mel and 5-MTO can be linked to th e binding sites described, it is unlikely that serotonin agonists like 5-MTA may act through the same sites to influence the mitogen-stimula ted lymphocyte proliferation.