Synergistic action of melatonin and vasoactive intestinal peptide in stimulating cyclic AMP production in human lymphocytes

In the present study we investigated the synergistic effect of melatonin and vasoactive intestinal peptide (VIP) on cyclic AMP production in human blood lymphocytes. As shown by our group previously, VIP alone behaved as a potent activator of cyclic AMP production in human lymphocytes. On the other hand, melatonin alone did not affect the intracellular levels of cyclic nucleotide at any time or dose studied. However, when cells were incubated with melatonin plus VIP, melatonin potentiated the effect of the peptide. This effect can be observed in the presence of physiological doses of both melatonin (10-100 pM) and VIP (1-100 pM). The effect is specific for VIP because with other peptides belonging to the secretin-VIP family the effect was not observed. Results suggest that melatonin, in conjunction with VIP, may directly participate in the regulation of immune function in the human.