Influence of melatonin on chicken lymphocytes in vitro: involvement of membrane receptors

Time-dependent melatonin effects on chicken lymphocyte proliferation in vitro and the involvement of cAMP in melatonin signal transduction were examined.

Splenocytes and peripheral blood mononuclear cells (PBMC) were cultured in vitro in the presence of melatonin, phytohemagglutinin, luzindole, dibutyrylcAMP (dbcAMP), forskolin and vasoactive intestine peptide (VIP). Proliferation was measured by [(3)H]-thymidine incorporation in cultures carried out for 24, 36, 48 and 72 h. Cyclic AMP formation was assessed by radioimmunoassay in cells incubated for 30 min. or 24 h.

Melatonin stimulated the spontaneous proliferation in short-term (36 and 48 h) splenocyte cultures and had no effect in 72 h cultures. It inhibited mitogen-stimulated proliferation already in 24 h cultures and this effect was observed regardless of the time of the culture. Both melatonin effects were antagonized by luzindole - membrane-bound melatonin receptor antagonist. Forskolin and dbcAMP caused a significant inhibition of proliferation of splenocytes and PBMC cultured for 24 or 72 h, respectively. Melatonin inhibited the cAMP formation (30 min. of incubation) stimulated by adenosine cyclase activators - forskolin and VIP, but added alone failed to affect the cAMP concentration. In mitogen-stimulated splenocytes cultured for 24 h Mel caused an increase in cAMP correlated with the inhibition of cell proliferation.

Melatonin effects on chicken splenocytes appears time- and activation-dependent: in short-term cultures it stimulates spontaneous and inhibits mitogen-activated proliferation, probably via membrane-bound, luzindole-sensitive melatonin receptors. Incubation with melatonin for 30 min. inhibits cAMP formation, but in 24 h cultures it increases cAMP concentration leading to inhibition of proliferation.