SPECIFIC BINDING OF 2-[I-125]IODOMELATONIN BY RAT SPLENOCYTES - CHARACTERIZATION AND ITS ROLE ON REGULATION OF CYCLIC-AMP PRODUCTION

In the present paper we show that pineal hormone melatonin interacts with rat splenocytes through high-affinity binding sites. Binding of 2-[I-125]iodomelatonin ([I-125]MEL) by splenocytes fulfills all criteria for binding to a receptor site. Binding exhibited properties such as dependence on time and temperature as well as reversibility, saturability, high affinity, specificity, and increased under constant light exposure. Results suggest binding to a single class of binding sites without cooperative interactions. The dissociation constant (K-d) for the single site was 0.34 nM with a binding capacity of 2.25 fmol/10(7) cells. These data are in close agreement with data obtained from kinetic studies, in which the kinetically derived value of the dissociation constant was 0.20 nM. The affinity of these binding sites suggests that they may recognize the physiological concentrations of melatonin in serum. Moreover, pharmacological doses of melatonin also inhibited cyclic AMP production stimulated by forskolin, a potent activator of adenylate cyclase system. The demonstration of [I-125]MEL binding sites in the spleen, in addition to those described in blood mononuclear cells and thymus, provides evidence to support a direct mechanism of action of melatonin on immune system.