High-affinity binding sites for melatonin in hamster spleen

Recent reports that the pineal gland hormone, melatonin, influences immune function in mammals, prompted an examination of the binding sites for this hormone in lymphoid tissue. Binding studies utilized the specific melatonin receptor probe, 2-[¹²⁵I]iodomelatonin ([¹²⁵I]MEL), and splenic homogenates from male Syrian hamsters. Scatchard analysis of saturation binding data indicated the presence of a high-affinity site with a dissociation constant (K(d)) of 2 nM and a density (B(max)) of 92 fmol/mg protein. Inhibition experiments indicated that 2-iodomelatonin has the highest affinity for this site, followed by melatonin and 6-chloromelatonin. The melatonin precursor, N-acetylserotonin, is about 6-fold weaker than melatonin, while serotonin exhibits no binding affinity. This report provides first evidence that specific, high-affinity binding sites for melatonin are present in lymphoid tissue. Moreover, these findings suggest that melatonin's immunomodulatory effects involve a direct action on these sites.