Location of melatonin receptors

Abstract

We attempted to locate melatonin receptors in different organs of rats. Experiments with plasma membranes and cytosol from rat liver revealed binding activity in both cell fractions. In both cases maximal binding was achieved after 3 hours at 20°C and after 6 hours at 4°C. Both binding sites had an optimum pH at 7.5 and were inhibited by trypsin. Plasma membrane receptors seem to be more Ca++ dependent than cytosol binding sites. Dissociation constants were 8·10-9M for membrane receptors and 6·10-8M for cytosol binding sites. Receptor concentration of liver, lung, spleen and heart cytosol varied between 50-200 fmoles/mg protein; that of testes, kidney and eyes was 2-3 times as high; that of pituitary gland, hypothalamus, epididymis and adrenals was 6-8 times as high. Melatonin receptor concentrations of plasma membranes from liver and testes were about 10 fold lower than in the cytosol fraction whereas spleen and lung membranes showed no melatonin binding activity. The search of melatonin in different organs of normal rats or animals treated with melatonin have shown significant concentration or accumulation in organs which showed also high concentrations of melatonin binding sites: eyes, brain, testes, epididymis, adrenals and kidney. These organs with the exception of kidney are known to be presumably targets for melatonin action.