2[125I]Iodomelatonin binding and interaction with beta-adrenergic signaling in chick heart/coronary artery physiology.

2[125I]Iodomelatonin ([125I]Mel) binding sites were characterized on membrane preparations of young chick hearts. [125I]Mel binding was rapid, saturable, stable, reversible, specific and of picomolar affinity and femtomolar density. Guanosine 5'-O-(3-thiotriphosphate) significantly lowered the binding affinity by one- to twofold, supporting G-protein linkage of melatonin receptors. Binding was detected as early as embryonic day-9 (E9), and increased steadily peaking at E13 before it slowly declined to about 15% of the peak level a week posthatch. Specific [125I]Mel binding was significantly increased by in ovo administration of inotropic agents dopamine and isoproterenol. Melatonin or 2-iodo-N-butanoyl-tryptamine inhibited isoproterenol-stimulated cAMP accumulation in primary heart cell cultures and the effect was attenuated after pretreatment with pertussis toxin (PTX). Localization of melatonin receptors using autoradiography showed intense labeling in the coronary arteries in all age groups whereas those in the myoblasts decreased as the heart matured. While the myoblasts and undifferentiated developing coronary arteries expressed melatonin MT1 receptor subtype in E11 hearts as detected by immunostaining with anti-MT1 receptor serum, immunoreactivities were observed mostly on the endothelium/subendothelium and smooth muscle cells of the well developed coronary vessels in posthatch hearts. Collectively, our data suggest the presence of PTX-sensitive, G protein-coupled melatonin receptors, whose expression is up-regulated by dopamine and isoproterenol, in the chick heart. Activation of these receptors, which include MT1 subtype, may modulate beta-adrenergic receptor-mediated cAMP signaling in the control of chick heart and coronary artery physiology.

2[125I]Iodomelatonin binding sites in guinea pig platelets.

Using 2[125I]iodomelatonin as the radioligand, we characterized 2[125I]iodomelatonin binding sites in guinea pig platelet membrane preparations. Saturation radioreceptor studies indicated that these 2[125I]iodomelatonin binding sites were of picomolar affinity and femtomolar density. The dissociation constant (Kd) and maximum number of receptor sites (Bmax) were 42.5 +/- 1.79 pM and 11.8 +/- 0.8 fmol/mg protein (n = 6), respectively. 2[125I]Iodomelatonin competition studies with indoles or drugs indicate the following rank order of potency: 2-iodomelatonin > melatonin > 6-chloromelatonin > 6-hydroxymelatonin > N-acetylserotonin > 5-methoxytryptophol, whereas serotonin and its analogs had less than 20% inhibition at 0.1 mM. Guanosine 5'-O-(3-thiotriphosphate) significantly increased the Kd by twofold suggesting that these binding sites are coupled to the guanine nucleotide binding proteins. Immunoblotting studies using anti-MT(1) IgG demonstrated one peptide blockable band with an apparent molecular mass of 37 kDa. Melatonin had no effect on prostacyclin or forskolin-stimulated intracellular 3',5'-cyclic adenosine monophosphate accumulation. A diurnal variation in binding density, which was abolished after the animals were adapted to constant light conditions, was observed. Age related studies demonstrated that Bmax increased as the animal matured. Physiological melatonin concentrations potentiated whereas those at pharmacological levels inhibited adenosine diphosphate- or arachidonic acid-stimulated platelet aggregation. Our study demonstrated G-protein coupled, saturable, reversible and highly specific picomolar affinity 2[125I]iodomelatonin binding sites in guinea pig platelets. Pharmocological and physiological data indicate that they may be different from the nanomolar [3H]melatonin binding sites in human platelets previously reported.

Molecular determinants for the differential coupling of Galpha(16) to the melatonin MT1, MT2 and Xenopus Mel1c receptors.

The pineal neurohormone melatonin modulates a variety of physiological processes through different receptors. It has recently been reported that the cloned melatonin receptors (MT1, MT2 and Mel1c) exhibit differential abilities to stimulate phospholipase C (PLC) via G(16). Here we examined the molecular basis of such differences in melatonin receptor signaling. Coexpression of MT1 or MT2 with the alpha subunit of G(16) (Galpha(16) ) allowed COS-7 cells to accumulate inositol phosphates in response to 2-iodomelatonin. In contrast, Mel1c did not activate Galpha(16) even though its expression was demonstrated by radioligand binding and agonist-induced inhibition of adenylyl cyclase. As Mel1c possesses an exceptionally large C-terminal tail, we further asked if this structural feature prevented productive coupling to Galpha(16). Eleven chimeric melatonin or mutant receptors were constructed by swapping all or part of the C-terminal tail between MT1, MT2 and Mel1c. All chimeras were fully capable of binding 2-[(125) I]iodomelatonin and inhibiting adenylyl cyclase. Chimeras containing the full-length Mel1c tail were incapable of activating Galpha(16), while those that contained the complete C-terminal region of either MT1 or MT2 stimulated PLC. Incorporation of the extra portion of the C-terminal tail of Mel1c to either MT1 or MT2 completely abolished the chimeras' ability to stimulate PLC via Galpha(16). In contrast, truncation of the C-terminal tail of Mel1c allowed interaction with Galpha(16). Our results suggest that Galpha(16) can discern structural differences amid the three melatonin receptors and provide evidence for functional distinction of Mel1c from MT1 and MT2 receptors.