Thermodynamic analysis of agonist and antagonist binding to the chicken brain melatonin receptor

Daily Rhythm in Plasma N-acetyltryptamine

Normal physiology undergoes 24-h changes in function that include daily rhythms in circulating hormones, most notably melatonin and cortical steroids. This study focused on N-acetyltryptamine, a little-studied melatonin receptor mixed agonist-antagonist and the likely evolutionary precursor of melatonin. The central issue addressed was whether N-acetyltryptamine is physiologically present in the circulation. N-acetyltryptamine was detected by LC-MS/MS in daytime plasma of 3 different mammals in subnanomolar levels (mean ± SEM: rat, 0.29 ± 0.05 nM, n = 5; rhesus macaque, 0.54 ± 0.24 nM, n = 4; human, 0.03 ± 0.01 nM, n = 32). Analysis of 24-h blood collections from rhesus macaques revealed a nocturnal increase in plasma N-acetyltryptamine (p < 0.001), which varied from 2- to 15-fold over daytime levels among the 4 animals studied. Related RNA sequencing studies indicated that the transcript encoding the tryptamine acetylating enzyme arylalkylamine N-acetyltransferase (AANAT) is expressed at similar levels in the rhesus pineal gland and retina, thereby indicating that either tissue could contribute to circulating N-acetyltryptamine. The evidence that N-acetyltryptamine is a physiological component of mammalian blood and exhibits a daily rhythm, together with known effects as a melatonin receptor mixed agonist-antagonist, shifts the status of N-acetyltryptamine from pharmacological tool to candidate for a physiological role. This provides a new opportunity to extend our understanding of 24-h biology.

Melatonin receptors in brain areas and ocular tissues of the teleost Tinca tinca: characterization and effect of temperature

The aim of the present study was to characterize the central melatonin receptors in brain areas and ocular tissues of the teleost Tinca tinca. We investigated the temperature-dependence of 2-iodo-melatonin ([(125)I]Mel) binding in the optic tectum-tegmentum area and the neural retina. The binding of [(125)I]Mel showed a widespread distribution in brain and ocular tissues, with the highest density in the optic tectum-thalamus and the lowest in hindbrain. The [(125)I]Mel affinity was similar in all the studied tissues, and it was on the order of the low pM range. Saturation, kinetic and pharmacological studies showed the presence of a unique MT(1)-like melatonin binding site. In addition, the non-hydrolysable GTP analog, the GTPgammaS, and sodium cations induced a specific binding decrease in the optic tectum and neural retina, suggesting that such melatonin binding sites in the tench are coupled to G protein. Thus, these melatonin binding sites in optic tectum and neural retina fulfil the requirements of a real hormone receptor, the specific binding is rapid, saturable, and reversible, and is inhibited by GTP analogs. Additionally, a clear effect of temperature on such central melatonin receptors was found. Temperature did not modify the B(max) and K(d), but the kinetics of [(125)I]Mel binding resulted in a highly thermosensitive process in both tissues. Both association and dissociation rates (K(+1) and K(-1)) significantly increased with assay temperature (15-30 degrees C), but the K(d) constant (estimated as K(-1)/K(+1)) remained unaltered. In conclusion, this high thermal dependence of the melatonin binding to its receptors in the tench central nervous system supports the conclusion that temperature plays a key role in melatonin signal transduction in fish.

Effects of temperature on 2-[125I]-iodomelatonin binding to melatonin receptors in the neural retina of the frog Rana perezi

The present study analyzes the effect of temperature-dependent modifications on the binding of the analog 2-[125I]-melatonin to melatonin receptors in isolated neural retina membranes from the greenfrog Rana perezi. Association and dissociation rate constants (K+1, K-1) were exponentially increased by the assay temperature. At 15 degrees C, association and dissociation required several hours; meanwhile, at 35 degrees C, rate constants were 100- and 34-fold faster, respectively. However, the Kd constant calculated as K-1/K+1 was unmodified by the assay temperature. When frogs were acclimated at either 5 or 22 degrees C for 1 month, K+1, and K-1 constants determined at 15 and 25 degrees C were identical in both cold- and warm-acclimated groups. Thus, the binding kinetics of melatonin receptors in frog retinas did not shown any thermal compensation. Results from saturation curves and pharmacological profiles of melatonin binding sites support a lack of effect of assay temperature on the affinity of melatonin receptors in the frog retina. The inhibition of [125I]Mel binding by GTPgammaS showed clearly that the coupling of melatonin receptors to G proteins is temperature-dependent. Higher concentrations of the GTP analog were needed to inhibit specific binding when temperature decreased. The temperature effect on binding kinetics and on the G protein coupling to melatonin receptors suggests that the melatonin signal could be transduced distinctly depending on the temperature. Thus, temperature plays a major role, not only on melatonin synthesis, but also in the transduction of melatonin signal in ectotherms.

Melatonin receptors: are there multiple subtypes?

There is now evidence for more than one site of action for the hormone melatonin (N-acetyl-5-methoxy-tryptamine). Recent pharmacological and molecular advances are providing the tools to address the characterization of melatonin receptor subtypes. The development of novel melatonin receptor agonists and antagonists, high-affinity radioligands, quantitative bioassays, and the recent cloning of melatonin receptors are furthering our understanding of native and recombinant melatonin receptors. In this article, Margarita Dubocovich discusses the properties of melatonin receptors, and the basis for their classification into at least two subtypes, the ML1 and ML2.

Structural requirements at the melatonin receptor

1. High affinity, specific binding sites for the pineal hormone, melatonin (5-methoxy N-acetyltryptamine) can be detected in chick brain membranes by use of the radiolabelled agonist, 2-[125I]-iodomelatonin (2-[125I]-aMT). 2. The affinity of a number of analogues of melatonin at the 2-[125I]-aMT binding site was determined and compared with an analysis of their electronic structure and significant quantitative relationships obtained. 3. The best correlations indicated that binding affinity was correlated with delta E, the difference between the frontier orbital energies, and QNH, the electron density in the highest occupied molecular orbital of the side-chain nitrogen atom. 4. These findings suggest that ligand binding may involve hydrogen bonding between the 5-methoxy and amide moieties of melatonin and complementary amino acid residues, and charge transfer interactions between the indole ring of melatonin and an aromatic amino acid in the receptor binding site. 5. A molecular model of a putative binding site is proposed based on the predicted amino acid sequence of the cloned Xenopus laevis melanophore melatonin receptor and the quantitative structure-affinity relationships observed in the present study.

Characterization of the chicken brain melatonin-binding protein using iodinated and tritiated ligands

The melatonin-binding protein in chicken brain membranes was characterized using both [125I]-2-iodomelatonin and [3H]-melatonin as radioligands. Saturation studies conducted at 25 degrees C revealed a single class of binding site with dissociation constants of 24 +/- 4.8 pM (n = 7) and 125 +/- 21 pM (n = 6) for the iodinated and tritiated ligands, respectively. Calculation of the affinity constant using data from kinetic experiments gave values of 2.2 +/- 0.4 pM and 135 +/- 15 pM for the iodinated and tritiated ligands, respectively. Competition studies showed that the rank order of inhibition of binding by melatonin analogues was similar for both radioligands (2-iodomelatonin > melatonin > 2,3-dihydromelatonin > N-acetyl-5-methoxykynurenamine > N-acetylserotonin > 5-methoxytryptamine). The calculation of Ki, which depends upon the affinity constant, was 22 +/- 4.9 pM and 129 +/- 21 pM for 2-iodomelatonin and melatonin, respectively, when the affinity constant derived from the [125I]-2-iodomelatonin saturation experiments was used, but 4.9 +/- 1.5 pM and 33 +/- 5.5 pM when the kinetically derived constant was used. When [3H]-melatonin was used, the Ki for melatonin was 72 +/- 8 pM and 20 +/- 4.6 pM for 2-iodomelatonin and melatonin. Binding of [125I]-2-iodomelatonin to the membranes was partially reversible at 25 degrees C in contrast to the complete reversibility of [3H]-melatonin. Examination of the effects of temperature on binding indicated that at 37 degrees C both association and dissociation of both ligands were accelerated. Closer examination showed that at 37 degrees C there was a loss of approximately 40% of the [125I]-2-iodomelatonin binding sites and little influence upon the affinity of binding with time. By contrast, when [3H]-melatonin was used, the affinity decreased fourfold, with only a slight change in the number of sites. If membranes were incubated at 37 degrees C and then switched 25 degrees C, binding increased, emphasizing the fact that the binding sites were not destroyed. Whereas there appears to be little doubt that 2-iodomelatonin is a biologically active melatonin agonist, the binding of the radioactive form of this agonist to the putative melatonin receptor binding site is quite different from that of the endogenous ligand. This may have serious consequences in studies where receptor content is determined following physiological or pharmacological interventions.