Characterization and mapping of melatonin receptors in the brain of three mammalian species: rabbit, horse and sheep. A comparative in vitro binding study.

This research study analyses and maps the melatonin receptors found in the brains of rabbits, horses, and sheep by using an in vitro binding technique with labeled iodomelatonin. Previous studies on this topic have not included rabbits and horses. The study characterizes these receptors’ properties and distribution in different brain regions, facilitating interspecies comparison under similar methodological conditions.

Study Methodology

• The experiment used 2-[125I]iodomelatonin, a radioactive form of iodomelatonin, as a labelled ligand to mark the melatonin receptors in the brain extracts from the three species.
• A series of kinetic and saturation experiments were conducted to understand the binding properties of the receptors.
• In addition, competition studies using a variety of indoles were used to measure the receptors’ binding strength and order of potency.

Key Findings

• The binding of the labelled iodomelatonin to the melatonin receptors was found to be quick, stable, reversible, and high affinity, showing high affinity for the ligand and low capacity.
• The competition studies demonstrated that the order of potency of different indoles used in inhibiting iodomelatonin binding was 2-iodomelatonin, followed by 6-chloromelatonin, melatonin, 5-methoxytryptophol, and then 5-methoxytryptamine, which was consistent across different brain regions in the three species.
• Prazosin, a potent melatonin analogue reported in hamster brains, showed no potency in these three mammalian species.
• The melatonin receptor distribution, although varies among different brain regions, was remarkably consistent across the three species, with the highest receptor density always observed in the Median Eminence/Pars Tuberalis (ME/PT) area.

Conclusion

• The research provides valuable insights into the nature and distribution of melatonin receptors in untested species – the horse and rabbit, in addition to providing more comprehensive information about the receptor in already studied species (sheep).
• Quantifying the order of potency of various indoles on these receptors enables better understanding of melatonin’s role and the interaction of other indoles with the receptor.
• The consistent regional distribution of receptor sites across species suggests common functional roles for melatonin in differing mammals, although the related biological consequences are yet to be fully understood.