Characteristics, day-night changes, subcellular distribution and localization of melatonin binding sites in the goldfish brain

Abstract

Melatonin binding sites in the goldfish brain were characterized by radioreceptor assay using 2-[ 125I]iodomelatonin as the radioligand. Specific binding of 2-[ 125I]iodomelatonin was rapid, stble, saturable and reversible. Saturation experiments demonstrated that 2-[ 125I]iodomelatonin binds to a single class of receptor site with an affinity constant ( K rmd) of 29.8±0.7pM and a total binding capacity ( B max)of11.47 ± 0.33fmol/mg protein at mid-dark, the B max value decreased significantly to 7.90±0.23fmol/mg protein ( Pˇ0.01) with no significant variation in the K d value (33.8±1.5pM). Competition experiments revealed the following order of pharmacological affinities: 2-iodomelatonin>melatonin> 6-hydroxymelatonin> N-acetyl-5-hydrxytryptamine> 5-methoxytryptamine> 5-methoxytryptophol> 5-methoxyindole-3-acetic acid. 5-Hydroxytryptamine, 5-hydroxytryptophol, 5-hydroxyindole-3-acetic acid, norepinephrine and acetylcholine exhibited no inhibition. Subcellular distribution of melatonin binding sites was demonstrated to be greatest in the P 2 and P 3 fractions as compared with the P 1 fraction. Localization of melatonin binding sites in discrete brain areas was determined to be highest in the optic tectum-thalamus and hypothalamus, intermediate in the telencephalon, cerebellum and medulla oblongata, and lowest in the olfactory bulbs and pituitary gland. These results suggest that characteristics of melatonin receptors are highly conserved during evolution and that in this species melatonin plays neuromodulatory roles in the central nervous system through specific receptors.