[I-125] IODOMELATONIN-BINDING SITES IN THE BURSA OF FABRICIUS OF BIRDS - BINDING CHARACTERISTICS, SUBCELLULAR-DISTRIBUTION, DIURNAL-VARIATIONS AND AGE STUDIES

The melatonin-binding sites in membrane preparations of the bursa of Fabricius of birds were studied using [I-125]iodomelatonin as a radioligand. The binding sites were stable, saturable, reversible and of high affinity. Scatchard analysis of specific binding revealed equilibrium binding constants (K(d)) of (means+/-S.E.M.) 43.1 +/- 5.9, 73.3 +/- 7.6 and 35.3 +/- 4.8 pmol/I respectively at the mid-point of the light period (mid-light) in chickens, pigeons and quail, with a total number of binding sites (B(max)) of 1.23 +/- 0.15, 1.33 +/- 0.18 and 0.94 +/- 0.07 fmol/mg protein. The diurnal variation in [I-125]iodomelatonin binding showed that the B(max) was 45, 115 and 70% higher at mid-light than at mid-dark in the bursae of chickens, pigeons and quail respectively. The K(d) value determined from kinetic analysis was 49.0 +/- 6.4 pmol/l at mid-light in the chicken bursa. The [I-125]iodomelatonin-binding sites of chicken bursal membranes had the following order of pharmacological affinities: 2-iodomelatonin > melatonin > 6-chloromelatonin > 6-hydroxymelatonin > N-acetyl-serotonin > 5-hydroxytryptamine, tryptamine, 5-methoxytryptophol, 1-acetylindole-3-carboxaldehyde, 3-acetylindole, L-tryptophan, 5-hydroxyindole-3-acetic acid, 5-hydroxytryptophan suggesting that the [I-125]iodomelatonin-binding sites were highly specific for melatonin. The subcellular distribution of binding sites in the chicken bursa was in the following descending order: nuclear>mitochondrial>microsomal>cytosolic fraction. There was an age-related decrease in [I-125]iodomelatonin-binding in chicken bursal membranes, with higher densities in the neonate. Our studies of [I-125]iodomelatonin-binding sites in the bird bursa indicate that this primary lymphoid tissue is a target organ for melatonin, and that melatonin has a direct effect on the immune system.