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The melatonin-binding sites in membrane preparations of the bursa of Fabricius of birds were studied using [¹²⁵I]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/l 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 [¹²⁵I]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 [¹²⁵I]iodomelatonin-binding sites of chicken bursal membranes had the following order of pharmacological affinities: 2-iodomelatonin > melatonin > 6-chloromelatonin > 6-hydroxymelatonin > N-acetylserotonin > 5-hydroxytryptamine, tryptamine, 5-methoxytryptophol, 1-acetylindole-3-carboxaldehyde, 3-acetylindole, L-tryptophan, 5-hydroxyindole3-acetic acid, 5-hydroxytryptophan suggesting that the [¹²⁵I]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 agerelated decrease in [¹²⁵I]iodomelatonin-binding in chicken bursal membranes, with higher densities in the neonate. Our studies of [¹²⁵I]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.

Journal of Endocrinology (1993) 138, 51–57