Specific binding of [125I]iodomelatonin in pigeon and quail spleen membrane preparations and effect with hydrocortisone-treatment

[125I]Iodomelatonin binding sites were studied in the spleens of pigeon and quail. Scatchard analyses of the membrane preparations collected at midday (12:00) revealed an equilibrium dissociation constant (Kd) of 250 +/- 74 pmol.L-1 and a maximal number of binding sites (Bmax) of 13.4 +/- 1.8 fmol/mg protein in the pigeon spleen and Kd = 390 +/- 132 pmol.L-1, Bmax = 21.3 +/- 3.7 fmol/mg protein in the quail spleen. Circadian studies indicated that spleens collected at midnight (24:00) had 53% and 70.9% less [125I]iodomelatonin binding sites than the samples collected at midday in the pigeon and quail, respectively. Specificity studies showed only melatonin and 6-chloromelatonin had significant inhibition to [125I]iodomelatonin binding. Hydrocortisone 15 mg.kg-1 x d-1 im for 5 d selectively increased [125I] iodomelatonin binding sites in pigeon spleen. These results suggest that spleen is a site of melatonin action outside the brain.