SPECIFIC BINDING OF MELATONIN IN BOVINE BRAIN

High affinity binding of melatonin in crude membrane preparations of bovine brain was examined by a rapid filtration procedure through Whatman GFB paper. Melatonin binding to medial basal hypothalamic (MBH) membranes attained its maximum at the first, third, and fifth hours of incubation at. 37, 18, and 0 C, respectively. Specific binding was linear up to 3 mg membrane protein, was thermolabile, and decreased after incubation with trypsin; it was also pH dependent, the maximum being observed at pH 7.4. Melatonin binding was affected by a variety of ionic manipulations; it was inhibited 55% and 62% after addition of 10 mM KC1 and 120 mM NaCl, respectively, and it was increased 40% and 50% after the addition of 4 or 6 mM CaCl2. Melatonin binding was increased 25% by 1.25 mM MgCl2, whereas it was depressed at higher concentrations. Among the various brain regions studied, melatonin binding was maximal in the MBH; indole binding in occipital and cerebellar cortexes was 73% and 34% that of MBH. Subcellular fractionation studies indicated that about 70% of the binding was located in the 27, 000 × g pellet. Scatchard analysis revealed a single population of binding sites with a Kd value of 1.2 ± 0.4 × 10-8M in three successive experiments; binding site concentration ranged from 8-14 fmol/mg protein. When various indole analogs were tested for their ability to inhibit [3H]-melatonin binding at different concentrations, the following half-maximal inhibition values were obtained: melatonin, 20 nM; 5-methoxytryptophol, 80 nM; 5-methoxyindoleacetic acid, 100 nM; serotonin, 160 nM; 5-hydroxytryptophol, 200 nM; 5-methoxytryptamine, 250 nM; Nacetylserotonin, 250 nM; tryptamine, 250 nM; 2-methyl indole, 1, 500 nM; 5-hydroxytryptophan, 1, 600 nM; 5-hydroxyindoleacetic acid, 2, 000 nM; 6-hydroxymelatonin, >10, 000 nM; and indomethacin, > 10, 000 nM. These results suggest that melatonin receptors are present in the brain. © 1979 by The Endocrine Society.