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Melatonin Receptor‐Mediated Inhibition of Cyclic AMP Accumulation in Chick Retinal Cell Cultures
Author(s) -
Iuvone P. Michael,
Gan Jiwei
Publication year - 1994
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1046/j.1471-4159.1994.63010118.x
Subject(s) - melatonin , luzindole , forskolin , medicine , endocrinology , melatonin receptor , pertussis toxin , biology , receptor , cyclic nucleotide , stimulation , biochemistry , g protein , nucleotide , gene
Melatonin receptors were characterized in cultured neurons and photoreceptors prepared from chick embryo retina. Cultured cells contained high‐affinity 2‐[ 125 I]iodomelatonin binding sites ( K D = 41.6 p M ), similar to those in intact retina. The effects of melatonin and related indoles on cyclic AMP accumulation were examined. Melatonin (10 −7 M ) had no effect on basal or K + ‐stimulated cyclic AMP accumulation, but inhibited forskolin‐stimulated cyclic AMP accumulation by approximately 50%. Melatonin inhibited forskolin‐stimulated cyclic AMP accumulation in the presence or absence of the cyclic nucleotide phosphodiesterase inhibitor 3‐isobutyl‐1‐methylxanthine, suggesting an effect on cyclic AMP synthesis rather than degradation. Half‐maximal inhibition was observed at 5.9 × 10 −10 M melatonin. The relative order of potency among melatonin analogues was 2‐iodomelatonin > melatonin ≈ 6‐chloromelatonin ≥ 6‐hydroxymelatonin > N ‐acetylserotonin ≈ 5‐methoxytryptophol > serotonin. The EC 50 value for inhibition of cyclic AMP accumulation by 2‐iodomelatonin (36.7 p M ) was comparable to the K D value for binding of the radioligand, suggesting that the binding sites represent functional receptors. The inhibitory effect of melatonin was antagonized by the putative melatonin antagonists luzindole, N ‐acetyltryptamine, and N ‐(2,4‐dinitrophenyl)‐5‐methoxytryptamine, with estimated K B values of 0.12, 0.17, and 1 µ M , respectively. At a concentration of 10 µ M , N ‐(2,4‐dinitrophenyl)‐5‐methoxytryptamine significantly inhibited forskolin‐stimulated cyclic AMP accumulation when added alone; at 30 µ M , luzindole and N ‐acetyltryptamine also had significant inhibitory effects. The inhibitory effect of melatonin was blocked by pretreatment with pertussis toxin. The results of this study indicate that melatonin receptors on retinal cells are coupled via inhibitory G proteins to cyclic AMP accumulation. Thus, some of the effects of melatonin on retinal physiology may be related to regulation of cyclic nucleotide metabolism.