[¹²⁵I]lodomelatonin Binding Sites in the Chicken Brain: Diurnal Variation and Effects of Melatonin Injection or Pinealectomy

Abstract

The diurnal variation of [¹²⁵I]iodomelatonin binding sites in the brain and serum melatonin levels were studied at 4-hour intervals under a 12h:12h light:dark cycle in 5-week-old chicks. There was a diurnal rhythm of [¹²⁵I]iodomelatonin binding capacities in brain membrane preparations with the peak in the light period and the trough in the dark period. However, a diurnal variation with high levels in the dark and low levels in the light was observed in serum melatonin concentrations. The binding capacity of [¹²⁵I]iodomelatonin was inversely related to the serum melatonin level. Four-week-old chicks were injected with melatonin (1 mg melatonin/kg; i.p.) twice a day (shortly after the light onset and shortly before light offset) for 3 weeks. When chickens were killed at 7 weeks, there was no diurnal rhythm of melatonin in the circulation. In addition, melatonin injection also abolished the diurnal variation of [¹²⁵I]iodomelatonin binding sites in brain membrane preparations in these animals. This was achieved by decreasing the binding capacity in the light period. Pinealectomy in 2-day-old chickens increased the density of [¹²⁵I]io-domelatonin binding sites in the brain membrane preparations both at midlight and middark when the chicks were later sacrificed at 6 weeks of age, although the rhythm of binding sites persisted. It is suggested that the [¹²⁵I]iodomelatonin binding capacity in the chicken brain is regulated by endogenous melatonin and influenced by exogenous melatonin. In addition, the rhythmicity of [¹²⁵I]iodomelatonin binding sites in the chicken brain is affected by, but not dependent on, pineal melatonin.

2-[¹²⁵I]Iodomelatonin Binding in the Brains of Four Salmonids

“…Our results are consistent with earlier findings in the Atlantic salmon and goldfish [11,12]. Kj in the 4 species were in the range of 9-13 pmol/1 and considerably lower than the values in the whole brains of several spe cies of fish (K<j = 35-68 pmol/1) [11,12] and birds (Kd = 165-705 pmol/1) [16,[22][23][24][25] and the pars tuberalis of rats (K^ = 58-60 pmol/1) [17]. The number of 2-[125I]iodomelatonin binding sites (fW ) in brain membrane prepa rations from the salmonid species (10-15 fmol/mg protein) tended to be similar to the fish brains (Bmax = 7-14 fmol/mg protein) Time (min) less than 50% inhibition at 10 nmol/1 AS = Atlantic salmon; AC = Arctic charr; RT = rainbow trout; CS = coho salmon; K| = inhibition constant.…”

“…6-Chloromelatonin was donated by Eli-Lilly (Indianapolis, Ind., USA); the remainder of the chemicals were purchased from Sigma (St. Louis, Mo., USA). The brain tissue was prepared and assayed using a modified method of Yuan and Pang [16]. Briefly, the frozen tissue was weighed and homogenized in 20 vol umes (w/v) of Tris-HCl buffer (0.05 mol/1; pH 7.4; 4°C), and the homogenate was centrifuged at 44,000 g for 20 min.…”

“…The dissociation equilibrium constants (K<j) and the maximum number of binding sites (Bmax) were cal culated by Scatchard analysis, and kinetic analyses were performed. Kinetic studies were analysed as de scribed [16]. The means and standard error of means of the data were determined.…”

“…In some rodents and birds there is an inverse relationship between plasma melatonin levels and the number of melatonin binding sites in nervous tissue [15][16][17][18][19][20], This study, which was under way when the papers of Ekstrom and Vanecek [11] and Martinoli et al [12] were published, is part of an ongoing study of the characteristics of melatonin binding to several organs in 4 salmonid species. Using radioligand binding assays, we describe the iodomelatonin binding in whole-brain preparations of Arctic charr (Salvelinus alpinus), coho salmon (Oncorhynchus kisutch), rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar); for Atlantic salmon, Arctic charr and rainbow trout we also examined the possibility of a diurnal change in the number of membrane binding sites.…”

Binding of [¹²⁵I]-Labelled lodomelatonin in the Duck Thymus

“…Melatonin action on the immune system also showed a diurnal varia tion [31] with the sensitive period recorded in the late afternoon. Studies on the brain tissues have demonstrated that there are diurnal variations [32][33][34][35] in the binding capacity for [l25I]-labelled iodomelatonin with higher lev els at midlight or evening and low levels at middark or morning [32,34,35], In this study, our finding of a diurnal variation in the binding capacity of duck thymus is consistent with the findings in other tissues [32,34,35]. The diurnal variation of melatonin in the blood with peak values in the dark period is inversely related to the binding capacity for Recent studies have shown that melatonin has a modulatory effect on the immune func tion including the cellular immune system [3,[5][6][7]36], The mechanisms of melatonin ac tion on the immune system remain unclear.…”