Age-related changes in 2-[125I]-iodomelatonin binding sites in the brain of sea breams (Sparus aurata, L.)

Abstract

The pineal organ of fish, through its 24h rhythmic release of melatonin, acts as a transducer of the photoperiod, influencing different physiological functions (e.g., reproduction, growth). The target sites for melatonin are poorly known in fish, especially marine species. A radioligand study was undertaken using the gilthead sea bream (Sparus aurata) maintained under natural temperature and photoperiod (at 28°N latitude). This species exhibits the property of changing sex during growth. Brains of one year-old males were collected at 16:00h and brains of three year-old females at 03:00, 10:00, 16:00 and 23:00h. Membrane homogenate receptor assays were run using 2-[(125)I]iodomelatonin as a ligand. Binding sites were detected in brains of young and old fish. In the younger, the exhibited a Bmax between 3.52 and 4.29 fmol mg protein(-1) and a KD between 358-380 pmol l(-1). In the older fish, the KD varied according to a daily pattern: values were three times higher at 03:00 and 10:00h (500-600 pmol l(-1)) than at 16:00 and 23:00h (150-300 pmol l(-1)). The number of sites also were higher at 03:00 and 10:00h (180-200 fmol mg protein(-1)) than at 16:00 and 23:00h (95-110 fmol mg protein(-1)). Melatonin and iodomelatonin displaced 2-[(125)I]iodomelatonin binding in a dose dependent manner, the second being more potent than the first. Binding was also inhibited by GTP. The results provide the first evidence for the presence of membrane melatonin binding sites in the brain of an exclusively marine fish. They suggest that their number and affinity varies during growth and throughout a light/dark cycle. Future experiments will aim to precise the anatomical location and role of these binding sites.

Effects of retinal light input on circadian rhythm genes in the yellowtail clownfish (Amphiprion clarkii) as determined using LED light spectra

“…This result agrees with various teleost studies, such as those on gilthead sea bream (Falcón et al 1996) and goldfish (Iigo et al 2003). In these studies, the teleost melatonin levels were significantly higher during the scotophase than during the photophase, similar to the results from this study.…”

Melatonin receptor of a reef fish with lunar‐related rhythmicity: cloning and daily variations

“…Analyses of Northern blot and real‐time PCR revealed a high Mel lb transcript in the brain and retina of the golden rabbitfish, suggesting that these neural tissues are primary targets of melatonin action. Daily variations of the density ( B max ) and/or affinity ( K d ) of melatonin‐binding sites under day–night cycles have been examined in the whole brain of goldfish, Carassius auratus [7, 45], gilthead seabream, Sparus aurata [6], and pike, Esox lucius [23], in which high density and affinity of melatonin‐binding sites during daytime were reported. In contrast, no daily variations of the B max and K d of melatonin‐binding sites were observed in the brain of rainbow trout, Atlantic salmon, Salmo salar , Arctic char, Salvelinus alpinus [5, 46] and European sea bass, Dicentrarchus labrax [47], although a day–night change in proportion of melatonin receptor subtypes, Mel 1a and Mel lb , was evident in the optic tectum‐thalamus of European sea bass [48].…”

“…This hormone plays an important role in regulating daily periodicities in various vertebrates [1–3]. Following transfer of melatonin through the blood, its action in peripheral tissues is mediated by activation of plasma membrane‐associated melatonin‐binding sites, ML1 and ML2, which have been identified with radioreceptor assay techniques using 2‐[ 125 I]‐iodomelatonin ([ 125 I]Mel) as a radioligand [4–7], and present widely in neural [8–10] and peripheral tissues [11–13]. ML1 shows a high affinity to [ 125 I]Mel and belongs to the family of G‐protein‐coupled receptors [14, 15], while ML2 has a low affinity to [ 125 I]Mel and is considered to be quinone reductase 2 [14, 16, 17].…”

Diurnal and circadian regulations by three melatonin receptors in the brain and retina of olive flounderParalichthys olivaceus: profiles following exogenous melatonin

“…(Sparus aurata) (Falco´n et al 1996). MT2 and MT3 were either not expressed or were expressed at very low levels in peripheral tissues such as the gill and liver.…”