Cloning and retinal expression of melatonin receptors in the European sea bass, Dicentrarchus labrax

Abstract

Melatonin contributes to synchronizing behaviors and physiological functions to daily and seasonal rhythm in fish. However, no coherent vision emerges because the effects vary with the species, sex, age, moment of the year or sexual cycle. And, scarce information is available concerning the melatonin receptors, which is crucial to our understanding of the role melatonin plays. We report here the full length cloning of three different melatonin receptor subtypes in the sea bass Dicentrarchus labrax, belonging, respectively, to the MT1, MT2 and Mel1c subtypes. MT1, the most abundantly expressed, was detected in the central nervous system, retina, and gills. MT2 was detected in the pituitary gland, blood cells and, to a lesser extend, in the optic tectum, diencephalon, liver and retina. Mel1c was mainly expressed in the skin; traces were found in the retina. The cellular sites of MT1 and MT2 expressions were investigated by in situ hybridization in the retina of pigmented and albino fish. The strongest signals were obtained with the MT1 riboprobes. Expression was seen in cells also known to express the enzymes of the melatonin biosynthesis, i.e., in the photoreceptor, inner nuclear and ganglion cell layers. MT1 receptor mRNAs were also abundant in the retinal pigment epithelium. The results are consistent with the idea that melatonin is an autocrine (neural retina) and paracrine (retinal pigment epithelium) regulator of retinal function. The molecular tools provided here will be of valuable interest to further investigate the targets and role of melatonin in nervous and peripheral tissues of fish.

The pineal complex of the European sea bass (Dicentrarchus labrax): I. Histological, immunohistochemical and qPCR study

“…This species has special interest for chronobiological studies because of the dual phasing capacity (diurnal/nocturnal) of its daily feeding rhythms, which vary according to the time of the year (Sá nchez-Vá zquez et al, 1995). The sea bass has also been used as a basic model for characterising the function of the pineal organ, its melatonin secretion, regulation by environmental factors and targets , 1997;Iigo et al, 1997;García-Allegue et al, 2001;Bayarri et al, 2002Bayarri et al, , 2003Bayarri et al, , 2004aMigaud et al, 2006Migaud et al, , 2007Sauzet et al, 2008;Herrera-Pé rez et al, 2010;Servili et al, 2010). In the present study, we extend our knowledge on the sea bass circadian system by providing information on the anatomical characteristics of its pineal organ, localising its photosensitive and presumed melatonin-secreting cells and determining daily rhythms in the expression of a key melatonin-synthesising enzyme.…”

Tryptophan hydroxylase and serotonin receptor 1A expression in the retina of the sea lamprey

“…With regards to HIOMT, its activity was reported in the lamprey pineal organ (Joss, 1977;Guerlott e et al, 1986), but, as far as we are aware, it has not been mentioned in the retina. In some teleosts, melatonin receptors have been cloned and they are expressed in all photoreceptors and also in cells of the inner nuclear and ganglion cell layers (Sauzet et al, 2008). Melatonin has been proposed to have a local paracrine function in the retina related to photic adaptation (revised in Cahill and Besharse, 1995), but its function in the lamprey retina needs to be investigated.…”

Interplay between the endocrine and circadian systems in fishes

“…These receptors are widely distributed in the fish central nervous system and peripheral tissues (Park et al 2006, Ikegami et al 2008, Sauzet et al 2008, allowing the melatonin circadian message to be broadcast throughout the organism (Fig. 1).…”