Abstract
Several evidences supported the existence of melatonin effects on reproductive system in fish. In order to investigate whether melatonin is involved in the modulation of GnRH systems in the European sea bass, we have injected melatonin (0.5 μg/g body mass) in male specimens. The brain mRNA transcript levels of the three GnRH forms and the five GnRH receptors present in this species were determined by real time quantitative PCR. Our findings revealed day–night variations in the brain expression of GnRH-1, GnRH-3 and several GnRH receptors (dlGnRHR-II-1c, -2a), which exhibited higher transcript levels at mid-light compared to mid-dark phase of the photocycle. Moreover, an inhibitory effect of melatonin on the nocturnal expression of GnRH-1, GnRH-3, and GnRH receptors subtypes 1c, 2a and 2b was also demonstrated. Interestingly, the inhibitory effect of melatonin affected the expression of hypophysiotrophic GnRH forms and GnRH receptors that exhibit day–night fluctuations, suggesting that exogenous melatonin reinforce physiological mechanisms already established. These interactions between melatoninergic and GnRH systems could be mediating photoperiod effects on reproductive and other rhythmic physiological events in the European sea bass.
Highlights
Melatonin (N-acetyl-5-methoxytryptamine) is an indolamine mainly synthesized by the pineal organ that is involved in the synchronization of many rhythmic physiological processes to the environmental cues [1,2,3,4]
Our quantitative real-time PCR analysis in sea bass brain revealed the existence of day–night variations in the expression of gonadotrophin-releasing hormone (GnRH)-1 and
We have reported the existence of day–night variations as well as melatonin inhibitory effects in the brain expression of GnRH-1, GnRH-3 and several GnRH receptors in the European sea bass
Summary
Melatonin (N-acetyl-5-methoxytryptamine) is an indolamine mainly synthesized by the pineal organ that is involved in the synchronization of many rhythmic physiological processes to the environmental cues [1,2,3,4]. Many evidences have shown a role of melatonin in fish reproduction, by acting at different levels of the reproductive axis. Melatonin has stimulatory effects on brain dopaminergic system in eel [8]. In the Atlantic croaker melatonin treatment increases luteinizing hormone levels, by activating the preoptic/hypothalamic areas and the pituitary gland [9]. Further evidences showing the pituitary gland as a target for melatonin actions are the presence of hypophysary melatonin receptors and/or actions in several fish species, including sea bass [10,11,12,13]. Melatonin binding sites and/or receptors were present in the gonads of sea bream [5], carp Catla catla [14], sea bass [15]
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