Melatonin Activates Brain Dopaminergic Systems in the Eel with an Inhibitory Impact on Reproductive Function

Abstract

In the eel, a deficit in gonadotrophin-releasing hormone (GnRH) and a strong dopaminergic (DA) inhibition are responsible for the blockade of gonad development if silver eels are prevented from their reproductive migration. Environmental factors that eels encounter during their oceanic reproductive migration are thought to play an important role in the stimulation of eel pubertal development. We investigated the potential role of melatonin, a known mediator of the effects of external factors on reproductive function in vertebrates. We demonstrated that a long-term melatonin treatment increased brain tyrosine hydroxylase (TH, the rate limiting enzyme of DA synthesis) mRNA expression in a region-dependent way. Melatonin stimulated the dopaminergic system of the preoptic area, which is involved in the inhibitory control of gonadotrophin [luteinising hormone (LH) and follicle-stimulating hormone (FSH)] synthesis and release. Moreover, we showed that the increased TH expression appeared to be consistent with melatonin binding site distribution as shown by 2[(125)I]-melatonin labelling studies. On the other hand, melatonin had no effects on the two eel native forms of GnRH (mGnRH and cGnRH-II) mRNA expression. Concerning the pituitary-gonad axis, we showed that melatonin treatment decreased both gonadotrophin beta-subunit (LHbeta, FSHbeta) mRNA expression and reduced sexual steroid (11-ketotestosterone, oestradiol) plasma levels. This indicates that melatonin treatment had a negative effect on eel reproductive function. To our knowledge, the results of the present study provide the first evidence that melatonin enhances TH expression in specific brain regions in a non-mammalian species. By this mechanism melatonin could represent one pathway by which environmental factors could modulate reproductive function in the eel.

Interplay between the endocrine and circadian systems in fishes

“…However, fish do not possess such an anatomically distinct pars tuberalis in the pituitary gland, and the involvement of melatonin in the fish reproductive cycle 232:3 is only supported by a few studies. However, effects of melatonin have been described at the hypothalamic (Popek et al 2005, Alvarado et al 2015, pituitary (Khan & Thomas 1996, Sébert et al 2008 and gonadal (Chattoraj et al 2005) levels.…”

Melatonin Inhibits GnRH-1, GnRH-3 and GnRH Receptor Expression in the Brain of the European Sea Bass, Dicentrarchus labrax

“…Thus, melatonin has stimulatory effects on brain dopaminergic system in eel [8]. Moreover, in the Atlantic croaker melatonin treatment increases luteinizing hormone levels, by activating the preoptic/hypothalamic areas and the pituitary gland [9].…”

Effects of melatonin on histomorphology and on the expression of steroid receptors, VEGF, and PCNA in ovaries of pinealectomized female rats

“…Melatonin acts on the reproductive system by acting on GnRH release (7,8) and thereby on gonadotropins (9). Some authors (10,11) have shown evidence of melatonin's direct action on the ovaries.…”