Current knowledge on the melatonin system in teleost fish

Abstract

Melatonin is a much conserved feature in vertebrates that plays a central role in the entrainment of daily and annual physiological rhythms. Investigations aiming at understanding how melatonin mediates the effects of photoperiod on crucial functions and behaviors have been very active in the last decades, particularly in mammals. In fish a clear-cut picture is still missing. Here we review the available data on (i) the sites of melatonin production in fish, (ii) the mechanisms that control its daily and annual rhythms of production and (iii) the characterization of its different receptor subtypes, their location and regulation. The in vivo and in vitro data on melatonin effects on crucial neuroendocrine regulations, including reproduction, growth, feeding and behavioral responses, are also reviewed. Finally we discuss how manipulation of the photic cues impact on fish circannual clock and annual cycle of reproduction, and how this can be used for aquaculture purposes.

Interplay between the endocrine and circadian systems in fishes

“…Plasma melatonin levels show a characteristic nocturnal peak in all studied species, which is an exceptional feature compared with other rhythmic hormones, whose daily profiles are more variable through the 24-h cycle depending on different factors, such as the active/rest phase and the feeding time, among others. Melatonin nocturnal rhythmic profiles are classified into three types: a discrete peak in the late dark phase, a discrete peak in the mid-dark phase and an expanded peak throughout the dark phase (reviewed in Falcón et al 2010). It is currently unclear whether these different profiles have functional implications.…”

“…1). In fish, pineal melatonin is acting on some diencephalic areas of the hypothalamus-pituitary neuroendocrine axis (Ekström & Vanecek 1992), particularly the rostral preoptic area, lateral tuberal nucleus, ventromedial thalamic nucleus and the anterior periventricular nucleus, the putative homologous to the mammalian SCN (reviewed in Falcón et al 2010). Retinal melatonin mainly has a local role, where it governs some of the daily rhythms, such as retinomotor movements (Zaunreiter et al 1998) or the modulation of neurotransmitter release (Ribelayga et al 2004).…”

“…The dependence of melatonin, pituitary hormone and glucocorticoid rhythms on the LD cycle has been studied for decades in mammals (Pevet & Challet 2011, Kalsbeek et al 2012, Lin et al 2015 and fishes (Falcón et al 2007(Falcón et al , 2010. However, it is known that many other hormones and neuropeptides (orexin, ghrelin, leptin and insulin) also exhibit daily oscillations (Patton & Mistlberger 2013, Tinoco et al 2014, Challet 2015, Sánchez-Bretaño et al 2015a, Schibler et al 2015.…”

“…We do not discuss other important endocrine rhythms in fishes, such as the reproductive hormones, because they have been reviewed previously (Bromage et al 1990, Falcón et al 2010, The fish circadian system: a net of circadian oscillators. The fish circadian system is composed of a net of oscillators that are widely distributed throughout the entire organism.…”

Changes in Sex Steroids, Growth Hormone, and Insulin‐Like Growth Factor‐I during Ovarian Development in Rainbow Trout Cultured in a Water Recirculating System with Continuous Light

“…This includes light provided 24-h to fish maintained outdoors (Taylor et al 2006). Previous studies have shown nighttime light levels as low as 10 lx can affect plasma melatonin in Rainbow Trout (Alvarino et al 1993;Bromage et al 2001), and spectrum and intensity matters to reproduction, growth, and stress response (Karakatsouli et al 2008;Falcón et al 2010). We recorded a minimum of 38 lx in our tanks and at least 10-fold changes in light intensity during the course of the day, which surely also changed in intensity with seasonal changes in ambient light intensities.…”