High Melatonin Conditions by Constant Darkness and High Temperature Differently Affect Melatonin Receptormt1and TREK Channeltrek2ain the Brain of Zebrafish

Abstract

Ambient light and temperature affect reproductive function by regulating kisspeptin and gonadotrophin-releasing hormone (GnRH) in vertebrates. Melatonin and melatonin receptors, as well as the two-pore domain K channel-related K (TREK) channels, are affected by light and/or temperature; therefore, these molecules could modulate kisspeptin and GnRH against ambient light and temperature. In this study, we investigated the effect of light and temperature, which affect melatonin levels in gene expression levels of TREK channels, kisspeptin, and GnRH. We first investigated the effects of different light and temperature conditions on brain melatonin concentrations by ELISA. Fish were exposed to either constant darkness, constant light, high temperature (35°C), or low temperature (20°C) for 72 h. Brain melatonin levels were significantly high under constant darkness and high temperature. We further investigated the effects of high brain melatonin levels by constant darkness and high temperature on gene expression levels of melatonin receptors (mt1, mt2, and mel1c), TREK channels (trek1b, trek2a, and trek2b), gnrh3, and kiss2 in the adult zebrafish brain by real-time polymerase chain reaction. Fish were exposed to constant darkness or elevated temperatures (35°C) for 72 h. trek2a, kiss2, and gnrh3 levels were increased under constant darkness. High temperature decreased gene expression levels of mt1, mt2, mel1c, and gnrh3 in the preoptic area, whereas other genes remained unchanged. Melatonin receptors, TREK channels, gnrh3, and kiss2 responded differently under high melatonin conditions. The melatonin receptors and the TREK channels could play roles in the regulation of reproduction by environmental cues, especially ambient light and temperature.

Melatonin and osmoregulation in fish: A focus on Atlantic salmon Salmo salar smoltification

“…Binding site abundance is high during the day and low at night, whereas the oscillations in mRNA display peaks at different times of the LD cycle, depending on the species and tissue investigated. These variations are maintained under constant light (LL) or dark (DD) conditions in most, but not all, tissues, 149,150,152 although the levels of expression may be modified 133,149 . A pineal factor, possibly melatonin, contributes to regulating these rhythms as they are abolished in pinealectomised goldfish C. auratus 150 .…”

“…Finally, in the zebrafish ( Danio rerio ), the abundance of brain MT1, MT2 and Mel1c mRNA was affected by high temperatures. The effects depend on the receptor and brain areas considered, being particularly marked in the hypothalamus 133 …”

“…Duplicates of some of these receptors are found in some fish species,121,[123][124][125][126] probably as a consequence of the gains and losses resulting from the teleost specific and salmonid specific whole genome duplications 127. As expected, the receptors display distinct binding characteristics for melatonin or melatonin analogues (e.g., O. latipes122 and S. salar 128 ) when tested in heterologous cell lines.One remarkable feature of melatonin receptors is their wide distribution in central and peripheral tissues [122][123][124][125][128][129][130][131][132][133][134][135][136][137][138][139][140][141][142]. Nervous tissues expressing these receptors include the brain, the retina and the pineal gland itself.…”

“…Almost all brain areas express one or another melatonin receptor subtype, and a good correlation exists between the information provided by mRNA localisation and the 2-[ 125 I]-iodomelatonin ([ 125 I]-Mel) binding-sites 126,132. The receptors are particularly abundant in the thalamic area and, to a lesser degree, in the optic tectum and cerebellum, associated with areas involved in sensory information (olfactory, auditory and visual), locomotor activity (cerebellum) or neuroendocrine regulation (including the pituitary gland) 118,128,130,[132][133][134]. Interestingly, some of the areas in the basal hypothalamus (e.g., the suprachiasmatic nuclei [SCN] and preoptic area [POA]) also receive additional photoperiodic information via nerve endings from both the pineal gland and the retina 144.…”

“…133,149 A pineal factor, possibly melatonin, contributes to regulating these rhythms as they are abolished in pinealectomised goldfish C. auratus 150. However, a more complex set of factors might be involved(e.g., in E. lucius brain membrane preparations, the circadian variations of [ 125 I]-Mel binding were maintained under constant LL, a condition that suppresses melatonin production).…”