Melatonin receptors in Atlantic salmon stimulate cAMP levels in heterologous cell lines and show season‐dependent daily variations in pituitary expression levels

Abstract

The hormone melatonin connects environmental cues, such as photoperiod and temperature, with a number of physiological and behavioural processes, including seasonal reproduction, through binding to their cognate receptors. This study reports the structural, functional and physiological characterization of five high‐affinity melatonin receptors (Mtnr1aaα, Mtnr1aaβ, Mtnr1ab, Mtnr1al, Mtnr1b) in Atlantic salmon. Phylogenetic analysis clustered salmon melatonin receptors into three monophyletic groups, Mtnr1A, Mtnr1Al and Mtnr1B, but no functional representative of the Mtnr1C group. Contrary to previous studies in vertebrates, pharmacological characterization of four receptors in COS‐7, CHO and SH‐SY5Y cell lines (Mtnr1Aaα, Mtnr1Aaβ, Mtnr1Ab, Mtnr1B) showed induction of intracellular cAMP levels following 2‐iodomelatonin or melatonin exposure. No consistent response was measured after N‐acetyl‐serotonin or serotonin exposure. Melatonin receptor genes were expressed at all levels of the hypothalamo‐pituitary‐gonad axis, with three genes (mtnr1aaβ, mtnr1ab and mtnr1b) detected in the pituitary. Pituitary receptors displayed daily fluctuations in mRNA levels during spring, prior to the onset of gonadal maturation, but not in autumn, strongly implying a direct involvement of melatonin in seasonal processes regulated by the pituitary. To the best of our knowledge, this is the first report of cAMP induction mediated via melatonin receptors in a teleost species.

New Insights Into the Evolutionary History of Melatonin Receptors in Vertebrates, With Particular Focus on Teleosts

“…In addition, light-dark cycles entrained the daily rhythms of oviposition and courtship in female medaka (91). Tissue distributions have been reported for the four receptor subtypes in several teleosts (7,10,15,92,93). In teleost fish, studies by semi-quantitative RT-PCR showed that the four melatonin receptors are widely expressed and show overlapping expression patterns.…”

“…The expression levels of three of the four melatonin receptors changed between day and night in medaka. We found different sets of mtnr genes expressed in the integument of teleost species: mtnr1b and mtnr1d in sole (Solea senegalensis) (94), mtnr1a and mtnr1d in mudskipper (Boleophthalmus pectinirostris) (95), mtnr1aaα, mtnr1aaβ, mtnr1ab, and mtnt1b in Atlantic salmon (10) and mtnr1b, mtnr1c, and mtnr1d in goldfish (8). Melatonin induces hypopigmentation in several ostariophysian species, by stimulating aggregation of melanophores partially through stimulation of Mtnr1a and Mtnr1b (111)(112)(113).…”

“…Previous studies have shown that mtnr gene expression varies depending on the time of day and photoperiod (10). It shows the importance of considering the photoperiod regime and the time of day when comparing expression profiles of mtnr and other daily and seasonal oscillator genes.…”

“…Recently, it was shown that the additional teleost mtnr1a-like gene is also present in some tetrapods (mtnr1a2 or mtnr1a1. 4) and consequently constitutes a fourth MTNR subtype named MTNR1D or MTNR1A-like (7)(8)(9)(10)(11)(12). The four MTNR monophyletic groups presumably derive from the two rounds of vertebrate tetraploidization early in vertebrate evolution (1R and 2R) (10)(11)(12).…”

“…4) and consequently constitutes a fourth MTNR subtype named MTNR1D or MTNR1A-like (7)(8)(9)(10)(11)(12). The four MTNR monophyletic groups presumably derive from the two rounds of vertebrate tetraploidization early in vertebrate evolution (1R and 2R) (10)(11)(12). Sakai et al reported that teleost species possess the four vertebrate MTNR subtypes but did not observe any further expansion of the mtnr gene repertoire in teleosts (11).…”