Analyses of signal transduction cascades reveal an essential role of calcium ions for regulation of melatonin biosynthesis in the light-sensitive pineal organ of the rainbow trout (Oncorhynchus mykiss)

Screening of Differentially Expressed Genes and miRNAs in Hypothalamus and Pituitary Gland of Sheep under Different Photoperiods

The reproduction of sheep is affected by many factors such as light, nutrition and genetics. The Hypothalamic-pituitary-gonadal (HPG) axis is an important pathway for sheep reproduction, and changes in HPG axis-related gene expression can affect sheep reproduction. In this study, a model of bilateral ovarian removal and estrogen supplementation (OVX + E₂) was applied to screen differentially expressed genes and miRNAs under different photoperiods using whole transcriptome sequencing and reveal the regulatory effects of the photoperiod on the upstream tissues of the HPG axis in sheep. Whole transcriptome sequencing was performed in ewe hypothalamus (HYP) and distal pituitary (PD) tissues under short photoperiod 21st day (SP21) and long photoperiod 21st day (LP21). Compared to the short photoperiod, a total of 1813 differential genes (up-regulation 966 and down-regulation 847) and 145 differential miRNAs (up-regulation 73 and down-regulation 72) were identified in the hypothalamus of long photoperiod group. Similarly, 2492 differential genes (up-regulation 1829 and down-regulation 663) and 59 differential miRNAs (up-regulation 49 and down-regulation 10) were identified in the pituitary of long photoperiod group. Subsequently, GO and KEGG enrichment analysis revealed that the differential genes and target genes of differential miRNA were enriched in GnRH, Wnt, ErbB and circadian rhythm pathways associated with reproduction. Combined with sequence complementation and gene expression correlation analysis, several miRNA-mRNA target combinations (e.g., LHB regulated by novel-414) were obtained. Taken together, these results will help to understand the regulatory effect of the photoperiod on the upstream tissues of HPG in sheep.

Melatonin synthesis and clock gene regulation in the pineal organ of teleost fish compared to mammals: Similarities and differences

The pineal organ of all vertebrates synthesizes and secretes melatonin in a rhythmic manner due to the circadian rhythm in the activity of arylalkylamine N-acetyltransferase (AANAT) - the rate-limiting enzyme in melatonin synthesis pathway. Nighttime increase in AANAT activity and melatonin synthesis depends on increased expression of aanat gene (a clock-controlled gene) and/or post-translation modification of AANAT protein. In mammalian and avian species, only one aanat gene is expressed. However, three aanat genes (aanat1a, aanat1b, and aanat2) are reported in fish species. While aanat1a and aanat1b genes are expressed in the fish retina, the nervous system and other peripheral tissues, aanat2 gene is expressed exclusively in the fish pineal organ. Clock genes form molecular components of the clockwork, which regulates clock-controlled genes like aanat gene. All core clock genes (i.e., clock, bmal1, per1, per2, per3, cry1 and cry2) and aanat2 gene (a clock-controlled gene) are expressed in the pineal organ of several fish species. There is a large body of information on regulation of clock genes, aanat gene and melatonin synthesis in the mammalian pineal gland. However, the information available on clock genes, aanat genes and melatonin synthesis in photoreceptive pineal organ of teleosts is fragmentary and not well documented. Therefore, we have reviewed published information on rhythmic expression of clock genes, aanat genes as well as synthesis of melatonin, and their regulation by photoperiod and temperature in teleostean pineal organ as compared to mammalian pineal gland. A critical analysis of the literature suggests that in contrast to the mammalian pineal gland, the pineal organ of teleosts (except salmonids) possesses a well developed indigenous clock composed of clock genes for regulation of rhythmic expression of aanat2 gene and melatonin synthesis. Further, the fish pineal organ also possesses essential molecular components for responding to light and temperature directly. The fish pineal organ seems to act as a potential master biological clock in most of the teleosts.

The hormonal Zeitgeber melatonin: role as a circadian modulator in memory processing

The neuroendocrine substance melatonin is a hormone synthesized rhythmically by the pineal gland under the influence of the circadian system and alternating light/dark cycles. Melatonin has been shown to have broad applications, and consequently becoming a molecule of great controversy. Undoubtedly, however, melatonin plays an important role as a time cue for the endogenous circadian system. This review focuses on melatonin as a regulator in the circadian modulation of memory processing. Memory processes (acquisition, consolidation, and retrieval) are modulated by the circadian system. However, the mechanism by which the biological clock is rhythmically influencing cognitive processes remains unknown. We also discuss, how the circadian system by generating cycling melatonin levels can implant information about daytime into memory processing, depicted as day and nighttime differences in acquisition, memory consolidation and/or retrieval.

Neurofibromatosis-1 heterozygosity impairs CNS neuronal morphology in a cAMP/PKA/ROCK-dependent manner

Children with the neurofibromatosis-1 (NF1) cancer predisposition syndrome exhibit numerous clinical problems that reflect defective central nervous system (CNS) neuronal function, including learning disabilities, attention deficit disorder, and seizures. These clinical features result from reduced NF1 protein (neurofibromin) expression in NF1+/- (NF1 heterozygosity) brain neurons. Previous studies have shown that mouse CNS neurons are sensitive to the effects of reduced Nf1 expression and exhibit shorter neurite lengths, smaller growth cone areas, and attenuated survival, reflecting attenuated neurofibromin cAMP regulation. In striking contrast, Nf1+/- peripheral nervous system (PNS) neurons are nearly indistinguishable from their wild-type counterparts, and complete neurofibromin loss leads to increased neurite lengths and survival in a RAS/Akt-dependent fashion. To gain insights into the differential responses of CNS and PNS neurons to reduced neurofibromin function, we designed a series of experiments to define the molecular mechanism(s) underlying the unique CNS neuronal sensitivity to Nf1 heterozygosity. First, Nf1 heterozygosity decreases cAMP levels in CNS, but not in PNS, neurons. Second, CNS neurons exhibit Nf1 gene-dependent increases in RAS pathway signaling, but no further decreases in cAMP levels were observed in Nf1-/- CNS neurons relative to their Nf1+/- counterparts. Third, neurofibromin regulates CNS neurite length and growth cone areas in a cAMP/PKA/Rho/ROCK-dependent manner in vitro and in vivo. Collectively, these findings establish cAMP/PKA/Rho/ROCK signaling as the responsible axis underlying abnormal Nf1+/- CNS neuronal morphology with important implications for future preclinical and clinical studies aimed at improving cognitive and behavioral deficits in mice and children with reduced brain neuronal NF1 gene expression.

Amplitude-encoded calcium oscillations in fish cells

The reaction of intracellular Ca(2+) to different agonist stimuli in primary hepatocytes from rainbow trout (Oncorhynchus mykiss) as well as the permanent fish cell line RTL-W1 was investigated systematically. In addition to "classical" agonists such as phenylephrine and ATP, model environmental toxicants like 4-nitrophenol and 3,4-dichloroaniline were used to elucidate possible interactions between toxic effects and Ca(2+) signaling. We report Ca(2+) oscillations in response to several stimuli in RTL-W1 cells and to a lesser extent in primary hepatocytes. Moreover, these Ca(2+) oscillations are amplitude-encoded in contrast to their mammalian counterpart. Bioinformatics and computational analysis were employed to identify key players of Ca(2+) signaling in fish and to determine likely causes for the experimentally observed differences between the Ca(2+) dynamics in fish cells compared to those in mammalian liver cells.

Neural regulation of dark-induced abundance of arylalkylamine N-acetyltransferase (AANAT) and melatonin in the carp (Catla catla) pineal: an in vitro study

In all the vertebrates, synthesis of melatonin and its rhythm-generating enzyme arylalkylamine N-acetyltransferase (AANAT) reaches its peak in the pineal during the night in a daily light-dark cycle, but the role of different neuronal signals in their regulation were unknown for any fish. Hence, the authors used specific agonist and antagonists of receptors for different neuronal signals and regulators of intracellular calcium (Ca(2+)) and adenosine 3',5'-cyclic monophosphate (cAMP) in vitro to study their effects on the abundance of AANAT and titer of melatonin in the carp (Catla catla) pineal. Western blot analysis followed by quantitative analysis of respective immunoblot data for AANAT protein, radioimmunoassay of melatonin, and spectrophotometric analysis of Ca(2+) in the pineal revealed stimulatory effects of both adrenergic (α(1) and β(1)) and dopaminergic (D(1)) agonists and cholinergic (both nicotinic and muscarinic) antagonists, inhibition by both adrenergic and dopaminergic antagonists and cholinergic agonists, but independent of the influence of any agonists or antagonists of α(2)-adrenergic receptors. Band intensity of AANAT and concentration of melatonin in the pineal were also enhanced by the intracellular calcium-releasing agent, activators of both calcium channel and adenylate cyclase, and phophodiesterase inhibitor, but suppressed by inhibitor of calcium channel and adenylate cyclase as well as activator of phophodiesterase. Moreover, an inhibitory effect of light on the pineal AANAT and melatonin was blocked by both cAMP and proteasomal proteolysis inhibitor MG132. Collectively, these data suggest that dark-induced abundance of AANAT and melatonin synthesis in the carp pineal are a multineuronal function, in which both adrenergic (α(1) and β(1), but not α(2)) and dopaminergic signals are stimulatory, whereas cholinergic signals are inhibitory. This study also provides indications, though arguably not conclusive evidence, that in either case the neuronal mechanisms follow a signal-transduction pathway in which Ca(2+) and cAMP may act as the intracellular messengers. It also appears that proteasomal proteolysis is a conserved event in the regulation of AANAT activity in vertebrates.

Changes in plasma melatonin levels and pineal organ melatonin synthesis following acclimation of rainbow trout (Oncorhynchus mykiss) to different water salinities

Melatonin has been suggested to play a role in fish osmoregulation, and in salmonids has been related to the timing of adaptive mechanisms during smolting. It has been described that acclimation to different environmental salinities alters levels of circulating melatonin in a number of fish species, including rainbow trout. However, nothing is known regarding salinity effects on melatonin synthesis in the pineal organ, which is the main source of rhythmically produced and secreted melatonin in blood. In the present study we have evaluated, in rainbow trout, the effects of acclimation to different salinities on day and night plasma melatonin values and pineal organ melatonin synthesis. Groups of freshwater (FW)-adapted rainbow trout were placed in tanks with four different levels of water salinity (FW, 6, 12, 18 p.p.t.; parts per thousand) and maintained for 6 h or 5 days. Melatonin content in plasma and pineal organs, as well as the pineal content of serotonin (5-HT) and its main oxidative metabolite (5-hydroxyindole-3-acetic acid; 5-HIAA) were measured by high performance liquid chromatography. In addition, day–night changes in pineal organ arylalkylamine N-acetyltransferase (AANAT2) activity and aanat2 gene expression were studied. Plasma osmolalities were found to be higher in rainbow trout exposed to all salinity levels compared with the control FW groups. A salinity-dependent increase in melatonin content was found in both plasma and pineal organs. This effect was observed during the night, and was related to an increase in aanat2 mRNA abundance and AANAT2 enzyme activity, both of which also occurred during the day. Also, the levels of indoles (5-HT, 5-HIAA) in the pineal organ were negatively affected by increasing water salinity, which seems to be related to the higher recruitment of 5-HT as a substrate for the increased melatonin synthesis. A stimulatory effect of salinity on pineal aanat2 mRNA expression was also identified. These results indicate that increased external salinity promotes melatonin synthesis in the pineal organ of rainbow trout by enhancing synthesis of AANAT protein independently of its regulation by light. The possibility that pineal melatonin is a target for hormones involved in the response of fish to osmotic challenge is discussed, as well as the potential role of melatonin in the timing of osmoregulatory processes.

Importance of light in temporal organization of photoreceptor proteins and melatonin-producing system in the pineal of carp Catla catla

The importance of light in the temporal organization of photoreceptor proteins and melatonin-producing system has been investigated for the first time in the pineal of a tropical fish. In this study, an identical experimental paradigm was followed during the four distinct phases of an annual cycle in adult carps (Catla catla) maintained either under natural photoperiod (NP) or continuous illumination (LL) or darkness (DD) for 30 days. At the end of each experiment, the pineal from fish in each experimental group was collected either at 06:00, 12:00, 18:00, or 24:00 in a daily cycle and assessed by Western blot analysis for pineal rod-like opsin, alpha-transducin, and AANAT. The same animals were also used for measurement of serum melatonin levels, and the serum as well as intra-pineal Ca(++) levels at each timepoint. The study revealed a daily rhythmicity with a peak at 12:00 h and nadir at 24:00 h in the band intensity of pineal rod-like opsin and alpha-transducin in NP fish, while the band intensities of these photo-pigment proteins remained high under LL and low under DD, irrespective of clock hour during the 24 h cycle. The band intensity of pineal AANAT, levels of serum melatonin, and both serum Ca(++) and intra-pineal Ca(++) were maximum at 24:00 h and minimum at 12:00h in NP fish, and they were significantly lower under LL and higher under DD at each point of study. The results showed loss of daily rhythm in each studied variable in both LL and DD carps, suggesting that their circadian organization is dependent on the external light-dark conditions, rather than an endogenous circadian oscillator in the pineal.

Neuronal regulation of photo-induced pineal photoreceptor proteins in carp Catla catla

In the present in vitro study on the pineal in carp Catla catla, specific agonist and antagonists of receptors for different neuronal signals and regulators of intra-cellular Ca(++) and cAMP were used to gather basic information on the neuronal signal transduction cascade mechanisms in the photo-induced expression of rod-like opsin and alpha-transducin-like proteins in any fish pineal. Western-blot analysis followed by quantitative analysis of respective immunoblot data for both the proteins revealed that photo-induced expression of each protein was stimulated by cholinergic (both nicotinic and muscarinic) agonists and a dopaminergic antagonist, inhibited by both cholinergic antagonists and a dopaminergic agonist, but not affected by any agonists or antagonists of adrenergic (alpha(1), alpha(2) and beta(1)) receptors. Moreover, expression of each protein was stimulated by voltage gated L type calcium channel blocker, adenylate cyclase inhibitor and phosphodiesterase activator; but suppressed by the activators of both calcium channel and adenylate cyclase, and by phosphodiesterase inhibitor. Collectively, we report for the first time that both cholinergic and dopaminergic signals play an important, though antagonistic, role in the photo-induced expression of photoreceptor proteins in the fish pineal through activation of a signal transduction pathway in which both calcium and cAMP may act as the intracellular messengers.

Effects of naphthalene, beta-naphthoflavone and benzo(a)pyrene on the diurnal and nocturnal indoleamine metabolism and melatonin content in the pineal organ of rainbow trout, Oncorhynchus mykiss

Polycyclic aromatic hydrocarbons (PAHs) have deleterious effects on neuroendocrine systems in teleost fish affecting, among other processes, reproductive function or stress responses. The hormone melatonin, mainly produced in the pineal organ of vertebrates, is involved in the regulation of biological rhythms as well as other important functions, and may also act as an antioxidant molecule. The effects of environmental pollutants on the endocrine and metabolic activity of the pineal organ have been studied only in mammals. We here evaluate the effects of the PAHs naphthalene (NAP) and benzo(a)pyrene (BaP) and the flavonoid beta-naphthoflavone (BNF) on the pineal organ of rainbow trout by quantifying the diurnal and nocturnal pineal content of some indoles and methoxyindoles, including melatonin. NAP mainly induced diurnal increases in the pineal content of melatonin and other methoxyindoles like 5-methoxytryptamine (5-MT), 5-methoxyindole-3-acetic acid (5-MIAA) or 5-methoxytryptophol (5-MTOL). Those increases did not occur at night, when even occasional decreases were observed compared with controls. NAP also induced some diurnal and nocturnal decreases in the levels of indolic compounds like serotonin (5-HT) and 5-hydroxyindole-3-acetic acid (5-HIAA), while pineal content of 5-hydroxytryptophan (5-HTP) was first decreased (few hours after injection) and then increased (few days after injection) during the day. BaP and BNF induced strong increases in diurnal levels of melatonin, whereas other pineal compounds were unaffected. It seems that an increase of the methylation capacity of the pineal organ takes place during the day, and a decrease occurs at night. Those effects could be mediated by changes in the activity of key enzymes involved in pineal melatonin biosynthesis, maybe as a result of the alteration of the cellular phototransduction mechanisms involved in the light-induced inhibition of melatonin synthesis in the pineal photoreceptor cells. These results demonstrate for the first time that environmental pollutants can disrupt the activity of the pineal organ of teleost fish. This disruption could be a threat for the survival of the animals in their natural environment, although the increases observed in melatonin levels could play a relevant role as a toxicity-protection factor.

Melatonin synthesis under calcium constraint in gilthead sea bream (Sparus auratus L.)

Brain or blood plasma melatonin was analysed as a measure for pineal melatonin production in sea bream. Access to calcium was limited by diluting the seawater to 2.5 per thousand and removing calcium from the diet or by prolonged feeding of vitamin D-deficient diet. Interactions/relations between melatonin and calcium balance and the hypercalcemic endocrines PTHrP and calcitriol were assessed. Restricting calcium availability in both water and diet had no effect on plasma melatonin, but when calcium was low in the water or absent from food, increased and decreased plasma melatonin was observed, respectively. Fish on a vitamin D-deficient diet (D- fish) showed decreased plasma calcitriol levels and remained normocalcemic. Decreased brain melatonin was found at all sampling times (10-22 weeks) in the D- fish compared to the controls. A positive correlation between plasma Ca2+ and plasma melatonin was found (R(2)=0.19; N=41; P <0.01) and brain melatonin was negatively correlated with plasma PTHrP (R(2)=0.78; N=4; P <0.05). The positive correlation between plasma levels of melatonin and Ca2+ provides evidence that melatonin synthesis is influenced by plasma Ca2+. The decreased melatonin production in the D- fish points to direct or indirect involvement of calcitriol in melatonin synthesis by the pineal organ in teleosts. The hypercalcemic factors PTHrP and calcitriol appeared to be negatively correlated with melatonin and this substantiates an involvement of melatonin in modulating the endocrine response to cope with hypocalcemia. It further points to the importance of Ca2+ in melatonin physiology.

Genetic aspects of melatonin biology

For decades, the important physiological roles of the pineal hormone have inspired scientific investigations. Research efforts have generated a broad amount of information relevant to various genetic aspects of melatonin biology. Nevertheless, our understanding of the effect of genetic factors upon melatonin biosynthesis and the mechanisms of gene expression regulation by melatonin in target tissues is far from complete. The present review makes an effort to summarize and systematize the existing information on the subject, sequentially discussing (i) the effect of genetic factors upon melatonin biosynthesis, (ii) melatonin receptor expression profiles, and (iii) the effect of melatonin upon expression of genes in target tissues.

Cloning, localization and functional properties of a cGMP-gated channel in photoreceptor cells from fish pineal gland

The perception of photic information and its translation into a rhythmic melatonin signal differ considerably among vertebrates. In the fish pineal gland, melatonin biosynthesis is controlled directly by the natural light/dark cycle. There are indications that the mechanisms of phototransduction are similar in the retinal and pineal photoreceptor cells. Here we report the molecular cloning of a novel ionic cyclic guanosine monophosphate (cGMP)-gated channel from trout pineal photoreceptors. The deduced amino acid sequence exhibits a high sequence homology to cyclic nucleotide-gated-3 (CNG) channels from retinal cones. In situ hybridization with sections of trout pineal gland revealed the expression of CNG channel in photoreceptor cells of the pineal organ. Electrophysiological studies by means of patch-clamp technique indicated that the native channel in photoreceptor cells and the expressed channel in a human cell line (HEK 293 cells) have properties similar to those of cone-CNG (cCNG)-3 channels. They are activated by cGMP, insensitive to cyclic adenosine monophosphate (cAMP) and blocked by intracellular Mg2+ ions at positive voltage values. They have a single-channel conductance close to 42 pS in negative voltage range. In transfected HEK cells loaded with the calcium indicator dye Fura 2, direct activation of CNG channels by 8-Br-cGMP increased fluorescence. The signal was blocked by the addition of Mg2+ ions. From these results, it is suggested that the pineal cyclic nucleotide-gated channel is a good candidate for mediating calcium entry into the pineal photoreceptors. It is most probably a key element in the signalling pathways that control the rhythmic production of melatonin.