Molecular cloning and brain distribution of three types of gonadotropin-releasing hormone from mummichog Fundulus heteroclitus

Localization of three forms of gonadotropin-releasing hormone in the brain and pituitary of the self-fertilizing fish, Kryptolebias marmoratus

The localization of gonadotropin-releasing hormone (GnRH) in the brain and pituitary of the self-fertilizing mangrove killifish Kryptolebias marmoratus was examined by immunohistochemistry and in situ hybridization to understand its neuroendocrine system. The genome assembly of K. marmoratus did not have any sequence encoding GnRH1, but sequences encoding GnRH2 (chicken GnRH-II) and GnRH3 (salmon GnRH) were found. Therefore, GnRH1 was identified by in silico cloning. The deduced amino acid sequence of the K. marmoratus GnRH1 (mature peptide) was identical to that of the medaka GnRH. GnRH1 neurons were detected in the ventral part of the preoptic nucleus by immunohistochemistry and in situ hybridization, and GnRH1-immunoreactive (ir) fibers were observed throughout the brain. GnRH1-ir fibers were in close contact with luteinizing hormone (LH)-ir cells in the pituitary using double immunohistochemistry. GnRH2 neurons were detected in the midbrain tegmentum by immunohistochemistry and in situ hybridization. Although GnRH2-ir fibers were observed throughout the brain, they were not detected in the pituitary. GnRH3 neurons were detected in the lateral part of the ventral telencephalic area by both methods. GnRH3-ir fibers were observed throughout the brain, and a few GnRH3-ir fibers were in close contact with LH-ir cells in the pituitary. These results indicate that GnRH1 and possibly GnRH3 are responsible for gonadal maturation through LH secretion and that all three forms of GnRH function as neurotransmitters or neuromodulators in the brain of K. marmoratus.

Cloning and effects of fasting on the brain expression levels of appetite-regulators and reproductive hormones in glass catfish (Kryptopterus vitreolus)

The regulation of feeding is a complex process that involves coordination between various signals. Feeding hormones can be described as orexigenic (stimulate food intake, e.g. orexin and neuropeptide Y - NPY) or anorexigenic (inhibit food intake, e.g. cocaine and amphetamine regulated transcript - CART). Reproduction and energy homeostasis are closely linked, as factors that affect appetite have also been shown to influence reproductive hormones and behaviors. Gonadotropin-releasing hormone (GnRH) is one of the most influential factors controlling reproduction. Although our understanding of the endocrine regulation of feeding and reproduction in fish is progressing, many gaps still remain, particularly in catfish. Glass catfish (Kryptopterus vitreolus) are freshwater fish known for their natural transparency. In this study, we isolated cDNA encoding reproductive hormones (GnRH1, GnRH2) and appetite regulators (orexin, NPY, and CART) from glass catfish and examined their distribution in various tissues. All peptides had wide distributions across various brain and peripheral tissues, except CART, which was only present in brain. In order to assess whether limited energy supply affects these peptides, we examined the effects of fasting on their brain mRNA expression levels. Fasting increased the expression of both the orexigenic (i.e. orexin and NPY) and anorexigenic (i.e. CART) hormones, and decreased expression levels of GnRH1, but did not affect GnRH2. Overall, our results suggest that fasting affects the expression of peptides involved in both feeding and reproduction, and provides new insights on the endocrine mechanisms that regulate feeding and reproduction in catfish.

Characterization of Schizothorax prenanti cgnrhII gene: fasting affects cgnrhII expression

In this study, the role of chicken gonadotropin-releasing hormone II (cgnrhII) in feeding regulation was investigated in Schizothorax prenanti. First, the full-length S. prenanti cgnrhII cDNA consisted of 693 bp with an open reading frame of 261 bp encoding a protein of 86 amino acids. Next, cgnrhII was widely expressed in the central and peripheral tissues. Last, there were significant changes in cgnrhII mRNA expression in the fasted group compared to the fed group in the S. prenanti hypothalamus during 24 h fasting (P < 0.05). Furthermore, the cgnrhII gene expression presented a significant decrease in the fasted group compared with the fed group (P < 0.05) on days 3, 5 and 7, after re-feeding, there was no significant changes in cgnrhII mRNA expression level between refed and fed group on day 9 (P > 0.05). Thus, the results suggest that cGnRH II expression is influenced by fasting and the gene may be involved in feeding regulation in S. prenanti.

Melanin-concentrating hormone (MCH) and gonadotropin-releasing hormones (GnRH) in Atlantic cod, Gadus morhua: tissue distributions, early ontogeny and effects of fasting

Melanin-concentrating hormone (MCH) is classically known for its role in regulating teleost fish skin color change for environmental adaptation. Recent evidence suggests that MCH also has appetite-stimulating properties. The gonadotropin-releasing hormone (GnRH) peptide family has dual roles in endocrine control of reproduction and energy status in fish. Atlantic cod (Gadus morhua) are a commercially important aquaculture species inhabiting the shores of Atlantic Canada. In this study, we examine MCH and GnRH transcript expression profiles during early development as well as in central and peripheral tissues and quantify juvenile Atlantic cod MCH and GnRH hypothalamic mRNA expressions following food deprivation. MCH and GnRH3 cDNAs are maternally deposited into cod eggs, while MCH has variable expression throughout early development. GnRH2 and GnRH3 mRNAs "turn-on" during mid-segmentation once the brain is fully developed. For both MCH and GnRH, highest expression appears during the exogenous feeding stages, perhaps supporting their functions as appetite regulators during early development. MCH and GnRH transcripts are found in brain regions related to appetite regulation (telencephalon/preoptic area, optic tectum/thalamus, hypothalamus), as well as the pituitary gland and the stomach, suggesting a peripheral function in food intake regulation. Atlantic cod MCH mRNA is upregulated during fasting, while GnRH2 and GnRH3 transcripts do not appear to be influenced by food deprivation. In conclusion, MCH might be involved in stimulating food intake in juvenile Atlantic cod, while GnRHs may play a more significant role in appetite regulation during early development.

Gonadotrophin-releasing hormone in winter flounder (Pseudopleuronectes americanus): molecular characterization, distribution and effects of fasting

Gonadotrophin-releasing hormone (GnRH) is primarily related to reproductive processes in vertebrates. However other physiological roles, including functions in food intake regulation and energy status, have been demonstrated for GnRH in animals. The ten amino acid active peptide is relatively conserved throughout chordates, more specifically in fish species. Teleosts generally have at least two variants of GnRH present in their genomes. GnRH2 (commonly termed chicken-GnRH) is common to all fish, whereas other prevalent forms include GnRH1 and/or GnRH3 (also known as salmon-GnRH). The mRNAs of all three forms were identified in winter flounder (Pseudopleuronectes americanus). Winter flounder GnRH1 appears to be ubiquitously and strongly expressed throughout the brain. GnRH2 mRNA is highly expressed in the optic tectum/thalamus. Finally, GnRH3 mRNA is expressed throughout the brain, but not in the pituitary, with apparent highest expression in the telencephalon/preoptic area. Flounder GnRH1 mRNA is found in most peripheral tissues examined, including the foregut, midgut and gonads. GnRH2 mRNA appears to be expressed throughout the periphery, with apparent highest transcript expression in male gonads. Finally, winter flounder GnRH3 transcript is found at low levels in the skin, heart, and gonads. The effect of fasting on the expression of each of the three isoforms was assessed. Fasting reduces GnRH2 and GnRH3 mRNA expression in the optic tectum/thalamus and hypothalamus, and telencephalon/preoptic area, respectively, compared with fed fish. GnRH1 mRNA expression does not appear to be altered by feeding status. GnRH mRNAs do not seem to regulate food intake peripherally through the gut based on our preliminary findings. Our preliminary results suggest that the GnRH system could play a central role in food intake regulation of winter flounder.

Molecular cloning, tissue distribution, and ontogeny of gonadotropin-releasing hormone III gene (GnRH-III) in half-smooth tongue sole (Cynoglossus semilaevis)

Gonadotropin-releasing hormone (GnRH) is a neuropeptide that plays a vital role in hypothalamus-pituitary-gonad (HPG) axis. In the present study, the GnRH-III gene was isolated from half-smooth tongue sole (Cynoglossus semilaevis). In the 1160 bp genomic sequence, four exons, three introns, and 5'-/3'-flanking sequences were identified. The putative peptide was 92 residues long, including a putative signal peptide containing 23 amino acids, the GnRH decapeptide, a proteolytic cleavage site of three amino acids and a GnRH associated peptide of 56 amino acids. The overall amino acid sequence of C. semilaevis GnRH-III (csGnRH-III) was highly conserved with other teleost GnRH-III genes. Phylogenetic analysis showed the evolutionary relationships of csGnRH-III with other known GnRH genes. A 320 bp promoter sequence of the csGnRH-III was also analyzed, and several potential regulatory motifs were identified which were conserved in the GnRH promoters of other teleosts. Quantitative real-time PCR analysis indicated csGnRH-III was expressed only in brain and gonads. In C. semilaevis, the csGnRH-III transcript was maternally deposited and appeared to be developmentally regulated during embryogenesis and early larval development. Comparing sequence and expression patterns of csGnRH-III with other teleosts GnRH-IIIs suggested that the main function of GnRH-III might be conserved in teleosts.

Fundulus heteroclitus: ovarian reproductive physiology and the impact of environmental contaminants

Fundulus heteroclitus, the mummichog or Atlantic killifish, is the dominant small-bodied fish species of the east coast estuaries and salt marshes of Canada and the USA, where it is present as two subspecies, the northern F. h. macrolepidotus and the southern F. h. heteroclitus. Recently identified as the premier teleost model in environmental biology, the species has long been of value in understanding evolved tolerance to toxicants and more lately in adding to our knowledge about reproductive effects of environmental endocrine disruptors. The body of literature on F. heteroclitus ovarian physiology and reproduction, from both field and laboratory studies, provides the foundation for present work focused on understanding the reproductive effects and modes of action of environmental toxicants. In this paper, we review the environmental and endocrine factors controlling ovarian and reproductive cycling in F. heteroclitus, noting specifics related to field and laboratory studies on the two subspecies as well as key research gaps compared to other fish species. We also summarize recent development of methodologies to study the effects of environmental contaminants on endocrine signalling and egg production in F. heteroclitus. Continued efforts to progress both our fundamental understanding of reproductive physiology in mummichog, coupled with studies focused on the modes of action of environmental contaminants, have high potential to further develop this teleost model. While the model may presently lag behind those based on other species of fish, the unique biochemical and physiological adaptations which allow F. heteroclitus to adapt to changing environmental and toxic conditions provide a valuable experimental system for comparative physiologists, ecotoxicologists and evolutionary biologists.