Ovarian melanotropic peptides and adaptation in two teleostean species: Sparus aurata L. and Dicentrarchus labrax L

Role of the Melanocortin System in Gonadal Steroidogenesis of Zebrafish

In teleost, as in other vertebrates, stress affects reproduction. A key component of the stress response is the pituitary secretion of the adrenocorticotropic hormone (ACTH), which binds to the melanocortin 2 receptor (MC2R) in the adrenal glands and activates cortisol biosynthesis. In zebrafish, Mc2r was identified in male and female gonads, while ACTH has been shown to have a physiological role in modulating reproductive activity. In this study, the hypothesis that other melanocortins may also affect how the zebrafish gonadal function is explored, specifically steroid biosynthesis, given the presence of members of the melanocortin signaling system in zebrafish gonads. Using cell culture, expression analysis, and cellular localization of gene expression, our new observations demonstrated that melanocortin receptors, accessory proteins, antagonists, and agonists are expressed in both the ovary and testis of zebrafish (n = 4 each sex). Moreover, melanocortin peptides modulate both basal and gonadotropin-stimulated steroid release from zebrafish gonads (n = 15 for males and n = 50 for females). In situ hybridization in ovaries (n = 3) of zebrafish showed mc1r and mc4r in follicular cells and adjacent to cortical alveoli in the ooplasm of previtellogenic and vitellogenic oocytes. In zebrafish testes (n = 3), mc4r and mc1r were detected exclusively in germ cells, specifically in spermatogonia and spermatocytes. Our results suggest that melanocortins are, directly or indirectly, involved in the endocrine control of vitellogenesis in females, through modulation of estradiol synthesis via autocrine or paracrine actions in zebrafish ovaries. Adult zebrafish testes were sensitive to low doses of ACTH, eliciting testosterone production, which indicates a potential role of this peptide as a paracrine regulator of testicular function.

Proopiomelanocortin gene expression and β-endorphin localization in the pituitary, testis, and epididymis of stallion

Proopiomelanocortin (POMC) is a precursor protein that contains the sequences of several bioactive peptides including adrenocorticotropin (ACTH), beta-endorphin (beta-EP), and melanocyte-stimulating-hormone (MSH). POMC is synthesized in the pituitary gland, brain, and many peripheral tissues. Immunoreactive POMC-derived peptides as well as POMC-like mRNA have been evidenced in several nonpituitary tissues, thus suggesting that POMC is actively synthesized by these tissues. The present study was aimed at evaluating if also in the case of stallion POMC-derived peptide, beta-EP, is produced locally in the testis, thus playing effects in a paracrine/autocrine fashion. To investigate this hypothesis the POMC gene expression was analyzed using 3' RACE-PCR and Northern Blot approaches in the testis and epididimys of stallion; moreover, immunocytochemical localization for beta-EP was also performed through confocal laser microscopy. The immunofluorescence results showed a positive beta-EP reaction not only in cellular nest of pituitary but also in the testis and genital tract of stallion, which function could be related with sperm mobility. Such role seem not to be no dependent on the peptide synthesized locally, because the molecular biology approach demonstrated the presence of POMC transcript in the pituitary only. In fact the Northern Blot analysis showed the presence of a single POMC transcript in the pituitary while no signal was detected in the testis and epididimys. The same results were obtained by applied 3' RACE-PCR analysis. In conclusion, opioid-derived peptide beta-EP is present in the genital tract of stallion, but is not locally produced as in other mammalian, and nonmammalian models; its possible biological function at testicular level could be linked to a long-loop feed-back mechanisms.

One melanocortin 4 and two melanocortin 5 receptors from zebrafish show remarkable conservation in structure and pharmacology

We report the cloning, genome mapping, functional expression, pharmacology and anatomical distribution of three melanocortin (MC) receptors from zebrafish (z). Phylogenetic analysis showed with high bootstrap support that these genes represent one MC4 receptor and two MC5 receptors. Chromosomal mapping showed conserved synteny between regions containing zMC4 and human (h) MC4 receptors, whereas the two zMC5 receptor genes map on chromosome segments in which the zebrafish has several genes with two orthologues of a single mammalian gene. It is likely that the two copies of zMC5 receptors arose through a separate duplication in the teleost lineage. The zMC4, zMC5a, and zMC5b receptors share 70-71% overall amino acid identity with the respective human orthologues and over 90% in three TM regions believed to be most important for ligand binding. All three zebrafish receptors also show pharmacological properties remarkably similar to their human orthologues, with similar affinities and the same potency order, when expressed and characterized in radioligand binding assay for the natural MSH) peptides alpha-, beta-, and gamma-MSH. Stimulation of transfected mammalian cells with alpha-MSH caused a dose-dependent increase in intracellular cAMP levels for all three zebrafish receptors. All three genes were expressed in the brain, eye, ovaries and gastrointestinal tract, whereas the zMC5b receptor was also found in the heart, as determined by RT-PCR. Our studies, which represent the first characterization of MC receptors in a nonamniote species, indicate that the MC receptor subtypes arose very early in vertebrate evolution. Important pharmacological and functional properties, as well as gene structure and syntenic relationships have been highly conserved over a period of more than 400 million years implying that these receptors participate in vital physiological functions.

Acetyl salmon endorphin-like and interrenal stress response in male gilthead sea bream, Sparus aurata

The present study investigates the role of melanotrope proopiomelanocortin-derived peptide in the interrenal stress response to different stressors in male gilthead sea bream, Sparus aurata. Plasma cortisol and acetyl salmon endorphin (acetyl s-EP), as well as pituitary acetyl s-EP contents, were measured during two stress paradigms: (a) long-term (1-month) confinement and crowding, and (b) short-term (60-min) confinement, crowding, and manipulation. In addition, naltrexone, a highly specific opioid receptor antagonist, was employed in some experimental groups to evaluate the adaptability of the opioid response to interrenal stress. In the long-term (1-month) confinement and crowding, higher plasma cortisol levels and acetyl s-EP concentrations than in the control group were found. However, although plasma cortisol levels significantly increased in both types of stress paradigm, a significant rise in plasma acetyl s-EP was observed only in the case of confinement plus crowding. These data seem to suggest a direct correlation of acetyl s-EP plasma levels exclusively in cases of specific stress, and support previous observations about the different nature of the pituitary-interrenal stress response in salmonids and in mammals. The results obtained in the short-term (60-min) experiments demonstrate the double activation of both the opioid and corticotrope systems when manipulation plus crowding was applied.

Expression of pro-opiomelanocortin (POMC) in the cerebral ganglion and ovary of a protochordate

The distribution of neurones expressing POMC mRNA in the cerebral ganglion of the protochordate ascidian, Styela plicata, was investigated using a non-radioactive in situ hybridization technique. Nerve cell bodies of mono and bipolar types expressing POMC mRNA, were observed mainly in the outer layer of the ganglion. Discrete groups of neurones containing POMC mRNA were also localized in the inner portion of the ganglion, and few small monopolar perykaria expressing POMC mRNA were visible at the emergence of the main nerve trunks. POMC mRNA labeling was also found at level of the cytoplasm of previtellogenic and vitellogenic oocytes, and of follicular cells. Our results demonstrate the expression of one or more genes in the cerebral ganglion and ovary, that may be similar to one or more regions of the mammalian POMC gene. Therefore POMC-related molecules seem to be involved in neuromodulatory pathways and regulatory mechanisms of the oogenesis of ascidians.

Acetyl salmon endorphin-like immunoreactivity in the ovary of two teleostean species: changes with environmental conditions

The presence of salmon acetylated endorphin (acetyl sEP) in the ovary of seabream and sea bass was investigated through immunocytochemical and biochemical techniques in order to compare aquatic species with terrestrial ones. Endorphin-like immunoreactivity was found in the cytoplasm of oogonia and similar immunostaining was present in the granulosa layer of mature follicles. In both pituitary and ovarian extracts of the two teleostean species, acetyl sEP-like immunoreactivity was distributed over three main peaks, the second one corresponding to the elution time of the reference synthetic peptide. Serial dilutions of HPLC fraction II of the ovaries of both fishes ran parallel with the standard curve obtained with reference peptide. The ovarian content of acetyl sEP, obtained by calculating the integrated area of the fraction II peak, indicates large and highly significant (p < 0.01) differences in the amount of peptide found in ovarian tissues of wild seabream in comparison with that of farmed fish. Increased peptide values in wild animals with respect to farmed fish were also found in the sea bass. These data indicate that not only the pituitary, but also the ovary is sensitive to environmental cues, and strongly suggest the role of opioid peptides in adaptation.

Bacterial lipopolysaccharide (LPS) and interleukin 1 (IL-1) exert multiple physiological effects in the tilapia Oreochromis mossambicus (Teleostei)

To gain insight in immuno-endocrine communication in teleosts the physiological effects of interleukin 1 and bacterial lipopolysaccharide in teleosts were investigated. Tilapia (Oreochromis mossambicus) were treated with murine interleukin 1 and E. coli lipopolysaccharide in vivo, and lipopolysaccharide was administered to pituitary lobes and head kidneys in vitro. The integument of the fish appeared to be a sensitive target for the preparations tested, since proliferation of chloride cells and of epidermal mucous cells was observed as well as an increase in epidermal thickness. These effects may relate to an acute phase-like reaction caused by the treatments. Lipopolysaccharide administration furthermore resulted in an increase in plasma free fatty acids levels. Lipopolysaccharide, but not interleukin 1, stimulated the interrenal axis of the fish, as judged by the increase in cortisol production measured in superfusion of head kidneys. In addition to these in vivo effects, lipopolysaccharide also displayed several effects in vitro. Pituitary adrenocorticotropic hormone, as well as alpha-melanocyte stimulating hormone, release was inhibited, and the head kidney responsiveness to adrenocorticotropic hormone was inhibited after pretreatment of the tissue with the E. coli product. This latter effect coincided with the release of an unidentified alpha-melanocyte stimulating hormone immunoreactive fraction by the head kidneys which could be stimulated by lipopolysaccharide. The data strongly support the notion that the immune system is involved in adaptive regulations in teleosts, and that immunoendocrine interactions are phylogenetically old mechanisms.

Immunocytochemical localization and biochemical characterization of melanotropin-like peptides in the gonads of a protochordate

The compound gonads of the protochordate ascidian Styela plicata were investigated by immunocytochemistry, HPLC, and radioimmunoassay to verify the presence of melanotropin-like peptides, alpha-MSH-like immunoreactivity is localized in the follicular cells and in the perinuclear cytoplasm of different types of ovaric follicles, as well as in the spermatogonia and spermatocytes of testicular lobules. The ascidian immunoreactive peptides occurring in the gonads consist of alpha-MSH and ACTH(1-13)-NH2 and their amounts are higher in summer than in winter.