GnRH stimulates LH release directly via inositol phosphate and indirectly via cAMP in African catfish

Differential involvement of cAMP/PKA-, PLC/PKC- and Ca2+/calmodulin-dependent pathways in GnRH-induced prolactin secretion and gene expression in grass carp pituitary cells

Gonadotropin-releasing hormone (GnRH) is a key stimulator for gonadotropin secretion in the pituitary and its pivotal role in reproduction is well conserved in vertebrates. In fish models, GnRH can also induce prolactin (PRL) release, but little is known for the corresponding effect on PRL gene expression as well as the post-receptor signalling involved. Using grass carp as a model, the functional role of GnRH and its underlying signal transduction for PRL regulation were examined at the pituitary level. Using laser capture microdissection coupled with RT-PCR, GnRH receptor expression could be located in carp lactotrophs. In primary cell culture prepared from grass carp pituitaries, the native forms of GnRH, GnRH2 and GnRH3, as well as the GnRH agonist [D-Arg6, Pro9, NEt]-sGnRH were all effective in elevating PRL secretion, PRL mRNA level, PRL cell content and total production. In pituitary cells prepared from the rostral pars distalis, the region in the carp pituitary enriched with lactotrophs, GnRH not only increased cAMP synthesis with parallel CREB phosphorylation and nuclear translocation but also induced a rapid rise in cytosolic Ca2+ by Ca2+ influx via L-type voltage-sensitive Ca2+ channel (VSCC) with subsequent CaM expression and NFAT2 dephosphorylation. In carp pituitary cells prepared from whole pituitaries, GnRH-induced PRL secretion was reduced/negated by inhibiting cAMP/PKA, PLC/PKC and Ca2+/CaM/CaMK-II pathways but not the signalling events via IP3 and CaN/NFAT. The corresponding effect on PRL mRNA expression, however, was blocked by inhibiting cAMP/PKA/CREB/CBP and Ca2+/CaM/CaN/NFAT2 signalling but not PLC/IP3/PKC pathway. At the pituitary cell level, activation of cAMP/PKA pathway could also induce CaM expression and Ca2+ influx via VSCC with parallel rises in PRL release and gene expression in a Ca2+/CaM-dependent manner. These findings, as a whole, suggest that the cAMP/PKA-, PLC/PKC- and Ca2+/CaM-dependent cascades are differentially involved in GnRH-induced PRL secretion and PRL transcript expression in carp lactotrophs. During the process, a functional crosstalk between the cAMP/PKA- and Ca2+/CaM-dependent pathways may occur with PRL release linked with CaMK-II and PKC activation and PRL gene transcription caused by nuclear action of CREB/CBP and CaN/NFAT2 signalling.

Pituitary luteinizing hormone synthesis starts in aromatase (cyp19a1b)-positive cells expressing esr1 and esr2b at the onset of puberty in Takifugu rubripes (fugu)

Unlike mammals, teleost fish have high aromatase activity (AA) in the pituitary. However, the cells responsible for oestradiol synthesis and the local physiological roles of this hormone remain unclear. Hence, we investigated the effects of age and development on steroidogenic activity, mRNA expression, and cyp19a1b localization in the pituitary gland of the Japanese pufferfish Takifugu rubripes. Under aquaculture conditions, AA was highest after puberty, and the mRNA expression levels of cyp19a1b and the oestrogen receptors esr1 and 2b and the level of serum testosterone (T) were significantly increased after puberty compared with the other developmental stages in male and female pufferfish. Immunohistochemistry using multiple antibodies and in situ hybridization analysis revealed that Cyp19a1b colocalizes with luteinizing hormone (LH) in pituitary cells. Furthermore, Esr1 was localized in the nuclei of all hormone-producing cells, whereas Esr2b was localized only in the nuclei of Cyp19- and LH-positive cells. The administration of an aromatizable androgen (T) or oestrogen (E2) to reproductively inactive females induced LH synthesis in vivo. We prepared spheroids from pituitary cells to investigate the role of local E2 in LH synthesis in vitro. Immunohistochemical analysis of spheroids showed that T-induced LH synthesis could be blocked by an aromatase inhibitor and/or an ER antagonist but not an AR antagonist. Taken together, these findings suggest that LH synthesis is initiated in cyp19a1b-, esr1-, and esr2b-expressing cells at the onset of puberty under the control of steroidal feedback, and both feedback and local oestrogen may be involved in controlling LH synthesis in these cells.

Pituitary Gonadotropin Gene Expression During Induced Onset of Postsmolt Maturation in Male Atlantic Salmon: In Vivo and Tissue Culture Studies

Precocious male maturation causes reduced welfare and increased production costs in Atlantic salmon (Salmo salar) aquaculture. The pituitary produces and releases follicle-stimulating hormone (Fsh), the gonadotropin triggering puberty in male salmonids. However, little is known about how Fsh production is regulated in Atlantic salmon. We examined, in vivo and ex vivo, transcriptional changes of gonadotropin-related genes accompanying the initial steps of testis maturation, in pituitaries of males exposed to photoperiod and temperature conditions promoting maturation (constant light and 16°C). Pituitary fshb, lhb and gnrhr2bba transcripts increased in vivo in maturing males (gonado-somatic index > 0.1%). RNA sequencing (RNAseq) analysis using pituitaries from genetically similar males carrying the same genetic predisposition to mature, but differing by responding or not responding to stimulatory environmental conditions, revealed 144 differentially expressed genes, ~2/3rds being up-regulated in responders, including fshb and other pituitary hormones, steroid-related and other puberty-associated transcripts. Functional enrichment analyses confirmed gene involvement in hormone/steroid production and gonad development. In ex vivo studies, whole pituitaries were exposed to a selection of hormones and growth factors. Gonadotropin-releasing hormone (Gnrh), 17β-estradiol (E₂) and 11-ketotestosterone (11-KT) up-regulated gnrhr2bba and lhb, while fshb was up-regulated by Gnrh but down-regulated by 11-KT in pituitaries from immature males. Also pituitaries from maturing males responded to Gnrh and sex steroids by increased gnrhr2bba and lhb transcript levels, but fshb expression remained unchanged. Growth factors (inhibin A, activin A and insulin-like growth factor 1) did not change gnrhr2bba, lhb or fshb transcript levels in pituitaries either from immature or maturing males. Additional pituitary ex vivo studies on candidates identified by RNAseq showed that these transcripts were preferentially regulated by Gnrh and sex steroids, but not by growth factors, and that Gnrh/sex steroids were less effective when incubating pituitaries from maturing males. Our results suggest that a yet to be characterized mechanism up-regulating fshb expression in the salmon pituitary is activated in response to stimulatory environmental conditions prior to morphological signs of testis maturation, and that the transcriptional program associated with this mechanism becomes unresponsive or less responsive to most stimulators ex vivo once males had entered pubertal developmental in vivo.

Comparative aspects of GnRH-Stimulated signal transduction in the vertebrate pituitary - Contributions from teleost model systems

Gonadotropin-releasing hormone (GnRH) is a major regulator of reproduction through actions on pituitary gonadotropin release and synthesis. Although it is often thought that pituitary cells are exposed to only one GnRH, multiple GnRH forms are delivered to the pituitary of teleost fishes; interestingly this can include the cGnRH-II form usually thought to be non-hypophysiotropic. GnRHs can regulate other pituitary cell-types, both directly as well as indirectly, and multiple GnRH receptors (GnRHRs) may also be expressed in the pituitary, and even within a single pituitary cell-type. Literature on the differential actions of native GnRH isoforms in primary pituitary cells is largely derived from teleost fishes. This review will outline the diversity and complexity of GnRH-GnRHR signal transduction found within vertebrate gonadotropes as well as extra-gonadotropic sites with special emphasis on comparative studies from fish models. The implications that GnRHR transduction mechanisms are GnRH isoform-, function-, and cell-specific are also discussed.

Responsiveness of pituitary to galanin throughout the reproductive cycle of male European sea bass (Dicentrarchus labrax)

The neuropeptide galanin (Gal) is a putative factor regulating puberty onset and reproduction through its actions on the pituitary. The present study investigated the pituitary responsiveness to galanin and the patterns of galanin receptors (Galrs) expression throughout the reproductive cycle of two years old male European sea bass (Dicentrarchus labrax), an important aquaculture species. Quantitative analysis of pituitary and hypothalamus transcript expression of four galr subtypes revealed differential regulation according to the testicular developmental stage, with an overall decrease in expression from the immature stage to the mid-recrudescence stage. Incubation of pituitary cells with mammalian 1-29Gal peptide induced significant changes in cAMP concentration, with sensitivities that varied according to the testicular development stages. Furthermore 1-29Gal was able to stimulate both follicle stimulating hormone (Fsh) and luteinizing hormone (Lh) release from pituitary cell suspensions. The magnitude of the effects and effective concentrations varied according to reproductive stage, with generalized induction of Fsh and Lh release in animals sampled in January (full spermiation). The differential expression of galrs in pituitary and hypothalamus across the reproductive season, together with the differential effects of Gal on gonadotropins release in vitro strongly suggests the involvement of the galaninergic system in the regulation the hypothalamus-pituitary-gonad axis of male sea bass. This is to our knowledge the first clear evidence for the involvement of galanin in the regulation of reproduction in non-mammalian vertebrates.

Effects of melanocortin-4 receptor agonists and antagonists on expression of genes related to reproduction in spotted scat, Scatophagus argus

Melanocortin-4 receptor (Mc4r) function related to reproduction in fish has not been extensively investigated. Here, we report on gene expression changes by real-time PCR following treatment with Mc4r agonists and antagonists in the spotted scat (Scatophagus argus). Using in vitro incubated hypothalamus, the Mc4r nonselective agonist NDP-MSH ([Nle⁴, D-Phe⁷]-α-melanocyte stimulating hormone; 10^-6 M) and selective agonist THIQ (N-[(3R)-1, 2, 3, 4-Tetrahydroisoquinolinium-3-ylcarbonyl]- (1R)-1-(4-chlorobenzyl)-2-[4-cyclohexyl-4-(1H-1,2,4-triazol-1-ylmethyl) piperidin-1-yl]-2-oxoethylamine; 10^-7 M) significantly increased the expression of gnrh (Gonadotropin releasing hormone), while the Mc4r nonselective antagonist SHU9119 (Ac-Nle-[Asp-His-DPhe/DNal(2')-Arg-Trp-Lys]-NH2; 10^-6 M) and selective antagonist Ipsen 5i (compound 5i synthesized in Ipsen Research Laboratories; 10^-6 M) significantly inhibited gnrh expression after 3 h of incubation. In incubated pituitary tissue, NDP-MSH and THIQ significantly increased the expression of fshb (Follicle-stimulating hormone beta subunit) and lhb (Luteinizing hormone beta subunit), while SHU9119 and Ipsen 5i significantly decreased fshb and lhb expression after 3 h of incubation. During the in vivo experiment, THIQ (1 mg/kg bw) significantly increased gnrh expression in hypothalamic tissue, as well as the fshb and lhb expression in pituitary tissue 12 h after abdominal injection. Furthermore, Ipsen 5i (1 mg/kg bw) significantly inhibited gnrh expression in hypothalamic tissue, as well as fshb and lhb gene expression in pituitary tissue 12 h after abdominal injection. In summary, Mc4r singling appears to stimulate gnrh expression in the hypothalamus, thereby modulating the synthesis of Fsh and Lh in the pituitary. In addition, Mc4r also appears to directly regulate fshb and lhb levels in the pituitary in spotted scat. Our study suggests that Mc4r, through the hypothalamus and pituitary, participates in reproductive regulation in fish.

Neuroendocrinology of reproduction in teleost fish

This review aims at synthesizing the most relevant information regarding the neuroendocrine circuits controlling reproduction, mainly gonadotropin release, in teleost fish. In teleosts, the pituitary receives a more or less direct innervation by neurons sending projections to the vicinity of the pituitary gonadotrophs. Among the neurotransmitters and neuropeptides released by these nerve endings are gonadotrophin-releasing hormones (GnRH) and dopamine, acting as stimulatory and inhibitory factors (in many but not all fish) on the liberation of LH and to a lesser extent that of FSH. The activity of the corresponding neurons depends on a complex interplay between external and internal factors that will ultimately influence the triggering of puberty and sexual maturation. Among these factors are sex steroids and other peripheral hormones and growth factors, but little is known regarding their targets. However, very recently a new actor has entered the field of reproductive physiology. KiSS1, first known as a tumor suppressor called metastin, and its receptor GPR54, are now central to the regulation of GnRH, and consequently LH and FSH secretion in mammals. The KiSS system is notably viewed as instrumental in integrating both environmental cues and metabolic signals and passing this information onto the reproductive axis. In fish, there are two KiSS genes, KiSS1 and KiSS2, expressed in neurons of the preoptic area and mediobasal hypothalamus. Pionneer studies indicate that KiSS and GPR54 expression seem to be activated at puberty. Although precise information as to the physiological effects of KiSS1 in fish, notably on GnRH neurons and gonadotropin release, is still limited, KiSS neurons may emerge as the "gatekeeper" of puberty and reproduction in fish as in mammals.

Regulation of fish gonadotropins

Neurohormones similar to those of mammals are carried in fish by hypothalamic nerve fibers to regulate directly follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Gonadotropin-releasing hormone (GnRH) stimulates the secretion of FSH and LH and the expression of the glycoprotein hormone alpha (GPalpha), FSHbeta, and LHbeta, as well as their secretion. Its signal transduction leading to LH release is similar to that in mammals although the involvement of cyclic AMP-protein kinase A (cAMP-PKA) cannot be ruled out. Dopamine (DA) acting through DA D2 type receptors may inhibit LH release, but not that of FSH, at sites distal to activation of protein kinase C (PKC) and PKA. GnRH increases the steady-state levels of GPalpha, LHbeta, and FSHbeta mRNAs. Pituitary adenylate cyclase-activating polypeptide (PACAP) 38 and neuropeptide Y (NPY) potentiate GnRH effect on gonadotropic cells, and also act directly on the pituitary cells. Whereas PACAP increases all three subunit mRNAs, NPY has no effect on that of FSHbeta. The effect of these peptides on the expression of the gonadotropin subunit genes is transduced differentially; GnRH regulates GPalpha and LHbeta via PKC-ERK and PKA-ERK cascades, while affecting the FSHbeta transcript through a PKA-dependent but ERK-independent cascade. The signals of both NPY and PACAP are transduced via PKC and PKA, each converging at the ERK level. NPY regulates only GPalpha- and LHbeta-subunit genes whereas PACAP regulates the FSHbeta subunit as well. Like those of the mammalian counterparts, the coho salmon LHbeta gene promoter is driven by a strong proximal tripartite element to which three different transcription factors bind. These include Sf-1 and Pitx-1 as in mammals, but the function of the Egr-1 appears to have been replaced by the estrogen receptor (ER). The GnRH responsive region in tilapia FSHbeta 5' flanking region spans the canonical AP1 and CRE motifs implicating both elements in conferring GnRH responsiveness. Generally, high levels of gonadal steroids are associated with high LHbeta transcript levels whereas those of FSHbeta are reduced when pituitary cells are exposed to high steroid levels. Gonadal or hypophyseal activin also participate in the regulation of FSHbeta and LHbeta mRNA levels. However, gonadal effects are dependent on the gender and stage of maturity of the fish.

Multiplicity of gonadotropin-releasing hormone signaling: a comparative perspective

GnRH regulation of GtH synthesis and release involves PKC- and Ca(2+)-dependent pathways. There are differential signaling mechanisms in different cells, tissues and species. Signaling mechanisms involved in GnRH-mediated GtH release appear to be more conserved compared to that of GnRH-induced GtH gene expression. This may in part be due to different 5' regulatory regions on the GtH-subunit genes. Cell type specific expression of various signaling and/or exocytotic components may also be responsible for the observed differences in signaling between gonadotropes and somatotropes in the goldfish and tilapia pituitaries. However, this can not explain the observed differences in post receptor mechanisms for sGnRH and cGnRH-II in gonadotropes which is more likely to result from the existence of GnRH receptor subtypes. Support for this hypothesis is also provided by observations on mechanisms of autocrine/paracrine regulation of ovarian function by sGnRH and cGnRH-II in the goldfish ovary in which GnRH antagonists only block GnRH stimulation of oocyte meiosis and do not affect inhibitory effects of sGnRH. It should be easier to explain observed variations concerning GnRH-induced responses as more information becomes available on different types of GnRH receptors, and their distribution and function in mammals and non-mammalian vertebrates.

Involvement of both G(q/11) and G(s) proteins in gonadotropin-releasing hormone receptor-mediated signaling in L beta T2 cells

The hypothalamic hormone gonadotropin-releasing hormone (GnRH) stimulates the synthesis and release of the pituitary gonadotropins. GnRH acts through a plasma membrane receptor that is a member of the G protein-coupled receptor (GPCR) family. These receptors interact with heterotrimeric G proteins to initiate downstream signaling. In this study, we have investigated which G proteins are involved in GnRH receptor-mediated signaling in L beta T2 pituitary gonadotrope cells. We have shown previously that GnRH activates ERK and induces the c-fos and LH beta genes in these cells. Signaling via the G(i) subfamily of G proteins was excluded, as neither ERK activation nor c-Fos and LH beta induction was impaired by treatment with pertussis toxin or a cell-permeable peptide that sequesters G beta gamma-subunits. GnRH signaling was partially mimicked by adenoviral expression of a constitutively active mutant of G alpha(q) (Q209L) and was blocked by a cell-permeable peptide that uncouples G alpha(q) from GPCRs. Furthermore, chronic activation of G alpha(q) signaling induced a state of GnRH resistance. A cell-permeable peptide that uncouples G alpha(s) from receptors was also able to inhibit ERK, c-Fos, and LH beta, indicating that both G(q/11) and G(s) proteins are involved in signaling. Consistent with this, GnRH caused GTP loading on G(s) and G(q/11) and increased intracellular cAMP. Artificial elevation of cAMP with forskolin activated ERK and caused a partial induction of c-Fos. Finally, treatment of G alpha(q) (Q209L)-infected cells with forskolin enhanced the induction of c-Fos showing that the two pathways are independent and additive. Taken together, these results indicate that the GnRH receptor activates both G(q) and G(s) signaling to regulate gene expression in L beta T2 cells.

Gonadotropin response to GnRH during sexual ontogeny in the common carp, Cyprinus carpio

This study was designed to reveal whether gonadotropic response to GnRH in the common carp (Cyprinus carpio) changes during sexual ontogeny and whether the response of FSHbeta and LHbeta subunits is uniform or differential. The study comprised fish at the following stages: juveniles (4-month-old females with primary oocytes and early spermatogenic males); maturing (9-month-old previtellogenic females and advanced spermatogenic males); and mature (16-month-old postvitellogenic females and spermiating males). Fish were injected with superactive salmon GnRH analogue (sGnRHa; 25 microg/kg) and blood was sampled 6, 12 and 24 h later for cGtH (LH) and sex steroid levels. Pituitaries were taken for determination of FSHbeta and LHbeta mRNA levels by slot-blot hybridization and for cGTH content in the same glands by radioimmunoassay (RIA). Values were compared with the levels prior to sGnRHa administration and with control fish sampled at the same intervals. Juvenile fish did not respond at all to sGnRHa. In maturing females, FSHbeta mRNA increased by >300%, while that of LHbeta increased by 200%. In maturing males, FSHbeta mRNA did not change and only a slight increase occurred in that of LHbeta. In 16-month-old postvitellogenic females, there was no response of FSHbeta mRNA, while that of LHbeta dramatically increased. In spermiating males of the same age, mRNA of both FSHbeta and LHbeta increased following sGnRHa injection. Immunoreactive cGtH was present in the pituitary and plasma of all fish examined, but in juveniles it did not change following sGnRHa injection. In maturing and mature fish of both genders, sGnRHa administration was followed by a marked increase in circulating cGtH, concomitant with a decrease in its pituitary content, indicating the limited amount of the hormone stored in the gland. In conclusion, the response of the gonadotropin subunit mRNAs in the common carp was found to be differential and dependent on the gender and the phase of sexual ontogeny.

Recombinant perciform GnRH-R activates different signaling pathways in fish and mammalian heterologous cell lines

Perciforms have three forms of gonadotropin-releasing hormone (GnRH) in their brain. All three GnRHs are potent secretogogues for luteinizing hormone (LH) from the pituitary. The pivotal role of GnRH-R-GnRH interactions in reproductive homeostasis is well established; however, there is a paucity of information on how a GnRH-R responds to the three endogenous GnRH forms in a perciform species. In this study, a recombinant pituitary GnRH-R from striped bass (stb) was expressed in a mammalian cell line (COS-7) and a fish cell line (CHSE-214). Activation of the signaling pathways was monitored by reporter gene (luciferase) based assays, which were specific for cAMP-PKA or Ca 2+/calmodulin kinase (activated via c-fos promoter) signaling pathways. The stbGnRH-R expressed in two different cell lines triggered different downstream signaling in response to the treatments with chicken (c) GnRH II. Interestingly, when endogenous GnRHs were used in combinations, the luciferase activity was significantly attenuated in transfected CHSE-214 cells.