Perspectives on fish gonadotropins and their receptors

Abstract

Teleosts lack a hypophyseal portal system and hence neurohormones are carried by nerve fibers from the preoptic region to the pituitary. The various cell types in the teleost pituitary are organized in discrete domains. Fish possess two gonadotropins (GtH) similar to FSH and LH in other vertebrates; they are heterodimeric hormones that consist of a common alpha subunit non-covalently associated with a hormone-specific beta subunit. In recent years the availability of molecular cloning techniques allowed the isolation of the genes coding for the GtH subunits in 56 fish species representing at least 14 teleost orders. Advanced molecular engineering provides the technology to produce recombinant GtHs from isolated cDNAs. Various expression systems have been used for the production of recombinant proteins. Recombinant fish GtHs were produced for carp, seabream, channel and African catfish, goldfish, eel, tilapia, zebrafish, Manchurian trout and Orange-spotted grouper. The hypothalamus in fishes exerts its regulation on the release of the GtHs via several neurohormones such as GnRH, dopamine, GABA, PACAP, IGF-I, norepinephrine, NPY, kisspeptin, leptin and ghrelin. In addition, gonadal steroids and peptides exert their effects on the gonadotropins either directly or via the hypothalamus. All these are discussed in detail in this review. In mammals, the biological activities of FSH and LH are directed to different gonadal target cells through the cell-specific expression of the FSH receptor (FSHR) and LH receptor (LHR), respectively, and the interaction between each gonadotropin-receptor couple is highly selective. In contrast, the bioactivity of fish gonadotropins seems to be less specific as a result of promiscuous hormone-receptor interactions, while FSHR expression in Leydig cells explains the strong steroidogenic activity of FSH in certain fish species.

Neurokinin Bs and neurokinin B receptors in zebrafish-potential role in controlling fish reproduction

“…2B). Moreover, increased levels of estradiol are a required characteristic of both follicular growth and final oocyte maturation in fish (3,35), pointing toward the involvement of the piscine NKB system in control of reproduction, probably in concert with kisspeptin and GnRH.…”

“…In teleosts, the pituitary is innervated directly by neurons projecting to the vicinity of the pituitary gonadotrophs (2). Among the neuropeptides released by these nerve endings are gonadotrophin-releasing hormones (GnRHs) and dopamine, which act as stimulatory and inhibitory factors on the release of luteinizing hormone (LH) and follicle-stimulating hormone (3). However, new actors have recently entered the field of reproductive physiology: kisspeptins, neurokinin, and dynorphin have all been implicated in controlling GnRH (4).…”

Developmental tracing of luteinizing hormone β‐subunit gene expression using green fluorescent protein transgenic medaka (Oryzias latipes) reveals a putative novel developmental function

“…In fish, as opposed to mammals, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), are synthesized and secreted from distinct cells in the pituitary. The synthesis and secretion of LH and FSH is largely controlled by gonadotropin releasing hormone (GnRH), gonadal feedback loops and a variety of stimulating and inhibiting factors, such as kisspeptin, neuropeptide y, dopamine, and gonadotropin inhibiting hormone (Levavi-Sivan et al, 2010;Clarke, 2011). Upon release, LH and FSH bind to their specific receptors, LHR and FSHR, in the gonads, stimulating steroidogenesis and gametogenesis.…”

Serotonin, GnRH-A, and dopamine interaction in the control of in vivo luteinizing hormone release in Prussian carp (Carassius gibelio Bloch) at the time of gonad recrudescence

“…Some of them, like GnRH, dopamine, γ-aminobutyric acid (GABA), pituitary adenylate cyclase activating peptide (PACAP), norepinephrine, neuropeptide Y (NPY), ghrelin, leptin, kisspeptin have been reviewed extensively (Lethimonier et al 2004;Levavi-Sivan et al 2010;Zohar et al 2010), however, potential interactions between them affecting the synthesis and release of gonadotropins have not been definitively elucidated yet. Of so many at least two are considered as the most important: GnRH and dopamine (Peter and Yu 1997).…”