Bernard Breton

7 The Gonadal Steroids

Publisher Summary This chapter discusses that the gonads potentiality to produce steroids, and the regulation of the syntheses. The nature, shape, and intensity of a hormonal signal, ready to be received by a target cell, is the result of an intricate series of positive and negative regulations. In the case of hormonal steroids in fish, only some aspects of this complex have been considered. Once a steroid is secreted, several mechanisms may inactivate it before it reaches its target. Little is known of catabolism of sexual steroids in teleosts. Most available data are concerned with the total radioactivity found in tissues after fish are fed labeled steroid. The biological significance of glucuronidation or sulfonation remains to be explored. Although the conjugated steroids are usually considered to be inactive, recent studies attribute a pheromonal role to glucuronides. In other respects, the binding to plasma proteins may lead to a reversible inactivation, although, in mammals, it has been suggested that steroid secretion may be enhanced by the presence of serum steroid-binding proteins. Finally, the conversion of plasma steroids into biologically active metabolites can occur in some target tissues. The actual physiological role of gonadal steroids in fish is discussed with emphasis on gametogenesis.

A reinvestigation of the Gn-RH (gonadotrophin-releasing hormone) systems in the goldfish brain using antibodies to salmon Gn-RH

SummaryThe organization of Gn-RH systems in the brain of teleosts has been investigated previously by immunohistochemistry using antibodies against the mammalian decapeptide which differs from the teleostean factor. Here, we report the distribution of immunoreactive Gn-RH in the brain of goldfish using antibodies against synthetic teleost peptide.Immunoreactive structures are found along a column extending from the rostral olfactory bulbs to the pituitary stalk. Cell bodies are observed within the olfactory nerves and bulbs, along the ventromedial telencephalon, the ventrolateral preoptic area and the latero-basal hypothalamus. Large perikarya are detected in the dorsal midbrain tegmentum, immediately caudal to the posterior commissure. A prominent pathway was traced from the cells located in the olfactory nerves through the medial olfactory tract and along all the perikarya described above to the pituitary stalk. In the pituitary, projections are restricted to the proximal pars distalis. A second immunoreactive pathway ascends more dorsally in the telencephalon and arches to the periventricular regions of the diencephalon. Part of this pathway forms a periventricular network in the dorsal and posterior hypothalamus, whereas other projections continue caudally to the medulla oblongata and the spinal cord. Lesions of the ventral preoptic area demonstrate that most of the fibers detected in the pituitary originate from the preoptic region.

Comparative distribution of somatostatin, LH-RH, neurophysin, and α-endorphin in the rainbow trout: An immunocytological study

Abstract Using immunofluorescence, evidence of a somatostatin (SRIF)-like antigen has been found in the brain and digestive tract of rainbow trout. In the diencephalon, periventricular SRIF immunoreactive hypendymocytes are located in the region dorsal to the nucleus preopticus (NPO). SRIF immunoreactive perikarya are concentrated anterior to the NPO in the nucleus preopticus periventricularis, scattered in small cells in the nucleus lateralis tuberis (NLT) pars anterior, and in a few cells located in an unnamed nucleus in the dorsomedial hypothalamus. In the pituitary SRIF immunoreactive material is located in the neurohypophysial tissue in the proximal pars distalis. In the gut, SRIF cells have been found in the endocrine pancreas and in the gastric mucosa. Comparatively, material immunoreactive for luteinizing hormone-releasing hormone has the same distribution in the pituitary as SRIF, whereas neurophysin immunoreactivity was found in only the neurophysial tissue of the neurointermediate lobe. A few cells reacting with anti-α-endorphin were seen in the NLT in the pituitary stalk region. All the pars intermedia cells in the neurointermediate lobe react with anti-α-endorphin.

Plasma 11-oxotestosterone and gonadotropin during the beginning of spermiation in rainbow trout (Salmo gairdneri R.)

Radioimmunoassays are used to follow plasma 11-oxotestosterone and glycoprotein gonadotropin levels related to sperm production in male rainbow trout at the onset of spermiation. The 11-oxotestosterone concentrations are higher in the plasma of males giving a measurable volume of sperm. When these levels increase as spermiation progresses, the gonadotropin level tends to decrease slightly. Furthermore, the quantities of collected sperm are positively correlated with the 11-oxotestosterone levels, but not with GTH secretion.

Advancement and synchronisation of spawning in Salmo gairdneri and S. trutta following administration of LRH-A combined or not with pimozide

Abstract Prior to the onset of the spawning season female rainbow trout were injected intraperitoneally (IP) with LRH-A ([des Gly 10 D-Ala 6 ] LHRH ethylamide) alone at a dose of 1 μg/kg body weight or in combination with pimozide (PIM, 10 mg/kg body weight), a dopamine antagonist, given 6 h before the LRH-A. Pimozide + LRH-A and pimozide alone induced a significant elevation in plasma GTH but only PIM + LRH-A consistently advanded ovulation. Egg quality was altered with PIM + LRH-A and PIM + saline, resulting in lower survival at the eyed stage. Prior to the onset of the spawning period, brown trout were injected IP with LRH-A alone at doses of 1, 10 and 20 μg/kg body weight. A single injection slightly initiated and synchronized ovulation at doses of 10 and 20 μg; a second injection given 6 days later did not modify the ovulation profile. There was no effect when the same doses were given in pelleted form in silicone rubber implants. When LRH-A and PIM were injected IP 6 h apart at doses of 10 and 1 mg/kg body weight PIM and 10 μg/kg body weight LRH-A, ovulation was slightly stimulated. The effects of LRH-A given in silicone rubber implants were potentiated by simultaneous injection of PIM (10 mg/kg body weight). No alteration of egg quality was observed after these various treatments on brown trout.

Effects of acute versus sustained administration of GnRHa on GtH release and ovulation in the rainbow trout, Oncorhynchus mykiss

Abstract The effects of different forms of GnRHa (sustained vs. acute) on the stimulation of gonadotropin secretion and the rate of ovulation were investigated in the rainbow trout, Oncorhynchus mykiss . A biodegradable sustained release form of DTrp 6 LH-RH induced a progressive rise in the gonadotropin plasma levels in a dose-dependent manner. The stimulation of gonadotropin secretion was maintained for 3 weeks. All fish ovulated within 5 days after injection using a high dosage (50 μg/kg body weight) whereas with the lower dosages (25 and 12.5 μg/kg body weight) 100% ovulation was reached after 8 days. In contrast, acute injections of GnRHa (20 μg/kg body weight) induced a transient stimulation of gonadotropin secretion. Ovulation rates were first accelerated within 8–10 days and then remained stable but they did not reach 100% 26 days after the injection. At that time the rate of ovulation in the control group had increased to 70%. These results are discussed, along with a comparison of variability of egg quality with treatments.

Gonadotropine glycoproteique maturante et oestradiol-17β pendant le cycle reproducteur chez la truite fario (Salmo trutta) femelle

Maturational glycoproteique gonadotropin and estradiol-17β during the reproductive cycle of the female brown trout (Salmo trutta). Female brown trout were sacrificed every month or twice a month during the reproductive season. Plasma gonadotropin (GtH) and estradiol-17β (E2-17β) were measured using radioimmunological techniques, in relation with the state of gametogenesis. From ovulation to July eggs diameters remained less than 1 mm, and ovogenesis was characterized from the histological appearance of three types of vitellus: glycoproteique (type I), lipidique (type II), lipidoproteique (type III), and the immunological plasma detection of the vitellogenin. During this period GtH levels remained lower than 1 ng/ml except in March, when they increased around 5 ng/ml, both with pituitary GtH and plasma E2 17β. This rise occurred just before the appearance of the type III vitellus within the oocyte, and might correspond to a critical phase of the reproductive cycle during which vitellogenin could not be detected in 70% of the animals. During rapid growth of the oocyte from 1 to 5 mm, mean GtH levels increased from 0.75 to 2.5 ng/ml when those of the E2-17β increased more rapidly but began to drop before the end of vitellogenesis. Maturation and ovulation were accompanied by a rise of the GtH level, in correlation with the lower levels of E2-17β measured during the cycle, GtH remained high even after ovulation, and more in fish which had kept their eggs within the body cavity. Correlation among GtH, E2-17β, and the diameters of eggs had been calculated. There was a positive correlation among GtH, E2-17β levels, and the diameters of eggs during exogenous vitellogenesis, and a negative between GtH and E2-17β at the end of the reproductive cycle. These results were discussed, in relation to the existence of a pulsatile mode of GtH secretion.

Neuropeptide Y stimulates in vivo gonadotropin secretion in teleost fish

The effects of the neuropeptide Y (NPY), alone or in combination with a gonadotropin-releasing hormone analogue, D-Ala6-desGly10-Pro9-Net LHRH (LHRHa), have been studied on the in vivo secretion of the maturational gonadotropin (GtH2) in the rainbow trout Oncorhynchus mykiss and the common carp Cyprinus carpio. Depending on the species, two routes of administration were used: in trout, intraperitoneal injection (20 micrograms/kg body wt); in carp, direct infusion (3 micrograms/kg body wt) into the third ventricle via a temporary brain cannula. In both cases NPY alone induced a twofold increase in GtH2 secretion and peaked 2 to 4 hr administration regardless of the route of injection. The plasma gonadotropin levels returned to basal within 8 hr. The relative increases (peaked secretion/basal secretion) did not differ with the route of injection. When the animals were first treated with NPY and then LHRHa (20 micrograms/kg) 1 hr later, the magnitude of the response to LHRHa was greater in the animals pretreated with NPY, indicating either a potentiation of LHRHa action by NPY or additive effects of the two peptides. The return to basal levels also took longer in fish receiving NPY first. NPY may act directly at the pituitary level or activate central neuromediatory systems.

Differential effect of insulin-like growth factor I on in vitro gonadotropin (I and II) and growth hormone secretions in rainbow trout (Oncorhynchus mykiss) at different stages of the reproductive cycle.

The short-term effect of insulin-like growth factor I (IGF-I) on GTH I (FSH-like), GTH II (LH-like), and GH production by cultured rainbow trout pituitary cells was studied in immature fish of both sexes, at early gametogenesis and in spermiating and periovulatory animals. IGF-I had no effect on basal GTH I and GTH II release, whereas it always inhibited basal GH, showing decreasing intensity with the gonad maturation. In absence of IGF-I, GTH I and GTH II cells were always responsive to GnRH, whereas no response was observed for GH cells whatever the sexual stage. The action of IGF-I on the sensitivity to GnRH differs between GTH and GH cells. The former requires a coincubation with IGF−I (10−6 m)/GnRH to show an increase in sensitivity, independent of the sexual stage. To be responsive to GnRH, the GH cells require longer exposure to IGF-I, the efficiency of which decreases with gonad maturation. The action of IGF-I (10−6 m) on GTH cell sensitivity to GnRH does not seem to be related to a mitogenic effe...

Use of pituitary cells in primary culture to study the regulation of gonadotropin hormone (GtH) secretion in rainbow trout: setting up and validating the system as assessed by its responsiveness to mammalian and salmon gonadotropin releasing hormone

To study the regulation of gonadotropin secretion in rainbow trout in vitro, a method for preparing primary cultures of dispersed pituitary cells is described. Cells were dispersed by collagenase 0.1% in Hanks saline solution for 20 hr at 12 degrees and a high yield of viable cells was obtained. Attempts to improve cell functioning were made by varying culture conditions (density of cells initially plated, age of the culture). Cell functioning was assessed by their ability to respond to increasing doses of mammalian and salmon GnRH. Pituitaries were collected from spermiating males whose pituitaries are known to be sensitive to mammalian GnRH in vivo. Using 96-well plates, optimal conditions for good biological activity, are initial plating with 6.2 X 10(4) cells, incubation with GnRH for 24 hr on the third day after plating. In these conditions mammalian analog and salmon GnRH induced an increase in GtH release for doses ranging from 10(-9) to 10(-6) M. The GtH released during the GnRH incubation period does not decrease the sensitivity of the system since addition of 20 ng of GtH at the beginning of incubation does not modify the response profile.