R.E. Peter

Endocrine changes during natural spawning in the white sucker, Catostomus commersoni: I. Gonadotropin, growth hormone, and thyroid hormones☆

White suckers (Catostomus commersoni; Cypriniformes, Teleosteii) spawning in a small stream in central Alberta were captured during different stages of their spawning migrations in 1981 and 1982, blood was sampled, and the fish were examined to determine their reproductive condition. Blood samples were analyzed for gonadotropin (GtH), growth hormone (GH), triiodothyronine (T3), and thyroxine (T4) by radioimmunoassay. GtH levels in both sexes were lowest prior to the onset of spawning, increased significantly in spawning males, females in which germinal vesicle migration had begun, and ovulated females and then dropped significantly in spent fish of both sexes. GH was lowest in prespawning females, increased significantly at ovulation, and remained high in spent females. In contrast, GH levels in males were relatively constant throughout spawning. In both sexes, highest T4 levels were found in prespawning fish, and T4 decreased significantly in spent fish. Although a similar decline was seen in T3 in 1981, in 1982 there were no T3 changes associated with changes in reproductive condition. No significant diurnal variations were detected in the levels of GtH or T3; T4 levels appeared to vary on a diurnal basis in prespawning males only. Spawning activity in both sexes therefore appears to be associated with increases in GtH occurring at ovulation in females and at the initiation of spawning in males. GH levels may also be related to reproductive condition in females, but not in males. The relationship of thyroid hormone levels to reproductive condition is less clear, however, and these levels may reflect both endocrine and environmental influences on thyroid function.

Onset of gonadotropic hormone accumulation in the immature trout pituitary gland in response to estrogen or aromatizable androgen steroid hormones

Abstract A study was conducted to determine the specificity of the steroids able to cause a stimulation of pituitary gonadotropic hormone (GtH) levels in sexually immature rainbow trout. Low pituitary GtH levels characteristic of juvenile trout were rapidly elevated following treatment with estrogen and aromatizable androgenic steroid hormones, but nonaromatizable androgens and progestogenic steroids were ineffective. An aromatase inhibitor, 1,4,6-androstatrien-3,17-dione, significantly reduced the pituitary GtH increase in response to treatment with testosterone. A central effect of steroid hormones on pituitary GtH was confirmed by the fact that pituitary steroid implants did not increase liver weight as observed after perivisceral hormone treatment. The possible participation of sex steroids in a mechanism of initiation of gonad recrudescence or onset of pubertal gonadal development in salmonids is discussed.

The endocrinology of growth in carp and related species

Abstract The focus of this review is on various aspects of the endocrine control of growth by the brain neuroendocrine-growth hormone (GH)-insulin-like growth factor (IGF) axis in carp and other cyprinids, with reference to other species, including mammals, if there are particular gaps in knowledge. The neuroendocrine regulation of GH secretion in goldfish and other carp is multifactorial, with a balance of stimulatory and inhibitory inputs to somatotrophs. Somatostatin is the primary inhibitor of basal and stimulated growth hormone (GH) secretion. GH secretion is stimulated by GH-releasing factor, gonadotropin-releasing hormone, dopamine, neuropeptide Y, thyrotropin-releasing hormone and cholecystokinin. Sex steroids, in particular estradiol, influence the responsiveness of the somatotrophs to neuroendocrine factors; the responsiveness to gonadotropin-releasing hormone, neuropeptide Y, and thyrotropin-releasing hormone is increased by estradiol, whereas the responsiveness to dopamine and cholecystokinin is greatest in sexually regressed goldfish. Growth hormone, a 188-amino-acid peptide with 5 cysteine residues, stimulates growth through direct actions on some tissues, as well as by stimulation of production of insulin-like growth factor (IGF) production. The liver has a high number of GH receptors and is a primary target organ; GH binding sites have also been demonstrated in gill, intestine, kidney and gonads. GH increases the efficiency of food conversion; GH stimulates intestinal amino acid transport and intestinal mass, which may be one mechanism for the effects on food conversion. A number of factors influence GH receptor number, including GH itself, nutritional status and other hormones. IGFs have been characterized in several salmonid species using molecular biology techniques. Production of IGFs is under stimulation of GH, insulin and other hormones, and is also influenced by nutritional status and metabolic factors. The liver contains the highest concentrations of IGF, although IGFs have been found in a number of other tissues. IGFs appear to travel in the blood bound to specific binding proteins. IGF receptors have been demonstrated in only one teleost species to date. IGF-I stimulates cartilage proteoglycan synthesis; GH is dependent on IGF-I for this action. Growth rates of cultured fish may be stimulated by neuroendocrine factors added to food, administration of GH or recombinant GH preparations, or by producing GH transgenic lines of fish. Techniques for enhancing growth rates of cultured fish are in an experimental stage.

Effects of pimozide and des Gly10,[d-Ala6]luteinizing hormone-releasing hormone ethylamide on serum gonadotropin concentrations, germinal vesicle migration, and ovulation in female goldfish, Carassius auratus

In gravid female goldfish held at 10-12 degrees, a single injection of des Gly10,[D-Ala6]luteinizing hormone-releasing hormone ethylamide (LHRH-A), or 2 injections of LHRH-A given 12 hr apart, increased serum gonadotropin (GtH) concentrations, but did not stimulate germinal vesicle migration or ovulation. Injection of a dopamine antagonist, pimozide (PIM), increased serum GtH levels. PIM injected with the second of two LHRH-A injections not only potentiated the LHRH-A-induced increase in serum GtH concentrations but also caused germinal vesicle migration and ovulation. PIM injected with, or in place of, the first of two LHRH-A injections increased the frequency of ovulation in fish injected with LHRH-A. Similarly, PIM injected simultaneously with a single LHRH-A injection also increased the number of ovulating fish. These results are consistent with the idea that dopamine inhibits GtH release in goldfish. The results also suggest that the preovulatory surge of GtH secretion is regulated by both a stimulation by GtH-releasing hormone and release from the dopamine inhibition on GtH secretion.

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.

The influence of mammalian and teleost somatostatins on the secretion of growth hormone from goldfish (Carassius auratus L.) pituitary fragments in vitro

The effect of various vertebrate somatostatins (SRIF) on basal growth hormone (GH) secretion from goldfish pituitary fragments was studied using an in vitro perifusion system. SRIF-14 caused a rapid and dose-dependent decrease in the rate of GH release from goldfish pituitary fragments. The half-maximal effective dose (ED50) of SRIF-14 was calculated as 1.3 nM following exposure to two minute pulses of increasing concentrations of SRIF-14, whereas the ED50 of SRIF-14 calculated after continuous exposure to sequentially increasing doses of SRIF-14 was 65 nM. This difference suggests that the pituitary fragments were less responsive to SRIF-14 in the latter experiment, possibly as a result of previous exposure to SRIF-14. SRIF-28 was found to be equipotent with SRIF-14 in decreasing basal GH secretion from the goldfish pituitary. In contrast, catfish SRIF-22, a uniquely teleost SRIF isolated from catfish pancreatic islets, did not alter GH secretion. These results provide further support for the hypothesis that SRIF-14 or a very similar molecule functions as a GH release-inhibiting factor in teleosts, indicating that this action of SRIF-14 has been fully conserved throughout vertebrate evolution.

Ovulatory surge of gonadotropin in the goldfish, Carassius auratus

The periovulatory pattern of circulating gonadotropin is described for the first time in a poikilothermic vertebrate. In sexually mature female goldfish (Carassius auratus) maintained on a 16L:8D photoperiod, ovulation occurs during the latter part of the dark phase, approximately 20 hr after the fish are warmed from 13 to 21°. Serum gonadotropin levels increase gradually during the first half of the light phase, and by the latter part begin to increase rapidly, reaching the peak of the gonadotropin surge by the onset of the dark phase. Gonadotropin levels remain high through the period of ovulation, and decrease markedly by the onset of the following light phase. The findings indicate that the goldfish provides a valuable model for the study of ovulation in teleosts.

Growth homone-dependent potentiation of gonadotropin-stimulated steroid production by ovarian follicles of the goldfish

The possible involvement of growth hormone (GH) in the regulation of ovarian function in the goldfish was investigated by determining the effects of common carp GH on steroid production by vitellogenic and preovulatory ovarian follicles incubated in vitro. Carp GH acts in a dose-dependent manner to potentiate the actions of common carp gonadotropin (GtH) on the production of 17 beta-estradiol and testosterone by vitellogenic ovarian follicles and the actions of human chorionic gonadotropin (hCG) on testosterone production by preovulatory ovarian follicles. Carp GH alone had no effect on basal steroid secretion by either class of ovarian follicles. Chum salmon GH but not bovine GH also enhanced carp GtH-induced production of 17 beta-estradiol by vitellogenic ovarian follicles. Common carp prolactin had no effects on basal or GtH-stimulated steroid production by vitellogenic or preovulatory ovarian follicles. The actions of carp GH on preovulatory follicles were not apparent when tested with the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine, suggesting that GH may act to enhance either the formation or actions of cAMP. In summary, these data demonstrate that GH has a direct modulatory effect on GtH-stimulated steroid production and suggest that GH may be an important regulator of follicular development in the goldfish.

In vitro release of gonadotropin-releasing hormone from the brain preoptic-anterior hypothalamic region and pituitary of female goldfish

In vitro release of gonadotropin releasing hormone (GnRH) from slices of the preoptic-anterior hypothalamic (P-AH) region and fragments of the pituitary of goldfish was studied using a static incubation system. Release of GnRH from both tissue preparations was stimulated by depolarizing concentrations of extracellular potassium ions (K+). Other putative secretagogues, calcium ionophore A23187 (1 microM), forskolin (100 microM), and prostaglandin E2 1 microM) also stimulated release of GnRH from both tissue preparations. Omission of Ca2+, or chelating the remaining remaining Ca2+ by EGTA (0.1 mM), abolished the release of GnRH stimulated by high K+ concentrations (60 mM), but did not reduce spontaneous release. Verapamil (1 microM), a voltage-sensitive calcium channel blocker, abolished the release of GnRH stimulated by high K+ or A21387 from both tissue preparations. The GnRH released in vitro from both the P-AH region and pituitary was concentrated by Sep-Pak and then separated by high-performance liquid chromatography. The major peak of the GnRH immunoreactivity was found to coelute with synthetic salmon GnRH [( Trp7,Leu8]-GnRH) and the minor peak with chicken GnRH-II [( Gln8]-GnRH). Dopamine (10 and 100 microM) inhibited GnRH release from both P-AH slices and pituitary fragments, while serotonin (1-100 microM) stimulated release from both. Norepinephrine (10-100 microM) stimulated GnRH release from P-AH slices but not from pituitary fragments. The results demonstrate that the release of GnRH from goldfish P-AH slices and pituitary fragments in vitro in response to various secretagogues and monoamines can be studied using a static incubation system.

Induction of ovulation in goldfish, Carassius auratus, by pimozide and analogues of LH-RH

Abstract The effects of intraperitoneal injections or pellet implantations of luteinizing hormone-releasing hormone analogues, des-Gly10-[D-ALA6]-LH-RH-ethylamide (LRH-Aa) or des-Gly10-[D-Trp6]-LH-RH-ethylamide (LRH-At), respectively, and the combination of these treatments with pimozide, a dopamine antagonist, on serum gonadotropin (GtH) and ovulation in goldfish held at 18–20°C were investigated. Injections of LRH-Aa (0.1 μg/g body weight) or pellet implants of LRH-At (25 and 125 μg/fish) alone increased serum GtH levels, but were relatively ineffective for inducing ovulation in goldfish (≅ 25% rate of occurrence). Pimozide greatly potentiated the GtH release response to LRH-Aa injections or LRH-At pellet implantations. A high rate of occurrence of ovulation (87%) was observed within 24 h when pimozide was injected prior to or with the first of two injections of LRH-Aa, or following implantations of LRH-At pellets that give continuous release of LRH-At. These results are consistent with the idea that dopamine acts directly at the pituitary level as a GtH release-inhibitory factor in goldfish.