GTH Secretion Research Articles

Control of gonadotropin release in the Atlantic croaker ( Micropogonias undulatus): Evidence for lack of dopaminergic inhibition

Gonadotropin (GTH) secretion is known to be under inhibitory dopaminergic control in several species of fish. To investigate whether this is also the case in the Atlantic croaker (Micropogonias undulatus), juvenile and adult croaker were treated with a gonadotropin-releasing hormone analog (des-Gly10D-Ala6Pro9 n ethylamide luteinizing hormone-releasing hormone (LHRHa), 1-100 ng/g body wt) in combination with various dopaminergic drugs (1-20 mg/kg body wt). None of the dopamine antagonists tested, metoclopramide, pimozide, haloperidol, and domperidone, were able to increase plasma GTH levels above those induced by treatment with LHRHa alone and in some cases the gonadotropin response to LHRHa was reduced. The dopamine agonists bromocriptine and apomorphine either had no effect on the normal response to LHRHa or increased it. None of the drugs tested had any detectable effect on GTH levels in the absence of LHRHa. These results provide evidence for a lack of dopaminergic inhibition in the control of GTH secretion in the Atlantic croaker.

Evidence That Gonadotropin-Releasing Hormone Also Functions as a Growth Hormone-Releasing Factor in the Goldfish*

The present study examined the influence of GnRH on the in vivo and in vitro secretion of GH in the goldfish (Carassius auratus). Intraperitoneal injection of several GnRH peptides, including a form native to goldfish, salmon GnRH (sGnRH), elevated circulating GH levels in female goldfish. An analog of mammalian GnRH (mGnRH), [D-Ala6,Pro9-NEt] mGnRH (mGnRH-A), at a dosage of 0.1 microgram/g BW increased serum GH levels for up to 48 h after a single ip injection. Goldfish receiving a series of injections of this dose of mGnRH-A also displayed an increased rate of body growth, indicating that the mGnRH-A-induced increase in the circulating GH level was sufficient to accelerate body growth. In vitro experiments using perifused pituitary fragments found that sGnRH stimulated the secretion of GH from the goldfish pituitary in a potent, dose-dependent, and reversible manner. The time course of response and half-maximally effective dose of sGnRH were very similar for both GH and gonadotropin (GTH) secretion in vitro, suggesting that the mechanism(s) mediating the stimulatory actions of GnRH in the goldfish may be similar for both GH and GTH secretion. However, GnRH-induced GH and GTH secretion from the goldfish pituitary can occur independently of each other, as demonstrated by the finding that somatostatin inhibited the GnRH stimulation of GH secretion in vitro, without influencing the GTH response, whereas the dopamine agonist apomorphine inhibited GnRH-induced GTH secretion in vitro, without influencing the GH response. Furthermore, the dopamine antagonist pimozide did not influence serum GH levels, although pimozide potentiated the stimulatory effect of GnRH on GTH secretion in vivo by blocking the endogenous GTH release inhibitory action of dopamine. Results of the present study suggest that the secretion of GH and GTH in the goldfish are regulated, at least in part, through a common releasing factor, GnRH, whereas somatostatin and dopamine appear to act independently as GH and GTH release inhibitory factors, respectively.

Effects of Aroclor 1254 and cadmium on reproductive endocrine function and ovarian growth in atlantic croaker

The effects of a polychlorinated biphenyl (PCB) mixture (Aroclor 1254) and cadmium on the reproductive endocrine function in female Atlantic croaker ( Micropogonias undulatus) were investigated. Fish were fed Aroclor 1254 in the diet (0·5 mg/100 g body wt/day) for 17 days or exposed to 1 mg cadmium/liter seawater (30% S) for 40 days during the period of ovarian recrudescence. Both Cd and the PCB disrupted reproduction, but they exerted opposite effects on reproductive endocrine function and ovarian growth. PCB treatment impaired ovarian growth which was accompanied by a significant decline in vitellogenesis as assessed by a decrease in plasma estradiol levels. In addition there was a decrease in spontaneous secretion of gonadotropin (GTH) in vitro by pituitaries from PCB-treated fish. In contrast, exposure to Cd accelerated ovarian growth and elevated plasma estradiol concentrations, suggesting a stimulation of vitellogenesis. In addition there was a marked increase in the spontaneous secretion of GTH from pituitaries of Cd-exposed fish in vitro. The results show that chemicals can disrupt reproductive processes in teleosts both by inhibiting and stimulating the activity of the hypothalamus-pituitary-gonadal-liver axis.

Serotonin stimulates gonadotropin release in female and male goldfish, Carassius auratus L

Effects of serotonin (5-HT) on serum gonadotropin (GtH) levels were studied in female and male goldfish, Carassius auratus L. Intraperintoneal (ip) administration of 5-HT (10 μ/g) produced a significant increase in serum levels of GtH at 0.5, 1, and 2 hr but not at 3–4 hours, after the drug was injected. Adminstration of different concentrations of 5-HT into the brain third ventricle (iv) did not affect serum GtH levels. In both sexes, the intraperitoneal administration of different doses of 5-HT (2.5–20 μg/g) caused a dose-dependent increase in serum GtH levels in fish at different gonadal stages; the greatest responses occurred in mature (=prespawning) fish. Fluoxetine (10 μg/g; ip), a 5-HT reuptake inhibitor, potentiated the stimulatory effects of 5-HT on GtH secretion. Ketanserin (10 μg/g; ip), a 5-HT antagonist, did not modify basal GtH levels; however, ketanserin blocked the 5-HT induced release of GtH. These results indicate that 5-HT has a stimulatory effect on GtH release in goldfish, most likely at the level of the pituitary gland.

Development of salmon GTH I and GTH II radioimmunoassays

Radioimmunoassays (RIAs) for the measurement of two gonadotropins, GTH I and GTH II, in the plasma and pituitary of salmonid fish were developed using a rabbit antiserum to β-subunits of chum salmon GTH I and GTH II. Intact GTH I and GTH II were used as standards and radioactive competitors. The displacement curves for plasma of salmonids including chum salmon, amago salmon, and rainbow trout were parallel to chum salmon GTH I and GTH II standards. Parallel displacement curves were obtained for pituitary extracts of chum salmon and amago salmon. The cross-reactivities of growth hormone, prolactin, and proopiomelanocortin (POMC)-related hormones were less than 1% in both RIAs. However, cross-reactivities of GTH I in the GTH II RIA and GTH II in the GTH I RIA were 10 and 12%, respectively. Plasma concentrations of both GTHs from salmonids at various stages of reproductive development were compared. In immature rainbow trout of both sexes (males: average (AV) gonadosomatic index (GSI) = 0.05; females: AV GSI = 0.24), plasma levels of GTH I and GTH II were low (less than 2 ng/ml). During prematurational stages of spermatogenesis and vitellogenesis in rainbow trout (males: AV GSI = 0.43; females: AV GSI = 2.8), the predominant GTH in the pituitary and plasma was GTH I. In contrast, plasma concentrations of GTH II were significantly higher than those of GTH I in postovulatory amago and chum salmon. Similarly, pituitary concentrations of GTH II were significantly higher than those of GTH I in postovulatory and spermiating amago salmon and postovulatory chum salmon. These data suggest that the synthesis and secretion of GTH I and GTH II vary during salmonid reproductive development.

Evidence for the involvement of D 2 receptors in the dopaminergic inhibition of gonadotropin release in the African catfish, Clarias gariepinus

In the African catfish, Clarias gariepinus, dopamine (DA) inhibits the gonadotropin (GTH)-releasing hormone (GnRH)-stimulated GTH release. The aim of the present investigations was to characterize the DA receptor, involved in the regulation of GTH secretion. Specific D 1 and D 2 agonistic and antagonistic drugs were administered to female African catfish, in combination with a luteinizing hormone-releasing hormone analogue (LHRHa). D 1 agonists and antagonists did not influence the LHRHa-induced GTH release. On the other hand, the LHRHa stimulation was inhibited by a D 2 agonist and enhanced by D 2 antagonists. It is concluded that in the African catfish the inhibiting action of dopamine on the GnRH-stimulated GTH release is mediated via the D 2 receptor.

Circadian changes in GtH secretion in carp males in contact or not with another male or hypophysed female

Abstract Changes in the maturational gonadotropin (c‐GtH) levels during 24 h in carp males, as a result of the presence or absence of other carp specimens, were investigated. Carp males were cannulated to ventral aortae. Gonadotropin levels were radioimmunologically determined. It has been found that in males two peaks of c‐GtH occur strictly in 2 periods during the day in which spawning takes place under natural conditions (early morning and late evening). Being in contact or not with another male or hypophysed female did not affect circadian changes in c‐GtH levels in investigated males.

Effect of osmotic pressure on prolactin release in rainbow trout: In vitro studies

To investigate a possible effect of osmotic pressure on prolactin (PRL) release in rainbow trout, we developed a technique for in vitro perifusion of trout pituitaries. Changes in osmotic pressure similar to those observed in fish plasma during transfer experiments did not induce significant modifications of PRL release. In contrast, high-amplitude variation of osmotic pressure resulted in clear modifications of PRL secretion: hyperosmotic medium caused a reduction in PRL release, while infusion of hyposmotic medium induced a transitory increase in PRL release. By using different concentrations of mannitol, we found that the modifications of prolactin secretion could not be ascribed to alterations of the ionic composition of the medium but actually resulted from variations in the osmotic pressure of the incubation medium. In further experiments osmotic pressure was decreased from 300 to 220 mOsm/kg or from 400 to 300 mOsm/kg; a similar transitory increase in PRL release was observed. Measurement of gonadotropin (GtH) in the perifusion effluent medium showed that PRL and GtH secretion followed similar patterns. Thus, our results suggest a possible mechanical effect of wide changes in osmotic pressure on pituitary cell membranes. These data indicate that the rainbow trout differs notably from nonsalmonid teleost species thus far studied in the lack of sensitivity of its PRL cells to osmotic pressure.

Effects of [ d-Arg 6, Trp 7, Leu 8, Pro 9 NEt]-luteinizing hormone-releasing hormone (sGnRH-A) and [ d-Ala 6, Pro 9NEt]-luteinizing hormone-releasing hormone (LHRH-A), in combination with pimozide or domperidone, on Gonadotropin release and ovulation in the Chinese loach and common carp

The effects of LHRH-A and sGnRH-A alone and in combination with the dopamine receptor antagonists pimozide (PIM) and domperidone (DOM) on stimulation of gonadotropin (GtH) secretion and ovulation in Chinese loach ( Paramisgurnus dabryanus) and common carp ( Cyprinus carpio) were studied. When tested in the absence of a dopamine receptor antagonist, sGnRH-A was about 10-fold more potent than LHRH-A in stimulating GtH secretion in loach, whereas these peptides were of similar effectiveness in common carp. Both peptides were effective alone in inducing ovulation in loach, but were ineffective in common carp. PIM and DOM stimulate a small but significant increase in serum GtH, and greatly potentiate the effects of LHRH-A and sGnRH-A on GtH secretion and ovulation in both species. DOM was of equal or somewhat greater potency than PIM in potentiating the action of releasing hormone agonists in loach; however, DOM had markedly greater effectiveness in common carp. sGnRH-A in the presence of DOM was more effective than LHRH-A in stimulating GtH secretion and ovulation in common carp. The present studies suggest that there are marked differences in the relative contribution of GnRH and dopamine in the control of GtH secretion in loach and common carp, and indicate in particular that the more intense dopaminergic inhibitory control of GtH release in common carp modifies the responsiveness of the pituitary to GnRH peptides.

Brain distribution of radioimmunoassayable gonadotropin-releasing hormone in female goldfish: Seasonal variation and periovulatory changes

A radioimmunoassay (RIA) for [Trp 7, Leu 8]gonadotropin-releasing hormone (sGnRH) was developed to determine the gonadotropin-releasing hormone (GnRH) content in discrete brain areas of female goldfish at different stages of ovarian development. Temporal changes in serum gonadotropin (GtH) and GnRH concentrations in discrete brain areas were measured during spontaneous ovulation. There were no clear parallel changes in brain GnRH with seasonal ovarian development in goldfish. However, under a 10° temperature acclimation regimen, the GnRH content in the hypothalamus and pituitary decreased as the ovary progressed from the regressed to the mature condition; on the other hand, GnRH content in the spinal cord increased in sexually mature fish compared with that in regressed fish. Significant decreases in GnRH concentration were observed in certain brain areas (olfactory bulbs, telencephalon, hypothalamus, and pituitary) of fish undergoing spontaneous ovulation compared with those of nonovulatory fish. The simultaneous changes of GnRH concentration in these brain areas suggested that the GnRH neuronal system may function as an integrated unit for the activation of GtH secretion during ovulation in goldfish.

Effects of (D-Ala6, Pro9N ethylamide) — LHRH on plasma levels of gonadotropin, 17α,20β-dihydroxy-4-pregnen-3-one and testosterone in male goldeye (hiodon alosoides Rafineque)

Male goldeye were treated with (D-Ala(6), Pro(9)-N ethylamide) - LHRH (LHRH-A) in saline or a silastic pellet (100 µg.kg(-1) body weight) and changes in plasma gonadotropin (GtH), 17α,20β-dihydroxy-4-pregnen-3-one (17α,20βP) and testosterone (T) levels were determined by radioimmunoassay. LHRH-A increased plasma GtH levels, with the response to LHRH-A in saline being of much greater magnitude and duration than the response to silastic pellet implants. Seasonal differences were found in the response to LHRH-A. Spermiated fish were the most responsive, recrudescing fish the least, and regressed fish displayed an intermediate response. Plasma 17α,20βP levels were elevated in response to LHRHA in fish of all sexual stages although the magnitude of the increase was not related to the magnitude of the increase in GtH levels. Treatment with LHRH-A also resulted in a transitory increase in plasma T levels. The endocrine control of GtH and steroid secretion in goldeye is discussed in relation to studies in cyprinids and salmonids.

The effects of aromatizable androgens, non-aromatizable androgens, and estrogens on gonadotropin release in castrated African catfish, Clarias gariepinus (Burchell)

To study the feedback mechanism of gonadal hormones on GTH secretion in male African catfish, the effects of castration and steroid replacement on GTH release, pituitary GTH content, and ultrastructural appearance of gonadotropes were investigated.

Endocrine profiles in the females of a twice-annually spawning strain of rainbow trout

Abstract Endocrine profiles of the reproductive phases in individual females were studied using forty-four 2.5-year-old fish for 1 year following their first ovulation in winter. Ovulation occurred from 24 April to 28 July and from 14 December to 26 January, in 24 individuals (68.6%) and 33 individuals (94.3%), respectively, out of 35 females which survived the whole experimental period. Ovulation occurred over a few months in the summer breeding season, whereas it was rather concentrated in the winter breeding season. In the fish that were successful ovulators in the summer, plasma estradiol-17β rapidly increased in January, attaining the highest values from March to May and then decreased before ovulation. Plasma gonadotropin (GtH) gradually increased from January followed by a second increase just before ovulation in summer. In the fish that failed to mature in summer, plasma estradiol and GtH showed little increase from January to February, and synchronously decreased in March. Lengthening daylength in spring probably inhibited GtH secretion in the once-spawners, but not in the twice-spawners which already had high plasma estradiol and GtH at the critical period between late February and late March. In almost all females, synchronous increases in plasma estradiol and GtH occurred in July, probably triggered by the shortening daylength. The highest values for estradiol were attained in October.

Effects of dopamine and apomorphine on gonadotropin release from the transplanted pars distalis in goldfish

Male goldfish bearing pars distalis transplants from other male goldfish have increased serum gonadotropin (GtH) levels, due to the high spontaneous release rate of GtH by the transplants. Intraperitoneal injection of dopamine or its agonist, apomorphine, each reduced the elevated serum GtH levels caused by the release from the transplanted pars distalis. These results suggest that dopamine has GtH-release-inhibitory activity and acts directly on gonadotrophs to inhibit spontaneous secretion of GtH.

Effects of catecholaminergic agonists and antagonists on serum gonadotropin concentrations and ovulation in goldfish: Evidence for specificity of dopamine inhibition of gonadotropin secretion

The elevated serum gonadotropin (GtH) levels in goldfish receiving two injections of des Gly 10, [ d-Ala 6] LH-RH ethylamide (LH-RH-A), given 12-hr apart, were reduced by apomorphine, a dopamine agonist, injected at either the first or the second LH-RH-A injection. Serum GtH concentrations in goldfish given two injections of LH-RH-A at a 3-hr interval were also depressed by bromocriptine, a specific D-2 dopamine receptor agonist, administered simultaneously with both LH-RH-A injections. Injections of dopamine antagonists, pimozide or metoclopramide (a specific D-2 antagonist) caused increased serum GtH concentrations in normal goldfish, but no changes were found following injections of the α-adrenergic antagonist phentolamine, the β-adrenergic antagonist propranolol, or the sympathomimetic agent octopamine. Injection of pimozide or metoclopramide at the time of the second of two LH-RH-A injections given at an interval of 12 hr potentiated the LH-RH-A-induced increase in serum GtH concentrations; injection of phentolamine, propranolol, or octopamine did not alter the response to LH-RH-A. Injections of pimozide or metoclopramide also increased the frequency of ovulation in LH-RH-A-injected gravid female goldfish. These results suggest that stimulation of dopamine receptors can block the potentiating effect of multiple injections of GtH-releasing hormone, as well as ongoing LH-RH-A-stimulated release. The results also indicate that the dopamine inhibition of GtH secretion is specific and may be mediated by receptors resembling the D-2 type receptors in mammals.

Twenty four-hour rhythm of gonadotropin release induces cyclic progesterone secretion by the ovary of prepubertal and adult golden hamsters.

Cyclic release of gonadotropin (GTH) begins on day 16 or 17 of age, about 2 weeks before ovulatory estrous cycles are initiated in female golden hamsters (Mesocricetus auratus). The daily cycles of LH and FSH are characterized by surges that peak at about 1700 h. The timing of this daily surge is similar to that observed in adult photoperiod-induced anovulatory or lactating females and to that of ovulatory females on the afternoon of proestrus. The purpose of this investigation was to determine if initiation of cyclic GTH release during the prepubertal period results in significant changes in ovarian function. A second objective was to examine the similarity between cyclic secretion of GTH in the prepubertal animal and that in the short day anovulatory adult. Groups of prepubertal females were killed throughout a 24-h period on days 13-14, 19-20, and 27-28, i.e. both before and after the expected onset of cyclic GTH release. LH did not vary significantly on days 13-14, but thereafter showed a significant rhythm, with a maximum at 1700 h. Levels of progesterone (P) were low and nonvarying on days 13-14, but a significant rhythm was measured in samples collected on days 27-28. When samples were collected from photoperiod-induced anovulatory adults, the rhythm of circulating LH and P was similar to that in the late prepubertal female. Ovariectomy of 22-day-old females resulted in low and tonic levels of P when the animals were killed 3 days later at 1400, 1700, and 2000 h, showing that the cyclical rhythm of P measured in these samples results from ovarian, not adrenal, secretion. The response of the daily cycle of LH to phenobarbital (PB) blockade in prepubertal females was identical to that in the adult: the LH surge did not occur on the afternoon of PB injection, but reappeared 24 h later at the expected time. The late afternoon surge of P was also blocked by PB administration. These results show 1) that the initiation of cyclic GTH secretion by the prepubertal female hamster influences ovarian maturation as reflected by changes in the levels and patterns of secretion of P; 2) that the rhythms of LH, FSH, and P secretion in the prepubertal female are similar to those in the photoperiod-induced anovulatory adult; and 3) that the response of the prepubertal female to barbiturate blockade is identical to that of the adult.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulation of spermiation by LHRH analogue in carp.

The effects of daily administration of LHRH analogue (D-Ala8-desGly10) LHRH ethylamide in NaCl or propylene glycol for five days was studied on spermiation, plasma gonadotropin and 11-ketotestosterone levels. Daily injection of 10 μg/kg body weight of LHRH analogue in 0.7% NaCl increased the volume of collectable milt, the number of spermatozoa, and serum gonadotropin and 11-ketotestosterone levels. Daily injection of 1 μg/kg in NaCl or 1 μg/kg and 10 μg/kg in propylene glycol did not stimulate these respnses. Theses results indicate that LHRH analogue can stimulate the secretion of GtH from pituitary gland which in turn stimulates androgen secretion and the spermiation in the testis.

Effects of pimozide and des Gly 10,[ d-Ala 6]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°, a single injection of des Gly 10,[ d-Ala 6]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.

Evolution of Neurohormonal Regulation of Reproduction in Lower Vertebrates

Immunological and chromatographic studies demonstrate that the gonadotropin (GtH) releasing hormone (GnRH) in the brain of chondrichthyes, teleosts, reptiles and birds is different from luteinizing hormone-releasing hormone (LH-RH) of mammals. LH-RH is present in brain extracts from amphibia. The alterations in the structure of LH-RH found in various of the non-mammalian vertebrates are at positions 7 and 8; the structure of chum salmon GnRH is Trp7-Leu8-LH-RH, and in chickens is Gln8-LH-RH. A response to injection of LH-RH or its agonistic analogues, in terms of increased blood levels of GtH or a gonadal response such asovulation indicative of increased GtH secretion, has been found in all classes of vertebrates, and the cephalochordate amphioxus. This suggests a basic similaritiy of the GnRH receptors throughout vertebrates, and that the ancestral origin of the system was in the invertebrate chordates. Immunohistochemical studies demonstrate that many lower vertebrates have perikarya containing LH-RH-like material in the preoptic and ventrobasal hypothalamic regions. Brain lesioning studies provide functional evidence for GnRH from both locations in amphibia and only the ventrobasal hypothalamus in teleosts. Brain lesioning studies on goldfish suggest the presence of a GtH release-inhibitory factor (GRIF). Dopamine has GRIF activity in goldfish and common carp to modulate the actions of LH-RH and spontaneous release of GtH. How widespread this system for dual neurohormonal regulation of GtH secretion is in vertebrates is not known.

Effects of synthetic LH-RH and analog on plasma gonadotropin levels and maturational response to 17α-hydroxy-20β-dihydroprogesterone

The effects of LH-RH and analog on the stimulation of gonadotropin (GtH) secretion have been studied in order to use them as priming, before inducing maturation with 17α-hydroxy-20β-dihydroprogesterone (17α-20βP) in the carp Cyprinus carpio. One injection of LH-RH (2 μg/kg body weight) or LH-RH analog (0.3 μg/kg) induces a transient rise in the plasma GtH concentration, but subsequent 17α-20βP treatment does not result in an increase in the number of oocytes with germinal vesicle breakdown or in ovulation. Two and three in ections of LH-RH or analog at the same dosages are followed by a stimulation of GtH secretion (to 30 ng/ml) which is maintained for 6–9 h, and lasts longer with LH-RH analog than with LH-RH. The same magnitude in the increase of GtH secretion is obtained with lower doses of LH-RH analog. Injection of 17α-20βP then increases the number of oocytes undergoing germinal vesicle breakdown and sometimes partial ovulation. Futher studies on the mechanism of action of GtH during priming hypophysation treatment are needed in order to develop a sequential treatment using: (1) priming by LH-RH or analog, (2) 17α-20βP induced maturation treatment.