R. de Leeuw

Pimozide modulates the luteinizing hormone-releasing hormone effect on gonadotrophin release in the African catfish, Clarias lazera

In the African catfish the effect of (i) the dopamine antagonist pimozide (PIM), (ii) a luteinizing hormone-releasing hormone analog (LH-RHa), and (iii) a combination of these substances on gonadotrophin (GTH) release and on ovulation was investigated. PIM alone increased plasma GTH levels in juvenile catfish but was ineffective in mature animals. LH-RHa increased plasma GTH levels in both juvenile and mature catfish and within 12 hr after the injection a high rate of ovulation (80%) was observed. PIM potentiated the LH-RHa effect on GTH release and ovulation. It was concluded that dopamine does not effect the GTH release directly, but modulates the effect of endogenous and exogenous gonadotrophin-releasing hormone.

Pituitary gonadotropin-releasing hormone (GnRH) receptor activity in goldfish and catfish: seasonal and gonadal effects

The goldfish pituitary contains two classes of gonadotropin-releasing hormone (GnRH) binding sites, a high affinity/low capacity site and a low affinity/high capacity site (Habibiet al. 1987a), whereas the catfish pituitary contains a single class of high affinity GnRH binding sites (De Leeuwet al. 1988a). Seasonal variations in pituitary GnRH receptor binding parameters, and the effect of castration on pituitary GnRH receptor binding were investigated in goldfish and catfish, respectively. In goldfish, GnRH receptors undergo seasonal variation with the highest pituitary content of both high and low affinity sites occurring during the late stages of gonadal recrudescence. The observed changes in pituitary GnRH receptor content correlate closely with responsiveness to a GnRH agonistin vivo in terms of serum gonadotropin (GTH) levels. In catfish, castration results in a two-fold increase in pituitary GnRH receptor content, which can be reversed by concomitant treatment with androstenedione, but not by the non-aromatizable androgen 11β-hydroxyandrostenedione; changes observed in GnRH receptor content correlate with variations in serum GTH levels and responsiveness to a GnRH agonist. In summary, the present study provides a clear evidence for seasonal variation in pituitary GnRH receptor activity in goldfish, and demonstrates a gonadal feedback mechanism regulating GnRH receptor activity in the catfish pituitary.

The dopminergic inhibition of the gonadotropin-releasing hormone-induced gonadotropin release: An in vitro study with fragments and cell suspensions from pituitaries of the African catfish, Clarias gariepinus (Burchell)

Pituitary fragments and pituitary cell suspensions of the African catfish in a perifusion system released gonadotropic hormone (GTH) spontaneously. After a few hours of perifusion a baseline of GTH release was established. The dopamine (DA) agonist apomorphine (APO) caused a decreased in the release from pituitary fragments but not from pituitary cell suspensions. APO abolished the luteinizing hormone-releasing hormone analog (LH-RHa)-induced GTH release from both pituitary fragments and cell suspensions. It was concluded that DA inhibits the GTH release induced by gonadotropin-releasing hormone.

Purification of gonadotropic hormone from the pituitary of the African catfish, Clarias gariepinus (Burchell), and the development of a homologous radioimmunoassay

Glycoprotein gonadotropic hormone (GTH) was purified from 2000 pituitaries of male and female African catfish, Clarias gariepinus. The first step was chromatography on concanavalin A-Sepharose followed by filtration on Ultrogel Aca 54, chromatography on DEAE-cellulose, and filtration on Ultrogel Aca 54, respectively. Finally, the purified fractions were analyzed by polyacrylamide gel electrophoresis. The gonadotropic activity in the different fractions was characterized using two tests: the radioimmunoassay for carp gonadotropin-beta subunit was used to quantify the immunoreactive GTH and a cAMP accumulation test was applied to measure the GTH biological activity. The purified glycoprotein GTH was used to raise antibodies and to develop a radioimmunoassay. This resulted in an assay with a variation between assays of approximately 4%, a precision of 4-8%, and an accuracy of 4-8%. GTH levels can be measured over a range of 0.8 to 12.5 ng/ml.

Pimozide-LHRHa-induced breeding of the African catfish, Clarias gariepinus (Burchell)

Abstract Reproduction of the African catfish under hatchery conditions depends on the artificial induction of egg maturation and ovulation. In this study the combination of pimozide/LHRHa as ovulation-inducing substance has been evaluated. At a dosage, given as one single injection, of 5 mg pimozide + 0.05 mg LHRHa per kilogram body weight, 100% ovulation was achieved. Most of the eggs, collected after stripping, could be fertilized and hatched normally.

The regulation of gonadotropin release by neurohormones and gonadal steroids in the African catfish, Clarias gariepinus

The secretion of gonadotropic hormone (GTH) from the pituitary of teleosts is considered to be regulated by neuropeptides and neuroamines of cerebral origin and steroid hormones from the gonads. This paper reviews our studies concerning the control of GTH release in the African catfish, Clarias gariepinus. It was demonstrated that luteinizing hormone releasing hormone (LHRH) stimulates GTH release and that the gonadotropin release-inhibiting activity of dopamine is restricted to the LHRH-induced GTH release. With regard to the inhibitory action of steroid hormones on GTH release, a hypothesis was postulated which links together the inhibitory actions of dopamines and gonadal steroids. According to this hypothesis, only aromatizable androgens should feed back on the release of GTH and the effects of catecholestrogens and dopamine on GTH release should be comparable.

Gonadotropic hormone-releasing hormone (GnRH) bioactivity in the brain of the immature rainbow trout, Salmo gairdneri: The effect of testosterone

The effect of exogenous testosterone on the synthesis and release of a bioactive gonadotropic hormone-releasing hormone (GnRH)-like substance has been studied in juvenile male rainbow trout. Long-term testosterone treatment caused an increased gonadotropic hormone (GTH) release and precocious sexual maturity. Tel- and diencephalon extracts from long-term testosterone-treated juvenile rainbow trout showed significantly higher GTH-releasing activity than extracts from control animals when injected in short-term testosterone-pretreated juvenile rainbow trout. It was concluded that long-term testosterone treatment caused an increased synthesis and release of GnRH-like material.

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

SummaryTo 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.Castration resulted in an increase in plasma GTH levels, a decrease in pituitary GTH content, and a degranulation of many gonadotropes. The aromatizable androgens testosterone and androstenedione were able to abolish the castration-induced increase in plasma GTH. This was accompanied with a restoration of pituitary GTH content and a regranulation of gonadotropes. The non-aromatizable androgens 5α-dihydrotestosterone and 11β-hydroxyandros tenedione did not have these effects. Replacement with estrone or estradiol resulted in an increase in pituitary GTH, however, without abolishing the elevated plasma GTH levels; ultrastructurally, many gonadotropes showed a welldeveloped granular endoplasmic reticulum together with a regranulation.The results of the present study indicate the significance of androgen aromatization in the feedback mechanism of gonadal steroids on the brain-pituitary axis.

Gonadotropin-releasing hormone-immunoreactive neuronal structures in the brain and pituitary of the African catfish, Clarias gariepinus (Burchell).

SummaryThe distribution of gonadotropin-releasing hormone (GnRH) immunoreactivity was studied in the African catfish, Clarias gariepinus, by means of immunofluorescence and immunoperoxidase techniques. Immunoreactive neurons were found throughout the preoptic nucleus (NPO). However, only a portion of the secretory perikarya in the NPO showed a positive reaction by use of an anti-LHRH serum. Numerous immunoreactive fibres were found to enter the pituitary and to terminate in its proximal pars distalis, the site of concentration of the gonadotropic cells. Since GnRH is present in the brain and pituitary of the African catfish, the lack of spontaneous ovulation in captivity is apparently due to an insufficient release of GnRH.

The effect of pimozide/LHRHa and 17α-hydroxyprogesterone on plasma steroid levels and ovulation in the African catfish, Clarias gariepinus

The effect was studied of a preovulatory gonadotropic hormone (GTH) surge, induced by pimozide/LHRHa treatment, and of 17α-hydroxyprogesterone on oocyte maturation and ovulation and on the plasma levels of 17α-hydroxyprogesterone, 17α-hydroxy-20β-dihydroprogesterone, testosterone and estradiol. GTH and 17α-hydroxyprogesterone initially caused an increase in plasma testosterone levels, followed by an increase in 17α-hydroxy-20β-dihydroprogesterone. Neither treatment affected estradiol levels. All fish treated with pimozide/LHRHa or 17α-hydroxyprogesterone showed oocyte maturation and ovulation.