sGnRH-A

Gonadotropin-releasing hormone (GnRH) binding sites have been characterized in the fully mature common carp ovary, using an analog of salmon GnRH ([D-Arg6,Trp7,Leu8,Pro9-NEt]-GnRH; sGnRH-A) as a labeled ligand. Binding of sGnRH-A to carp follicular membrane preparation was found to be time-, temperature-, and pH-dependent. Optimal binding was achieved after 40 min of incubation at 4 degrees C at pH 7.6; binding was found to be unstable at room temperature. Binding of radioligand was a function of tissue concentration, with a linear correlation over the range of 8.0-40.0 micrograms membrane protein per tube. Incubation of membrane preparations with increasing levels of [125I]sGnRH-A revealed saturable binding at radioligand concentrations greater than 400 nM. The binding of [125I]sGnRH-A to the carp ovary was also found to be reversible; addition of unlabeled sGnRH-A (10(-6) M) after reaching equilibrium resulted in complete dissociation of [125I]sGnRH-A within 30 min, and the log dissociation plot indicated the existence of a single class of binding sites. Addition of unlabeled sGnRH-A displaced the bound [125I]sGnRH-A in a dose-related manner. Hill plot as well as Scatchard analysis suggested the presence of one class of high affinity GnRH binding sites. Bound [125I]sGnRH-A was also found to be displaceable by other GnRH peptides, including sGnRH ([Trp7,Leu8]-GnRH), cGnRH-II ([His5,Trp7,Tyr8]-GnRH) and a GnRH antagonist ([D-pGlu1,D-Phe2,D-PTrp3,6]-GnRH; GnRH-ANT) in a parallel fashion, indicating that these peptides bind to the same class of binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

In fish, like in mammals, ghrelin affects gonadotropin release acting at the level of the hypothalamus as well as directly on the pituitary gland. In the present study, enzymatically dispersed pituitary cells obtained from sexually mature male and female carp (Cyprinus carpio L.) were incubated in the presence of human ghrelin at the concentration of 10(-7) or 10(-6) M, salmon GnRH analogue (Des-Gly(10), D-Arg(6), Trp(7), Leu(8), Pro(9))-LHRH (sGnRH-A) at the concentration of 10(-8) M or the combination of ghrelin (both concentrations) and sGnRH-A. ELISA method was used for carp LH levels determination in the media collected after 10 or 24 h of incubation. Ghrelin at the concentration of 10(-6) M caused the increase of the spontaneous LH secretion from female pituitary cells only. The combination of ghrelin (both concentrations) with sGnRH-A resulted in the significant elevation of LH levels in the incubations of both male and female pituitary cells in comparison with control incubations as well as with sGnRH-A alone treated cells. The results obtained in this study show that ghrelin functions as LH-stimulating hormone in common carp and that it acts directly on gonadotrophic cells, potentiating also the action of GnRH.