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The hypothalamic regulation of gonadotropin secretion in Tilapia: mechanism of GnRH action and sites of dopamine interaction
Levavi-Sivan, B.; Yaron, Z. (1992). The hypothalamic regulation of gonadotropin secretion in Tilapia: mechanism of GnRH action and sites of dopamine interaction, in: Progress in aquaculture research: proceedings of the 4th German-Isreali Status Seminar held on October 30-31, 1990. Spec. Publ. Eur. Aquacult. Soc, 17: pp. 167-196
In: (1992). Progress in aquaculture research: proceedings of the 4th German-Isreali Status Seminar held on October 30-31, 1990. Spec. Publ. Eur. Aquacult. Soc, 17. European Aquaculture Society: Oostende. ISBN 90-71625-11-7. 360 pp., more
In: Spec. Publ. Eur. Aquacult. Soc, more

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Keyword
    Marine

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  • Levavi-Sivan, B.
  • Yaron, Z.

Abstract
    The paper summarizes our findings on the transduction of GnRH stimulatory effect on gonadotropin (GTH) secretion from the pituitary of tilapia (Oreochromis niloticus x O. aureus) and the possible sites of its dopaminergic inhibition. The baseline secretion rate of tilapia GTH (taGTH) from perifused pituitary fragments, and its secretion in response to a pulse of a GnRH superactive analog (D-Ala6, Pro9 -GnRH-N ethylamide=GnRHa), were both attenuated in Ca defficient medium. Ca²+ ionophore (A23187) mimicked GnRH effect only in the presence of Ca. Reintroduction of Ca was followed by dramatic increase in taGTH secretion. A 5 min pulse of OAG (1 oleyl-2-acetyl-rac-glycerol; 2 -200 µM) resulted in a dose-dependent increase in taGTH release. These results, combined with those previously reported in tilapia, indicate that the relay of GnRH effect on GTH release in this fish involves, as in mammals, the increase in intracellular Ca²+ .phosphoinositide turnover, activation of protein kinase C and arachidonic acid or its metabolic products. In order to test the hypothesis that cAMP is also involved in the mediation of GTH release, pituitary cells were dispersed and cultured for 7 days and then challenged for 24 h with agonists of cyclic AMP. Dibutyryl cAMP (dbcAMP) or 8-Br-cAMP caused a dose dependent increase in taGTH secretion. Activation of adenylate cyclase by forskolin or cholera toxin also evoked an increase in taGTH secretion. Furthermore, inhibition of phosphodiesterase activity by IBMX augmented the response to GnRHa. Kinetic studies in perifused pituitary fragments showed that the response to a cAMP agonist occurs within less than 6 min. Determination of cAMP production by perifused pituitaries showed a sharp peak of cAMP in the effluent medium within the first 15 min after a pulse of GnRHa, which was followed by a peak in taGTH. The results are consistent with the hypothesis that in the fish, the adenylate cyclase-cAMP system is involved as a mediator in the transduction of GnRH signal and that this involvement is associated with an early phase of the effect. Dopamine (DA) suppressed both the basal and the GnRH-stimulated secretion of GTH from perifused pituitary fragments. Withdrawal of DA invariably resulted in a dramatic increase in the secretion rate of GTH, far above 2+ the baseline. DA curtailed the rise in GTH release in response to Ca²+ ionophore (A23187) or in response to the activation of PKC by OAG. The stimulatory effect of dbcAMP (3 mM) or forskolin (10 µM) was greatly diminished in the presence of 0.5 or 1.0 µm DA, respectively. However, the stimulated production of cAMP in response to GnRHa or forskolin was not reduced in the presence of DA. Arachidonic acid (0.5 mM) circumvented, to a great extent the dopaminergic inhibition of GTH release. It is concluded that DA exerts its inhibitory effect on the Ca²+ -DAG-PKC transduction system at a site distal to the Ca in flux and distal to the activation of PKC, but proximal to the formation of arachidonic acid. At the other transduction route of adenylate cyclase-cAMP system, the dopamine inhibition of GnRH-stimulated GTH release is exerted distal to the formation of the nucleotide, possibly at the level of protein kinase A, or perhaps on the mobilization of some internal Ca²+ pools.

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