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BOR - Papers in Press, published online ahead of print January 28, 2004.
Biol Reprod 2004, 10.1095/biolreprod.103.021998
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Submitted August 6, 2003
Returned for revision August 27, 2003
Accepted January 14, 2004

Pituitary


Regulation of Gonadotropin-Releasing Hormone (GnRH) Receptor Gene Expression in Tilapia: Effect of GnRH and Dopamine

Berta Levavi-Sivan *, Helena Safarian , Hanna Rosenfeld , Abigail Elizur , and Ayelet Avitan

* To whom correspondence should be addressed. E-mail: sivan{at}agri.huji.ac.il.

Abstract
This work was designed to study certain aspects of the endocrine regulation of gonadotropin-releasing hormone receptor (GnRH-R) in the pituitary of the teleost fish tilapia. A GnRH-R (taGnRH-R) was cloned from the pituitary of hybrid tilapia and was identified as a typical seven transmembrane receptor. Northern blot analysis revealed a single GnRH-R transcript in the pituitary, of approximately 2.3 kilobases (kb). taGnRH-R mRNA levels were found to be significantly higher in females than in males. Injection of the salmon GnRH analog (sGnRHa; 5-50 µg/kg) increased the steady-state levels of taGnRH-R mRNA, with the highest response recorded at 25 µg/kg and at 36 h. At the higher dose of sGnRHa (50 µg/kg), taGnRH-R transcript appeared to be down-regulated. Exposure of tilapia pituitary cells in culture to graded doses (0.1-100 nM) of seabream (sbGnRH = GnRH I), chicken II (cGnRH- II), or salmon GnRH (sGnRH = GnRH III) resulted in a significant increase in taGnRH-R mRNA levels. The highest levels of both LH (luteinizing hormone) release and taGnRH-R mRNA levels were recorded after exposure to cGnRH II, and the lowest after exposure to sbGnRH. The dopamine agonist quinpirole suppressed LH release and mRNA levels of taGnRH-R, indicating an inhibitory effect on GnRH-R synthesis. Collectively, these data provide evidence that GnRH in tilapia can up-regulate, while dopamine down-regulates, taGnRH-R mRNA levels.

Key words: Pituitary • Dopamine • Gonadotropin-releasing hormone • Gonadotropin-releasing hormone receptor


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