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BOR - Papers in Press, published online ahead of print March 17, 2004.
Biol Reprod 2004, 10.1095/biolreprod.104.027615
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Submitted January 22, 2004
Returned for revision February 12, 2004
Accepted March 4, 2004

Pituitary


No Evidence for Pituitary Priming to Gonadotropin-Releasing Hormone in Relation to Luteinizing Hormone (LH) Secretion Prior to the Preovulatory LH Surge in Ewes

J. L. Crawford *, J. R. McNeilly , and A. S. McNeilly

* To whom correspondence should be addressed. E-mail: janet.crawford{at}agresearch.co.nz.

Abstract
The purpose of this study was to determine the occurrence of, and the regulatory mechanisms involved in priming of the pituitary to GnRH prior to the preovulatory LH surge in sheep. Expt 1: Forty-two ewes had progestagen devices removed after 14 days and were assigned to luteal (Lut) or follicular (Foll) groups. Fifteen days later, blood sampling was initiated either immediately or 36h after induced luteolysis in Groups Lut and Foll, respectively. After 4h, ewes were administered either saline (n=5), 250ng (n=8) or 10µg (n=8) of GnRH. Five ewes per treatment group were killed 1h later whilst remaining animals were blood sampled for a further 7h. Expt 2: Eighteen ewes were allocated to Lut and Foll groups (described above). Blood samples were collected from 2h prior to GnRH (10µg) treatment, until 7h thereafter. Despite up-regulated GnRH-R mRNA levels in Foll ewes, pituitary content and plasma levels of LH and LH{beta} mRNA levels were similar between groups. Mean FSH{beta} mRNA and plasma FSH levels were elevated in Lut ewes but declined after GnRH treatment. Inversely, plasma estradiol and inhibin-A concentrations were higher in Foll ewes and declined after GnRH treatment. Fewer LH+ve/secretogranin II-ve (SgII-ve) granules were present in gonadotropes of Foll ewes, coincident with increased basal LH levels. Fewer smaller-sized granules were present after GnRH treatment. In conclusion, there was no evidence of self-priming prior to onset of the preovulatory LH surge. Constitutive release of LH+ve/SgII-ve granules may maintain basal LH levels whilst smaller-sized, presumably mature granules may be preferentially released after GnRH stimulation.

Key words: Pituitary • Follicle-stimulating hormone • Gonadotropin-releasing hormone • Luteinizing hormone


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[Abstract] [Full Text] [PDF]


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