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Studies on ovarian GnRH-R and GnRH mRNA, and the direct effects of GnRH on ovarian function in the bovine Ramakrishnappa, Nagaraja

Abstract

In the bovine species, available information from a limited number of studies has resulted in contradicting opinions with respect to the intra-ovarian presence of GnRH-GnRH receptor system or direct effects of GnRH on ovarian function. Therefore, experiments were carried out to examine: if GnRH-R and GnRH mRNA are expressed in bovine ovary; the direct effects of a GnRH agonist (buserelin) on steroid hormone secretion from granulosa cells, luteal cells and luteal tissues; the effects of buserelin on mRNA expression for steroidogenic enzymes (StAR protein, P450scc, 3β-HSD) and the apoptotic genes (Bcl2, Bax), and; the effects of post-breeding GnRH administration on corpus luteum (CL) function and pregnancy outcome in Holstein cows. Results from present study revealed GnRH-R mRNA expression in granulosa cells o f small, medium, and large follicles as well as in the CL. The sequence analysis of RT-PCR-amplified products from granulosa cells and CL tissues revealed a complete homology to that of bovine pituitary GnRH receptor cDNA sequence. RT-PCR studies also revealed the possible evidence for presence of GnRH mRNA expression in granulosa cells from different size follicles. Buserelin elicited a dose-dependent biphasic response on E2 production from granulosa cells. A similar trend in P4 secretion from luteal cells and luteal tissues was observed following buserelin treatment. GnRH antagonist alone (P = 0.07) or in combination with buserelin resulted in a significant (P = 0.004) stimulatory responses on P4 output from CL tissues. In terms of luteal steroidogenic machinery, GnRH-a treatment of luteal tissues showed a mild (nonsignificant) stimulatory response on mRNA levels for StAR protein and P450scc mRNA although; tendency for significance (P = 0.12) could be seen only in the case of 3β-HSD. Buserelin treatment did not affect mRNA levels of Bax and Bcl2 in CL tissues. In response to post-breeding GnRH administration, despite the slight (P > 0.05) elevation in P4 levels, no improvement in pregnancy rates was observed. In conclusion, the present findings reveal evidence for the presence of GnRH-receptor mRNA expression in bovine ovarian follicles and CL. Buserelin (GnRH-a) caused a dose dependent biphasic response on steroid output from bovine granulosa cells, luteal cells and luteal tissue. However, the mRNA levels of StAR protein, P450scc, and 3β-HSD observed following buserelin treatment do not provide the definitive evidence for the direct interaction of GnRH with its receptor in the above cell types and tissue. Post-breeding GnRH administration did not result in improved CL function or pregnancy outcome in diary cattle.

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