PARENT SESSION
Ovulation

REGULATION OF PROGESTERONE RECEPTOR mRNA EXPRESSION IN BOVINE PERIOVULATORY FOLLICULAR AND LUTEAL TISSUE FOLLOWING THE LH SURGE.

Cassar, Carolyn¹, Dow, Mark^1,2, Pursley, J. Richard¹, Smith, George^1,2, ^{1 2}

ABSTRACT- Our long-term objective is to understand the intrafollicular regulation of the ovulatory process in dairy cattle. A requirement of progesterone receptor signaling pathways for follicle rupture in rodents has been well established. Such pathways may help mediate LH surge-induced increases in extracellular matrix degrading proteinases that break down the preovulatory follicle wall prior to ovulation. However, the intrafollicular localization and regulation of progesterone receptor expression during the periovulatory period in cattle are not known. Thus, our objectives were to determine the effect of the preovulatory LH surge on localization and expression of progesterone receptor mRNA in bovine periovulatory follicular and luteal tissue. Follicular growth and timing of the preovulatory LH surge were synchronized in dairy cows using the Ovsynch procedure (GnRH-7d-PGF2-36h-GnRH). Ovaries containing preovulatory follicles or corpora hemorrhagica (CH) were collected at 0, 6, 12, 18, 24 and 48 h (CH) after the second GnRH injection (n = 5-6 per timepoint). Expression of progesterone receptor mRNA was detected in periovulatory follicular and luteal tissue collected at all timepoints following the LH surge. Multiple progesterone receptor mRNA transcripts of 14.0, 12.0, 4.0, and 1.5 kb were observed. Relative levels of progesterone receptor mRNA were dramatically upregulated within 6 h after the LH surge compared to all other time points (P < 0.0001). In situ hybridization analysis revealed that the significant increase in progesterone receptor mRNA expression was localized to the granulosal layer of preovulatory follicles. Our results indicate that progesterone receptor mRNA expression is upregulated in the granulosal layer of bovine preovulatory follicles following the LH surge. Progesterone receptor signaling pathways may help mediate the effects of the preovulatory LH surge on follicle rupture in dairy cattle. Supported by USDA 98-35203-6226.

KEY WORDS: Progesterone Receptor, Ovulation, Bovine