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The interaction of GnRH with its receptor represents a crucial point for control of reproduction. Additionally, the porcine GnRH receptor (GnRHR) gene is a positional candidate for genes influencing reproductive efficiency, residing in a similar chromosomal region as a QTL for ovulation rate (OR). Gonadotropic gene expression in lines of swine divergent for OR may provide insight into the phenomena of increased litter size. Chinese Meishan (OR=14.5), a white crossbred line selected based on an index of OR and embryonic survival (OR=17.8), and a Control line (OR=13.2) were evaluated. Following three (Index and Control) or six (Meishan) estrous cycles, gilts were ovariectomized and treated with a GnRH antagonist (SB-75) or 0.9% saline at 60, 36 and 12 hours prior to sacrifice. Blood samples were collected prior to treatment and at sacrifice. Serum LH and FSH levels were determined by RIA and RNA was extracted from anterior pituitary tissue. In all lines, LH was reduced to basal levels by SB-75 treatment; however, FSH was reduced in the Control line only. Levels of GnRHR and gonadotropin subunit gene expression were measured by quantitative real-time PCR. Expression of the GnRHR gene prior to treatment was highest in Meishan, intermediate in Index and lowest in Control. Levels of GnRHR gene expression were reduced after treatment in Control and Index, but not Meishan gilts. Initial amounts of glycoprotein α-subunit mRNA were highest in Control and Index compared to Meishan. Further, values decreased following treatment in pituitaries from Control and Index lines only. Prior to SB-75 treatment, FSHβ-subunit mRNA levels were higher in Control and Index compared to Meishan, decreasing in Control and Index, but not Meishan after treatment. Expression of the LHβ-subunit gene was highest in Control, intermediate in Index and lowest in Meishan; however amounts of mRNA decreased in Control and Index following treatment and remained unchanged in Meishan. Affymetrix GeneChip® Porcine Genome Arrays were run on pooled RNA from each breed x treatment group in duplicate and gene expression analyzed. Treatment with the GnRH antagonist, SB-75, decreased or increased expression of 30, 17 and 33 genes in Control, Index and Meishan gilts, respectively, of which 14, 7 and 11 genes were expressed at greater than 2-fold over gilts treated with vehicle. Twenty-three genes were down-regulated in Control gilts and 7 genes had increased expression levels compared to vehicle treated females. Expression of 12 genes was decreased in the anterior pituitary glands of Index line gilts, whereas only 5 genes were up-regulated following treatment with SB-75. Gene expression in ovariectomized Meishan gilts was increased in 25 and decreased in 8 genes. Thus, gene expression patterns in response to treatment with a GnRH antagonist suggest differential mechanisms underlying gonadotrope function between lines of swine.