title: Proopiomelanocortin mRNA levels in ovine hypothalamus are not reduced at the time of the preovulatory luteinising hormone surge.
abstract: A reduction in inhibition (disinhibition) of gonadotropin-releasing hormone (GnRH) secretion by endogenous opioid systems in the hypothalamus is thought to be permissive of the preovulatory surge of GnRH and luteinising hormone (LH). In rats, proopiomelanocortin (POMC) mRNA levels in the arcuate nucleus are reduced at the time of the LH surge, and this is thought to be part of the neural mechanism of oestrogen positive feedback. There are no studies of POMC mRNA levels at the time of the LH surge in other species. POMC mRNA levels were measured by in-situ hybridization using a 35S-labelled cRNA probe and computer-assisted grain counting in the arcuate nucleus of ovary-intact ewes (n=4) killed on day 10 of the luteal phase or 24 or 48 h into the follicular phase (experiment 1), and ewes killed on day 10 of the luteal phase or during the preovulatory LH surge (experiment 2). Grain counts per cell, the proportion of cellular area covered by grains and the number of labelled cells per section were not significantly different (P > 0.1) between animals killed in the luteal phase and animals killed during the follicular phase or during the LH surge. We conclude that in sheep, POMC mRNA levels are not reduced at the time of the preovulatory LH surge, and reduced POMC gene transcription does not appear to be part of the neural mechanism of oestrogen positive feedback in this species.
authors.affiliation: Prince Henry's Institute of Medical Research, Clayton, Victoria, Australia.
authors.forename: J P
authors.initials: JP
authors.lastname: Walsh
authors.forename: A
authors.initials: A
authors.lastname: Rao
authors.forename: D M
authors.initials: DM
authors.lastname: Simmons
authors.forename: I J
authors.initials: IJ
authors.lastname: Clarke
mesh_terms: D000818:Animals; D001111:Arcuate Nucleus of Hypothalamus / Q000378:metabolism; D004971:Estrus / Q000378:metabolism; D005260:Female; D007031:Hypothalamus / Q000378:metabolism*; D007091:Image Processing, Computer-Assisted; D007986:Luteinizing Hormone / Q000378:metabolism*; D011333:Pro-Opiomelanocortin / Q000096:biosynthesis*; D012333:RNA, Messenger / Q000096:biosynthesis*; D011863:Radioimmunoassay; D012756:Sheep
chemical_list: D012333:RNA, Messenger; D011333:Pro-Opiomelanocortin; D007986:Luteinizing Hormone