title: Effect of constant light exposure on circulating gonadotrophin levels and hypothalamic gonadotrophin-releasing hormone (GnRH) content in the ovariectomized pony mare.
abstract: Melatonin is thought to play a role in relaying photic information to the central nervous system as part of the seasonal reproductive cycle of the mare. However, the mechanisms by which melatonin may act are unknown. Therefore, this study was designed to determine whether exposure to constant light would, by reducing circulating melatonin concentrations, have any effect on hypothalamic gonadotrophin-releasing hormone (GnRH) content and circulating levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Blood samples were collected for 12 h at 15-min intervals from 8 ovariectomized (OVX) pony mares under ambient light conditions (12 h light, 12 h dark, October 6). Animals were then placed under representative ambient light (12 h light, 12 h dark; control, n = 4) or constant light (24 h light, 0 h dark; treatment, n = 4) in light-controlled rooms. Blood samples were collected daily and on experimental Days 14, 21 and 28 samples were collected at 15-min intervals for 12 h for analysis of circulating LH and melatonin (Bleed 2, 3 and 4, respectively). All animals were killed on Day 28 (following Bleed 4) and the hypothalami were collected. Mares exposed to constant light had significantly higher (P less than 0.05) LH concentrations in daily blood samples and showed significantly (P less than 0.05) higher LH concentrations during frequent sampling periods on Days 14 and 21 (Bleeds 2 and 3) compared with control mares. FSH did not differ significantly among groups in the daily samples. GnRH content was 1.5994 +/- 0.325 and 0.9457 +/- 0.193 pg/mg protein (treatment and control respectively).(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Animal Science Department, University of Florida, Gainesville 32611.
authors.forename: B D
authors.initials: BD
authors.lastname: Cleaver
authors.forename: W R
authors.initials: WR
authors.lastname: Grubaugh
authors.forename: S D
authors.initials: SD
authors.lastname: Davis
authors.forename: P C
authors.initials: PC
authors.lastname: Sheerin
authors.forename: K J
authors.initials: KJ
authors.lastname: Franklin
authors.forename: D C
authors.initials: DC
authors.lastname: Sharp
mesh_terms: D000818:Animals; D005260:Female; D005640:Follicle Stimulating Hormone / Q000097:blood; D007987:Gonadotropin-Releasing Hormone / Q000032:analysis*; D006062:Gonadotropins / Q000097:blood*; D006064:Gonadotropins, Equine / Q000097:blood; D006736:Horses / Q000097:blood / Q000502:physiology*; D007031:Hypothalamus / Q000737:chemistry / Q000502:physiology*; D008027:Light; D007986:Luteinizing Hormone / Q000097:blood; D008550:Melatonin / Q000097:blood; D010052:Ovariectomy
chemical_list: D006062:Gonadotropins; D006064:Gonadotropins, Equine; D007987:Gonadotropin-Releasing Hormone; D007986:Luteinizing Hormone; D005640:Follicle Stimulating Hormone; D008550:Melatonin