title: The pituitary and testicular responses to GnRH challenge between 4 and 14 months of age in thoroughbred colts born in spring and autumn.
abstract: Gonadotropin releasing-hormone analogue (buserelin) challenges were carried out every 8 weeks from 4 to 14 months of age on thoroughbred colts born in the spring (n = 6) or autumn (n = 5) to define the onset of puberty. In all colts, luteinizing hormone (LH) secretion followed a seasonal pattern, with high baseline and maximal concentrations in the spring and summer and low concentrations in the winter. Testosterone concentrations were undetectable before spring and, thus, autumn-born colts were younger than spring-born colts when a testosterone response to buserelin was first observed. Mean weights of the autumn-born colts were 300 kg (282-327 kg) at the time of the first detectable testosterone response in the following spring (October). Spring-born colts had reached this weight in the winter (May and June, before day length had increased) but did not exhibit a significant testosterone response until the spring at a mean weight of 352 kg (327-403 kg). It is proposed that colts must achieve a threshold body weight concurrently with stimulatory photoperiod for onset of puberty to occur.
authors.affiliation: Institute of Veterinary, Animal and Biomedical Sciences, Massey University, Palmerston North, New Zealand.
authors.forename: C G
authors.initials: CG
authors.lastname: Brown-Douglas
authors.forename: E C
authors.initials: EC
authors.lastname: Firth
authors.forename: T J
authors.initials: TJ
authors.lastname: Parkinson
authors.forename: P F
authors.initials: PF
authors.lastname: Fennessy
mesh_terms: D000375:Aging; D000818:Animals; D001724:Birth Weight; D001835:Body Weight; D002064:Buserelin / Q000008:administration & dosage*; D007987:Gonadotropin-Releasing Hormone / Q000031:analogs & derivatives; D006736:Horses; D007986:Luteinizing Hormone / Q000097:blood / Q000378:metabolism; D008297:Male; D017440:Photoperiod; D010902:Pituitary Gland / Q000187:drug effects* / Q000502:physiology; D012621:Seasons; D013737:Testis / Q000187:drug effects* / Q000502:physiology; D013739:Testosterone / Q000097:blood
chemical_list: D007987:Gonadotropin-Releasing Hormone; D013739:Testosterone; D007986:Luteinizing Hormone; D002064:Buserelin