Stress and the control of LH secretion in the ewe.
Stress influences the activity of the reproductive system at several sites. One of the most significant effects is at level of the GnRH secretory system to reduce GnRH pulsatility and thus LH pulsatility. This in turn reduces the oestradiol signal that stimulates the GnRH-LH surge in the follicular phase. Three sequential phases have been identified in the induction of the GnRH-LH surge by oestradiol: (i) activation, (ii) transmission and (iii) surge secretion. There is evidence that administration of endotoxin prevents activation but not transmission, hypoglycaemia blocks both activation and transmission, whereas truck transport is effective during the late, but not early, transmission phase. Opioids mediate the suppressive effects of hypoglycaemia on both LH pulsatility and the delayed onset of the LH surge in ewes. The exact neurocircuitry used in sheep is yet to be identified but many of the connections that are proposed as important in rats are present in sheep. Corticotrophin-releasing hormone (CRH) neurones in the paraventricular nucleus that project axons to the median eminence probably do not directly inhibit GnRH, but either afferent or parallel central pathways are involved. New members of the CRH peptide and receptor families have been identified, but roles in the control of reproduction have yet to be determined.
Smith
R F
RF
Department of Veterinary Clinical Science, University of Liverpool, Leahurst, Neston, Cheshire CH64 7TE, UK. robsmith@liverpool.ac.uk
Ghuman
S P S
SP
Evans
N P
NP
Karsch
F J
FJ
Dobson
H
H
D000324:Adrenocorticotropic Hormone / Q000097:blood; D000818:Animals; D001127:Arginine Vasopressin / Q000097:blood; D003346:Corticotropin-Releasing Hormone / Q000097:blood; D004731:Endotoxins / Q000378:metabolism; D004958:Estradiol / Q000097:blood; D005260:Female; D005498:Follicular Phase / Q000097:blood*; D007987:Gonadotropin-Releasing Hormone / Q000097:blood*; D006706:Homeostasis; D006854:Hydrocortisone / Q000097:blood; D007003:Hypoglycemia / Q000378:metabolism; D007986:Luteinizing Hormone / Q000097:blood*; D009294:Narcotics / Q000097:blood; D006080:Ovarian Follicle / Q000502:physiology; D011374:Progesterone / Q000097:blood; D012636:Secretory Rate; D012757:Sheep Diseases / Q000097:blood*; D013312:Stress, Physiological / Q000097:blood*
D004731:Endotoxins; D009294:Narcotics; D001127:Arginine Vasopressin; D007987:Gonadotropin-Releasing Hormone; D011374:Progesterone; D004958:Estradiol; D000324:Adrenocorticotropic Hormone; D007986:Luteinizing Hormone; D003346:Corticotropin-Releasing Hormone; D006854:Hydrocortisone