I've got a question in regards to maternal recognition of pregnancy in the mare. Oxytocin from the posterior pituitary causes the uterus to release prostaglandins, which then regress the CL, right? What is the trigger for this release of oxytocin from the PP?
Also, in cattle and sheep, I know there is a postive feedback loop, where uterine PGF causes the CL to release Oxytocin, causing more uterine PGF... I've been told that the CL of the mare does NOT release oxytocin, so how does the mare provide enough prostaglandin to regress the CL?
Finally, how does the conceptus block the production of prostaglandins?
I am a second year vet student, studying reproduction at the moment. I realise that I am no expert in this area, but this is the information that i got straight from my notes. This is the most up to date information as we are being lectured by Prof.C.Wathes who has done a lot of research into the field of maternal recognition of pregnancy, especially in Mares. Hope this is of help!
In most species, Prostaglandins cause luteal regression. In the mare, Luteolysis is effected by the pulsatile release of endometrial Prostaglandin F2alpha between days 12-16 of the cycle.
Exogenous (i.e.administered) Oxytocin (OT) can cause luteolysis(regression of the Corpus luteum). The pattern of OT release and uterine Oxytocin receptor formation during the cyle and early pregnancy has not yet been elucidated.
According to my notes, the Corpus luteum does not, indeed produce oxytocin. This suggests that OT is not as important in the mare during pregnancy and that other hormones may be more important.
In all species, the effect of the OT depends on the number of OT receptors present.e.g. in the cow, Increased oxytocin receptors develop in the endometrium around day 16-17. OT of luteal origin can then bind to these, causing pulsatile release of PGF2alpha. OT given to a cow on day 15 will have no effect on PGs because the OT receptors are not present. Given on day 17, when they are present, there is an increase in PG release.
Oxytocin receptor formation can be stimulated by oestradiol, so oestrogens in ruminants are luteolytic. In ruminants, the conceptus secretes trophoblast interferon from about day10-21. This has local action on the endometrium to prevent oxytocin receptor formation.
Contrary to belief, only a small amount of OT is required, as it is highly potent.
The conceptus, is small, spherical and highly mobile and it is through its interaction with the uterus, this mechanism is prevented. However, the mechanism is not clear.
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