Relationships among LH, FSH and prolactin secretion, storage and response to secretagogue and hypothalamic GnRH content in ovariectomized pony mares administered testosterone, dihydrotestosterone, estradiol, progesterone, dexamethasone or follicular fluid.

The research focuses on how certain treatments, both steroidal and nonsteroidal, impact hormone levels in female ponies who have had their ovaries removed. This includes hormones such as luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL). The study found that these treatments can affect the secretion, storage, and response of these hormones to artificial stimulus, as well as the content of gonadotropin releasing hormone (GnRH) in certain areas of the brain.

Research Design and Treatment

• The study involved thirty-five pony mares that have been ovariectomized, meaning their ovaries were removed.
• The ponies were treated daily with various substances, including testosterone, dihydrotestosterone, estradiol, progesterone, dexamethasone, a form of equine follicular fluid, and vegetable shortening as a control group.
• The injections to stimulate hormone secretion, namely GnRH and thyrotropin releasing hormone (TRH), were given one day prior to treatment and again after 15 days of treatment.

Results and Observations

• It was found that testosterone, dihydrotestosterone and dexamethasone treatments reduced the levels of LH, while estradiol increased its level.
• Progesterone treatment, on the other hand, only reduced LH secretion. Testosterone, dihydrotestosterone, estradiol and dexamethasone, however, reduced FSH secretion whereas progesterone increased it. Follicular fluid treatment had no effect on FSH secretion.
• All treatments increased the storage of FSH in the pituitary gland but only treatments with testosterone and dihydrotestosterone increased the FSH response to exogenous GnRH.
• PRL secretion didn’t significantly differ among the treatment groups except for a brief increase in mares treated with estradiol. Only estradiol treatment increased PRL storage, but increases in the PRL response to exogenous TRH were observed in groups treated with testosterone or dihydrotestosterone.
• Notably, most of the treatments did not affect the content of GnRH in the body and pre-optic area of the brain. However, testosterone, estradiol, and dexamethasone treatments increased the GnRH content in the median eminence, a part of the brain.

Conclusion and Correlations

• The study revealed a high correlation (r >= .77) for LH secretion, storage and response to exogenous GnRH.
• For FSH, only storage and responses to exogenous GnRH were related. PRL characteristics, however, were not significantly related to each other.
• Interestingly, the amount of GnRH in the median eminence was not found to be related to any LH or FSH characteristic.
• This study contributes to our understanding of hormone regulation under various steroidal and nonsteroidal conditions in equine species particularly in mares.