The effect of pulsatile gonadotropin-releasing hormone and estradiol administration on luteinizing hormone and follicle-stimulating hormone concentrations in pituitary stalk-sectioned ovariectomized pony mares.

This study investigates the hormonal changes in pony mares that have undergone specific surgery in the pituitary gland—an area vital in hormone regulation. The researchers studied the effects of two treatments: pulses of a hormone called GnRH, and injections of estradiol, a type of estrogen. Their findings suggest the former treatment may maintain specific hormone levels, while the latter seems ineffective.

Research Methodology

• This study involved ovariectomized (ovary-removed) anestrous (not in heat) pony mares that were tested during the summer months. The mares were split into three groups. Group 1 served as the control, Group 2 received pulsed doses of GnRH, and Group 3 received estradiol injections.
• All mares underwent a surgery known as pituitary stalk-section (PSS) that effectively cuts the connection between the hypothalamus and the pituitary gland, both of which are essential in regulating hormones in the body. Treatments began immediately after this surgery.
• Blood samples were taken every 30 minutes for 8 hours on the day before the surgery, and 5 days after. These samples allowed researchers to compare hormone levels before and after PSS. Further sampling took place every 12 hours starting from the evening of the surgery up until the 10th day, permitting an analysis of hormone secretory time trends. After 10 days, the mares were euthanized to confirm PSS and for tissue sample analysis.

Results and Findings

• The evidence showed that a type of hormone known as luteinizing hormone (LH) decreased in the control group and the estradiol group from Day 0 to Day 5. However, LH tended to decrease less prominently in mares that received repeated doses of GnRH.
• On Day 5, the amount of LH in the bloodstream was significantly higher in the GnRH group compared to both the control and estradiol groups, suggesting that GnRH pulses helped maintain LH levels.
• On the other hand, the injections of estradiol did not appear to stimulate either LH or another hormone known as follicle-stimulating hormone (FSH).
• All groups saw decreases in FSH levels after surgery, but the GnRH group had significantly higher FSH concentrations compared to the other two groups.
• Regression analysis, a statistical measure, showed that the time trends for LH and FSH levels were not parallel.

Conclusions

• The PSS surgery seemed to reduce LH and FSH to undetectable levels within five days.
• The pulsed administration of GnRH appeared to maintain LH and FSH secretion, although concentrations did tend to be lower than on Day 0.
• Contrarily, estradiol did not stimulate LH or FSH secretion.