The slope of the LH surge is more acute than that of the FSH surge. Serum estradiol levels are peaking at about the beginning of the rapid ascent phase of the LH surge, and progesterone levels increase from approximately 12 hours before the onset of the LH surge and continue at a plateau level until approximately 36 hours after the peak of the surge. .
The progressive rise in progesterone may be the signal to terminate the LH surge, as high concentrations of progesterone exert a negative feedback effect on the pituitary. The fall in estradiol as LH is rising also may have an effect on the subsequent descent of LH, possibly by an acute downregulation of GnRH receptors. The LH short-loop regulation (pituitary-hypothalamic) of GnRH secretion also has been demonstrated. The LH has been postulated to have an ultrashort-loop feedback on its own secretion, another possible signal for the termination of the surge. Such a feedback could not be demonstrated by hCG administration.
The threshold duration for LH surge levels required to reinitiate meiosis appears to be 14 to 18 hours, whereas metaphase II oocytes are obtainable some 28 hours after the onset of the LH surge. The threshold amplitude of the LH surge is not known, but only 5 percent of the normal LH surge amplitude is necessary for the oocyte maturation, but ≥ 85 percent of peak levels is needed to activate actual ovulation.
The Dopamine Tract.
Cell bodies for dopamine synthesis can be found in the arcuate and paraventricular nuclei. The dopamine tuberoinfundibular tract arises within the medial basal hypothalamus and projects to the median eminence. Although the exact chemical nature of the endogenous Prolactin inhibiting hormone is still not known. Evidence is rather overwhelming that dopamine is the hypothalamic inhibitor of Prolactin secretion. It is directly secreted into the portal blood, thus behaving like a neurohormone. Therefore, dopamine may directly suppress arcuate GnRH activity, and also be transported via the portal system to directly and specifically suppress pituitary Prolactin secretion.