The LH surge in the midcycle is triggered by positive feedback of estradiol on the hypothalamic-pituitary axis. Estradiol above a threshold (~200 pg/mL for >36 hours) switches from negative to positive feedback at the pituitary level. The mechanism of this positive feedback involves upregulation of which pituitary component?

• A Estradiol increases kisspeptin secretion from hypothalamic arcuate neurons to amplify GnRH pulse frequency
• B Estradiol directly stimulates LH secretion by activating membrane estrogen receptors (GPER1) on gonadotrophs bypassing GnRH
• C Estradiol upregulates GnRH receptors on gonadotrophs and increases LH synthesis and storage, amplifying the response to each GnRH pulse ✓
• D Estradiol inhibits progesterone synthesis in the corpus luteum, disinhibiting the LH surge

Explanation

Estradiol positive feedback on gonadotrophs involves upregulation of GnRH receptor (GnRHR) expression, which sensitizes gonadotrophs to GnRH. Additionally, estradiol enhances LH synthesis and increases the size of the readily releasable pool of LH-containing vesicles. When the preovulatory GnRH surge occurs (itself amplified by hypothalamic kisspeptin neurons via the AVPV kisspeptin population rather than the arcuate nucleus), the sensitized gonadotroph produces a massive LH surge. Progesterone (from the granulosa cells already exposed to LH) further amplifies the LH surge in the late follicular phase.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.