How to measure GnRH in menstrual cycle?

Question

I am interested in pulse frequencies of GnRH (gonadotropin-releasing hormone) in the menstrual cycle (MC) i.e. 28 +/- 7 days. I would like to study its pulsatile nature i.e. within one MC, not same as cyclic nature within many MCs; but possible cyclic dynamics could be found in one MC too but not focus there.

GnRH releases human chorionic gonadotropin (hCG) which can be monitored by beta hCG but also by (source DynaMed Plus on Chorionic gonadotropin)

• serum progesterone, Luteinizing hormone (LH)
• estradiol levels
• ovarian US 7-14 MCD i.e. in late follicular phase
• onset of ovulation at 14 MCD
• pelvic exam
• cryptorchidism: signs of precocious puberty

Two articles on GnRH monitoring:

• Monitoring gonadotropin-releasing hormone administration by measurement of urinary steroid conjugates in the morning urine samples.
• some gene studies: Episodic gonadotropin-releasing hormone gene expression revealed by dynamic monitoring of luciferase reporter activity in single, living neurons

Possible cases to study

• Amenorrhea: Kallmann syndrome where congenitally deficiency of GnRH but how low?

However, I did not find any publication describing direct/indirect continuous measurement of GnRH.

---

How can GnRH be measured continuously during the menstrual cycle?

Answer 1

GnRH is found in very low concentrations (2-10 pg/mL)

Options:

1. A highly sensitive analytical method (e.g. radioimmunoassay)
2. To sample directly from the hypophyseal-portal system
3. A surrogate measure

1. This abstract: http://www.ncbi.nlm.nih.gov/pubmed/9749566 describes direct measurement.

2. Direct sampling could only be done under general anaesthesia and in a short window, so I'd avoid that altogether unless you're talking about using an animal model and can surgically impact a cannula.

3. This abstract: http://www.ncbi.nlm.nih.gov/pubmed/21646369 describes use of the $\alpha$-subunit as a surrogate of GnRH (the $\alpha$-subunit is the same for FSH, LH, and TSH and is released under the control of GnRH).

By the way, GnRH does not promote secretion of hCG so much as it does LH and FSH (LH is fairly homologous to hCG). For the most part hCG is secreted by trophoblasts (in the placenta) and is quite low except in pregnancy or the presence of certain germ cell tumors, so wouldn't be worth measuring in non-pregnant women or males anyway.

The bigger point is why do you want to measure GnRH pulsatility? This has been done and the you know that it is pulsatile is because it has been done.