Hayes et al., Aromatase Inhibition in the Human Male Reveals a Hypothalamic Site of Estrogen Feedback, JCEM Vol. 85, No. 9 3027-3035
http://jcem.endojournals.org/cgi/content/full/86/1/53
From the discussion:
...The increase in LH pulse amplitude, observed after aromatase inhibition, could potentially reflect an increase in the amplitude of GnRH pulses stimulating the pituitary, and/or enhanced pituitary sensitivity to the same amount of endogenous GnRH. Previous studies have attempted to distinguish between these two mechanisms by examining pituitary responsiveness to pharmacological doses of exogenous GnRH before and during antiestrogen therapy (11, 31, 32).
These studies paradoxically demonstrated that clomiphene blunted pituitary responsiveness to exogenous GnRH despite increasing both mean LH levels and the amplitude of spontaneous LH pulses (11, 31, 32). The mechanism proposed for this divergence between spontaneous pulse height and acute pituitary responsiveness to exogenous GnRH was that clomiphene was having tissue-specific mixed agonist/antagonist effects. The authors concluded that clomiphene was acting as an estrogen antagonist at the hypothalamus, resulting in an increase in endogenous GnRH secretion, but as an estrogen agonist at the pituitary, causing decreased responsiveness to exogenous GnRH (11).
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Some studies showing that Nolvadex increases pituitary responsiveness to LHRH:
Andrologia 1985 Jul-Aug;17(4):369-78 Effect of lower versus higher doses of tamoxifen on pituitary-gonadal function and sperm indices in oligozoospermic men.
Dony JM, Smals AG, Rolland R, Fauser BC, Thomas CM.
Administration of the antiestrogen
tamoxifen for one month to 12 patients with idiopathic oligozoospermia significantly
increased the mean basal testosterone (T) level and the responses of luteinizing hormone (LH) and follicle stimulating hormone (FSH) to constant luteinizing hormone releasing hormone (LHRH) infusion but did not significantly influence the mean oestradiol (E2) levels or the E2 over testosterone ratio...
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Fertil Steril 1978 Mar;29(3):320-7 Hormonal effects of an antiestrogen, tamoxifen, in normal and oligospermic men.
Vermeulen A, Comhaire F.
The administration of tamoxifen, 20 mg/day for 10 days, to normal males produced a moderate increase in luteinizing hormone (LH), follicle-stimulating hormone (FSH), testosterone, and estradiol levels, comparable to the effect of 150 mg of clomiphene citrate (Clomid).
However, whereas Clomid produced a decrease in the LH response to LH-releasing hormone (LHRH), no such effect was seen after the administration of tamoxifen. In fact, prolonged treatment (6 weeks) with tamoxifen significantly increased the LH response to LHRH...