Physiology of Gonadotropin-Releasing Hormone (Gnrh): Beyond the Control of Reproductive Functions

Evaluation by metabolic profiling and in vitro autoradiography of two promising GnRH-receptor ligands for brain SPECT imaging

The increased expression of gonadotropin releasing hormone receptor (GnRH-R) in brain has been strongly linked to Alzheimer disease. Therefore, the development of radiolabeled imaging agents for GnRH-R is relevant for early diagnosis of Alzheimer disease. We have recently disclosed the discovery of two promising compounds displaying nanomolar-range affinity for the GnRH-R. In the present study, a preclinical evaluation of the compound properties was performed to evaluate their potential as single photon emission computed tomography (SPECT) radiotracers for imaging the GnRH-receptor. The compounds were assessed in vitro by performing serum stability analysis by human and rat serum, metabolic profiling by human liver microsomes, and exploratory rat brain autoradiography. The investigated compounds displayed satisfactory stability against human, rat serum, and liver microsomal metabolism, which favors their potential as SPECT-imaging agents. Additionally, we identified and quantified the formation rate of the metabolites by fragmentation of up to five mass spectrometric stages. The GnRH-R rat brain specificity of these compounds was tested in competition with a known ligand for the receptor and the in vitro autoradiography confirmed that compounds 3 and 4 binds to rat GnRH-R in different rat brain regions.

GnRH in the Human Female Reproductive Axis

Gonadotropin-releasing hormone (GnRH) is recognized as the central regulator of the functions of the pituitary-gonadal axis. The increasing knowledge on the mechanisms controlling the development and the function of GnRH-producing neurons is leading to a better diagnostic and therapeutic approach for hypogonadotropic hypogonadisms and for alterations of the puberty onset. During female life span, the function of the GnRH pulse generator may be affected by a number of inputs from other neuronal systems, offering alternative strategies for diagnostic and therapeutic interventions. Moreover, the identification of a GnRH/GnRH receptor system in both human ovary and endometrium has widened the spectrum of action of the peptide outside its hypothalamic functions. The pharmacological use of GnRH itself or its synthetic analogs (agonists and antagonists) provides a valid tool to either stimulate or block gonadotropin secretion and to modulate the female fertility in several reproductive disorders and in assisted reproduction technology. The use of GnRH agonists in young female patients undergoing chemotherapy is also considered a promising therapeutic approach to counteract iatrogenic ovarian failure.

Systematic review of hormone replacement therapy in the infertile man

Objectives

To highlight alternative treatment options other than exogenous testosterone administration for hypogonadal men with concomitant infertility or who wish to preserve their fertility potential, as testosterone replacement therapy (TRT) inhibits spermatogenesis, representing a problem for hypogonadal men of reproductive age.

Materials and methods

We performed a comprehensive literature review for the years 1978–2017 via PubMed. Also abstracts from major urological/surgical conferences were reviewed. Review was consistent with the Preferred Reporting Items for Systemic Reviews and Meta-Analyses (PRISMA) criteria. We used Medical Subject Heading terms for the search including ‘testosterone replacement therapy’ or ‘TRT’ and ‘male infertility’.

Results

In all, 91 manuscripts were screened and the final number used for the review was 56. All studies included were performed in adults, were written in English and had an abstract available.

Conclusions

Exogenous testosterone inhibits spermatogenesis. Hypogonadal men wanting to preserve their fertility and at the same time benefiting from TRT effects can be prescribed selective oestrogen receptor modulators or testosterone plus low-dose human chorionic gonadotrophin (hCG). Patients treated for infertility with hypogonadotrophic hypogonadism can be prescribed hCG alone at first followed by or in combination from the start with follicle-stimulating hormone preparations.