REGULATION OF GONADOTROPIN RELEASING HORMONE GENE EXPRESSION

The main objective of our research projects is to determine the involvement of different neurotransmitter/neuromodulator systems in the regulation of gonadotropin-releasing hormone (GnRH) in the rat brain. We have already shown that GnRH mRNA levels are positively regulated by dopamine. The role of serotoninergic (5-HT) system was also investigated and the results obtained strongly suggest that 5-HT negatively regulates GnRH gene expression via activation of 5-HT2 receptors. We have also established, using specific agonists of the different subtypes of the neuropeptide Y (NPY) receptor, that NPY positively influences GnRH gene expression via the Y1 NPY receptor subtype. Recently, we have demonstrated that activating the GABAA receptor by using the agonist muscimol, barbiturates and benzodiazepines (BZD) decreases the GnRH RNA levels, indicating a negative regulation by the GABAergic system.

We have also investigated the role of neurosteroids, such as 5ß-pregnan-3a-ol-20-one (5ß3aP), and found that this progesterone (P) metabolite exerts a profound depressing on the levels of GnRH gene expression. We have recently shown that the endozepine diazepam-binding inhibitor (DBI) can also depress the levels of GnRH mRNA by activating the GABAA complex, an effect which was prevented by the BZD antagonist flumazenil.

In the next few years, we are planning to study in detail the sites of action of the different monoaminergic systems by determining the anatomical connections between adrenergic, noradrenergic and serotoninergic systems and GnRH neurons. Double-labeling techniques involving double-immunostaining or a combination of immunocytochemistry and in situ hybridization will be used at both light and electron microscopic levels. Another approach will be to localize receptor subtypes for each neurotransmitter in GnRH neurons. These studies will allow to determine whether the monoamines are acting directly on GnRH neurons and also help to classify which subtype(s) of the receptor is (are) the most likely involved. The role of NPY will be further investigated using a similar approach. So far only one subtype of NPY receptor, the Y1 receptor, has been cloned. Since we have shown that this subtype is involved in GnRH regulation, we will attempt to localize this receptor subtype in GnRH neurons. The role of neurosteroids will be pursued by investigating the effects of neurosteroids such as the GABAA agonist 5a, 3aP and pregnenolone sulphate a GABAA antagonist. Since the effects of the BZD take place after activation of the GABAA receptor complex, we will determine whether the nerve cells which produced the steroidogenic enzymes such as 3ß-hydroxysteroid dehydrogenase (3ß-HSD) and 5a-reductase also contain BZD receptors. These studies will ultimately lead to a better understanding of the roles of neurotransmitter/neuromodulator systems and neurosteroids in the regulation of the GnRH neurons.

These projects are supported by the Medical Research Council Group in Molecular Endocrinology.

Li S, Pelletier G (1995) Inhibitory effect of the endogenous benzodiazepine receptor ligand, octadecaneuropeptide (ODN), on gonadotropin-releasing hormone gene expression in the male rat brain. Neuroreport 6 : 1354-1356.