Role of the GABA(A) receptor gamma 2 subunit in the development of gonadotropin-releasing hormone neurons in vivo

We have employed transgenic mouse models to examine the functional signific ance of the gamma 2 subunit of the GABA(A) (gamma-aminobutyric acid) recept or to the correct development of gonadotropin-releasing hormone (GnRH) neur ons in vivo. In the first experiment, the expression of gamma 2 subunit pro tein by the GnRH phenotype was determined using transgenic mice in which Gn RH gene sequences direct the expression of the LacZ reporter to the nucleus of the GnRH neurons. This greatly facilitates the immunocytochemical ident ification of non-nuclear-located antigens within GnRH neurons and revealed that similar to 25% of juvenile GnRH neurons were immunoreactive for the ga mma 2 subunit and that this increased to 40% in pubertal mice. In the secon d experiment, GnRH mRNA expression was examined in the brains of gamma 2 su bunit knockout mice (gamma 2(0/0)) and their wild-type (gamma 2(+/+)) litte rmates at embryonic day 15 and postnatal days (P) 0 and 11-16 using in situ hybridization. The distribution and numbers of cells expressing GnRH mRNA in gamma 2(+/+) and gamma 2(0/0) mice were not found to differ at any age. However, the GnRH mRNA content of medial septal cells was significantly low er in gamma 2(0/0) compared with gamma 2(+/+) mice at P11-16 (P < 0.05) and the same trend was observed for preoptic area neurons. These results demon strate that while the gamma 2 subunit of the GABA(A) receptor is expressed by postnatal GnRH neurons, their embryonic development does not require a f unctional gamma 2 subunit. In contrast, postnatal GnRH mRNA expression was found to be dependent upon signalling through the GABA(A) receptor.