Role of the GABA(A) receptor γ2 subunit in the development of gonadotropin-releasing hormone neurons in vivo

We have employed transgenic mouse models to examine the functional significance of the γ2 subunit of the GABA(A) (γ-aminobutyric acid) receptor to the correct development of gonadotropin-releasing hormone (GnRH) neurons in vivo. In the first experiment, the expression of γ2 subunit protein by the GnRH phenotype was determined using transgenic mice in which GnRH gene sequences direct the expression of the LacZ reporter to the nucleus of the GnRH neurons. This greatly facilitates the immunocytochemical identification of non-nuclear-located antigens within GnRH neurons and revealed that ~ 25% of juvenile GnRH neurons were immunoreactive for the γ2 subunit and that this increased to 40% in pubertal mice. In the second experiment, GnRH mRNA expression was examined in the brains of γ2 subunit knockout mice (γ2(0/0)) and their wild-type (γ2(+/+)) littermates 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 γ2(+/+) and γ2(0/0) mice were not found to differ at any age. However, the GnRH mRNA content of medial septal cells was significantly lower in γ2(0/0) compared with γ2(+/+) mice at 11-16 (P< 0.05) and the same trend was observed for preoptic area neurons. These results demonstrate that while the γ2 subunit of the GABA(A) receptor is expressed by postnatal GnRH neurons, their embryonic development does not require a functional γ2 subunit. In contrast, postnatal GnRH mRNA expression was found to be dependent upon signalling through the GABA(A) receptor.