Late postnatal reorganization of GABA(A) receptor signalling in native GnRH neurons

The molecular and cellular characteristics of the gonadotropin-releasing ho rmone (GnRH) neurons have been difficult to ascertain due to their scattere d distribution within the basal forebrain. Using morphological criteria cou pled with single cell RT-PCR postidentification, we have developed a method for investigating native GnRH neurons in the mouse brain and used it to ex amine the development of GABA(A) receptor signalling in this phenotype. Fol lowing the harvesting of the cytoplasmic contents of individual GnRH neuron s, single cell multiplex RT-PCR experiments demonstrated that GABA(A) recep tor alpha 1-5, beta 1-3 and gamma 2 & 3 subunit transcripts were expressed by both neonatal (postnatal day 5) and juvenile (day 15-20) GnRH neurons in a heterogeneous manner. Following puberty, this profile was reduced to a p redominant alpha 1, alpha 5, beta 1, gamma 2 subunit complement in rostral preoptic area GnRH neurons of the adult female. Whole-cell patch-clamp reco rdings revealed little difference between juvenile and adult GnRH neurons i n their resting membrane potential and spontaneous firing rates. All GnRH n eurons were found to be subjected to a tetrodotoxin-insensitive, tonic GABA ergic barrage signalling through the GABA(A) receptor. However, marked hete rogeneity in the sensitivity of individual juvenile GnRH neurons to GABA wa s revealed and, in parallel with the change in subunit mRNA profile, this w as dramatically reduced in the reproductively competent adult GnRH neurons. These findings provide the first electrical and molecular characterization of the GnRH phenotype and demonstrate a novel pattern of late postnatal re organization of native GABA(A) receptor gene expression and signalling in t he GnRH neuronal population.