Late postnatal reorganization of GABAA receptor signalling in native GnRH neurons

The molecular and cellular characteristics of the gonadotropin-releasing hormone (GnRH) neurons have been difficult to ascertain due to their scattered distribution within the basal forebrain. Using morphological criteria coupled with single cell RT-PCR postidentification, we have developed a method for investigating native GnRH neurons in the mouse brain and used it to examine the development of GABA(A) receptor signalling in this phenotype. Following the harvesting of the cytoplasmic contents of individual GnRH neurons, single cell multiplex RT-PCR experiments demonstrated that GABA(A) receptor 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 predominant alpha 1, alpha 5, beta 1, gamma 2 subunit complement in rostral preoptic area GnRH neurons of the adult female. Whole-cell patch-clamp recordings revealed little difference between juvenile and adult GnRH neurons in their resting membrane potential and spontaneous firing rates. All GnRH neurons were found to be subjected to a tetrodotoxin-insensitive, tonic GABAergic barrage signalling through the GABA(A) receptor. However, marked heterogeneity in the sensitivity of individual juvenile GnRH neurons to GABA was revealed and, in parallel with the change in subunit mRNA profile, this was 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 reorganization of native GABA(A) receptor gene expression and signalling in the GnRH neuronal population.