Late postnatal reorganization of GABA_A receptor signalling in native GnRH neurons

Abstract

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 GABAA receptor alpha1-5, beta1-3 and gamma2 & 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 alpha1, alpha5, beta1, gamma2 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.

Endogenous Activation of Metabotropic Glutamate Receptors Modulates GABAergic Transmission to Gonadotropin-Releasing Hormone Neurons and Alters Their Firing Rate: A Possible Local Feedback Circuit

“…The present results suggest a novel role for mGluRs: to inhibit GABAergic transmission directly to GnRH neurons. Because of the dominance of GABAergic contacts and synaptic transmission to these neurons (Sim et al, 2000;Jansen et al, 2003;, mGluRs may substantially impact GnRH neuron activity.…”

“…Functional GABAergic synaptic transmission to GnRH neurons via ionotropic GABA A receptors varies as a function of reproductive state, and these changes may help drive reproductive cycles and transitions between fertile and infertile conditions (Sim et al, 2000;Moenter, 2004a,b, 2005). Although the consequence of GABA A receptor activation in GnRH neurons is controversial (Sim et al, 2000;DeFazio et al, 2002;Han et al, 2002Han et al, , 2004Moenter et al, 2004;Moenter, 2004a, 2005), mechanisms regulating this key synaptic input are poised to play a major role in the central regulation of fertility (Jarry et al, 1988;Kasuya et al, 1999;Bilger et al, 2001).…”

Steroid modulation of GABAA receptor-mediated transmission in the hypothalamus: Effects on reproductive function

“…Specifically, changes in synaptic regulation at puberty (both excitatory, via glutamate and the peptide, kisspeptin, and inhibitory, via GABA) leads to activation of the hypothalamic pulse generator, which promotes enhanced, pulsatile GnRH release that, in turn, regulates the secretion of luteinizing hormone (LH) from the pituitary (for review, Freeman, 1994;Ojeda and Urbanski, 1994;Ojeda et al, 2006). In adult subjects, inhibitory GABAergic inputs onto GnRH neurons promotes diminished action potential bursting activity and subsequent pulsatile GnRH and LH release (Sim et al, 2000;Nunemaker et al, 2003;Han et al, 2004;for review, Herbison et al, 1991;Moenter et al, 2003). A very interesting facet with respect to GABAergic control of GnRH neuron activity is that the developmental change in the chloride reversal potential (E Cl ), which causes GABA to switch from imparting depolarizing to hyperpolarizing effects, is dramatically delayed in GnRH neurons.…”

Anatomical location of mature GnRH neurons corresponds with their birthdate in the developing mouse

“…For example, in rodents, it is quite clear that the population of GnRH neurons expressing c-fos at the time of the GnRH/luteinizing hormone (LH) surge resides predominantly, if not exclusively (Wintermantel et al, 2006), within the POA (Hoffman et al, 1993). Furthermore, studies in our own lab have found numerous instances where the morphology of (Cottrell et al, 2006), expression of receptors for neuromodulators (Jasoni et al, 2005;Grattan et al, 2007) on, or neurotransmitter effects (Sim et al, 2000;Pape et al, 2001;Clarkson and Herbison, 2006) on, adult GnRH neurons appears to be dependent upon their rostrocaudal location in the mouse; in particular whether they are located in the MS or POA. The underlying basis for this topographical heterogeneity remains unknown.…”