Altered electrophysiology and transcriptome of GnRH neurons in middle-aged female mice

Abstract

Aging affects the reproductive system, although its impact on GnRH neurons is mainly unexplored. Thus, we compared the transcriptome and electrophysiology of GnRH neurons obtained from female middle-aged (MA, 400-430 days) and young (Y, 70 days), diestrous mice, respectively. Transcriptomic analysis revealed reproductive senescence-related molecular changes in one-third of the MA mice. The upregulated genes (n=225) were linked to immune signalling, olfactory- and vomero-nasal receptors. The downregulated genes (n=233) were related to mRNA processing, G-protein-coupled receptors, oxidative phosphorylation, electron transport, and estrogen signalling. In addition, ion channel (Na, K, Ca), neurotransmitter (ACh, GABA, glutamate)- and various neuropeptide receptor-coding genes showed differential expression indicating functional alterations of the cells. Accordingly, whole-cell patch-clamp recordings revealed a two-fold increase in spontaneous firing frequency of MA GnRH neurons. Significant changes were also observed in characteristics of action potentials and afterhyperpolarization amplitudes. Conspicuously, miniature postsynaptic currents were absent in 72% of MA-GnRH neurons and pharmacological blockade of GABA-A and glutamate receptors didn't affect firing rate. However, administration of their ligands evoked inward currents and facilitated firing in both animal groups, although with a decreased efficacy in MA animals. MA-GnRH neurons sustained responsiveness to estradiol, G-protein inhibition and kisspeptin (KP) like those of young animals. While KP receptor antagonist, KP-234 diminished firing frequency of MA GnRH neurons, it had no effect on young GnRH cells. Collectively, these findings revealed that both the transcriptome and electrophysiological activity of GnRH neurons change at middle-age and the explored alterations are hallmarks of early phase of reproductive senescence.

Significance statement Aging reduces gonadal hormone production, which is centrally regulated by GnRH neurons. Understanding how the early phase of reproductive senescence affects the GnRH system is crucial. Using middle-aged (MA) female mice, we demonstrate that GnRH neurons undergo transcriptomic changes indicating altered, house-keeping mechanisms and modification of cellular activity. Accordingly, MA-GnRH neurons showed increased firing activity despite the attenuated GABA and glutamate neurotransmission, and sustained estradiol responsiveness. G-protein-coupled receptor signalling remained functional, and kisspeptin (KP) further increased the firing frequency. Contrasting young GnRH neurons, the MA cells received a tonic KP modulation. These findings also highlight the heterogeneity of middle-aged mice population and provide molecular and functional markers of GnRH neurons characteristic for the early phase of reproductive senescence.