In middle‑aged female mice, the brain cells that make the hormone GnRH (the same hormone in the drug gonadorelin) show big changes: they fire twice as fast, many of their gene messages shift, and they lose some tiny synaptic signals. Some drug responses stay the same, but others, like the effect of a kisspeptin blocker, only appear in older mice. This tells us that the GnRH system starts to remodel well before full reproductive aging.

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&#x2005;d) and young (Y, 70&#x2005;d), diestrous mice, respectively. Transcriptomic analysis revealed reproductive senescence-related molecular changes in one-third of the MA mice. The upregulated genes (<i>n</i>&#x2009;=&#x2009;225) were linked to immune signaling, olfactory- and vomeronasal receptors. The downregulated genes (<i>n</i>&#x2009;=&#x2009;233) were related to mRNA processing, G-protein-coupled receptors, oxidative phosphorylation, electron transport, and estrogen signaling. 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 twofold 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 the 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.