Mechanistic Basis

We propose that the pulsatile secretion of gonadotropin‑releasing hormone (GnRH) from the hypothalamus functions as a master timer that coordinates the circadian‑hypothalamic axis described in recent work. GnRH pulse frequency sets the downstream release of pituitary hormones—growth hormone (GH), luteinizing hormone (LH), and thyroid‑stimulating hormone (TSH)—which in turn entrain the suprachiasmatic nucleus (SCN) and peripheral clocks through hormone‑dependent signaling pathways (e.g., GH‑STAT5, LH‑cAMP). Altered GnRH pulsatility with advancing age reduces GH/IGF‑1 signaling, diminishes SIRT1 activity, and weakens the molecular clock, thereby increasing oxidative stress and DNA damage [1,2]. Simultaneously, low‑frequency GnRH pulses disinhibit NF‑κB signaling in hypothalamic microglia, fostering a chronic inflammatory state that suppresses GnRH neurons themselves—a positive feedback loop [3]. This dual influence places GnRH pulsatility upstream of both circadian disruption and hypothalamic inflammation, positioning it as a candidate controller of the six hallmarks of aging.

Testable Predictions

1. Restoring youthful GnRH pulse frequency in aged mice will re‑synchronize central and peripheral circadian rhythms, reduce hypothalamic NF‑κB activity, improve DNA‑repair markers, and extend median lifespan.
2. Inducing irregular or high‑frequency GnRH pulses in young mice will precipitate premature circadian desynchronization, elevate hypothalamic inflammation, accelerate senescence biomarkers, and shorten lifespan.
3. Genetic ablation of the GnRH receptor specifically in SCN neurons will uncouple hormonal feedback from the clock, leading to circadian dysfunction without directly affecting peripheral hormone levels, thereby distinguishing central versus peripheral actions of GnRH.

Experimental Design

• Animals: C57BL/6J mice, young (3 mo) and aged (20 mo); additional cohorts with GnRH‑R floxed alleles crossed to SCN‑specific Cre (Vgat‑Cre) or global Kisspeptin‑Cre for pulse manipulation.
• Interventions:
  • Rescue group: Aged mice receive subcutaneous osmotic pumps delivering GnRH in a pulsatile pattern mimicking juvenile secretion (1 pulse h⁻¹, 0.5 µg per pulse) for 8 weeks.
  • Challenge group: Young mice receive pumps delivering GnRH at either supra‑physiological frequency (4 pulses h⁻¹) or continuous infusion to blunted pulsatility.
  • Control groups: Vehicle‑filled pumps or wild‑type littermates.
• Readouts:
  • In vivo bioluminescence of PER2::LUC reporters in SCN and liver to assess circadian amplitude and phase coherence.
  • Hypothalamic tissue immunoblots for phospho‑IKKα/β and nuclear NF‑κB p65.
  • Serum IGF‑1, LH, TSH levels; peripheral clock gene expression (Bmal1, Cry1) in muscle and adipose.
  • DNA damage markers (γH2AX, 8‑oxo‑dG) in leukocytes.
  • Frailty index, grip strength, and survival monitoring.
• Statistical plan: Power analysis targeting 80 % power to detect a 15 % lifespan change (α = 0.05). Mixed‑effects models for longitudinal rhythm data; Cox proportional hazards for survival.

If the predictions hold, GnRH pulsatility would constitute a single upstream regulator whose age‑driven decline orchestrates the observed hallmarks, offering a focal point for interventions that target the neuroendocrine timer rather than downstream pathways individually.