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1 Department of Statistics, University of Virginia and 2 Center for Biomathematical Technology, Health Sciences Center, Charlottesville 22903; and 3 Division of Endocrinology, Department of Internal Medicine, General Clinical Research Center, Charlottesville, Virginia 22908
The incremental nature of neuroendocrine aging suggests that subtle system dysregulation may precede overt axis failure. The present analyses unmask threefold disruption of pulsatile gonadotropin-releasing hormone (GnRH)-luteinizing hormone (LH) secretion in the aging male. First, by way of random effects-based deconvolution analysis, we document an elevated daily GnRH-LH pulse frequency in healthy older men [namely, mean (±SE) 23 ± 1 (older) vs. 15 ± 1 (young) LH secretory bursts/24 h, P < 0.001] and lower mean LH pulse mass [3.73 ± 0.58 (older) vs. 5.46 ± 0.66 (young) IU/l, P = 0.038]. However, total LH secretion rates and two-compartment LH elimination kinetics were comparable in the two age cohorts. Second, using the approximate entropy statistic, we show an equivalently random order-dependent succession of LH interpulse-interval lengths in young and older men, but a marked age-related deterioration of the ad seriatim regularity of LH pulse mass series in older individuals (P = 0.0057). Third, by modeling GnRH pulse-generator output as a Weibull renewal process (generalized Gamma density) to emulate loosely coupled GnRH neuronal oscillators, we identify an age-related reduction in the frequency-independent and order-independent variability of GnRH-LH interpulse-interval sets (P = 0.08). These findings indicate that the GnRH-LH pulsing mechanism in healthy older men maintains an increased mean frequency and lower amplitude of bursting activity, a reduced uniformity of serial LH pulse-mass values, and an impaired variability among interpulse-interval lengths. Thereby, the foregoing order-dependent and order-independent alterations in GnRH-LH signal generation in the aging human suggest a general framework for exploring subtle disruption of time-sensitive regulation of other neurointegrative systems.
age; reproduction; human; male; pituitary; hypothalamus; androgen
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