Operating characteristics of the hypothalamo-pituitary-gonadal axis in men: circadian, ultradian, and pulsatile release of prolactin and its temporal coupling with luteinizing hormone.

Abstract:  To investigate the circadian, ultradian, and pulsatile nature of PRL release in the human, we sampled blood at 10-min intervals for 24 h in each of 12 normal young men. The subsequent serum PRL time series were subjected to contemporary techniques of rhythmic and episodic peak detection. Fourier analysis revealed a significant circadian rhythm in serum PRL concentrations in all 12 men. The mean circadian amplitude was 2.1 +/- 0.3 ng/mL (micrograms/L), which accounted for an average of 30 +/- 4% of the 24-h mean PRL concentration. In addition, multiple ultradian PRL rhythms were found with periodicities ranging from 22-242 min. Spectral analysis disclosed ultradian cycles with periodicities of 30-32, 51-59, 90-98, and 234 min. Assessment of episodic PRL pulsatility by Cluster analysis revealed 14 +/- 1 PRL peaks/24 h (P less than 0.01 vs. signal-free noise), which occurred at an interpulse interval of 95 +/- 6 min. The average duration of a serum PRL peak was 67 +/- 5 min, and its incremental amplitude was 4.0 +/- 0.3 ng/ml (micrograms/L), which represented a 58 +/- 6% increase above the preceding nadir. Discrete PRL peaks were separated by nonpulsatile valleys, with a mean duration of 27 +/- 1 min. Analysis of the temporal coupling between LH and PRL release by bivariate autoregressive modeling in six men revealed significant cross-correlations between LH and simultaneous PRL concentrations as well as between LH and PRL concentrations that lagged LH by 10 or 20 min. Cross-spectral analysis demonstrated significantly correlated PRL and LH cycles with periodicities of 33-37, 47-52, and 84-106 min. In summary, PRL release in normal young men is characterized by significant circadian and ultradian periodicities, by discrete episodic pulsations that occur approximately every 95 min, and by a close temporal coupling with LH (temporal lag between LH and PRL of 0-20 min).