The subjects were considered to be physically and mentally

healthy, but no standard questionnaire or scale was used to provide a formal confirmation. Other hormones showed a wide range of secretory patterns, but within the range considered to be normal This was the case with LH, with nocturnal PHA-739358 cell line concentration patterns showing 1 to 6 secretory pulses depending on subject, together with a Inhibitors,research,lifescience,medical 3-fold range in mean LH concentration. These interindividual differences in LH secretion in men were stable over time. Several authors have studied the chronobiology of hormones in normal subjects or in patients in protocols where measurements were repeated. Thus, in 10 normal subjects studied over 24 hours on three occasions, the coefficients of variation of the parameters for melatonin, prolactin, LH, and testosterone secretion varied less within Inhibitors,research,lifescience,medical subjects than between subjects, indicating that the individual secretion patterns were stable over time, particularly with LH,1 as also seen in our subjects. Coincidnt secretory pulses of hormones Hormones that belong to the same endocrine axis are often secreted with similar temporal patterns, for exampie Cortisol and adrenocorticotropic hormone Inhibitors,research,lifescience,medical (ACTH)

or p-endorphin,2 LH and estradiol or progesterone.3 This coincidence in hormone secretory pulses is to be expected.

However, temporal coincidence is also found between Inhibitors,research,lifescience,medical hormones that belong to different axes. Examples are LH and prolactin,4 testosterone and melatonin,5 TSH and leptin,6 LH and leptin in women,7 and the amino acid L-arginine and insulin.8 In addition, more than two hormone secretions can be coupled temporally, as described for Cortisol, leptin, LH, and GH.9 Finally, ultradian coupling can occur between events Inhibitors,research,lifescience,medical that belong to different physiological systems. An illustration is the finding the by Brandenberger11 of the coupling between prolactin and electroencephalographic sleep waves. In this particular case, the coupling was so tight that the normalized hormone concentration and delta wave power (expressed as Z-scores) followed almost exactly the same value versus time curve.9 Ultradian rhythms Ultradian and circadian changes in blood hormone concentrations are an indirect marker of the activity of central nervous system pacemakers, but how biological clocks govern ultradian rhythms of hormone secretion is still not well understood. Gonadotrophin-releasing hormone (GnRH, also called luteinizing-releasing hormone or LHRH) stands as an exception and has been well described for several decades.