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Abstract
Pulsatility and circadian rhythmcity characterize neuroendocrine hormone secretion. Melatonin is secreted at night by the pineal gland and its circadian rhythm of secretion influences many biological rhythms. The rhythm of CRH and ACTH secretion produces the rhythm of cortisol secretion with peak secretion in the morning. GRH and GH are secreted with circadian rhythmicity with higher levels during the night, but IGF1 production remains steady during the 24-hour period. In this paper we have considered a possible role of reciprocal hormone modulation of the hypothalamic-pituitary-adrenal axis, hypothalamic-pineal axis and GH-IGF1 axis function in humans. Melatonin, cortisol, GH and total IGF1serum levels were determined on blood samples obtained every four hours for 24 hours from 10 healthy males, aged 36–51 (mean age ± SEM 44.3 ± 1.02, body mass index ± SEM 25.2 ± 0.45). We correlated hormone serum levels at each sampling time and evaluated the presence of circadian rhythmicity of hormone secretion. Pearson Product Moment Correlations showed that at 1000 h melatonin correlated negatively with IGF1 (t = −2.112, p = 0.02), cortisol correlated positively with GH (t = 7.471, p < 0.001), and at 2200 h melatonin correlated negatively with IGF1 (t = −2.206, p = 0.03). A clear circadian rhythm was validated for the time-qualified changes of melatonin and GH secretion (with acrophase at night), for cortisol serum levels (with acrophase in the morning), but not for total IGF1 serum level changes. We conclude that melatonin and cortisol may modulate GH-IGF1 axis function