[Puberty and melatonin]

Melatonin is the main hormone secreted by the pineal gland and secretion is maximal at night. Although researchers disagree, numerous data suggest that elevated melatonin levels during the prepubertal age maintain the hypothalamic-pituitary-gonadal axis in quiescence, thus exerting an inhibitory effect on pubertal development. The decrease in serum melatonin with advancing age activates hypothalamic pulsatile secretion of gonadotropin-releasing hormone and consequently the reproductive axis, which results in the onset of puberty.

To evaluate urinary melatonin levels in children of different ages and the characteristics of its rhythmic excretion and to determine whether puberty is associated with a significant reduction in urinary melatonin levels.

Thirty-two children were studied (17 boys and 15 girls). Concentrations of 24-h urinary melatonin were quantified by radioimmunoassay in daytime samples (collected between 9.00 and 21.00) and nighttime samples (collected between 21.00 and 9.00 on the following day). Blood levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH), estradiol, testosterone and dehydroepiandrosterone sulfate (DHEAS) were calculated. Circadian rhythms and melatonin secretion in the various Tanner stages were assessed (ANOVA).

No significant differences were found between day- and nighttime secretion of melatonin among boys (daytime melatonin: 1.38 0.52 pg/ml; nighttime melatonin: 6.92 2.06 pg/ml) and girls (daytime melatonin: 1.15 0.43 pg/ml; nighttime melatonin: 11.41 4.32 pg/ml). Highly significant differences were found (p < 0.001) between the day and night rates of melatonin secretion in both genders. Highly significant differences (p < 0.001) were also found in day-, nighttime and total secretion among the different Tanner stages. Comparison among groups revealed a significant decrease in secretion rates in stages I and II in both boys and girls. Melatonin significantly decreased with age in both sexes (lineal relationship). This decrease was greater at night. No relationship was found between the secretion of melatonin and estradiol, testosterone, LH, FSH and DHEAS.

Melatonin secretion follows a circadian pattern, with greater secretion at night. The change in this rhythm was significantly greater in girls, due to greater nighttime secretion. Secretion significantly decreases in Tanner stages I and II with subsequent decreases in the later stages.