Suppression of human melatonin by light over the course of the rising phase of the synthesis

Melatonin Ingestion Prevents Liver Damage and Improves Biomarkers of Renal Function Following a Maximal Exercise

Background: While the promotion of the beneficial effects of melatonin (MEL) ingestion on the modulation of oxidative stress is widespread, less attention is given to the biological influence that it could exert on the results of hematology and clinical chemistry parameters. This study was undertaken to assess the effects of acute MEL ingestion on these parameters during a maximal running exercise. Methods: In double blind randomized design, 12 professional soccer players [age: 17.54 ± 0.78 yrs, body mass: 70.31 ± 3.86 kg, body height: 1.8 ± 0.08 m; maximal aerobic speed (MAS): 16.85 ± 0.63 km/h; mean ± standard deviation], all males, performed a diurnal (17:00 h ± 30 h) running exercise test (RET) at 100% of their MAS following either MEL or placebo ingestion. Blood samples were obtained at rest and following the RET. Results: Compared to placebo, MEL intake decreased post-exercise biomarkers of liver damage (aspartate aminotransferase, p<0.001; alanine aminotransferase, p<0.001; gamma-glutamyltransferase; p<0.05) and improved post-exercise renal function markers (i.e., creatinine, p<0.001). However, lipid profile, glucose, lactate and leukocyte were not affected by MEL ingestion. Regarding the time to exhaustion, no difference was found between MEL (362.46 ± 42.06 s) and PLA (374.54 ± 57.97 s) conditions. Conclusion: The results of this investigation clearly attest that MEL ingestion before a maximal running exercise might protect athletes from liver damage and perturbation in renal function biomarkers. However, this study comprises an acute MEL supplementation and no assessment on chronic effects or circadian rhythm the day before was done.

Suppression of melatonin by 2000-lux light in humans with closed eyelids

BACKGROUND

In order to clarify the role of light in regulating body functions in sleeping humans, we studied whether the light-sensitive pineal hormone melatonin can be suppressed by facial light exposure in subjects with closed eyelids.

METHODS

Eight healthy volunteers participated in 3 nightly sessions: a dim-light control session (< 10 lux) and two light-exposure sessions (2000 lux, 60 min between 2400 and 0200 h). One light exposure occurred with eyes open and the other with eyes closed. Saliva samples were collected at least every hour from 1900 to 0300 h. Melatonin concentrations were measured by radioimmunoassay.

RESULTS

Salivary melatonin concentrations decreased only in 2 of the 8 volunteers during light-exposure sessions with eyes closed. On average, light exposure did not decrease the salivary melatonin concentration.

CONCLUSIONS

Because indoor illuminance is usually much lower than 2000 lux, light is probably ineffective in regulating the neuroendocrine hypothalamic functions in people during their sleep. Nevertheless, the possibility remains that higher illuminances, often used for therapeutic purposes, can inhibit the secretion of melatonin even in sleeping patients.

Suppression of melatonin secretion by bright light in seasonal affective disorder

Eleven patients with winter seasonal affective disorder and 10 healthy controls were exposed to light of 3300 lux for 5 min and for 1 hour respectively on consecutive evenings at 22:00 hours during winter and summer. In the winter, the measurements were undertaken both before and after the treatment with bright light for 2 weeks. In the summer, there was no treatment. Melatonin concentration in saliva and subjective sleepiness were measured at 22:00 and 23:00 hours on each test. There was no significant difference in the suppression of melatonin in response to the light tests between the patients and the controls. Exposure to light reduced the level of subjective sleepiness more among the patients compared to the control subjects. This reduction was not associated with the change in melatonin secretion nor the improvement in depressive symptoms.

Exogenous melatonin fails to counteract the light-induced phase delay of human melatonin rhythm

Salivary melatonin levels were measured in 6 healthy volunteers in order to determine whether the phase shift caused by a single 60-min light pulse of 2000 lux might be inhibited by maintaining high melatonin concentration. In the control sessions, the samples were collected at 60-min intervals under lighting of < 10 lux from 18.00 to 11.00 h. In the light-exposure sessions, placebo or 0.5 mg melatonin was administered orally 60 min prior to the light pulse, timed at the rising phase of the melatonin synthesis. The after-light sessions, one day after the light exposure, were like the control sessions. The average delays of the melatonin half-rise and half-decline times were equal (about 0.7 h) in the placebo and melatonin replacement experiments. The maintenance of high melatonin levels during the light exposure did not counteract the influence of bright light on the melatonin rhythm. Thus, in the adjustment of the melatonin rhythm, light is a stronger regulator than melatonin itself.