The correlation between serum and salivary melatonin concentrations and urinary 6-hydroxymelatonin sulphate excretion rates: two non-invasive techniques for monitoring human circadian rhythmicity

Urinary 6-sulfatoxymelatonin profiles in male Djungarian hamsters (Phodopus sungorus) responding and not responding to short-day photoperiods: possible role of elevated daytime levels

The lack of endocrine and physiological responses of some Djungarian hamsters (Phodopus sungorus) to the transition from long to short photoperiods (L:D 16:8 --> L:D 8:16) has been known for a long time but is not yet understood. We investigated the role of melatonin synthesis in this context because melatonin, as part of the circadian system, may play a role in non-responsiveness. In ten responding and ten non-responding male hamsters, the urinary 24 hr 6-sulfatoxymelatonin (aMT6s) profiles under L:D 8:16 and L:D 16:8 were measured. Both short day responding and non-responding hamsters showed diurnal aMT6s excretion rhythms. Whereas responders reacted to the transition L:D 16:8 --> L:D 8:16 with a marked elevation of aMT6s excretion, in non-responders no adjustment of the melatonin rhythm to the change of the photoperiod was seen. Furthermore, under L:D 16:8 the daytime levels of aMT6s were significantly (P<0.001) lower in responders compared to non-responders whereas under L:D 8:16 these levels were higher (P<0.01). It is speculated that high daytime levels of aMT6s under long-day photoperiods in non-responders result in down-regulation of melatonin receptors of the nucleus suprachiasmaticus, the pacemaker for the pineal gland, leading to a lack of response to the transition to short-day photoperiods.

Prediction of nocturnal plasma melatonin from morning urinary measures

A growing literature indicates that blood levels of the hormone melatonin may have important implications for human health and well-being. Melatonin is synthesized and released into the general circulation at night, however, and it is seldom feasible to draw blood samples at night in epidemiological studies. There is some evidence that levels of urinary melatonin and of 6-sulfatoxymelatonin (aMT6s), the major metabolite of melatonin, accurately reflect nocturnal plasma melatonin. If this is the case, urinary assays could be powerful tools for epidemiological studies. A laboratory-based study was performed to examine the relationships between nocturnal plasma melatonin, morning urinary melatonin, and morning urinary aMT6s levels in 78 men. The relationship between total nocturnal plasma melatonin and both urinary aMT6s corrected for creatinine and urinary melatonin is significant. Combining the two urinary measures accounts for 72% of the variance in total plasma melatonin. Peak nocturnal plasma melatonin also was significantly related to urinary melatonin and to aMT6s. The urinary measures show good sensitivity and specificity in identifying individual differences in nocturnal plasma melatonin levels. These results support the inclusion of morning urine samples to assess the contribution of the hormone melatonin in occupational or residential studies involving healthy, young men.

Sulphatoxymelatonin excretion in older people: relationship to plasma melatonin and renal function

In order to validate measurement of urinary sulphatoxymelatonin as an accurate method of estimating plasma melatonin secretion in older people, we compared 24 h plasma melatonin secretion and sulphatoxymelatonin excretion with renal function in 20 subjects 62-89 years of age. There was a good correlation between plasma and urinary sulphatoxymelatonin over the same 24 h period (R2 = 0.797) and no relationship between creatinine clearance and sulphatoxymelatonin excretion (R2 = 0.075). The results suggest that sulphatoxymelatonin excretion estimation is a good surrogate measurement of plasma melatonin secretion in older people, at least across the range of creatinine clearance for the subjects in the study, 0.41-1.81 ml/sec.

Creatinine is an appropriate reference for urinary sulphatoxymelatonin of laboratory animals and humans

In our studies on diurnal 6-sulphatoxymelatonin (aMT6s) rhythms in various species, we have sometimes obtained fluctuating patterns. In most of these, the volume of individual urine fractions was not accurately measured because of methodological problems. Here, we report a simple method to overcome these problems by using urinary creatinine to estimate urine volume. The benefit of this method is demonstrated in two representative examples of the diurnal aMT6s rhythms of rats, domestic pigs and humans. Because the human urine fractions were collected accurately, the qualitative pattern of the aMT6s rhythm was not altered by using urinary creatinine as a substitute for urine volume. The total creatinine excretion (urine volume x creatinine concentration) was constant within a small range and showed no diurnal rhythm. In rats and pigs, the highly variable aMT6s concentrations relative to urine volume throughout the 24-hr period were changed drastically by referring to creatinine. All aMT6s patterns became stable and qualitatively similar to those of the rest of the group. From these results it can be concluded that creatinine is an adequate substitute for urine volume and a beneficial parameter with which to overcome technical problems with urine collection from laboratory animals or unknown urine volumes in human studies.

Salivary melatonin as a circadian phase marker: validation and comparison to plasma melatonin

There are many situations in which it would be useful to know the phase state of the biological clock. It is recognized that measurement of melatonin levels can provide this information, but traditionally blood has been used for the analysis, and there are many problems in extending the measurements into the home or field situations. The aim of this study was to develop and validate a salivary melatonin radioimmunoassay and to compare results obtained against a plasma assay for determining the onset of melatonin secretion. The assay developed was sensitive (4.3 pM) and required only 200 microliters of sample. A rhythm in melatonin was detected in saliva, peaking at approximately 120 pM or 30% of the plasma levels. Using an objective criterion for determining the onset of secretion (mean +/- 2 standard deviations of three daytime samples), the time of onset was shown to exhibit low intraindividual variability (coefficient of variation = 1.5%-4.3%). The time of onset determined using saliva was significantly correlated with the plasma onset (r = .70, p < .05). The onsets determined were 22:30 h +/- 22 min for the saliva and 21:50 h +/- 16 min for plasma for 17 subjects. Similarly, the acrophases of the saliva and plasma melatonin rhythms were significantly correlated. Neither posture alone nor changes in posture affected the calculation of the onset of melatonin secretion using the saliva approach. Very high saliva flow rates induced by citric acid resulted in lower melatonin concentrations compared to the gentle chewing on parafin film. These results firmly establish the use of salivary melatonin measurements for phase typing of the melatonin rhythm in humans.

Exposure to long summer days affects the human melatonin and cortisol rhythms

Exposure of 8 human subjects in summer to a natural 16 h bright light photoperiod phase advanced the morning salivary melatonin decline and cortisol rise and shortened the nocturnal melatonin signal by 2 h relative to the winter patterns of the same subjects followed under a combined artificial and natural light 16 h photoperiod. The data suggest that summer days experienced from sunrise till sunset and not winter days with a combined artificial and natural light long photoperiod evoke a true long day response of the human circadian system.

A fixed morning awakening coupled with a low intensity light maintains a phase advance of the human circadian system

The next day after an early morning awakening of human subjects coupled with a single exposure to bright light from 0600 to 0900 h, the salivary melatonin rhythm was phase advanced by about 1 h as compared with the pre-treatment profile. The phase advance was maintained for 4 days when the subjects adhered to the 0600 h awakening coupled with just low intensity light, but vanished when they slept till 0900 h. Similarly, 3 days following a twice repeated exposure to bright light from 0600 to 0900 h coupled with an early morning awakening, the salivary melatonin rhythm was phase advanced relative to the pre-treatment profile only in the subjects who adhered to the 0600 h awakening but not in those who slept till 0900 h. The findings may be used for treatment of chronobiological disorders.

Urinary 6-sulfatoxymelatonin cycle-to-cycle variability

For either clinical or research purposes, the timing of the nocturnal onset in production of the urinary melatonin metabolite 6-sulfatoxymelatonin (UaMT6s-onset), has been proposed as a reliable and robust marker of circadian phase. However, given that most circadian rhythms show cycle-to-cycle variability, the statistical reliability of phase estimates obtained from a single study using UaMT6s-onset remains to be determined. Following 2 weeks of sleep diary and wrist actigraphy, 15 young, healthy good sleepers participated in four UaMT6s sampling sessions spaced 1 day apart. During the sampling sessions subjects remained indoors under low light conditions and hourly urine samples were collected from 19:00 to 02:00 h. Samples were subsequently assayed for UaMT6s using standard radioimmunographic techniques. UaMT6s-onset was determined by the time at which melatonin production exceeded the average of three proceeding trials by 100%. Sleep onset times were derived from sleep diary and actigraphic measures taken before the melatonin collection nights. We found that there was no significant variation between nights in group mean UaMT6s-onset times, and intraindividual variability was small. In addition, UaMT6s-onset times were highly and significantly correlated between nights (grand mean r = 0.804). Our results suggest that within 95% confidence interval limits, individual UaMT6s-onset estimates obtained from a single night UaMT6s-onset study can be used to predict subsequent UaMT6s-onset times within +/- 97 min. A close temporal relationship was also found between the timing of UaMT6s-onset and sleep onset. Overall, our results suggest that under entrained conditions single-session UaMT6s-onset studies can provide reliable individual UaMT6s-onset phase estimates and that the protocol described in this study is a practical and noninvasive methodology.

Circadian rhythms of early morning awakening insomniacs

It has been suggested that two types of insomnia, sleep onset insomnia and early morning awakening insomnia, may be caused by delays and advances respectively of circadian rhythms. Evidence supports the circadian rhythm phase delay of sleep onset insomniacs. The present study investigated the phase timing of circadian rhythms of early morning awakening insomniacs compared with a group of age matched good sleepers. A 24-h bed rest laboratory session was used to evaluate the endogenous core body temperature and urinary melatonin rhythms. Objective and subjective sleepiness were also measured every 30 min across the session with 10 min multiple sleep latency tests and Stanford Sleepiness Scale. Maximum and minimum phases of each individual's rhythm were identified using two-component cosine curve fitting. Compared with the good sleepers, the insomniacs had significant phase advances of 2-4 h for the temperature and melatonin rhythms. However, the 0-4 h advances of the sleepiness rhythms were not significant. This latter unexpected result was explained on the basis of variability of sleepiness measures. It was suggested that early morning awakening insomnia arises from phase advanced circadian rhythms which evoke early arousal's from sleep.

Effect of light therapy on salivary melatonin in seasonal affective disorder

To investigate the role of a light-induced advance in the timing of the melatonin rhythm in seasonal affective disorder, 11 depressed patients underwent 2 weeks of light therapy with full spectrum or cool white light. Evening saliva samples were collected before and after each week of treatment and assayed for melatonin to determine the time of onset of nocturnal secretion. Both treatments reduced depression scores, advanced the timing of the melatonin rhythm, and increased melatonin concentrations. Time of onset of the nocturnal increase in melatonin did not differ between clinical responders and nonresponders, suggesting that a phase advance in the onset of nocturnal melatonin secretion is not sufficient to induce clinical remission in seasonal affective disorder.

Melatonin and cortisol assessment of circadian shifts in astronauts before flight

Melatonin and cortisol were measured in saliva and urine samples to assess the effectiveness of a 7-day protocol combining bright-light exposure with sleep shifting in eliciting a 12-hr phase-shift delay in eight U.S. Space Shuttle astronauts before launch. Baseline acrophases for 15 control subjects with normal sleep-wake cycles were as follows: cortisol (saliva) at 0700 (0730 in urine); melatonin (saliva) at 0130 (6-hydroxymelatonin sulfate at 0230 in urine). Acrophases of the astronaut group fell within 2.5 hr of these values before the treatment protocols were begun. During the bright-light and sleep-shifting treatments, both absolute melatonin production and melatonin rhythmicity were diminished during the first 3 treatment days; total daily cortisol levels remained constant throughout the treatment. By the fourth to sixth day of the 7-day protocol, seven of the eight crew members showed phase delays in all four measures that fell within 2 hr of the expected 11- to 12-hr shift. Although cortisol and melatonin rhythms each corresponded with the phase shift, the rhythms in these two hormones did not correspond with each other during the transition.

Urinary 6-sulphatoxymelatonin excretion reflects pineal melatonin secretion in the Djungarian hamster (Phodopus sungorus)

To monitor pineal function in the Djungarian hamster (Phodopus sungorus), we measured the urinary excretion of the melatonin metabolite 6-sulphatoxymelatonin (aMT6s) at 3-hr intervals by radioimmunoassay. Hamsters maintained in either long photoperiod (LP, LD 16:8) or short photoperiod (SP, LD 8:16) showed marked daily rhythms in aMT6s excretion, with elevated levels during the dark phase. In both photoperiods, we found large interindividual differences, mainly in the amplitude of the signal. However, the amplitude as well as the duration of nocturnal aMT6s excretion was higher in SP than in LP. Light exposure at night (180 mW/m2, 30 min) caused a decrease in aMT6s excretion, indicating that the pineal gland is the major source of urinary aMT6s. Moreover, there was a significant correlation between nocturnal pineal/plasma melatonin contents and 24-hr aMT6s excretion. We conclude that, measurements of aMT6s provide a valid and quantitative index of pineal melatonin synthesis in this hamster species. As an advantage in determining pineal melatonin contents, this approach will allow noninvasive long-term studies of individual animals under varying environmental conditions.

Melatonin response in active epilepsy

Urinary excretion of 6-sulfatoxymelatonin (aMT.6S), the hepatic metabolite of melatonin, was measured for three consecutive 8-h intervals, beginning at 0600 h, in 30 patients with untreated active epilepsy and in 19 healthy subjects. Excretion of aMT.6S in a 24-h period in patients with active epilepsy was 77.3 +/- 55 nmol (median 68.0, range 8.7-280 nmol), significantly higher (p < 0.05) than that of healthy subjects (49.1 +/- 14 nmol, median 49.0, range 19.7-68.0 nmol). Sequential 8-h urinary aMT.6S excretion rates in patients with active epilepsy were 2.45 +/- 2.8 nmol/h (0600-1400 h), 0.83 +/- 0.5 nmol (1400-2200 h) and 6.38 +/- 5.0 nmol/h (2200-0600 h) as compared with 1.43 +/- 0.8, 1.10 +/- 0.8 and 3.81 +/- 1.3 nmol/h, respectively, in healthy subjects. Analysis of variance (ANOVA) indicated that the difference in total output resulted from greater nocturnal excretion (F = 5.58, p = 0.018). Melatonin production in untreated patients with active epilepsy is increased and has a circadian pattern with a phase difference as compared with that of normal subjects.

Phase-shifts in melatonin, 6-sulphatoxymelatonin and alertness rhythms after treatment with moderately bright light at night

OBJECTIVES

Shift work and rapid travel across several time zones leads to desynchronization of internal circadian rhythms from the external environment and from each other with consequent problems of behaviour, physiology and performance. Field studies of travellers and shift workers are expensive and difficult to control. This investigation concerns the simulation of such rhythm disturbance in a laboratory environment. The main objectives are to assess the ability of controlled exposure to moderately bright light and darkness/sleep to delay circadian rhythms in volunteers without environmental isolation and, secondly, to evaluate the use of different indices of melatonin (MT) secretion together with self-rated alertness as marker rhythms.

PATIENTS

Six normal volunteers aged 22-26 years (mean +/- SD 24.3 +/- 1.4).

DESIGN

Subjects were exposed to the following periods of moderately bright light (1200 lux) on three consecutive days in early December 1991: Day (D)1: 2000-0200 h, D2: 2200-0400 h and D3: 2400-0600 h. Each period was followed by 8 hours of darkness (< 1 lux). Hourly blood, sequential 4-hourly urine (8-hourly when asleep) and hourly saliva (except when asleep) samples were taken throughout a 24-hour period on D0 (baseline), D4 (1 day post-light treatment) and D7 (4 days post-light treatment). During waking hours, subjective alertness was rated every 2 hours on a visual analogue scale.

MEASUREMENTS

MT was measured in plasma and saliva, and its metabolite, 6-sulphatoxymelatonin (aMT6s), was measured in urine. MT, aMT6s and alertness scores were analysed by ANOVA and a cosinor analysis program.

RESULTS

A delay shift was present in the aMT6s, plasma MT and salivary MT rhythms (degree of shift: 2.67 +/- 0.3 h (P < 0.001, n = 5); 2.35 +/- 0.29 h (P < 0.001, n = 6); and 1.97 +/- 0.32 h (P < 0.01, n = 6), mean +/- SEM, respectively) 1 day post-light treatment compared to baseline. Adaptation to the initial phase position was apparent by the 4th post-treatment day. Significant correlations were obtained between plasma MT onset (degree of shift: 3.12 +/- 0.74 h (P < 0.001, n = 6, mean +/- SEM)) and the acrophases (calculated peak times) of plasma MT (P < 0.001), salivary MT (P < 0.05) and urinary aMT6s (P < 0.01). A significant phase delay in the alertness rhythm was also evident 1 day post-treatment (3.08 +/- 0.67 h (P < 0.01, n = 6, mean +/- SEM)) with adaptation by the 2nd post-treatment day.

CONCLUSIONS

This study suggests that these methods of determining MT secretion are comparable and give reliable assessments of the MT circadian phase position even after a phase-shift. Significant phase-shifts of similar magnitude can be induced in both MT and alertness rhythms using moderate intensity bright light at night.

Posture influences melatonin concentrations in plasma and saliva in humans

Melatonin is extensively used as a circadian marker rhythm and thus any factors influencing its concentrations other than endogenous rhythmicity must be assessed. We report here the effects of posture on melatonin concentrations in plasma and saliva. The study was performed during the rising phase of the melatonin rhythm between 21:00 and 01:30 h. From 23:00 h, 7 healthy subjects remained sitting until 24:00 h and in dim light (< 10 lux) until 01:30 h. From 24:00 h, they assumed a different postural position in one of the following stages: Stage 1a: 24:00-00:30 h-standing, 00:30-01:00 h-supine, 01:00-01:30 h-standing; Stage 1b-reverse of Stage 1a; Stage 2a: 24:00-01:30 h-supine; and Stage 2b: 24:00-01:30 h-standing. Blood and saliva samples were obtained every hour from 21:00-24:00 h and then every 10 min from 24:00-01:30 h. Plasma and salivary melatonin concentrations, measured by radioimmunoassay, increased when moving from a supine to a standing position and decreased when these positions were reversed. These changes, as seen with other blood components, can be explained through the influence of gravity which causes a decrease in plasma volume on standing and an increase in plasma volume on lying down.

Maintenance of a circadian phase adjustment of the human melatonin rhythm following artificial long days

In winter, a 5-day exposure of 4 human subjects to a skeleton photoperiod, with 3 h of bright light in the evening and again in the morning, phase advanced the morning serum melatonin offset by 1-3 h as compared with the original winter melatonin rhythm pattern. The phase advance persisted for 3 days even after the bright light withdrawal. The data indicate that the long skeleton photoperiod had a prevailing phase-advancing effect on the melatonin rhythm and its underlying pacemaker. The maintenance of the phase advance might be due to the fixed sleep-wake schedule.

Urinary 6-sulphatoxymelatonin excretory rhythms in laboratory rats: effects of photoperiod and light

The excretion rhythm of the melatonin metabolite, 6-sulphatoxymelatonin, was determined in rats maintained on contrasting 14 h L:10 h D and 10 h L:14 h D photoperiods. The novel use of a high protein liquid diet together with an automatic urine collection system facilitated the monitoring of the onset, offset and total production of metabolite before, during and after a 57.5% reduction in the dark period (5.75 h and 8 h delay in lights off). In 14 L:10 D, melatonin metabolite excretion increased 2.7 +/- 0.2 h after lights off, whereas in 10 L:14 D, the onset occurred 5.1 +/- 0.2 h after lights off. Duration of melatonin metabolite excretion was not different between the two photoperiods. Reduction of the dark period by 57.5% delayed the appearance of melatonin metabolite excretion until 2.15 +/- 0.4h and 2.9 +/- 0.4h after darkness in both long and short photoperiods. Upon restoration of the original photoperiod on the third day, the onset of metabolite excretion was significantly delayed by 1.4 +/- 0.4 h (P < 0.05) in long day animals and 2.2 +/- 0.2 h (P < 0.05) in short day-length compared to the first day. This study highlights the utility of urinary 6-sulphatoxymelatonin determinations in rats and provides new information on the acute effects of extended evening light on pineal melatonin secretion.

Influence of temperature and photoperiod on plasma melatonin in the mudpuppy, Necturus maculosus

A melatonin (MEL) radioimmunoassay employed previously only in mammals was used to estimate plasma MEL in a salamander, Necturus maculosus. Validation procedures included thin-layer chromatography of plasma extracts, parallel inhibition curves of authentic MEL and serially diluted plasma and plasma extracts and quantitative recovery of authentic MEL added to pooled Necturus plasma. A diel cycle of plasma MEL was demonstrated in mudpuppies acclimatized for a minimum of 3 weeks under a 12L:12D photoperiod and 15 +/- 1 degrees and sampled in late March. The MEL cycle persisted under a reversed photoperiod (lights on, 1800 hr), but the amplitude of the MEL peak was diminished, and the peak was more than 180 degrees out of phase with the corresponding peak under a normal photoperiod. In animals acclimated to 5 degrees in mid-June under a 12L:12D photoperiod, the diel cycle of plasma MEL continued, but both midphotophase and midscotophase concentrations were reduced compared with 15 degrees controls. The diel cycle was also present in animals acclimated to 25 degrees, but the decrease in MEL concentrations was less marked than that in 5 degrees animals. Photoperiod is apparently the primary cue for cycles in plasma MEL in Necturus, but the cycle can be influenced by temperature.

Human circadian rhythm in serum melatonin in short winter days and in simulated artificial long days

Serum melatonin rhythm was studied in 6 human subjects experiencing short winter days resembling light/dark (LD) 8:16 h and in 6 subjects exposed at the same time to a long, LD 16:8 h skeleton photoperiod, with 3 h of bright light in the evening and again in the morning; 4 out of the 6 subjects entrained to the simulated summer photoperiod within 3 days. In the synchronized subjects, the nocturnal melatonin signal was 3 h shorter than in those experiencing just winter days. The data indicate that humans are able to respond to environmental day length by forming a proper endogenous photoperiodic signal.

Effects of evening bright light exposure on melatonin, body temperature and sleep

Five male subjects were exposed to a single 2-h period of bright (2500 lux) or dim (<100 lux) light prior to sleep on two consecutive nights. The two conditions were repeated the following week in opposite order. Bright light significantly suppressed salivary melatonin and raised rectal temperature 0.3 degrees C (which remained elevated during the first 1.5 h of sleep), without affecting tympanic temperature. Bright light also increased REM latency, NREM period length, EEG spectral power in low frequency, 0.75-8 Hz and sigma, 12-14 Hz (sleep spindle) bandwidths during the first hour of sleep, and power of all frequency bands (0.5-32 Hz) within the first NREMP. Potentiation of EEG slow wave activity (0.5-4.0 Hz) by bright light persisted through the end of the second NREMP. The enhanced low-frequency power and delayed REM sleep after bright light exposure could represent a circadian phase-shift and/or the effect of an elevated rectal temperature, possibly mediated by the suppression of melatonin.

Urinary 6-sulphatoxymelatonin, an index of pineal function in the rat

The objective of this study was to apply the radioimmunoassay for 6-sulphatoxymelatonin (aMT6s) to rat urine, and use it to study the source of aMT6s. The radioimmunoassay was found to have acceptable within- and between-assay variation, excellent specificity, and good parallelism between the standard and unknown. Because urine is highly contaminated we assessed whether preliminary purification was required and established that it was unnecessary. Using this assay a 24-hr rhythm in 6-sulphatoxymelatonin output was seen in pools of urine harvested at 3-hr intervals from Wistar rats on LD 12:12. The nocturnal rise in aMT6s was abolished by constant light. In contrast pinealectomy lowered aMT6s output significantly throughout both dark and light. This study confirms previous studies indicating that the pineal is the major source of 6-sulphatoxymelatonin. It is concluded that urinary 6-sulphatoxymelatonin as measured by radioimmunoassay is a valid measure of pineal gland activity in the rat.

Early morning bright light phase advances the human circadian pacemaker within one day

One day after a single exposure to bright light from 03.00 to 09.00 h the morning declines in the serum melatonin concentration were phase-advanced in all subjects relative to pre-exposure patterns by 1.2-2.6 h. The evening melatonin rise was phase-advanced in 5 out of 6 subjects by 0.6-2.2 h. The data suggest that an underlying human circadian pacemaker controlling the melatonin rhythm may be phase-advanced within one day; however, the evening melatonin rise and the morning decline do not necessarily phase-shift by the same amount.

Factors influencing urinary 6-sulphatoxymelatonin, a major melatonin metabolite, in normal human subjects

6-Sulphatoxymelatonin (aMT6s) has been measured, by a direct radioimmunoassay, in urine from 130 normal volunteers aged 2-80 years. Its relationship to a number of physiological parameters has been assessed. Total urinary excretion of aMT6s did not vary in a group of 40 children aged 2-20 years (24 boys and 16 girls) except when expressed as a function of body weight. In this case, total aMT6s excretion over 24 h decreased as a function of age. In 90 adult volunteers (44 men and 46 women) aged 20-80 years, there was an age-related decline in total 24 h aMT6s excretion with significantly lower values in elderly subjects. In this same adult group no relationships were found between total aMT6s excretion and body weight or height. No sex differences were found either in the 2-20 years or the 20-80 years groups. Pineal calcification was assessed by lateral skull X-ray in 26 adult volunteers (17 men and 9 women) aged 20-50 years. No significant differences in aMT6s excretion were found as a function of pineal calcification. In 16 of these subjects plasma melatonin and aMT6s also showed no relationship to pineal calcification. These studies confirm the usefulness of aMT6s as an index of melatonin secretion in normal volunteers.

Correlation between salivary and serum melatonin: dependence on serum melatonin levels

Saliva and serum samples were collected from eight healthy volunteers every two hours during a 26-hour period. Melatonin concentrations were measured by radioimmunoassay after chloroform extraction using radioiodinated melatonin as a tracer. Five of the subjects had high serum melatonin levels at night (peak levels higher than 75 pg/ml); in three subjects the highest serum melatonin concentration was 20-40 pg/ml. All subjects had low levels (less than 10 pg/ml) during the day. The correlations between salivary and serum levels were calculated. The regression line y = 0.33x + 3.7 pg/ml, r = 0.95, P less than 0.001, was obtained for all detectable value pairs (n = 73). The regression and correlation coefficients were almost equal for the peak values of melatonin and during the rising and descending phases of the secretion patterns. However, no significant correlation was found between low daytime salivary and serum concentrations when calculated separately. In the five high-secretors the melatonin levels in saliva reflected reliably the changes in serum, but in the three low-secretors the correlation between salivary and serum melatonin was not significant. The proportion of melatonin found in saliva decreased with increasing serum melatonin levels. Circadian rhythm parameters were estimated by single cosinor analysis. The acrophases did not differ significantly within a subject in the concomitant measurements of serum and salivary melatonin. The measurements of salivary melatonin levels seem valid for studies on melatonin rhythms, but the melatonin concentrations measured in saliva do not always consistently reflect the absolute concentrations in blood.

The dim light melatonin onset as a marker for circadian phase position

Masking is known to affect a variety of circadian rhythms, making it difficult to use them as reliable markers of circadian phase position. Melatonin may be unique in that it appears to be masked only by (bright) light. Sleep and activity do not appear to influence the melatonin rhythm. By measuring the onset of melatonin production, a clearly demarcated event, we can reliably assess circadian phase position, provided blood is sampled under dim light (the dim light melatonin onset, or DLMO). The DLMO has been useful in assessing the phase-shifting properties of bright light and in phase typing patients with chronobiologic disorders, such as winter depression.

Melatonin: perspectives in the life sciences

The pineal gland hormone melatonin is now considered an important neuroendocrine component of animal physiology. Although the functional status of melatonin has been well described for subhuman species, there is a paucity of data concerning the physiological role of this hormone in man. This paucity of data has much to do with the limitations of experimental design imposed by the practical and ethical difficulties associated with the study of a nocturnally secreted hormone. The recent advent of salivary melatonin assay has provided a very practical means of monitoring melatonin secretion in long-term longitudinal type community based studies of pineal gland function in human health and disease. The efforts to describe key chronobiological changes in melatonin secretion of possible functional significance have been accompanied by a seemingly less enthusiastic search to describe the nature of the melatonin receptor, another highly important component of the 'melatonin message'. The functional relevance of specific chronobiological changes in melatonin secretion cannot be completely understood without an increased knowledge of melatonin action at the receptor level. The present work describes the recent methodological advance in the investigation of human pineal gland physiology represented by salivary melatonin assay, and discusses the present status of our knowledge of the melatonin receptor.