Aino Alila-Johansson

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.

No evidence for extraocular light induced phase shifting of human melatonin, cortisol and thyrotropin rhythms

The view that light affects the mammalian circadian clock only through the eyes was recently challenged by a study in which the phases of human circadian rhythms were shifted by extraocular light exposure. This finding has not been confirmed, however. We studied the effects of light exposure (3 h, broad spectrum fluorescent white light, 13 000 lux) on abdomen and chest on the circadian rhythms of serum melatonin, cortisol and thyrotropin in six subjects. The protocol consisted of two 3-day sessions in a dimly lit (< 10 lux) experimental unit. In both sessions hourly serum samples were collected for hormone analysis on days 1 and 3. The skin light exposure was delivered on day 2 from 22.00 to 01.00 h in one of the two sessions in a randomized order. In both sessions all three rhythms tended to delay, presumably due to the endogenous circadian cycle length being slightly longer than 24 h. However, the phase shifts did not differ significantly between the sessions. Thus, the present study does not support the existence of extraocular photic regulation of the circadian rhythms in humans.

Serum Cortisol Levels in Goats Exhibit Seasonal But Not Daily Rhythmicity

The aim of the study was to find out whether there is a daily rhythm in goat serum cortisol concentrations, whether the concentration profiles differ between normal light:dark and constant dark conditions, and whether any seasonal variations might be detected in daily cortisol secretion patterns. Seven Finnish landrace goats were kept at indoor temperature (18–23°C) under artficial lighting that approximately simulated the annual changes of daylength at 60°N. Blood samples were collected for cortisol measurements by radioimmunoassay at 2h intervals during six times of the year: winter (light:dark 6:18h), early spring (10:14h), late spring (14:10h), summer (18:6h), early fall (14:10h), and late fall (10:14h). Cortisol profiles were determined for two consecutive days, first in light:dark (LD) conditions and then in continuous darkness (DD). There was no significant daily rhythm in serum cortisol levels in any time of the year, nor did the profiles in LD and DD conditions show any differences. A significant seasonal variation was, however, detected among the overall cortisol levels. In winter, the concentrations were higher than in any other season, and from early spring to summer they were at their lowest. Under equal photoperiods, the cortisol levels were higher in fall than spring. The difference between winter and summer was confirmed in the following year in LD conditions. There was no correlation between the serum cortisol and progesterone levels. The results suggest that the possible circadian variation of cortisol secretion in goats is completely masked by external factors, and the lighting conditions do not have immediate effects on the daily secretion patterns. The seasonal variation in the overall cortisol levels is most probably related to the changes in photoperiod, because other conditions were relatively constant during the experiment.

Seasonal variation in endogenous serum melatonin profiles in goats: a difference between spring and fall?

The pineal hormone melatonin serves as a signal of day length in the regulation of annual rhythms of physiological functions and behavior. The duration of high melatonin levels in body fluids is proportional to the duration of the dark period of the day. Due to the direct suppression of melatonin by light, the overt melatonin rhythm may differ from the endogenous rhythm driven by the hypothalamic circadian clock. The aim of this study was to find out possible differences between the overt and endogenous melatonin rhythms in goats during the course of a year. Seven Finnish landrace goats (nonlactating females) were kept under artificial lighting that approximately simulated the annual changes of day length at 60°N. Blood samples for melatonin measurements by radioimmunoassay were collected at 2-h intervals during six seasons: winter (light:dark 6:18 h), early spring (10:14), late spring (14:10), summer (18:6), early fall (14:10), and late fall (10:14). Melatonin profiles were determined for 2 consecutive days, first in light-dark (LD) conditions and then in continuous darkness (DD). In LD conditions, the profiles matched the dark period with one exception: in winter, the mean peak duration was significantly shorter than the scotoperiod. In DD conditions, two types of endogenous melatonin patterns were found: a “winter pattern” (peak duration 13–15 h) in winter, early spring, early fall, and late fall, and a “summer pattern” (duration about 11 h) in late spring and summer. Thus, in equal habitual LD conditions in late spring and early fall (LD 14:10), the endogenous melatonin rhythms were not quite similar: the pattern in late spring resembled that in summer, and the pattern in early fall that in winter. These results suggest that, in addition to the light-adjusted overt melatonin rhythm, the endogenous rhythm of melatonin secretion varies during the course of a year.

Bright light exposure of a large skin area does not affect melatonin or bilirubin levels in humans

BACKGROUND Light treatment through the eyes is effective in alleviating the symptoms of some psychiatric disorders. A recent report suggested that skin light exposure can affect human circadian rhythms. Bilirubin can serve as a hypothetical blood-borne mediator of skin illumination into the brain. We studied whether bright light directed to a large body area could suppress the pineal melatonin secretion or decrease serum total bilirubin in conditions that could be used for therapeutic purposes. METHODS Seven healthy volunteers participated in two consecutive overnight sessions that were identical except for a light exposure on the chest and abdomen in the second night from 12:00 AM to 6:00 AM (10,000-lux, 32 W/m(2) cool white for six subjects and 3000-lux, 15 W/m(2) blue light for one subject). Hourly blood samples were collected from 7:00 PM to 7:00 AM for melatonin radioimmunoassays. Bilirubin was measured by a modified diazo method in blood samples taken at 12:00 AM and 6:00 AM and in urine samples collected from 7:00 PM to 11:00 PM and from 11:00 PM to 7:00 AM. RESULTS The skin light exposure did not cause any significant changes in serum melatonin or bilirubin levels. The excretion of bilirubin in urine was also the same in both sessions. CONCLUSIONS Significant melatonin suppression by extraocular light does not occur in humans. Robust concentration changes of serum total bilirubin do not have a role in mediating light information from the skin to the central nervous system.

Pineal melatonin and brain transmitter monoamines in CBA mice during chronic oral nicotine administration

The effects of chronic oral nicotine administration on the pineal melatonin and brain transmitter monoamines were studied in male CBA mice, which possess a clear daily rhythm of melatonin secretion. On the 50th day of nicotine administration, pineal melatonin as well as cerebral dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), norepinephrine (NE), 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG), serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations were determined at various times. The chronic nicotine treatment did not alter the timing of the pineal melatonin peak, which occurred at 10 h after the light offset. However, in mice drinking nicotine solution, the nocturnal pineal melatonin levels were lower than in control mice drinking tap water. The chronic nicotine treatment increased the striatal DA, DOPAC, HVA and 5-HIAA levels, the hypothalamic NE, MHPG and 5-HIAA and the cortical MHPG. Most prominent effects of nicotine were found at 8 h after the light offset, when the striatal levels of DA and HVA, hypothalamic NE and MHPG as well as cortical MHPG were significantly elevated in the nicotine-treated mice compared with the control mice. No direct correlation between nicotines effects on brain transmitter monoamines and on pineal melatonin levels was apparent. The results suggest that chronic nicotine treatment slightly suppresses the melatonin production but does not alter the daily rhythm of pineal melatonin in mice maintained on a light-dark cycle. However, the results indicate that nicotinic receptors might be involved in the regulation of pineal function.

The daily rhythms of melatonin and free fatty acids in goats under varying photoperiods and constant darkness

The purpose of the study was to explore parallel and divergent features of the daily rhythms of melatonin and plasma free fatty acids (FFA) in goats exposed to different lighting conditions. From these features, we attempted to analyze whether the endogenous melatonin rhythm plays any role in the maintenance of the FFA rhythm. Seven Finnish landrace goats were kept under artificial lighting that simulated the annual changes of photoperiod at 60°N (longest photoperiod, 18 h; shortest, 6 h). The ambient temperature and feeding regimen were kept constant. Blood samples were collected 6 times a year at 2 h intervals for 2 d, first in the prevailing light‐dark (LD) conditions and then after 3 d in constant darkness (DD). In LD conditions, the melatonin levels always increased immediately after lights‐off and declined around lights‐on, except in winter (18 h darkness), when the low daytime levels were restored clearly before lights‐on. The FFA levels also displayed a consistent rhythmicity, with low levels at night and a transient peak around lights‐on. In DD conditions, the melatonin profiles were very similar to those found in the habitual LD conditions, but the rhythm tended to advance. The FFA rhythm persisted also in DD, and the morning peak tended to advance. There was an overall parallelism between the two rhythms, with one significant exception. In winter in LD conditions, the morning rise in FFA levels coincided with lights‐on and not with the declining phase of melatonin, whereas in DD conditions, the FFA peak advanced several hours and coincided with the declining phase of melatonin. From this finding and comparisons of the calculated rhythm characteristics, i.e., phase‐shifts, phase differences, and correlations, we conclude that the daily rhythm of FFA levels is most probably generated by an endogenous oscillator, primarily adjusted by dawn, whereas the melatonin rhythm in this species is regulated by an oscillator primarily adjusted by dusk. The results did not exclude a modulatory effect of melatonin on the daily FFA profiles, but melatonin secretion, alone, does not explain the patterns sufficiently.

Evidence against alpha2-adrenoceptor involvement in the regulation of rat melatonin synthesis by ambient lighting

This study was carried out to clarify the role of alpha2-adrenoceptors in the regulation of pineal melatonin synthesis. Medetomidine, a selective alpha2-adrenoceptor agonist, was previously found to be a potent suppressor of nocturnal melatonin levels in rats. Medetomidine and alpha2-adrenoceptor antagonists atipamezole and yohimbine were injected into rats in different conditions, and their pineal melatonin contents were measured by radioimmunoassay. Experiment 1: Blocking the alpha2-adrenoceptors and possible non-adrenergic binding sites with atipamezole did not counteract the light-induced suppression of nocturnal melatonin. These receptors are, thus, not essential for the suppression of melatonin by light. Experiment 2: Blocking the alpha2-adrenoceptors with atipamezole or yohimbine did not sensitize the pineal melatonin synthesis to daytime darkness in the light/dark-entrained rats. The binding sites are not involved in keeping the daytime melatonin levels low, even in darkness. Experiment 3: The rats were sensitized to daytime darkness by keeping them for seven days in constant light. The dark-elicited melatonin rise was suppressed by a lower dose of medetomidine than the normal nocturnal rise in light/dark-entrained rats, while atipamezole had no effect. The results showed that alpha2-adrenoceptor insufficiency is not involved in the constant light-induced pineal supersensitivity. In summary, the experiments indicated that the physiological regulation of melatonin synthesis by ambient lighting in rats does not depend on alpha2-adrenergic mechanisms.

Supersensitivity with reduced capacity for pineal melatonin synthesis in constant light-treated rats

Summary. Melatonin is synthetized from serotonin in two steps driven by the enzymes N-acetyltransferase and hydroxyindole-O-methyltransferase. Constant light treatment reduces rat pineal hydroxyindole-O-methyltransferase activity while the activation of N-acetyltransferase becomes supersensitive to adrenergic stimulation. We studied the effect of this discrepancy on the production of melatonin. Male rats were kept under 12/12-h light/dark (LD) conditions or for 7 days under constant light (LL). They received subcutaneous injections of isoproterenol or methoxamine in the middle of the light period (LD-rats) or the estimated rest phase (LL-rats). A low dose of isoproterenol (0.1 mg/kg) increased pineal melatonin only marginally in LD-rats, while a maximum effect was found in LL-rats. A medium dose (0.2 mg/kg) produced similar levels in both groups. A high dose (0.4 mg/kg) elevated pineal melatonin contents significantly more in normal than light-treated rats. Methoxamine (0.8 mg/kg) had no effects alone nor combined with isoproterenol. The results suggest supersensitivity with reduced capacity for melatonin formation in constant light-treated rats.

Alpha2-adrenoceptor agonist medetomidine affects the melatonin rhythm in rats

We investigated whether alpha2-adrenergic mechanisms participate in the regulation of the daily melatonin rhythm. Female Wistar rats, living under 12:12 h light-dark conditions, received a subcutaneous injection of saline or medetomidine (alpha2-adrenoceptor agonist; 100 microg/kg) 1 h after lights off. Thereafter they were kept in continuous darkness. Pineal glands were collected for melatonin measurements at 2-h intervals during the first and second subjective nights. During both nights, a significant elevation of melatonin levels in medetomidine-injected rats was found 2 h later than in control rats. We interpret the first-night delay to be a sign of medetomidines suppressive effect on melatonin synthesis, and the second-night delay a medetomidine-induced resetting of the circadian clock controlling the melatonin onset.