Melatonin rhythms in Arctic urban residents

Sustained response of pineal melatonin synthesis to a single one-minute light pulse during night in Djungarian hamsters (Phodopus sungorus)

Previous experiments have demonstrated that short light pulses during the night suppress pineal melatonin formation almost completely for the remainder of the night. Here, the effects of a single 1 min light pulse during the night on melatonin synthesis during the following night was investigated in Djungarian hamsters (Phodopus sungorus). It is shown that the melatonin pattern during the consecutive night is suppressed to a greater extent than that caused by acute light exposure, indicating a very effective light memory of the melatonin-generating neuronal network, possibly a key to understanding the reliability of the photoperiod-measuring system.

The midline pineal "eye": MR and CT characteristics of the pineal gland with and without benign cyst formation

The purpose of this study was to evaluate the magnetic resonance imaging characteristics of pineal cysts and pineal calcifications and to determine the incidence of benign pineal cysts. Two-hundred-fifty magnetic resonance examinations were retrospectively examined for the incidence of pineal cysts. In addition, 60 collected cases of pineal cysts were evaluated with regard to cross sectional diameter and magnetic resonance signal characteristics. Finally, the magnetic resonance signal characteristics of pineal tissue in 50 patients were compared to companion computed tomographic scans that were scrutinized for the presence or absence of calcification. The incidence of pineal cysts as revealed by magnetic resonance imaging in this study was 10.8%. The minimal and maximal measurements ranged from a low of 2 x 2 x 2 mm to a high of 10 x 12 x 10 mm. The magnetic resonance signal intensities of pineal cyst as compared to cerebrospinal fluid were iso- or hyperintense on all magnetic resonance sequences in the majority of cases. Calcifications of the pineal gland as revealed by computed tomography tended to be isointense to gray matter if the calcifications were small and hypointense to gray matter if large on all magnetic resonance acquisitions. A careful analysis of the magnetic resonance signal characteristics enables the recognition of moderate- to large-sized pineal calcifications and their differentiation from large pineal cysts. However, small cysts of the pineal gland can be difficult or impossible to distinguish on magnetic resonance imaging from calcifications without comparison with computed tomography.

Effect of photoperiod on the diurnal melatonin and 5-methoxytryptophol rhythms in the human pineal gland

The mammalian pineal organ is a major endocrine component of the circadian timing system, primarily concerned with the regulation of photoperiodic responses. The effect of different photoperiods on the diurnal variation in the concentration of melatonin and 5-methoxytryptophol (5-ML) was studied in the pineal glands, obtained at autopsy, of 38 human subjects, and measured by radioimmunoassay. A diurnal rhythm in pineal melatonin was evident only in the long photoperiod (April-September) with melatonin concentrations being 4.2 times higher at night (22.00-10.00 h) than during the day (10.00-22.00 h). In contrast, diurnal variations in the pineal 5-ML content were only observed in the short photoperiod (October-March) with high concentrations during day-time and low concentrations during night-time. In general, night-time concentrations of both melatonin and 5-ML were higher in summer than in winter. These data suggest that the synthesis of indoleamines in the human pineal exhibits a diurnal rhythm which is affected by seasonal changes in day length.

Twenty-four-hour rhythms in relation to the natural photoperiod: a field study in humans

The daily rhythms of salivary melatonin, salivary cortisol, and axillary body temperature were measured in nine healthy volunteers in midsummer, around the autumn equinox, and in midwinter, at a latitude of 60 degrees N. The aim was to find out whether these rhythms were dependent on variations of the natural daylength. The samples were collected every 2 hr during 24-hr periods in everyday conditions. The individual rhythms were characterized with the acrophase estimates of the best-fitting cosine curve models and with the half-rise and half-decline times calculated from the raw data. The melatonin and cortisol rhythms were delayed significantly (about 1 hr) in midwinter as compared with summer and autumn. The most advanced rhythms were found in autumn. The shifts of the melatonin and cortisol rhythms could be explained as a result of the changes of natural illumination. The overt temperature rhythms did not differ significantly among the sampling months. The lack of seasonal patterns in temperature rhythms probably primarily reflected the socially determined rest-activity cycles of the subjects.