Circadian misalignment is associated with a high cardiovascular risk among shift workers: is this an opportunity for prevention in occupational settings?

Funding Acknowledgements

Type of funding sources: Public grant(s) – EU funding. Main funding source(s): Fundação para a Ciência e Tecnologia (FCT) and Fundo Social Europeu (FSE)

Introduction

Atypical work schedules encompass more than 20% of the European workforce. The link between shift work and cardiovascular disease (CVD) has been extensively studied being lifestyle behaviours, sleep disruption and circadian misalignment the key mechanisms involved. Social Jetlag (SJL) has been proposed as a proxy for circadian misalignment in epidemiological studies, once it takes into account  individual’s chronotype and working schedules. Therefore we hypothesize that, among  workers under fixed atypical work schedules, those with a greater SJL have a higher CVD risk.

Methods

A cross-sectional observational study was conducted among blue-collar workers of one retail company. Fixed working schedules were early morning, late evening, and night work. Sociodemographic, occupational, lifestyle and sleep data were collected through questionnaire. SJL was quantified by the difference for mid-sleep points on work- and free-days. Even though SJL is a continuous variable, 3 categories have been used (≤2h; 2-4h; ≥4h). Blood pressure (BP) and the total cholesterol (TC) were assessed. The CVD risk was estimated according to the relative risk SCORE chart. A relative risk≥3 was considered "high CVD risk". Descriptive statistics and bivariate analysis according to the CVD risk (high vs other) was performed. The relationship between SJL and high CVD risk was analysed through logistic binary regression using generalized linear models adjusted for age, sex, education, Body Mass Index, consumptions, sleep duration and quality plus work schedule and seniority.

Results

Of the 301 workers, 56.1% were male with a mean age of 33.0 ± 9.4years. Average SJL was 1:57 ± 1:38hours with the majority of workers experiencing ≤2h (59.4%) and 8% (n = 24) more than 4h. Less than a half had hypercholesterolemia (48.8%), overweight (37.9%)or hypertensive values (10.6%), however 50.5% were currently smokers. We found a significant trend for hypertension (p = 0.006) and smoking prevalence (p = 0.043) among ordinal SJL categories. A relative "high CVD risk" was found in 20.3% of the sample (n = 61). These workers were significantly older (p < 0.001), less educated (p = 0.003) and slept less hours on workdays (p = 0.021). In the multiple regression analysis, SJL was an independent risk factor for a "high CVD risk" (p = 0.029).The odds of having a "high CVD risk" increased almost thirty per cent  per each additional hour of SJL (OR = 1.29; 95% CI:1.03-1.63), even after adjusting for sociodemographic, lifestyle, sleep and working features.

Conclusions

We found compelling evidence that a greater SJL was associated with a bigger chance of high CVD risk. From this innovative perspective, the focus is not just on the working schedule itself but also on the worker’s chronotype. These findings suggest that interventions aimed to reduce Social Jetlag, especially in extreme chronotypes and working schedules, poses a great opportunity to minimize the cardiovascular health impact of shift work.

Sleep timing is more important than sleep length or quality for medical school performance

Overwhelming evidence supports the importance of sleep for memory consolidation. Medical students are often deprived of sufficient sleep due to large amounts of clinical duties and university load, we therefore investigated how study and sleep habits influence university performance. We performed a questionnaire-based study with 31 medical students of the University of Munich (second and third clinical semesters; surgery and internal medicine). The students kept a diary (in 30-min bins) on their daily schedules (times when they studied by themselves, attended classes, slept, worked on their thesis, or worked to earn money). The project design involved three 2-wk periods (A: during the semester; B: directly before the exam period--pre-exam; C: during the subsequent semester break). Besides the diaries, students completed once questionnaires about their sleep quality (Pittsburgh Sleep Quality Index [PSQI]), their chronotype (Munich Chronotype Questionnaire [MCTQ]), and their academic history (previous grades, including the previously achieved preclinical board exam [PBE]). Analysis revealed significant correlations between the actual sleep behavior during the semester (MS(diary); mid-sleep point averaged from the sleep diaries) during the pre-exam period and the achieved grade (p = 0.002) as well as between the grades of the currently taken exam and the PBE (p = 0.002). A regression analysis with MS(diary) pre-exam and PBE as predictors in a model explained 42.7% of the variance of the exam grade (effect size 0.745). Interestingly, MS(diary)--especially during the pre-exam period-was the strongest predictor for the currently achieved grade, along with the preclinical board exam as a covariate, whereas the chronotype did not significantly influence the exam grade.

A K(ATP) channel gene effect on sleep duration: from genome-wide association studies to function in Drosophila

Humans sleep approximately a third of their lifetime. The observation that individuals with either long or short sleep duration show associations with metabolic syndrome and psychiatric disorders suggests that the length of sleep is adaptive. Although sleep duration can be influenced by photoperiod (season) and phase of entrainment (chronotype), human familial sleep disorders indicate that there is a strong genetic modulation of sleep. Therefore, we conducted high-density genome-wide association studies for sleep duration in seven European populations (N=4251). We identified an intronic variant (rs11046205; P=3.99 × 10(-8)) in the ABCC9 gene that explains ≈5% of the variation in sleep duration. An influence of season and chronotype on sleep duration was solely observed in the replication sample (N=5949). Meta-analysis of the associations found in a subgroup of the replication sample, chosen for season of entry and chronotype, together with the discovery results showed genome-wide significance. RNA interference knockdown experiments of the conserved ABCC9 homologue in Drosophila neurons renders flies sleepless during the first 3 h of the night. ABCC9 encodes an ATP-sensitive potassium channel subunit (SUR2), serving as a sensor of intracellular energy metabolism.

Entrainment of the human circadian clock

Humans are an excellent model system for studying entrainment of the circadian clock in the real world. Unlike the situation in laboratory experiments, entrainment under natural conditions is achieved by different external signals as well as by internal signals generated by multiple feedbacks within the system (e.g., behavior-dependent light and temperature changes, melatonin levels, or regular nutrient intake). Signals that by themselves would not be sufficient zeitgebers may contribute to entrainment in conjunction with other self-sufficient zeitgeber signals (e.g., light). The investigation of these complex zeitgeber interactions seems to be problematic in most model systems and strengthens the human system for circadian research. Here, we review our endeavors measuring human entrainment in real life, predominantly with the help of the Munich ChronoType Questionnaire (MCTQ). The large number of participants in our current MCTQ database allows accurate quantification of the human phase of entrainment (chronotype) and how it depends on age or sex. We also present new data showing how chronotype depends on natural light exposure. The results indicate the importance of zeitgeber strength on human entrainment and help in understanding the differences in chronotype, e.g., between urban and rural regions.

Circadian entrainment of Neurospora crassa

The circadian clock evolved under entraining conditions, yet most circadian experiments and much circadian theory are built around free-running rhythms. The interpretation of entrainment experiments is certainly more complex than that of free-running rhythms due to the relationship between exogenous and endogenous cycles. Here, we systematically describe entrainment in the simplest of the traditional eukaryotic model systems in circadian research, Neurospora crassa. This fungus forms a mass of spores (bands of conidia) each day. Over a wide range of photoperiods, these bands begin to appear at midnight, suggesting integration of neither dawn nor dusk signals alone. However, when symmetrical light/dark cycles (T cycles, each with 50% light) are applied, dusk determines the time of conidiation with a uniform, period-dependent delay in phase. This "forced" synchronization appears to be specific for the zeitgeber light because similar experiments, but using temperature, result in systematic entrainment, with bands appearing relatively later in shorter cycles and earlier in longer cycles. We find that the molecular mechanism of entrainment primarily concerns posttranscriptional regulation. Finally, we have used Neurospora to investigate acute effects of zeitgeber stimuli known as "masking."

The effects of light on the Gonyaulax circadian system

The circadian system of the marine unicellular alga Gonyaulax polyedra consists of at least two separate circadian oscillators. One of these controls the rhythm of bioluminescence, the other the rhythm of swimming behaviour. These two oscillators have separate light input mechanisms. The bioluminescence oscillator responds mainly to blue light whereas the aggregation oscillator is also sensitive to red light. Therefore, one of the chlorophylls is a likely candidate for the light receptor of the aggregation oscillator. Owing to their differences in spectral sensitivity, the two oscillators can be internally desynchronized when frequent dark pulses (e.g., five minutes every 20 min) are given in otherwise constant red light. Single bright red light pulses interrupting a constant dim blue background shift the bioluminescence oscillator similarly to dark pulses. They also lead to aftereffects in the period of the bioluminescence rhythm, indicating that the aggregation oscillator has a different phase response to red light pulses. In contrast, blue light pulses interrupting a dim red background shift both oscillators in a similar way and do not significantly alter the circadian period following the light pulse. The mammalian phosphagen creatine shortens the period of the bioluminescence rhythm significantly in blue light but not in red. Because it also increases the sensitivity of the phase response of the bioluminescence oscillator, we propose that creatine acts on its blue-sensitive light input mechanism.

Circadian systems: different levels of complexity

After approximately 50 years of circadian research, especially in selected circadian model systems (Drosophila, Neurospora, Gonyaulax and, more recently, cyanobacteria and mammals), we appreciate the enormous complexity of the circadian programme in organisms and cells, as well as in physiological and molecular circuits. Many of our insights into this complexity stem from experimental reductionism that goes as far as testing the interaction of molecular clock components in heterologous systems or in vitro. The results of this enormous endeavour show circadian systems that involve several oscillators, multiple input pathways and feedback loops that contribute to specific circadian qualities but not necessarily to the generation of circadian rhythmicity. For a full appreciation of the circadian programme, the results from different levels of the system eventually have to be put into the context of the organism as a whole and its specific temporal environment. This review summarizes some of the complexities found at the level of organisms, cells and molecules, and highlights similar strategies that apparently solve similar problems at the different levels of the circadian system.

A fungus among us: the Neurospora crassa circadian system

Neurospora crassa is the only molecular genetic model system for circadian rhythms research in the fungi. Its strengths as a model organism lie in its relative simplicity--compared to photosynthesizing and vertebrate organisms, it is a stripped-down version of life. It forms syncitial hyphae, propagates and reproduces, and the circadian clock is manifest in numerous processes therein. As with other model circadian systems, Neurospora features a transcription/translation feedback loop that is fundamental to an intact circadian system. The molecular components of this loop converge with those of blue light photoreception, thus bringing the clock and one of its input pathways together.

Seasonality and photoperiodism in fungi

This review gives a retrospective of what is known about photoperiodism in fungi, which is largely based on reports about seasonal spore concentrations. Relatively few species have been investigated under laboratory conditions, so that our knowledge whether seasonal reproduction in fungi is mainly a direct response to environmental conditions or whether it involves a photoperiodic machinery with memory capacities and a relationship to the circadian system is extremely limited. To form a basis for further experimental endeavors into fungal photoperiodism, we review the reports about endogenous rhythms and photobiology. Finally, we will look at the possibilities of using the fungal circadian model system of Neurospora crassa for future work on photoperiodism.

Assembling a clock for all seasons: are there M and E oscillators in the genes?

The hypothesis is advanced that the circadian pacemaker in the mammalian suprachiasmatic nucleus (SCN) is composed at the molecular level of a nonredundant double complex of circadian genes (per1, cry1, and per2, cry2). Each one of these sets would be sufficient for the maintenance of endogenous rhythmicity and thus constitute an oscillator. Each would have slightly different temporal dynamics and light responses. The per1/cry1 oscillator is accelerated by light and decelerated by darkness and thereby tracks dawn when day length changes. The per2 /cry2 oscillator is decelerated by light and accelerated by darkness and thereby tracks dusk. These M (morning) and E (evening) oscillators would give rise to the SCN's neuronal activity in an M and an E component. Suppression of behavioral activity by SCN activity in nocturnal mammals would give rise to adaptive tuning of the endogenous behavioral program to day length. The proposition-which is a specification of Pittendrigh and Daan's E-M oscillator model-yields specific nonintuitive predictions amenable to experimental testing in animals with mutations of circadian genes.

Daily rhythm of vigilance assessed by temporal resolution of the visual system

To assess the daily distribution of temporal resolution in visual detection, binocular double-pulse resolution (DPR) was measured over a 24 h period in six healthy subjects. DPR showed a significant daily variation with an amplitude for the foveal stimulus of up to 60%. Like in other vigilance-dependent daily rhythms, optimal performance occurred around midday. The DPR measurements described here are an excellent method for assessment of vigilance and mental alertness (e.g. in pharmacological studies). They show strong time-of-day differences, are highly reliable across successive measurements, and can be fully automated.

The circadian cycle: is the whole greater than the sum of its parts?

The term 'circadian rhythm' describes an oscillatory behavior in the absence of exogenous environmental cues, with a period of about a day. As yet, we don't fully understand which biological mechanisms join together to supply a stable and self-sustained oscillation with such a long period. By chipping away at the molecular mechanism with genetic approaches, some common features are emerging. In combining molecular analyses and physiological experiments, those features that are crucial for structuring a circadian day could be uncovered.

Circadian clocks: Omnes viae Romam ducunt

The circadian clock in all organisms is so intimately linked to light reception that it appears as if evolution has simply wired a timer into the mechanism that processes photic information. Several recent studies have provided new insights into the role of light input pathways in the circadian system of Arabidopsis.

Circadian systems and metabolism

Circadian systems direct many metabolic parameters and, at the same time, they appear to be exquisitely shielded from metabolic variations. Although the recent decade of circadian research has brought insights into how circadian periodicity may be generated at the molecular level, little is known about the relationship between this molecular feedback loop and metabolism both at the cellular and at the organismic level. In this theoretical paper, we conjecture about the interdependence between circadian rhythmicity and metabolism. A mathematical model based on the chemical reactions of photosynthesis demonstrates that metabolism as such may generate rhythmicity in the circadian range. Two additional models look at the possible function of feedback loops outside of the circadian oscillator. These feedback loops contribute to the robustness and sustainability of circadian oscillations and to compensation for long- and short-term metabolic variations. The specific circadian property of temperature compensation is put into the context of metabolism. As such, it represents a general compensatory mechanism that shields the clock from metabolic variations.

Assignment of circadian function for the Neurospora clock gene frequency

Circadian clocks consist of three elements: entrainment pathways (inputs), the mechanism generating the rhythmicity (oscillator), and the output pathways that control the circadian rhythms. It is difficult to assign molecular clock components to any one of these elements. Experiments show that inputs can be circadianly regulated and outputs can feed back on the oscillator. Mathematical simulations indicate that under- or overexpression of a gene product can result in arrhythmicity, whether the protein is part of the oscillator or substantially part of a rhythmically expressed input pathway. To distinguish between these two possibilities, we used traditional circadian entrainment protocols on a genetic model system, Neurospora crassa.

Circadian clocks--from genes to complex behaviour

Circadian clocks control temporal structure in practically all organisms and on all levels of biology, from gene expression to complex behaviour and cognition. Over the last decades, research has begun to unravel the physiological and, more recently, molecular mechanisms that underlie this endogenous temporal programme. The generation of circadian rhythms can be explained, at the molecular level, by a model based upon a set of genes and their products which form an autoregulating negative feedback loop. The elements contributing to this transcriptional feedback appear to be conserved from insects to mammals. Here, we summarize the process of the genetic and molecular research that led to 'closing the molecular loop'. Now that the reductionist approach has led to the description of a detailed clock model at the molecular level, further insights into the circadian system can be provided by combining the extensive knowledge gained from decades of physiological research with molecular tools, thereby reconstructing the clock within the organism and its environment. We describe experiments combining old and new tools and show that they constitute a powerful approach to understanding the mechanisms that lead to temporal structure in complex behaviour.

Daily rhythm of temporal resolution in the auditory system

Over a period of 24 hours, fusion thresholds (click durations 100 micros) were assessed in 7 subjects. Over the same period, order thresholds (click duration of 1 ms) were measured in 10 subjects (12 independent sessions). Auditory fusion thresholds showed a diurnal rhythm with a maximum performance (shortest intervals) around midnight. In contrast, order thresholds appear to be independent on the time of day. Sex specific differences in threshold levels were only observed in order thresholds but not in fusion thresholds.

Molecular circadian oscillators: an alternative hypothesis

Results from experiments in different organisms have shown that elements of input pathways can themselves be under circadian control and that outputs might feed back into the oscillator. In addition, it has become clear that there might be redundancies in the generation of circadian rhythmicity, even within single cells. In view of these results, it is worth reevaluating our current working hypotheses about the pacemaker's molecular mechanisms and the involvement of single autoregulatory genes. On one hand, redundancies in the generation of circadian rhythmicity might make the approach of defining a discrete circadian oscillator with the help of single gene mutations extremely difficult. On the other hand, many examples show that components of signal transduction pathways can indeed be encoded by single genes. The authors have constructed a model placing an autoregulatory gene and its products on an input pathway feeding into a separate oscillator. The behavior of this model can explain the majority of results of molecular circadian biology published to date. In addition, it shows that different qualities of the circadian system might be associated with different cellular functions that can exist independently and, only if put together, will lead to the known circadian phenotype.

Nitrate, a nonphotic signal for the circadian system

Recent advances in circadian biology have brought insights into the molecular mechanisms involved in the generation of circadian rhythmicity. However, little is known about the relationship between these molecular oscillators and the organism's temporal adaptation to complex daily environmental changes. We have studied the effects of nutrients on the circadian system of a marine unicellular alga and found that nitrate influences all three canonical properties of circadian rhythmicity: amplitude, phase, and period. The effects of nitrate are part of a complex feedback loop involving both circadian input and output. These new aspects of circadian nutrient metabolism have important implications for our understanding of the ecological significance of circadian programs.

Phased protein synthesis at several circadian times does not change protein levels in Gonyaulax

The synthesis rates of 13 individual proteins in Gonyaulax polyedra, resolved by two-dimensional gel electrophoresis, were estimated from the amount of 35S methionine incorporated during in vivo pulse labeling at 2-h intervals over one circadian period. The synthesis rates of three proteins, taken as controls, varied 2-3 fold, and no systematic pattern to these variations was apparent. In contrast, the synthesis rates of 10 other proteins varied at least tenfold and in a smooth and systematic pattern. The patterns of protein synthesis were placed into three different groups, the first occurring during the late day/early night phase, the second during the middle of the night phase, and the third during the late night/early day phase. The length of time that individual proteins within each group could incorporate radiolabel was variable, raising the possibility that additional groups might be present. However, both a replicate experiment in continuous light and a light:dark experiment confirmed the presence of at least three different groups of protein synthesis patterns. Unlike the circadian changes in the synthesis rate of the luciferin binding protein, which produces variations in protein levels that correlate with the bioluminescence rhythm, no substantial changes were found in the levels of any other rhythmically synthesized proteins examined.

Several clocks may simplify the circadian system of Gonyaulax

The circadian system of Gonyaulax involves at least two pacemaking oscillators. These oscillators have been shown to be distinct at the physiological level by an examination of their rhythmic periods under different conditions, their phase response to pulses of light and darkness and their ability to compensate the rhythms for changes in temperature. It is also known that Gonyaulax can restrict synthesis of different proteins to at least three different times of day. We suggest that the two known oscillators may each control synthesis of a differently timed class of proteins.

Different phase responses of the two circadian oscillators in Gonyaulax

Two circadian oscillators have been previously shown to exist in the unicellular alga Gonyaulax polyedra by virtue of different periods for two different rhythms, which occur under certain experimental conditions. Here we show that phase shifts in response to light pulses also differ for these two rhythms. The phase response curves (PRCs) for white or blue light pulses are similar in shape and amplitude, but are somewhat displaced in time and have a slightly larger delay portion for the aggregation rhythm, corresponding to the A oscillator. In contrast, the aggregation rhythm PRC for dark pulses in a red light background has a much larger amplitude than that for the bioluminescence rhythm (B oscillator). These results suggest that the light input mechanism of the B oscillator is mainly blue-sensitive, whereas that of the A oscillator is sensitive to both red and blue light. This is supported by entrainment experiments: Dark pulses given as a skeleton 24-hr scotoperiod in a blue light background act as a strong zeitgeber for both oscillators. But if red light is used as background, there is bistability in the entrainment of the B oscillator, whereas the A oscillator is entrained to a definitive phase angle, regardless of the cells' prior phase of entrainment. Finally, the two oscillators can be differentially entrained with 22-hr T cycles using 3-hr dark pulses interrupting red light (19 hr), whereas both oscillators are entrained when the dark pulses interrupt blue light.

Light-induced phase responses in Gonyaulax are drastically altered by creatine

The mammalian phosphagen creatine has been shown to shorten the circadian period (tau) of the bioluminescence rhythm in the marine alga Gonyaulax polyedra from 23 to 18 hr. The studies described here concern the interactive effects of creatine and light, mainly on the bioluminescence rhythm. We have found that the tau-shortening effect of creatine is greater in blue light, suggesting that it acts on a blue-sensitive light input pathway. In addition, creatine affects the phase response mechanism in Gonyaulax, which is also known to be mainly blue-sensitive. The responses to 4-hr light pulses are dramatically increased under the influence of creatine. The unusual phase response curve (PRC) of the Gonyaulax circadian system, which has no phase delays in the early night, is changed in the presence of creatine to a more typical type 0 PRC, with delays of up to 12 hr. Creatine also amplifies the cells' phototactic response, suggesting that the blue-sensitive light input pathway is shared by the phase-shifting mechanism of the bioluminescence oscillator and the mechanism responsible for phototaxis.

Are the effects of light on phase and period of the Gonyaulax clock mediated by different pathways?

The effects of different wavelengths of light on period, phase shifting, entrainment and after-effects of the circadian clock of the motile marine dinoflagellate Gonyaulax polyedra are described. Phase shifting and entrainment by light can be explained by the action of a single blue sensitive light input pathway. However, tonic effects of light on the period under free-running conditions, and also after-effects on period resulting from single 4 h light exposures, appear to involve two input pathways with different absorption and temperature characteristics. These results suggest different mechanisms for the control of phase and period of the circadian clock.

Annual rhythm of human reproduction: I. Biology, sociology, or both?

The annual rhythm of human reproduction was analyzed on the basis of more than 3000 years of monthly birth rates covering 166 regions of the globe. The following variables were used to characterize the annual rhythm of human conception (birth minus 9 months): amplitude, phase of maximum and minimum, phase and length of the time span when rates are above the annual mean (alpha), and the steepest upward slope (delta max) of the curve fitted to the rates. The waveform of the annual rhythm is characteristic for geographical regions (e.g., unimodal or bimodal) and persists as such for many years. In most countries, the onset of alpha coincides with delta max and lies close to the spring equinox. This phase of the rhythm is the most stable over time. In many populations, the rhythm has changed in recent years, specifically in amplitude and phase. The phase of the rhythm depends on latitude, with a 6-month difference between the Northern and Southern Hemispheres. The latitudinal distribution of amplitudes is less systematic. In spite of the many social influences on timing of conceptions, we conclude that the seasonal component in human reproduction is based on biological factors.

Annual rhythm of human reproduction: II. Environmental correlations

Annual rhythms of human conception rates (based on a worldwide selection of statistics) were correlated with photoperiod, monthly averages of daily hours of sunshine, minimum and maximum temperature, and humidity. Our results show for the first time on a global scale that photoperiod, as shown for many animals, may also influence the physiology of human reproduction. At higher latitudes, where changes in daylength are pronounced, a steep increase in human conceptions coincides with the vernal equinox. Temperature also appears to be a major influencing factor. Conception rates are above the annual mean at temperatures between 5 degrees and 20 degrees C, and temperature extremes decrease the probability of conceptions. In regions with cold winters and moderate summers, the number of conceptions correlates positively with temperature; close to the equator, where winters are more moderate than the hot summers, this correlation is negative. Regions with both hot summers and cold winters tend to have a bimodal conception rhythm. The influence of photoperiod is dominant before 1930, whereas the impact of temperature dominates in later years. With industrialization, people are increasingly shielded from both photoperiod (by indoor work) and temperature (by heating and air conditioning), which may explain the deseasonalization of the human conception rhythm. Photoperiod and temperature and the specific changes in their impact can account for several features of the annual human conception rhythm: latitude dependence, waveform, phase and amplitude, and their specific changes over time.

Creatine accelerates the circadian clock in a unicellular alga

The circadian clock is considered to be a universal feature of eucaryotic organisms, controlling the occurrence and rates of many different aspects of life, ranging from single enzymatic reactions and metabolism to complex behaviours such as activity and rest. Although the nature of the underlying cellular/biochemical oscillator is still unknown, many substances are known to influence either phase or period of circadian rhythms in different organisms. These include D2O, electrolytes and ion channel inhibitors, small organic molecules such as alcohols and aldehydes, inhibitors of protein synthesis and amino-acid analogues. Certain transmitter and neurochemical drugs also influence the circadian clock in higher animals. We report here that the period of free-running circadian rhythms in the unicellular marine alga Gonyaulax polyedra is shortened by extracts from mammalian cells. The effect is dose-dependent, accelerating the circadian clock by as much as 4 hours per day. The substance responsible for this effect has been isolated from bovine muscle and identified as creatine. Authentic creatine has identical biological effects at micromolar concentrations and is known in animal systems for its involvement in cellular energy metabolism. A period shortening substance with similar chemical properties is also present in extracts of Gonyaulax itself.

Topographical distribution of the summation property of Y-ganglion cells in the cat retina

The stimulus response characteristics (SRCs) of 33 phasic retinal ganglion cells were established on the basis of extracellular recordings in the optic tract of anaesthetized and paralyzed cats. All SRCs were best described with two straight lines in double logarithmic coordinates. The near threshold light intensity summation was found to be linear, on the average up to 4.8 times threshold. The cells' threshold, defined as smallest response outside the 95%-confidence interval of the spontaneous activity (SpA) distribution, is dependent on the slope of the gain near threshold (linear gain) and the standard deviation of the spontaneous activity (SpA-scatter) prior to stimulation. The slope of the double logarithmic relationship at higher intensities (non-linear gain) - corresponding to the exponent of the power function - increases with threshold intensity. The linear, near threshold gain was used to describe the retinotopic distribution of the cells' threshold- and suprathreshold sensitivity. This sensitivity is high in the center of the retina decreasing steadily towards the periphery. Threshold, as well as linear and non-linear gain are interdependent parts of the SRC, specific for each ganglion cell and, furthermore, the geometrical mean between threshold activity and the response activity at the intersection point of the two regression lines is constant around 30 imp/s, irrespective of the cell's range of operation. The entire course of the SRC can therefore be predicted on the basis of the SpA-scatter and threshold intensity. The homogeneous population of investigated Y-ganglion cells proved to be a set of cells with summation characteristics, changing systematically with threshold and the distance of the receptive field from area centralis.