Photic sensitivity for circadian response to light varies with photoperiod

The response of the circadian system to light varies markedly depending on photic history. Under short day lengths, hamsters exhibit larger maximal light-induced phase shifts as compared with those under longer photoperiods. However, effects of photoperiod length on sensitivity to subsaturating light remain unknown. Here, Syrian hamsters were entrained to long or short photoperiods and subsequently exposed to a 15-min light pulse across a range of irradiances (0-68.03 μW/cm(2)) to phase shift activity rhythms. Phase advances exhibited a dose response, with increasing irradiances eliciting greater phase resetting in both conditions. Photic sensitivity, as measured by the half-saturation constant, was increased 40-fold in the short photoperiod condition. In addition, irradiances that generated similar phase advances under short and long days produced equivalent phase delays, and equal photon doses produced larger delays in the short photoperiod condition. Mechanistically, equivalent light exposure induced greater pERK, PER1, and cFOS immunoreactivity in the suprachiasmatic nuclei of animals under shorter days. Patterns of immunoreactivity in all 3 proteins were related to the size of the phase shift rather than the intensity of the photic stimulus, suggesting that photoperiod modulation of light sensitivity lies upstream of these events within the signal transduction cascade. This modulation of light sensitivity by photoperiod means that considerably less light is necessary to elicit a circadian response under the relatively shorter days of winter, extending upon the known seasonal changes in sensitivity of sensory systems. Further characterizing the mechanisms by which photoperiod alters photic response may provide a potent tool for optimizing light treatment for circadian and affective disorders in humans.     

Effects of Red and Far Red Light on the Initiation of Cold Acclimation in Cornus stolonifera Michx

Red and far red light distinctly influence the initial phytochrome-mediated phase of cold acclimation in red-osier dogwood (Cornus stolonifera). Under controlled conditions, short days and end-of-day far red light exposure after long days promote growth cessation, cold acclimation, and subsequent cold hardening of dogwood stems in response to low temperature. Nuclear magnetic resonance absorption spectra of the water in internode stem sections imply that the short day-induced phase of cold acclimation involves a change in tissue hydration, at least in part, due to a substantial reduction in bulk phase water as a result of senescence and loss of water from the pith. Seasonal responses to light and an attempt to induce early acclimation under natural conditions with end-of-day far red light are discussed.     

An Endogenous Rhythm in the Rate of CO2 Output of Bryophyllum: II. THE EFFECTS OF LIGHT AND DARKNESS ON THE PHASE AND PERIOD OF THE RHYTHM

The effects of light, darkness, and changes in light intensityon the phase and period of the endogenous rhythm in the rateof CO₂ output of excised leaves of Bryophyllum fedtschenkoihave been examined. The duration, intensity, and wavelength of a short light treatment,and the point in the cycle at which it is administered, determinethe degree of phase shift induced in a rhythm persisting indarkness. When light treatments of 3 and 6 hours' duration,at an intensity of 3,000 lux, are applied between the peaksthe phase is completely reset, the first post-treatment peakoccurring 18–19 hours after the end of the treatment.The degree of phase shift is therefore determined not by theduration of the treatment but by the time at which the treatmentterminates. One hour's illumination has little or no effect.The phase is unaffected when light treatments of up to 5 hours'duration at an intensity of 3,000 lux are applied at the crestof a peak. Over the range 8-3,000 lux the intensity of lightduring a 6-hour treatment applied between the peaks does notaffect the efficiency with which that treatment completely resetsthe phase. At an intensity of 2 lux, however, the phase delayis equal to the duration of the treatment. A 6-hour red-light treatment (850 ergs/cm.²/sec.) applied betweenthe peaks completely resets the phase whereas blue light (10,860ergs/cm.²/sec.) has no effect on the phase but induces a slightprotraction of the period. Moreover, continuous red light inhibitsthe rhythm, which recommences in blue light. A rhythm is induced in illuminated leaves when the light intensityis either gradually or suddenly reduced by at least 80 per cent.Whether a given intensity of illumination inhibits or permitsthe persistence of a rhythm depends upon the light intensityby which it is immediately preceded. A rhythm will persist in illuminated leaves for approximatelyas long as in leaves in darkness and the phase shows no correlationwith time of day. The period is unaffected by the intensityof white light (from 0-500 lux) to which the leaves are subjected.The duration of a short dark treatment, and the point in thecycle at which it is applied, determine the degree of phaseshift induced in a rhythm in illuminated leaves. The phase isreset when 3-, 6-, and 9-hour dark treatments are applied atthe crest of a peak, the amount of phase shift increasing toa maximum with 9 hours' darkness. The phase shift is not equalto the duration of the treatment. The phase is unaffected when3- and 6-hour dark treatments are applied between the peaks. The variation in the sensitivity of the phase of a rhythm persistingin darkness to short light treatments is in the opposite senseto that of a rhythm persisting in light to short dark treatments.The phase of a rhythm in illuminated leaves is completely resetwhen the leaves are transferred to continuous darkness commencingeither at the crest of, or between, the peaks. The results are discussed and compared with those of other authors.     

Evidence for a circadian oscillation in the gonadal response of the tropical weaver bird (Ploceus philippinus) to programmed photoperiod.

The effect of an asymmetrical skeleton photoperiod scheduleee was studied on the gonadal development in a tropical finch, the weaver bird (Ploceus philippinus). The schedul comprised a short nonstimulatory primary photoperiod of 6 hrs and a secondary much shorter lightperiod given as a 15-min light pulse at different times in the dark period. The light pulse 11 hrs after the basic period resulted in gonadal stimulation, while light pulse in contimuation with the basic period or 8, 14 or 21 hrs after the basicperiod was not stimulatory. The "photoinducible phase" was much more precisely outlined by shifting the birds from an 8-h to a 10-h pulse, and from an 11-h to 12-h pulse and was found to be very short, lasting about an hour falling between 11 and 12 hrs after the primary light period. The short photoinducible phase may be of an adaptive value since in the tropics the difference between the shortest and the longest daylength is also rather small (3 hrs and 15 min at Varanasi, 25degrees N). Clearly the weaver bird possesses a fine time-measuring device involving an endogenous circadian rhythm in photosensitivity. In nature, spermatogenesis in this bird also begins in March when the daylength exceeds 11 hrs (thus perhaps coinciding with the photoinducible phase). In rather small, photoperiod may not serve as a cue to trigger seasonal reproductive periodicity, it seems that photoperiod can act as a Zeitgeber for the initiation of spermatogenesis in the weaver bird at least.     

Behavioral inhibition of circadian responses to light.

Circadian locomotor rhythms in rodents may be synchronized by either photic or nonphotic events that produce phase shifts of the rhythm. Little is known, however, about how these two types of stimuli interact to produce entrainment. The well-characterized circadian photic response of the golden hamster was examined in situations where a short light pulse and locomotor activity, a nonphotic event, occurred simultaneously. Light-induced phase advances were attenuated when animals were active during light exposure. The results show that circadian responses to light depend upon the environmental situation in which the light is given, and call into question the implicit assumption in circadian rhythm research that phase shifting and entrainment to light-dark cycles depend simply on photic activation of well-known retinofugal pathways. Moreover, since light therapy is becoming an important component in the treatment of circadian-based disorders in humans, the results emphasize the need for evaluation of the behavioral aspects of light therapy protocols.     

Effects of Intermittent and Continuous Light on the Chlorophyll Formation in Etiolated Plants at Various Ages

The effect of the tissue age of dark-grown bean plants on the chlorphyll formation under continuous illumination or short impulses of white light has been studied. It was found that the protochlorophyllide present in the tissue is age-dependent and reaches a plateau at about 10 days of age, as judged by the chlorophyll formed in etiolated plants of various ages after 5 min illumination. The amount of chlorophyll a and chlorophyll b formed under short impulses of while light increases up to about 9 days of age and thereafter decreases. However, the decrease in chlorophyll a is sharper than that of chlorophyll b, the amount of which remains almost constant. The ratio of chlorophyll a lo chlorophyll b under the short impulses of white light is higher in the younger plant. Similar results are obtained after transfer of the plants from the flashing light to continuous illumination In the young plant there is no lag phase in the chlorophyll biosynthesis while as the age is increased the lag phase is evident and its duration increases as the plant ages. After protochlorophyllide phototransformation under continuous illumination the lag phase in chlorophyll biosynthesis is also age-dependent. Leaves up to 5 days old show no lag phase in chlorophyll synthesis; after this point, however, the lag phase's duration increases continuously with age.     

Phase shifts in the circadian rhythm in plasma concentrations of melatonin in rams induced by a 1-hour light pulse

The effect of a 1-hr light pulse, given at night, on the timing of the circadian rhythm in the plasma concentration of melatonin was examined in Soay rams to investigate the mechanisms involved in determining the duration of the nocturnal peak in melatonin secretion. Animals (n = 8) were housed under short days (LD 8:16) or long days (LD 16:8) and received a light pulse at various times of night. They were released into constant dim red light (DD) on day 1. Blood samples were collected hourly for 30 hr from 1000 hr on day 3, and the plasma concentration of melatonin was determined by radioimmunoassay to assess the timing of the melatonin peak. Control animals (n = 8) were maintained under the same conditions but received no light pulse. Under short days, a light pulse given early in the night caused a phase delay in the melatonin peak, and a light pulse given in the late night caused a phase advance. The mean duration of the melatonin peak was slightly reduced following a light pulse in the early or late night, and slightly increased following a pulse given near the middle of the night. Under long days, both light-pulse treatments given at night caused a phase delay in the melatonin peak, but there was no significant change in duration of the melatonin peak. The duration of the melatonin peak at day 3 under DD in the control animals was similar for all treatments, regardless of the previous entraining photoperiod (mean duration: 12.6-14.8 hr) and was similar to that under short days (14.6 hr), but was significantly longer than that under long days (8.2 hr). Information on the phase response curve in the Soay ram and on the period of the circadian oscillator governing the melatonin rhythm (c 23.0 hr under DD) predicts a close phase relationship between the end of the light phase and the onset of the melatonin peak as observed under normal 24-hr LD cycles. The current results also indicate that light acts to entrain the circadian rhythm influencing the onset and offset of melatonin secretion, and thus dictates the duration of the melatonin peak.     

Phytochrome Regulation of Flowering in the Long-Day Plant, Hyoscyamus niger

A daylength extension with incandescent light is more effective in promoting flowering of long-day plants like Hyoscyamus niger than fluorescent light. A low phytochrome photoequilibrium (Pfr/P_tot), attained by a far-red irradiation at the close of long days under fluorescent light, also promotes flowering. Moreover, if flower initiation processes are initiated by several long days, a low phytochrome photoequilibrium at the end of short, postinduction photoperiods also enhances flowering. The initiation phase of flowering requires Pfr to be present whereas the development phase proceeds more rapidly in the absence of Pfr. Spectral dependence studies, therefore, could be misinterpreted if the initiation and development stages are combined into a single audit of flowering.     

Photoperiod differentially modulates photic and nonphotic phase response curves of hamsters

Circadian pacemakers respond to light pulses with phase adjustments that allow for daily synchronization to 24-h light-dark cycles. In Syrian hamsters, Mesocricetus auratus, light-induced phase shifts are larger after entrainment to short daylengths (e.g., 10 h light:14 h dark) vs. long daylengths (e.g., 14 h light:10 h dark). The present study assessed whether photoperiodic modulation of phase resetting magnitude extends to nonphotic perturbations of the circadian rhythm and, if so, whether the relationship parallels that of photic responses. Male Syrian hamsters, entrained for 31 days to either short or long daylengths, were transferred to novel wheel running cages for 2 h at times spanning the entire circadian cycle. Phase shifts induced by this stimulus varied with the circadian time of exposure, but the amplitude of the resulting phase response curve was not markedly influenced by photoperiod. Previously reported photoperiodic effects on photic phase resetting were verified under the current paradigm using 15-min light pulses. Photoperiodic modulation of phase resetting magnitude is input specific and may reflect alterations in the transmission of photic stimuli.     

Intrinsic variability in the frequency of embryonic diapauses of the annual fish Nothobranchius korthausae,regulated by light:dark cycle and temperature

Synopsis The influence of the light:dark cycle and temperature on the embryonic development, especially diapause, of the annual fish Nothobranchius korthausae was investigated. The variability of the frequency of diapause II during constant L:D cycle and temperature, but at different times of the year, was also studied. In agreement with previous studies it appeared that diapause I does practically not occur in N. korthausae (and other Nothobranchius species), even under sub-optimal conditions which are known to induce diapause II and III. Only at very low temperatures, a first developmental arrest could be induced during the dispersed phase, as well as during the reaggregation phase. Diapause II and III can be induced more easily, by exposing the embryos to low (sub-optimal) temperatures and short light periods (or constant darkness). Both diapauses are inhibited or terminated at high temperature and long light periods (12L:12D). The occurrence of an indirect light:dark response via the adult fishes could not be demonstrated. At moderate temperatures and in constant darkness (or short light periods) considerable variability in diapause II-frequency was observed. This could be an intrinsic feature of annual fish development, constituting a strategy for better survival of the species.     

THE SIGNIFICANCE OF THE LIGHT AND DARK PHASES IN THE PHOTOPERIODIC CONTROL OF DIAPAUSE IN METATETRANYCHUS ULMI KOCH

The daily cycle of illumination is one of several agencies which control the onset of diapause in Metatetranychus ulmi. Both light and dark phases in the cycle are concerned in the determination process.
In general, a long light phase tends to suppress and a long dark phase to induce a diapause. In any combination, the path of development is decided by the balance between diapause-preventing (light phase) and diapause-inducing (dark phase) stimuli. However, as their effectiveness does not increase linearly with duration, the existing balance changes with the phase duration.
The effectiveness of the light phase in suppressing diapause increases most rapidly between 8 and 16 hr.; that of the dark phase rises very sharply between 8 and 12 hr. Longer dark periods of up to several days duration also induce diapause but are no more effective than a 13 hr. phase. The inclusion in the cycle of very long periods of light or darkness may also influence diapause by reducing the number of complementary phases experienced by the mite during the sensitive period of development.
M. ulmi is highly insensitive to the interruption of effective light and dark phases by short intervals of darkness or light—a further indication of the slow inception of the light-and dark-phase reactions.
These findings are discussed in terms of hypothetical mechanism involving cumulative synthesis and removal of some active substance, but the experimental results cannot yet be fully reconciled with a simple hypothesis of this kind.     

Biological adaptability under seasonal variation of light/dark cycles

3A substantial amount of experimental models designed to understand rhythms entrainment and the effects of different regimens of light exposure on health have been proposed. However, many of them do not relate to what occurs in real life. Our objective was to evaluate the influence of “seasonal-like” variation in light/dark cycles on biological rhythms. Twenty adult male Wistar rats were assigned to three groups: control (CT), kept in 12:12 light/dark (LD) cycle; long photoperiod/short photoperiod (LP/SP), kept in 16.5:7.5 LD cycle for 18 days (phase A), then 17 days of gradual reductions in light time (phase B), then 18 days of shorter exposure (7.5:16.5 LD cycle, phase C); short photoperiod/long photoperiod (SP/LP) group, with same modifications as the LP/SP group, but in reverse order, starting phase A in 7.5:16.5 LD cycle. Activity and temperature were recorded constantly, and melatonin and cortisol concentrations were measured twice. Activity and temperature acrophases of all groups changed according to light. The correlation between activity and temperature was, overall, significantly lower for SP/LP group compared with LP/SP and CT groups. Regarding melatonin concentration, LP/SP group showed significant positive correlation between phase A and C (p = 0.018). Animals changed temperature and activity according to photoperiod and demonstrated better adaptability in transitioning from long to short photoperiod. Since this model imitates seasonal variation in light in a species that is largely used in behavioral experiments, it reveals promising methods to improve the reliability of experimental models and of further environmental health research.     

Relationship between phase resetting and the free-running period in Djungarian hamsters.

A data set of 293 phase shifts was analyzed in order to determine the relationship between phase resetting and the free-running period (tau) in Djungarian hamsters. Phase shifts in response to a 15-min light pulse were assigned to one of two groups (tau short, less than 24 hr; tau long, greater than 24 hr), and two phase response curves (PRCs) were constructed. The two PRCs differed predominantly in the advance region, which extended so far into the subjective day of PRClong that a dead zone was lacking. The functional significance of PRC differences was assessed by computer simulations of entrainment to varying skeleton photoperiods and entrainment to a 12-hr skeleton photoperiod with varying tau's. Results from these simulations confirmed the theoretical predictions by Pittendrigh and Daan: Stability of entrainment under varying photoperiods depended on the ratio of the PRC slopes at the phases illuminated by light (SE/SM). This ratio was always larger than 1 for PRClong. It approached 0 for PRCshort as soon as the evening light illuminated the dead zone; this occurred for entrainment to very short photoperiods. Stability of entrainment to lights-off was in general better for PRClong than for PRCshort, especially if PRClong was used in combination with tau long. This suggests that it can be advantageous for stability of entrainment to lights-off to express a tau greater than 24 hr in combination with a PRC lacking a dead zone. Stability of entrainment under varying tau's was not much different for PRClong or PRCshort. However, stability of entrainment deteriorated for PRClong in combination with short tau's, whereas it deteriorated for PRCshort in combination with long tau's.     

Light Adaptation in the Ventral Photoreceptor of Limulus

Light adaptation in both the ventral photoreceptor and the lateral eye photoreceptor is a complex process consisting of at least two phases. One phase, which we call the rapid phase of adaptation, occurs whenever there is temporal overlap of the discrete waves that compose a light response. The recovery from the rapid phase of adaptation follows an exponential time-course with a time constant of approximately 75 ms at 21°C. The rapid phase of adaptation occurs at light intensities barely above discrete wave threshold as well as at substantially higher light intensities with the same recovery time-course at all intensities. It occurs in voltage-clamped and unclamped photoreceptors. The kinetics of the rapid phase of adaptation is closely correlated to the photocurrent which appears to initiate it after a short delay. The rapid phase of adaptation is probably identical to what is called the "adapting bump" process. At light intensities greater than about 10 times discrete wave threshold another phase of light adaptation occurs. It develops slowly over a period of ½ s or so, and decays even more slowly over a period of several seconds. It is graded with light intensity and occurs in both voltage-clamped and unclamped photoreceptors. We call this the slow phase of light adaptation.     

Different circadian rhythms regulate photoperiodic flowering response and leaf movement in Pharbitis nil (L.) Choisy

The phase shifting effect of red light on both the leaf movement rhythm, and on the rhythm of responsiveness of photoperiodic flower induction towards short light breaks (10 min red light), has been studied in Pharbitis nil, strain Violet, and comparisons between the two rhythms have been made. The phase angle differences between the rhythms after a phase shift with 2 or 6 h of red light given at different times during a long dark period were not constant. The results indicate the involvement of two different clocks controlling leaf movement and photoperiodic flower induction.Abbreviations DD continuous darkness - l:D x:y light/dark cycles with x hours of light and y hours of darkness - PPR rhythm of photoperiodic responsiveness towards light break     

Latitudinal clines in the properties of a circadian pacemaker

The circadian rhythm of eclosion activity and its pacemaker were analyzed in a series of latitudinal races of Drosophila auraria ranging from 34.2 degrees to 42.9 degrees N in Japan. The phase of the rhythm (psi EL) to the daily photoperiod (PP) changes as daylength is increased, and the slope of psi EL (PP) changes with latitude. Is is sufficiently greater in the north to cause a phase reversal of northern and southern races on long versus short photoperiods. This reversal is found in assays of the pacemaker's phase (psi PL) as well as that of the rhythm (psi EL). Assay of the pacemaker shows that its period (tau) is longer in northern than in southern races, and that the amplitude of its phase response curve (PRC) is lower in the north. The period of the rhythm in all latitudinal races is longer than 24 hr in short photoperiods (LD 1:23), but is probably less than 24 hr (as an aftereffect of photoperiod) in longer days such as LD 14:10. The observed north-south differences in the phase relation of both pacemaker and rhythm to the light cycle are explained by the latitudinal clines in pacemaker properties and a postulated aftereffect of photoperiod on tau. It is suggested that the latitudinal cline in PRC amplitude has functional significance in conserving the amplitude of the pacemaker's signal to the rest of the system it times. Computer simulation shows that without such a reduction in the perceived light intensity, pacemaker amplitude will be lowered by the increase in duration of the daily light at higher latitudes.     

Characterization of circadian function in Djungarian hamsters insensitive to short day photoperiod

Summary Djungarian hamsters (Phodopus sungorus sungorus) depend mainly on day length to cue seasonal adjustments. However, not all individuals respond to short day conditions. A previous study from this laboratory proposed that nonresponsiveness to short day conditions rests with a defect in the circadian organization of these hamsters.In this study we found pronounced differences between responsive and nonresponsive hamsters in the expression of circadian rhythmicity under constant darkness and under constant illumination. While responsive hamsters showed a free-running activity pattern with a period of 23.86+0.04 h and responded to brief light pulses with the expected phase delays and phase advances, nonresponsive hamsters exhibited a period of 24.04+0.05 h and responded to light pulses with phase advances. Furthermore, 9 out of 15 responsive hamsters showed a clear split in the activity pattern within 8 weeks under constant light (80–100 lux), while only 1 of the 7 nonresponsive hamsters exhibited a split activity pattern. As a result of these differences in circadian function, nonresponsive Djungarian hamsters are incapable of proper photoperiod time measurement and photoperiod-induced seasonality.Abbreviations PRC phase response curve - ct circadian time - DD constant dark - LL constant light     

Some Observations on a Circadian Rhythm in Carbon Dioxide Compensation in Bryophyllum fedtschenkoi

Detached shoots of Bryophyllum fedtschenkoi maintained in continuouslight and at a constant temperature exhibit a circadian rhythmin CO₂ compensation. The rhythm has a period of 21.6±0.1h at 23 °C and its phase can be set and shifted by suitabletreatments. These observations suggest that the ryhthm is trulyendogenous in nature. The phase is set by a light-on stimulus, or a drop in lightintensity. Phase shift can be induced by short periods (3 h)of reduced light intensity as well as similar periods of darknessgiven during the day phase of the rhythm. A change from whitelight to the same number of incident quanta of monochromaticlight of 450 nm (blue), 550 nm (green), or 652 nm (red) alwaysshifted the phase, but the degree of shift was less in red lightthan in blue or green. The plant reacted to achange to blueor green light as though it had been placed in the dark. Althbughred light was apparently ‘recognized’, the changefrom white to red was still sufficient to alter the phase ofthe rhythm. The possible role of phosphoenolpyruvate carboxylase activityin controlling the rhythm is discussed.     

The Circadian Rhythm of Leaf Movement of Coleus blumei x C. frederici, a Short Day Plant. II. The Effects of Light and Temperature Signals

The phase response curve for the circadian rhythm of leaf movement of Coleus blumei x C. frederici, a short day plant, is generally similar to those reported for other organisms. An increase in the duration of the light signal caused an increase in the extreme values of the phase response curve and shortened the time for transition from maximum delays to maximum advances. Experiments with 2 light signals showed that the overt rhythm of leaf movement represents the rhythm of the light sensitive oscillator even during the transient period that followed the first light signal. A temperature decrease of 7° for 8 hr caused only a transient phase shift in the following 2 cycles but not in the steady state. The combination of such a temperature decrease and a light signal showed that only the overt rhythm of leaf movement was disturbed by the temperature decrease whereas the light sensitive oscillator was free running. A temperature decrease of 11° for 10 hr caused a steady state phase shift and affected the light sensitive oscillator as well.     

Action Spectrum for Resetting the Circadian Phototaxis Rhythm in the CW15 Strain of Chlamydomonas: I. Cells in Darkness

We have developed protocols for phase shifting the circadian rhythm of Chlamydomonas reinhardtii by light pulses. This paper describes the photobiology of phase-resetting the Chlamydomonas clock by brief (3 seconds to 15 minutes) light pulses administered during a 24 hour dark period. Its action spectrum exhibited two prominent peaks, at 520 and 660 nanometers. The fluence at 520 nanometers required to elicit a 4 hour phase shift was 0.2 millimole photon per square meter, but the pigment that is participating in resetting the clock under these conditions is unknown. The fluence needed at 660 nanomoles to induce a 4 hour phase shift was 0.1 millimole photon per square meter, which is comparable with that needed to induce the typical low fluence rate response of phytochrome in higher plants. However, the phase shift by red light (660 nanometers) was not diminished by subsequent administration of far-red light (730 nanometers), even if the red light pulse was as short as 0.1 second. This constitutes the first report of a regulatory action by red light in Chlamydomonas.